Trans 1MarE, Vol 107, Parr?,pp 27-45
Ships and offshore cabling
where to now?
REVIEW OF CABLE STANDARDS
Naval cables
Understandably, perhaps, the Royal Navy took the lead in the late 1970s and 1980s when they introduced a new family of Naval Engineering Standards (NES). What is perhaps not so readily understood is the fact that the most radical change involves that of the sheathing compound. NES 518 covered its sheathing compound requirements, later to become De-fence Standard 61-12 Pt 31, with actual cable constructions covered by NES525, 526 and 527. Included in this family of standards are the specialised tests required to check compli-ance, such as NES711 and 713.
These standards introduced the term 'Low Fire Hazard' (LFH). NES525 provided thin wall insulation systems for up to 2.5 mm2 conductors, based upon engineering polymers or irradiation technology with zero halogen, low smoke (LFH) sheaths. It included a toxicity requirement covering all the materials used in the construction of these cables.
These cables save on both weight and space when used to
connect the complex electronics installed high up in the
superstructure of modern warships or throughout the hulls of modern submarines.
NES525 was itself converted to a Defence Standard: Def
Stan 61-12 Pt 25 in 1990.
NES526 replaces DGS212 covering a more standard, eas-ily recognised ship's cable, but still having such advanced features as dual wall, oil resistant, flame retardant insulations
P Waterworth
Delta Crompton Cables Ltd, UK
Fortune has not smiled kindly upon the UK Shipbuilding Industry, either the commercial or the military aspects, and the Offshore Industry, once seen as the salvation, is now itself at a
fairly low ebb. Driven by buoyant markets and sales, developments in both materials and the actual construction of electric cables advanced during the 1980s. Many new standards
were prepared and issued, existing standards were updated - NES525 and 526, BS6883, the
IEC92-35X and 27X series being typical examples. These are reviewed and their merits
discussed. During this review the reaction to fire that each type of cable exhibits must also be considered. What, contentiously, needs to be asked is: Has the industry gone too far when
addressing this particular aspect? Has it sacrificed too many of the virtues, that it perhaps
took for granted, ie ease of installation, toughness and performance in fluids, in the search
for the smallest, lightest, 'best' fire performance cable, or is the industry just selecting the
wrong type of cable for the particular application? One area not tackled very scientifically
during the advances of the 1980s is that of current rating relating to cables. A comparison
between onshore and offshore practices is made. Finally, an attempt will be made to answer
the following questions: Whereto now? What new materials and manufacturing processes are becoming available? Does a subdued Marine Industry want them or need them and,
finally, can it afford them?
(see Fig 1) in place of standard butyl or ethylene propylene rubber (EPR); these cores are contained within the same type
of LFH sheath and covered by the same overall toxicity
requirement.
The Navy's current circuit integrity, fire survival, cable is
still based upon the well tried and tested silicone rubber extrusion, glass fibre braided core or, in the case of larger
sizes, multiple layers of a silicone rubber impregnated glass
fibre tape. The sheath is the same as in the former two
standards, complying with Def Stan 61-12 Pt 31.
Perhaps surprisingly, the current test for fire survival
specified by the Ministry of Defence (Navy) (MOD(N)) is similar to the popular IEC331 but with a flame temperature
of 650°C.
Reviewing the sheathing material in more detail the
MOD(N) drew up quite stringent requirements when com-piling NES518, the combination of requirements and bal-ance of properties exceeding those of any other material in service throughout the world.
The requirements laid down cover virtually all situations a shipboard cable is ever likely to encounter. The physical properties specified match those of the successful heat, oil and flame retardant (HOFR) compounds but have, in addi-tion, excellent resistance to fuel oil, hydraulic and lubricat-ing oils, as well as water, both distilled and salt.
If this in itself was not enough the compound, as the term LFH suggests, exhibits a very useful balance of properties in the event of a fire; flame retardancy, low evolution of smoke
which is halogen free, as well as fumes considered less
harmful to humans.
Author's Biography
Peter Waterworth is the Technical Manager for the Special Cables Division of Delta Crompton Cables Ltd, based at Derby. He has over 25 years experience of the Cable Industry, having been employed by several manufacturers of ships and offshore cabling. He is a convenor or member of IEC, CENELEC, BSI and TEE committees and working groups relating to cables.
P Ware rwoeth
28
Offshore cables
The Oil Industry in its growth period produced a variety of in-house standards and specifications supplementedby, bu t
regrettably not replaced with, United Kingdom Offshore
Operators Association (UKOOA) Guidance Documents
cov-ering both instrumentation and control cables, as well as
power and control cables.
Table II BS 6883 thickness of insulation: comparison of 600/1000V cables to 1969 and1991 editions
Fig 1 Large three-core naval cable with dual wall insulation system
NB: The values in the1991edition now align with those inIEC92-350.
BS 7655 TypeRS 3 (ie HOER)compound. Typically, this material was standard CSP or CPE.
BS 6883: 1991offers a choice of the following four sheathing materials, which possess different degrees of improvement overRS 3:
Type A As RS 3 Type B MRS 3 Type C As RS 3 Type D As RS 3
+ enhancedoilresistance
+ enhanced oil resistance + halogen acid gas emission5 5%
+ tear resistance55.0N/mm
+ halogen acid gas emission5 0.5%
+ reduced smoke emission + elongation at break 5 150%
+ enhanced oil resistance
+ halogen acid gas emission 5 0.5% + reduced smoke emission + elongation at break n 50%
Cable types introduced during the 1980s have tended to follow the trend set in the particular practice applicable to
the actual installation, fixed platforms broadly following land based practice, with 'mobiles' and 'floaters' firmly in
the Marine Engineering camp.
Therefore, as has happened onshore, simple PVC/ SWA/
PVC BS6346 armoured cables have given way to XLPe/
OHLS/SWA/OHLS BS6724 types. The shortcomings of the
early varieties of thermoplastic zero halogen, low smoke
(OHLS) compounds have now largely been addressed and properties currently match those of standard PVCs -except-ing that of oil resistance. It should be noted that this thermo-plastic OHLS should not be confused with elastomeric OHLS
or LFH.
Being insulated with cross-linked polyethylene (XLPe)
the maximum allowable conductor temperature can be as
high as 90°C, against that of 70°C for PVC, with the conse-Table I Summary of important new features of BS 6883: 1991 Table III Insulation and sheathing in accordance with BS 6883:
1991 edition
IL 150/250Vpairs and triples with individual and/or collective
metalled tape screening. Insulation
2. 600/1000V 2,3and4core power cables and controlcables
containingup to48cores with or without metallic braid. High voltage cables up to 10/15kV.
TheEPRinsulation required by the1991edition of BS6883enables the cable to be operated at a continuous conductor temperature of90°C.
4.
5.
EVEinsulation now rated at90°C.
Choice of four sheathing materials, including two low smoke zero
The 5°C increase over the previous issue of this specification is reflected in higher current ratings.
halogentypes.
Detailedcable identification on outer surface.
Cables made to BS6883: 1969were required to be sheathed with a
7. Tests for flame propagation on bunched cables(BS 4066:Part 3).
Nominal area of conductor Thickness of insulation BS6883:1969 BS6883: 1991 inm2 min 1)1771 1.0 0.8 0.8 1.5 BS 0.8 2.5 0.8 0.8 4 1.0 1.0 6 1.0 1.0 10 1.2 1.0 16 1.2 1.0 25 1.4 1.2 35 1.4 1.2 50 1.6 1.4 70 1.6 1.4 95 1.8 1.6 120 1.8 1.6 150 2.0 IS 185 22 2.0 240 2.4 2.2 300 2.6 2.4 400 2.8 2.6 500 3.0 2.8 630 3.0 2.8 Sheath
materials
Fig 2 Operating temperature envelope for therrnoplastics
Materials
Fig 3 Operating temperature envelope for elastomers
quential beneficial effect upon the current rating assigned to a particular cable.
This family of cables is lighter and smaller than either PVC or marine types but has much higher minimum bend radii due,
in the main, to the single wire armour (SWA), termination techniques which are comparable to standard ship types.
Commercial ships' cables
In the commercial Marine Engineering camp, BS6883: 1969 has been radically updated, with the publication and issue of the 1991 edition.
This much enlarged document now covers 150/250V
pairs and triples, high voltage power cables for use in
earthed neutral systems, up to and including 15 kV,as well
Trans 1MarE, Vol 107, Part 1, pp 27-45
BR
PAIn Temp, Reel
0 Max Operating Temp.
0 Min. Temp. Fixed 0 Mex. Operating Temp.
as retaining the 600/1000V power cables of the superseded standard. Opportunity was also taken to uprate the insula-tion material, EPR, from 85°C continuous conductor tem-perature to 90°C and align radial thicknesses with the Inter-national Electrotechnical Commission (IEC) standards. Four
elastomeric sheathing compounds are now available,
in-cluding two OH LS types. Flame propagation, as determined
by IEC 332-3/BS4066: Pt 3, is also addressed. Tables 1,11 and
III give a more detailed review.
Internationally, IEC TC 18 SC 18A is making gradual but slow progress in converting the old IEC92-3 standard into a new multipart 1EC 92-35X and IEC 92-37X series of stand-ards.
These standards now derive the radial thickness of insu-lation used from IEC502. These are less than those tradition-ally found in the marine environment and certain countries 29
P Waterloo*
Table IV Status of tests on cables for reaction to fire
30
IECpublication Further work inIEC CENELECTC20 position British Standard position
Tests for flame propagation IEC 332- Tests on electric cables under fire conditions
Part 1(1993):
-
HD 405.1 to be BS 4066: Part 1Test on a single vertical rewritten as an EN implements HD 405.1
insulated wire or cable
Part 2(1989): HD 405.2 endorses BS 4066: Part 2
Test on a single small IEC 332-2 implements HD405.2
vertical insulated copper wire or cable
Part 3 (1992): 20(SEC)20 HD 405.3 endorses BS4066: Part 3
Tests on bunched (amendments plus conversion IEC 332-3 implements HD 405.3
wires or cables to International Standards) Tests for fire resistance
IEC 331 (1970): Proposed revision Draft for requirements New edition of
Fire resisting 20(SEC)23 relating to construction 856387 published
characteristics Products Directive
of electric cables (CLC/TC20(SEC)972)
approved for UAP Tests for evolution of acidic and corrosive gases
IEC 754 - Test on gases evolved during combustion of materials from cables
Part 1(1994): Revision published Agreed to issue as an BS 6425: Part 1 is
Determination of the EN based on the IEC
amount of halogen acid gas document
Part 2 (1991): Minor amendments HD 602 endorses BS6425: Part 2
Determination of degree proposed in IEC 754-2 implements HD602
of acidity of gasesevolved'
duringthe combustion of materials taken from electric cables by measuring pH and conductivity
Tests for smoke opacity
IEC 1034:
Measurement of smoke density of electric cables burning under defined conditions
Part 1(1990): Revision proposed - HD 606.1 endorses BS7622: Part 1
Test apparatus 20C(SEC)21 IEC 1034-1 implements HD 606.1
Part 2 (1991): Revision proposed Published as HD606.2 BS 7622: Part 2
Test procedure and 20C(SEC)22 with modification to implements HD 606.2
requirements include cable sizes down
to 2 mm overall diameter
AM D1-1993
5. ''Tests for toxicity
None approved IEC SC20C requested by ACOS to examine subject in collaborationwith ICTC89 IEC TC89 WG7 has
working drafts
Awaits 1EC. Some existing
(national) specifications called up for PPD documents
WC;12 (Railways) looking at
UITP method in interim
Will reflect IEC/
CENELEC work
-NB: Publication 1liC695-7: Guidance on the minimisation of toxic hazards due to fires involving electrotechnical products - Section 1: General, is recommended reading for those having an interest in this topic.
2.
3.
have commented unfavourably upon this. As would be ex-pected perhaps, being truly international, the scope of these standards goes beyond that of BS6883 even in its latest form.
Materials allowed for insulation include PVC, EPR, XLPe, silicone rubber and, at present in draft form, an irradiated
polyolefin insulation system favoured by France.
For sheaths two PVCs, polychloroprene (PCP),
chlorosulphonated polyethylene/chlorinated polyethylene(CSP/CPe) and, again in draft form, two OHLS materials, a thermoplastic favoured by France and an elastomeric pro-moted by the UK. Appendix A lists the standards issued to
date and the new work being considered. Figures 2 and 3
show the operating temperature envelopes for thermoplas-tics and elastomers, respectively.
Various thin wall cables have been offered and have
found use in commercial shipping and in the offshore indus-try, however high their costs and, unless special techniques
are adopted, problems with terminations at present out-weigh any size or out-weight savings, and has limited
wide-spread adoption of these types.
REACTION TO FIRE
It would, perhaps, be a good idea to take this opportunity to review the arena of 'Reaction to fire' and try to arrange the cables previously discussed into an order of merit.
In the UK, Europe and to a lesser extent the rest of the world, cable makers and users have, through the vehicle of BSI CIL/ 20,CENELECTC 20 (European Committee forElectrotechnical
Standardization Technical Committee 20) and IEC TC 20, split a cable's reaction to fire into five distinct categories:
Flame propagation. The property that determines the rate at which fires develop and spread.
Fire resistance. The maintenance of electrical circuit in-tegrity under fire conditions.
Smoke opacity. The property that determines the time
personnel have to evacuate a ship or installation and, most
importantly, mount an effective fire-fighting operation.
Acidic and corrosive gas evolution. The property that de-termines the propensity to corrosion of structure and
equipment during or after a fire.
Toxicity. In a joint paper' presented some two years ago,
it was stated that toxicity is: 'a very emotive subject, whereby any discussion of toxic emissions usually
generates many conflicting views'. Nothing has
happened in the intervening period to change this
view.
Table IV gives a list of some common standards relating to reaction to fire properties.
The principle presently adopted by Technical
Commit-tees' (TC) dealing with cables is one whereby the main
committee (TC20) lays down the type of test, the equipment
to be used and general methodology to be followed. The actual product working party then lays down the pass/fail criteria, based upon their more expert knowledge of the
application and duty.
Flame propagation
The 1980s saw a move from flame retardant to reduced
propagation, the Limiting Oxygen Index (LOT) being
aug-mented with the Flammability Temperature Index (FTI).
Both are useful laboratory scale indicators but on their own,
as a means of assessing a cable's fire performance, their value
is limited.
The simple, single sample, flame propagation test
re-mains IEC332-1BS4066-1, although in the 1993 edition the
burner is more closely defined and its output is now a
nominal 1 kW flame.
The LOT test is included in the updated and revised IEC332-3: 1992 BS4066 Pt 3: 1994 but only as a relatively
simple routine check on the components of a cable, to
re-move the need to repeat the large full-scale type test.
Reviewing the changes to this document it is perhaps
wise to point out and highlight certain sections: within the National Foreword:
'It should be noted that IEC332-3: 1992 is a Technical Report of Type 2. It is not to be regarded as an international standard. '(Author's italics.)
within the Scope:
'Three categories are defined and distinguished by test duration, and the volume of non-metallic material of
the sample under test; they are not necessarily related to different safety levels in actual cable installations.'
(Au-thor's italics.)
The Introduction should also be read in its entirety as it offers guidance on features to be considered, which have a bearing upon propagation of flame along a bunch of cables.
The main changes to this document in its latest form
concern the manner in which the cable samples are mounted on the ladder.
Category A, the 7 1/m of non-metallic material now
having two designations for the method of mounting, now becomes:
332-3A F/R 300 mm wide ladder
332-3A F 600 mm wide ladder
332-A3B F 300 mm wide ladder
332-3C F 300 mm wide ladder
with the F/R signifying cables are mounted on both the front
and rear of the ladder, and F signifying front mounted
only. As can be seen, Categories B and C remain
substan-tially unchanged. Table V gives a diagrammatic explana-tion.
The document rather usefully now includes Table I: 'Sum-mary of test conditions' and Table II: 'Sum'Sum-mary of guidance data for the selection of cables for type approval tests'. Both are reproduced as Appendices B and C.
What has not changed markedly is the mistaken belief that Category A is in some way the most severe test. For
certain cables this would be the case: typically large cables
mounted and spaced, front and rear, thus providing the
classic 'chimney effect' for the 70 000 BTU flame. For other cables, however, mounted more akin to ships and offshore Trans 1MarE, Vol 107, Part 1, pp 27-45
31
-3., 4, 5.-P Waterworlh
-Table V Revised IEC 332-3installation configurations,
practice, that is to say in one large bundle on the front of the ladder, it is not.
This leaves the specifier or installer with the age old
dilemma does the test one intended to simulate and repro-duce accurately, provide actual installation conditions and therefore gives an exact measure of how a cable will perform in service, or is it merely a test to confirm the cable has been
manufactured to the correct specification? Certainly the
extracts mentioned previously would point away from the 'former.
Practically speaking, what is clear is that extensive testing by both manufacturers and approval authorities2has shown that where cable is installed in large groups or bundles with little if any spacing, as is usually the case in the majority of marine installations, then propagation is greatly reduced.
Also, elastomeric insulated and sheathed cables tend to
perform better than XLPe insulated, thermoplastic zero halogen sheath types and that the inclusion of a metallic
.layer,, be it SWA or braid, also helps.
Fire resistance
as
far as ships are concerned not much has happened duringthe past 10 years, the test for electrical circuit integrity
remaining IEC331, which is a single temperature (750°CX fixed time test not involving impact or water spray.
Driven by the European Construction Products Directive (CPD) work is in hand internationally to correct what is, in
'essence, a fairly simple, almost crude test. Opportunity is
N32
being taken to allow the flame application time and
tempera-ture to vary by the adoption of a standard burner fed with
carefully controlled gas and air flow rates. The burner is also repositioned to avoid blockage by falling debris.
Cables meeting Naval Standards based upon silicone/
glass insulations would meet a standard 750°C, IEC331 test but, partly because of a lack of metallic braid armour would not always pass the enhanced 1000°C test.
In the offshore arena the spectre of a hydrocarbon fire caused the simple IEC331 test method to be upgraded to
increase the flame temperature from 750 to 1000°C (actually 950°C ± 50°C). Also, as the distance between new oil fields and land increased, so too did the required levels of safety. BP with their Magnus Field were one of the first compa-nies to address fire, impact and water, which resulted in the
development and issue of BP Engineering Standards 235
and 236 which remain current to this day.
These standards should not be confused with BS6387 in that one sample of cable is subject to a 1000°C flame for 3h, whilst subject to mechanical impact directly onto the cable, followed by a water spray of 151/min for 5 min. Needless to
say cables meeting these extremely onerous requirements are of a costly special design, incorporating metallic tape barriers or thermal barrier tapes under the braid armour.
BS6387 was developed and issued for onshore use in fire alarm, detection and emergency lighting circuits. Updated and re-issued in 1994 this standard allows a matrix of flame
temperatures, ie 650°C, 750°C and 950°C as a stand alone fire
test, with symbols A, B,C and S, the latter symbol signifying
a short 20 min test. A test is included of a 650°C fire with
_
Category
<
35mm2> 35mm2
, 1 1AJF
_ t _00 Cl UGULI 0
taillmr,
1 I:A/F/R
=
reammetei
111.0.111.111.111,11111116MIS
Oft),
lifeellifilo 0realt
_ 1 I Iree66565etallUU00
f6M6656644651C
. LCCOCOrri ri 1 { I0 0
, ,Fig 4 Small two-core circuit integrity cable with mica/glass taped conductors
water spray, symbol W, and finally fire, with impact at the three temperatures mentioned previously with symbols X, Y, Z. The most severe requirement is categories C, W, Z.
It has to be stated, however, that these three tests are
conducted on three separate samples of cable. Also, a further limitation of the standard is that it is only applicable to cables up to 450/750V and of small diameter, typically up to 21 mm overall diameter. Where cables are to be subject to impact they are mounted on a fire resisting board, and it is this board that is subject to the impact upon its upper edge, no direct contact being made with the cable.
Where cables are to be subject to water spray they are clipped to the front edge of two parallel metal strips and
water is applied via a commercial fire sprinkler.
This standard has now found its way into both ships and
the offshore field but, as stated, its value to ships and
offshore cabling is limited due to its size limitation and
careful thought is required when calling it up.
To obtain circuit integrity in the offshore market, early
designs relied upon the silicone/glass insulation system. Its shortcomings were very quickly highlighted and alternative
systems sought. From this early work emerged the almost universally accepted mica/glass taped fire barrier applied
directly onto the conductor over which is extruded an EPR or XLPe insulation (see Fig 4).
At various times people have made claims that a cheaper
cable has been produced using PVC as the insulating layer over the mica/glass tape. In the author's opinion this is highly improbable due to the fact that halogen ions are conductive in the extreme, and, enclosed within the confines of a cable
sheath, dramatically lower the insulation resistance by form-ing a conductive path through the mica/glass tape.
Perfectly acceptable for use in 600/1000V and 1900/
3300V cables, and even 4150V systems, problems start to
appear when this construction is applied to 3600/6000V cables and above. The inclusion of the mica/glass tape
prevents the application of the normal semiconducting con-ductor screen and therefore unwelcome discharges can make their presence felt over a prolonged period of use.
Fortuitously the effect of these discharges has little effect in the short term and so this design of cable can be considered for
'short life' applications, typically emergency fire pumps and
the like. For cables which are energised continually there
would appear to be problems in addition to those mentioned
previously, but these can be overcome. In the main, these high
voltage cables tend to be quite large by their very nature, and the consequential high thermal inertia can be used to good
effect; thermal barrier layers can be incorporated into theouter layers of these cables and a 3h rating achieved withoutcorn
pro-mising the electrical integrity of the inner insulation.
Fire testing of these cables at high voltage can also be problematic due to safety and equipment considerations.
Smoke opacity
In the early days of this inexact science levels of smoke were measured by testing small samples of individual cable com-ponents, such tests being the Arapahoe Smoke Chamber test and the NBS Smoke Chamber test.
Essentially both of these tests subject a small plaque of
material (100 mm x 100 mm) to a source of heat, either
flaming or radiant. In the case of the Arapahoe test the smoke evolved is passed through a simple filter and the amount of
debris collected, thereon weighed and expressed as a
per-centage of the initial plaque. In the case of the more popular and widely accepted N BS Smoke Chamber test the level of smoke evolved is measured optically.
Although still accepted and used by both the MOD(N) and the USA to predict and classify the smoke evolution
performance of complete cables, within the UK and Europe their use, although valuable, has been relegated to that of a laboratory development tool.
The European Cable Industry is now quite firmly
com-mitted to the use of the '3m cube' to test samples of the
complete cable as would be supplied to the user. IEC 1034 Pt 1 and Pt 2 are now reflected in both a CENELEC Harmoni-sation Document (HD) and a British Standard, BS7622 Pt 1
and Pt 2.
In simplistic terms when a cable burns it is the actual base polymer and processing aids (oils and waxes) that contrib-ute to the evolution of smoke. The materials used as fillers, typically chalks or whiting, do not contribute appreciably to
the levels of smoke generated. To make a low smoke'
compound therefore, one has merely to remove polymer and process aids, to be replaced with a higher loading of inert filler.
Unfortunately, as history has repeatedly proven, life is
never so simple. Whilst following this route would produce
a 'low smoke' cable other properties would suffer. The compound produced by this action would have much re-duced physical properties, tensile strength, elongation at break and, importantly in the marine environment, tear
strength.
Also, performance in fluids would suffer, with fluid uptake increased due to the hygroscopic nature of simple
untreated fillers.
Because of this a more complex approach is taken,
involv-ing the selection of polymers that produce light, white
smoke rather than the dense black smoke exhibited by such
materials as natural rubber or the chlorinated polymers.
Polymers that will accept higher filler loadings with
mini-mal effect upon ultimate physical properties are also very
useful. Active fillers such as aluminium trihydrate are cho-sen, as these also contribute to reducing flame propagation and can aid reduction of acidic and corrosive gas evolution Trans IMarE, Vol 107, Part Ic pp 27-45
P Waterworth
by way of dilution. Filler particle size, as well as active
surface area, all play a part, as do various coupling agents. Finally, the size of the smoke particles produced also has an important part to play in preventing obscuration, a few large agglomerated particles being preferred to a myriad of small particles, the analogy being heavy rain and fog.
What is clear is that the traditional cable sheathing
mate-rials, such as
polychloroprene
(PCP/OFR),
chlorosulphonated polyethylene or chlorinated polyethylene
(CSP-CPe/HOFR), in unmodified form do produce dense
black smoke when involved in a fire scenario, although they have good physical properties. This smoke evolution can be
reduced by the careful selection of compounding
ingredi-ents, but ultimate physical properties do suffer. These
mate-rials can also never be compounded in a practical form to match the low levels of smoke emission exhibited by the ethylene vinyl acetates (EVAs), ethylene methacrylates
(EM_As) and similar polymers.
Various 'low smoke' materials are now commonly avail-able in both the military and commercial market places.
Acidic and corrosive gas evolution
This property is closely linked to flame propagation, smoke opacity and toxicity.
Early test protocols (IEC 754 Pt 1 BS6425 Pt 1) pyrolised a small sample of material in a known and controlled air flow; the gases evolved were bubbled through a sodium hydroxide solution and then back titrated to express the
result in terms of hydrochloric acid (HCI). A limit of 0.5% maximum acid gas is usually applied.
More modem thinking has produced IEC754 Pt 2 BS6425
Pt 2, which follows a similar test protocol, but in this case the gases are bubbled through distilled water from which the
pH and conductivity are determined. Recommended values are a pH not less than 4.3 and a maximum conductivity of
10 microsiemens/ mm.
All of the halogenated polymer systems fail both parts of this test, ie PCP, CSP, CPe as well as PVC. Insulations based upon EPR and XLPe pass, when properly compounded, as do sheaths based upon EVA or EMA, as well as
hydrogen-ated nitrile butadiene rubber (HNBR) or nitrile butadiene
rubber (NBR), again when properly compounded. The
rea-son for stressing this latter statement is that although the
base polymer itself is halogen free and meets test protocols, certain synergistic flame retardant packages available to the cable maker contain halogens, typically bromine, and these can be difficult to detect by Pt 1 of the above standards. As with smoke opacity, materials are now commonly available to meet this requirement.
Toxicity
There are presently more test methods in existence to meas-ure 'toxicity' than any other reaction to the fire parameter, yet none are recognised or approved internationally.
All suffer from two basic failings in that the method used
to pyrolise the sample does not reflect a true fire scenario and that once collected, not in itself an easy task, the gases that are
34
detected are subject to a highly subjective weighing proce-d u re.
Various tests, such as NES713, can be used for laboratory screening procedures, but it is most unwise to rely on them to give a true and accurate indication of what happens in a real fire situation.
In practice, if common sense is applied, then cables and
materials which liberate copious amounts of dense smoke and acidic gases should be avoided, as there is reasonable
correlation between the levels of acidic gases generated and Immediately Dangerous to Life and Health (IDLE) values.
CURRENT RATINGS
If the IEE Ships Regulations,' the Blue Book, and LEE Off-shore Recommendations,' the Green Book, have a weakness it is that the current ratings given in Section 12 have a basis
that is not accurately known. Derived from the ratings
contained within IEC92 standards their origins go back so far as to make tracing them difficult, if not impossible.
They are known to contain 'safety factors' in that
applica-tion of the tabulated rating will not cause the temperature rise stated but a slightly lower value. These safety factors were included to reflect the nature of the application, eg a ship far from land and assistance. But, conversely, the
in-creasing level of electrification onboard ships and offshore installations results in bunches of cables far larger than any
present, or considered, when the ratings were first pub-lished. For this reason the simple 0.85 bunch correction
factor must be viewed with caution.
Finally, the present tables fail to differentiate between the types of circuit protection used: coarse-fuses or close-circuit breakers.
Naval vessels have their own ratings based upon NES Standards and associated supplementary reports. Onshore ratings have benefited from major reviews in, first, the Fifteenth Edition of the I EE Wiring Regulations and, latterly,
the Sixteenth Edition, now 5S7671: 1992. These are supple-mented by a wide variety of papers and reports published by ERA Technology Ltd of Leatherhead.
Table VI gives the size of cable required for various calculated currents, based upon IEE ships and offshore
ratings compared with 1EE Wiring Regulations; as can be
seen these vary with no apparent pattern.
Whichever basis is chosen to calculate current ratings
what cannot be avoided are the basic laws of physics. When a voltagepotential difference is applied to a cable then a current will flow. Copper conductors are not perfect conductors and have a finite resistance, therefore a
tempera-ture rise and a power loss occur when current flows. The
greater the current flow for a given conductor the greater the temperature rise (power loss and also voltage drop).
Modern materials will run quite happily at high tempera-tures and so they can be used with advantage to allow higher current ratings per given conductor size. However, there is a practical limit.
The heat generated can only escape from the cable either by conduction through the terminals or mainly by radiation from the outside surface of the cable. The optimum rating for
Table VI Comparison of conductor sizes for BS68 83 multi-core braided cables for land and offshore applications
Conditions
Ambient temperature 45°C; Conductor operating temperature 90°C. Cables installed on perforated metal tray.
Multi-circuits based on cables touching. 600/1000V, 3 phase operation.
Protection
'Close' defined as an enclosed fuse to BS88 or BS1361, or a mcb to BS3871.
'Coarse' defined as a semi-enclosed fuse to BS3036.
Land ratings based on 16th edition of TEE Regulations. For electrical installations (B5767:1992).
Ships ratings based on 6th edition of the WE Regulations for Electrical and Electronic Equipment of Ships.
a cable is one where the amount of energy capable of being radiated away equals the energy generated, ie power loss, in the conductor.
This needs to be carefully considered when current rat-ings are assigned to thin wall cables or standard wall cables are installed in large bunches. In the former, reduced surface area can negate increase in temperature due to new
materi-als; in the latter, surrounding cables act as thermal insula-tion. Overheating, premature ageing and ultimate failure could result. The actual power loss at higher conductor
temperatures also needs to be considered against installed generated capacity.
CABLE MAKING PROCESSES
The late 1970s and 1980s saw a virtual revolution within the
cable making industry. Up until this period only thermosets (rubber, etc) and thermoplastics (PVCs, etc) were available, both confined within clearly defined boundaries of perform-ance. The arrival of irradiation technology, silane
technol-ogy, engineering polymers and thermoplastic elastomers
has breached these once solid walls.
In the elastomeric field the old taped finish batch
process-ing methods have given way to continuous production
techniques. Liquid Curing Medium (LCM) and Pressurised Liquid Continuous Vulcanisation (PLCV) machines give to elastomers a smooth finish approaching that seen on PVC
cables. The substitution of a eutectic salt as the curing
medium, in place of steam, removes the risk of core
deforma-tion and braid push back due to applied process pressure. Cables produced by the LCM method, although meeting current standards, do not quite equal the performance in fluids of the PLCV method due to the need to incorporate desiccants into the compound to prevent porosity during
Trans 1MarE, Vol 107, Part 1, pp 27-45
processing. Superheated steam vulcanisation, the latest proc-ess development, now matches that of a PLCV in producing
smooth, tape mark free, braided cables, the smooth finish
aiding installation and glanding.
Higher vulcanisation temperatures have allowed the sub-stitution of sulphur cure systems with those of peroxide, a halogen free system.
Elastomers can also be cross-linked by means of an elec-tron beam - irradiation technology - and, although
special-ised and relatively costly, it opens the door to composite
thermoplastic /elastomeric cables due to the absence of heat
inherent in conventional steam or salt vulcanisation proc-esses.
An electron beam also allows the cross-linking of
materi-als that would be impossible to cross-link chemically by
heat, and this has greatly assisted the introduction of the thin wall insulation systems seen.
Silane technology has made available cost effective XLPe insulated cables, as well as EPR types, although it must be stressed these are not the true flexible elastomeric types of EPR given in BS6883 and the like.
Improvements in raw materials, mixing and extrusion of compounds has allowed the reduction in radial thicknesses seen, without compromising electrical performance of
ca-bles.
Engineering polymers are gaining in popularity and are used extensively in the thin wall insulation system market.
Although having a very high raw material cost, the small amount used per metre run of cable, allied to weight and
volume savings, makes for a more acceptable total installed
cost.
Thermoplastic elastomers have yet to penetrate the ship
and offshore markets but are starting to find applications onshore in, at this stage, low technology products. As the
name suggests, these materials have certain characteristics in common with conventional elastomers, yet they can be
No of circuits
on line Protection 15A 50A 100A 300A
Land Sea Land Sea Land Sea Land Sea
Close, 1.02 1.5' 102 102 252 352 1502 1852 1 Coarse 1.5' 1.52 102 BY 352 352 1852 185' 6 Close 1.5= 1.52 162 102 352 352 2402 1852 6 Coarse 2.52 1.52 162 102 502 352 3002 1852 10 Close 2.5' 2.52 16' 162 502 502 2402 2402 10 Coarse 2.5= 2.52 252 162 702 502 3002 2402
P Waterworth
Table VII Elastomeric materials selection guide
36 PVC Polyethylene EVA Polyamide PEEK Pe/PVDF" PVC Polyethylene EVA Polyamide PEEK Pe/PVDF4 Maximum operating operating temperature temperature "C installation Maximum operating temperature "C
Minimum Minimum Insulation Sheath Tensile
operating strength
temperature as fixed installation
processed on conventional PVC plant, ie no vulcanisation process. Whilst the advent of these materials could provide some opportunities in the future, it is important not to forget
that they remain essentially thermoplastic in nature and
Table VIII Thermoplastic materials selection guide Minimum Minimum Installation Sheath Tensile
operating operating strength
temperature temperature installation/ as fixed handling installation 70-85 +5 -15 Y V 10-20 60 -40 -40 Y N 10-20 70 -0 -20 YIN Y 8-15 200 -100 -200 Y YIN 165 200 -40 -70 Y YIN 70 150 -40 -65 Y N 23
Limited Flammability Smoke Flammability Halogens
oxygen temperature emission
index index 25-40 200-250 n E P 21 50 E P E 20-40 250-300 E VG E 60 360 E VG E 43 325 E E E 27 400 G VG F Abrasion resistance Oil Flexibility resistance
thus retain some of the inherent problems associated with the use of thermoplastic materials in these applications.
Their possible use will be investigated as their performance envelope expands. EPR 90 -35 -40 Y N 8 CSP/CPe 85 -20 -30 YIN Y 10-20 VG VG Silicone 105-150 -35 -40 Y N 5-15 XLPe 90 -35 -40 Y N 10-20 VG PCP 70 -25 -35 N Y 10-15 VG VG EVA 85-125 -40 -40 Y/N Y 5-15 VG EMA 85 -40 -40 NI, Y 5-15 HNBR 100-125 -30 -35 N Y 8-12
Limited Flammability Smoke Flammability Halogens
oxygen temperature emission
index index
Key: Y = Yes suitable E = Excellent VG = Very good
N = No not suitable G = Good F = Fair P = Poor
Key: Y = Yes suitable E = Excellent VG = Very good
N = No not suitable G = Good F = Fair P = Poor
Thin wall engineering polymer Irradiation cross-linked thin wall
EPR 21 55 F-P CSP/CPe 30-40 250-350 Silicone 20-25 100-150 VG XLPe 21 50 VG PCP 25-35 200-250 EVA 20-40 250-350 EMA 20-40 250-300 VG HNBR 30-40 250-300 VG
Abrasion Oil Flexibility
resistance resistance VG VG VG VG VG VG VG P F E E IE P
CONCLUSION THE FUTURE
The number of materials and processing techniques avail-able to the cavail-able maker has never been greater. As a
conse-quence the number and variety of cables available to the
specifier and installer are quite large.
Far from being a panacea this variety is creating its own problems by way of confusion, giving rise to the very real possibility of the wrong cable being specified and installed.
Although physical properties have been compromised
slightly in achieving improved performance in the event of a fire, the current overall balance of properties will ensure that tried and tested 'traditional' elastomeric cables are used
in power and control circuits onboard ships and offshore
installations for many years to come. These cables are still 'user friendly' and forgiving of abuse.
Encroaching into this domain, especially offshore, are the types of cable used onshore, typically XLPe insulated with
either PVC or OHLS sheaths. Although on paper a cost
effective alternative to traditional elastomeric types, care is needed in deciding where and how they are to be installed and used.
The operating temperature limits, during both normal and overload conditions, the minimum bending radii and
performance near liquids need to be considered when speci-fying this type of cable.
In instrumentation and low current circuits the move
towards the new generation of small, lightweight thin wall cables will continue. With this type of cable potential dam-age during installation, termination techniques and ultimate performance in the event of a fire, due to their small size, are areas that require consideration.
If cost were not a consideration then the range of materi-als and cable designs developed by, and for, the Royal Navy would reign supreme. With the ever present drive to reduce costs to a minimum these cables will never be as widely used in the commercial field as traditional EPR-CSP types.
How-ever, they should not be discounted out of hand for, in
problematic or demanding applications, they could provide a cost effective solution. Hopefully, increasing use will drive
down the cost of what is currently the speciality polymer
upon which they are based.
With regard to performance in the event of a fire,
al-though the Royal Navy are looking towards developing and improving their current cables into the ultimate, universal
cable, cost would preclude its widespread adoption in the
commercial marine industry.
Commercially, where circuit integrity is required, the most widely used types of cable will rely upon the mica/ glass taped conductor with a variety of further finishes.
Mineral Insulated Copper Covered (MICC) cables will still continue to find limited use due, in the main, to termination procedures.
If, at this stage, the move towards higher flame
tempera-tures of 1300°C and above continues, the use of MICC will be
put in doubt, but mica/glass can still be considered. Evolution of smoke and corrosive gases will continue to have the highest priority within accommodation areas, pub-lic spaces and control rooms and, as materials are developed
further, the current low levels of light obscuration will
reduce even further. Where a reduction in physical
proper-ties and fluid resistance can be tolerated, these very low
smoke emitting materials are available now. Tables VII and VIII give a generalised elastomeric and thermoplastic
mate-rials selection guide.
The area of greatest debate concerns that of flame propa-gation. Until the limitations of TEC332 Pt 3BS4066 Pt 3 are formally recognised and international regulations and rec-ommendations reflect this, confusion and acrimonious ex-changes between manufacturer, installer, owner and
certify-ing authority will continue.
Hopefully the work of [EC SC18A WG1 when started will lead to a speedy recognition and resolution of this
long-standing problem.
Gaining more and more importance as a subject is that of
toxicity; this topic more than any other is one where the more one knows the more one realises one does not know. The
sheer scope and depth of this subject leads to the many well intended but conflicting test methods and interpretations.
The work of IEC TC89 will not resolve all the problems but should give an internationally recognised and accepted basis for a test and presentation of results. For anyone with
an interest in this subject then IEC 695 Part 7 Section 1
produced by TC89 makes invaluable reading, in reviewing and comparing the more commonly existing test methodol-ogy and presentation of results.
Finally, although the present fortunes of the UK Maritime Industry are clouded, its engineers, engineering practices, rules and regulations are recognised and respected through-out the world, although of course not always followed.
The future format of the IEE Ships Regulations and Off-shore Recommendations will remain as now, with no plans to convert them to British Standards, as was the case with the onshore regulations.
Continuously updated to match current practices these
two publications are a valuable source of information and
guidance, their prominent position on any Marine
Engi-neer's desk being highly recommended.
REFERENCES
T L Joumeaux and P Waterworth, 'Update on cable standards',
ERA Conference (1992).
F D Sydney-McCrudden, 'Fire performance of electric cables',
Trans IMarE, Vol 101 (1989).
The Institution of Electrical Engineers Regulations for the
Elec-trical and Electronic Equipment of Ships with Recommended Practice for their Implementation, Sixth Edition (1990) plus
Supplement (1994).
The Institution of Electrical Engineers Recommendations for the
Electrical and Electronic Equipment of Mobile and Fixed
Off-shore Installations, Second Edition (1992).
BIBLIOGRAPHY
NES518: Requirements for Limited Fire Hazard Sheathing for
Electric Cables.
NES525: Requirements for Electric Cables, Thin-wall Insulated, Limited Fire Hazard.
NES526: Requirements for Cables Electric, Rubber Insulated,
Limited Fire Hazard Sheathed for General Services.
Trans 1MarE, Vol 107, Part], pp 27-45
1..
2
3
P Waterworth
NES527: Requirements for Cables Electric, Fire Survival, High
Temperature Zones and Limited Fire Hazard Sheathed.
NES711: Determination of Smoke Index of Products of Combus-tion from small specimens of material.
NES713: Determination of the Toxicity Index of the Products of Combustion from small specimens of materials.
Defence Standard 61-12 (Part 25). Cables, Electrical Limited Fire Hazard, up to 2.5 mm Cross-Sectional Area.
Defence Standard 61-12 (Part 31). Sheaths Limited Fire Hazard. BS6346: 1989 Specification for PVC-insulated cables for electric-ity supply.
BS6387: 1994 Specification for performance requirements for cables required to maintain circuit integrity under fire
condi-tions.
BS6724: 1990 Specification for armoured cables for electricity
supply having thermosetting insulation with low emission of
smoke and corrosive gases when affected by fire.
BS6883: 1991 Specification for elastomer insulated cables for fixed wiring in ships and on mobile and fixed offshore units.
BS7655: 1991 Specification for insulating and sheathing mated-als for cables (replaces BS6746 and BS6899).
BS7671: 1992 Requirements for electrical installations. IEE Wir-ing Regulations. Sixteenth Edition.
IEC695 Part 7: Guidance on the minimisation of toxic hazards due to fires involving electrotechnical products - Section 1:
General Guidance.
BP Engineering Standard 235: Fire resistant power cables. BP Engineering Standard 236: Fire resistant instrument cables.
APPENDIX A
IEC PUBLICATIONS PREPARED BY SUB-COMMITTEE NO 18A
92: Electrical installations in ships.
92-3 (1965) Part 3: Cables (construction, testing and
in-stallations). Amendment No 1(1969) Amendment No 2 (1971) Amendment No 3 (1973) Amendment No 4 (1974) Amendment No 5 (1979) Amendment No 6 (1984).
An ongoing, albeit slow, programme of work was, until recently, attempting to update this document and convert it into a new multipart series of specifications as given below. A challenge to this programme has been lodged and the issue is now under consideration by IEC Central Office and
IEC TC 18.
92-350 (1988) Part 350: Low-voltage shipboard power
ca-bles.
General construction and test requirements.
38
A draft document intended to extend the voltage range up to and including 10/15 kV has been agreed but not yet
published.
92-351 (1983) Part 351: Insulating materials for shipboard power cables.
Amendment 1 (1992).
Recently the concept of low smoke zero halogen insula-tion systems was actively discussed and this standard will
be revised to reflect this. Also to be included will be an
irradiated polyolefin insulation system.
92-352 (1979) Part 352: Choice and installation of cables for low-voltage power systems.
Amendment No 1 (1987).
A new Working Group has been formed, but has not yet met, to revise this specification. A major part of this work will concern reaction to fire.
92-353 (1994) Part 353: Single and multi-core cables with
extruded solid insulation for rated voltages
0.6/1 kV.
This now follows the general move internationally to include 3 kV cables with 600/1000V. Amendments have
been agreed but not yet published.
92-354 (1994) Part 354: Single -and three-core power cables
with extruded solid insulation for rated
voltages 6 kV, 10 kV and 15 kV.
92-359 (1987) Part 359: Sheathing materials for shipboard power and telecommunication cables. Low smoke zero halogen sheathing materials are now to be included, France favouring a thermoplastic type, the UK a more conventional elastomeric material.
Amendment agreed but not yet published.
92-373 (1977) Part 373: Shipboard telecommunication
ca-bles and radio-frequency caca-bles. Shipboard
flexible coaxial cables.
92-374 (1977) Part 374: Shipboard telecommunication
ca-bles and radio-frequency caca-bles. Telephone
cables for non-essential communication
serv-ices.
92-375 (1977) Part 375: Shipboard telecommunication ca-bles and radio-frequency caca-bles. General
in-strumentation, control and communication
cables.
92-376 (1983) Part 376: Shipboard multi-core cables for con-trol circuits.
IEC92-3XX
The UK has submitted a proposal and
draft specification covering 150/250V
screened cables for control and
instrumenta-tion circuits - pairs and triples based upon BS6883: 1991. SC18A to circulate an
'At least one conductor greater than 35 mm2 tNo conductor cross section exceeding 35 mm'
APPENDIX C
Table II Summary of guidance data for the selection of cables for type approval tests
*Examples for category A, designation F:
Example 1: Single core cable, 1 x 70 mm' conductor cross section; outside diameter 17 rim; 0.2 litres per metre of non-metallic material. Maximum width available for test sample is 600 mm. To achieve 7 litres per metre would require 35 test pieces giving a total width of: 35 x 17 mm 34 x 8.5 mm = 884 mm.
This cable cannot comply with the limitations on choice. Type approval testing arrangements would therefore be made by agreement between the manufacturer and customer or test authority.
Example 2: Three-core cable: 3 x 50 min' conductor cross section; outside diameter 29 mm; 0.55 litres per metre of non-metallic material. Maximum width available for test sample is 600 mm. To achieve 7 litres per metre would require 12.7 test pieces. 13 test pieces give a total width of: 13 x 29 mm + 12 x 14.5 mm = 551 mm.
This cable complies with the limitations on choice.
Trans 1MarE, Vol 107, Part 1, pp 27-45
APPENDIX B
Table II Summary of test conditions
39
Categoryanddesignation A FIR AF BF CF
Range of conductor cross sections (mm 2) >35" 35t >35* 35t >35" 35t >35"
Non-metallic volume per metre
of test sample (I)
7 7 7 7 3.5 3.5 1.5 1.5
Number of layers: For the standard ladder:
Maximum width of test sample: 300 mm 2 1 1 1 1 1
(front and rear of ladder) For the wide ladder:
Maximum width °fittest sample: 600 mm
Positioning of test pieces Spaced Touching Spaced Touching Spaced Touching Spaced
Flame application time (min) 40 40 40 40 20
Number of burners 1 1 1 2 1 1 1
A FIR AF BE CF
Maximum Maximum Maximum Maximum
two layers one layer one layer one layer
(front and rear) 600 mm wide 300 mm wide 300 mm wide 300 mm wide
including specified gaps
including specified gaps" including specified gaps including specified gaps At least
two test pieces
At least
two test pieces
Size of cable cross section Cables with conductors Cables having conductors with at least one cross section > 35 mm
having cross sections 35 mm 2and telecommunication cables Category and designation AF BF CF
Limitation on cable choice At least
to provide the required two test pieces nominal volume of
non-metallic material
P Waterworth
Discussion
J S Williams (BP Shipping) Electric cabling for the marine
industry is probably one of those topics that all too often comes within the 'fit and forget' category and yet, as this
paper has amply illustrated, it is an area in which there has been considerable change within the last decade or so. I was particularly interested to see the data presented in Tables WI and VIII of the paper. A few years ago a paper was presented
at this Institute on behalf of the late Frank McCrudden,
which reported on a considerable amount of research under-taken by Lloyd's Register into how the fire performance of cables varied according to the manner of installation. That
paper is now very much a standard reference within our
industry. I suspect that certain aspects of this paper, espe-cially these two tables, could attain a similar status.
When talking to cable manufacturers' marketing people
one is often presented with seemingly conflicting views
concerning the most appropriate cable for a particular instal-lation or application. The information contained in the paper provides a useful basis for making an impartial decision, and
I would like to thank Peter Waterworth for providing us
with such a tool. Selection of cables does, however, need to consider cost, something which can vary remarkably around the world.! wonder if the author could give an indication of relative costs for different cable types manufactured within
the UK?
The paper rightly cautions us to remember that the stand-ard laboratory testing of cables may not be indicative of their actual performance once installed. In the early to mid-1980s my own company was concerned with the specification and
building of the oil production vessel Seillean. This vessel
contains vast quantities of cable, many times more than in a conventional merchant vessel, frequently installed in
con-fined locations. The choice and installation of cable was, therefore, an important cost and safety consideration. At that time considerable emphasis was being given to the
gases produced during a fire. Analysis of video recordings
of the then IEC 332 Part 3 Category A propagation test
conducted on various cables by Queen Mary College, Lon-don University, assisted us in making our decision. It was found that the flame propagation of EPR insulated, halogen free sheathed cables was dramatically worse than the EPR/ CSP eventually chosen, and that this cable could re-ignite some time after the fire had apparently extinguished follow-ing removal of the burner flame. By comparison EPR/CSP displayed virtually no fire propagation by the cable, extin-guishing almost as soon as the source was removed.
Could the author comment on how cable manufacturers achieve a sensible balance between conflicting requirements for insulation and sheathing requirements, eg flame propa-gation and gas evolution.
A characteristic of merchant shipbuilding is that,
particu-larly within the machinery spaces, virtually everything,
including cables, will be painted. For Seillean a typical run of cabling was assembled by Harland and Wolff and tested at Queen Mary College, the burner being repositioned to give
the same clearance as in the normal IEC test. Although largely a subjective assessment, much to our surprise and
relief the painting did not affect the fire performance at all. 40
There has been considerable development in interna-tional standards for cables. However, many countries still have their own seemingly independent national standards
which can become an accepted regional standard, eg J1S in the Far East. Figure 1 of the discussion shows the aftermath
of a 20-30s flash fire in a steering gear flat with JIS EPR /PVC cable having an outer steel braid, the standard ship wiring
cable in thatpart of the world. The shot was taken after a high pressure detergent wash had been carried out and the cable
clips removed. It may not be particularly clear from the
photograph, but the PVC sheathing has bubbled through the
braiding. Despite the short duration of the fire and the
absence of any damage to the deck head paint work all the cabling within the space was completely written off.
Com-prehensive data for such cables is not readily available. Could the author please comment on any moves on the
international scene to get nations, who are members of IEC,
to adopt agreed international standards.
P Waterworth (Delta Crompton Cables Ltd) On the
ques-tion of relative cost it is somewhat difficult to carry out a
direct cost comparison due to the different cable construc-tions. For example, cables contained within NES 526 have flexible class 5 conductors in place of the traditional stranded class 2 conductors of BS 6883 cables.
It would perhaps be more helpful, therefore, to try to give an indication of actual material costs, but I must stress these are indicative only (see Table I of discussion). Commercial
decisions could change the ranking, such as if a manufac-turer were determined they were going to win a particular
project, regardless of cost implications.
Since the presentation of this paper in November 1994 the industry has been rocked by severe shortages and very large increases in the price of CSP. Current world manufacturing capacity of the actual base polymer is believed to be 35 000t, whereas demand is running at 38 000t. The author is unable to predict what the outcome of this supply shortage will be. However, common-sense dictates it could be the start of the process to phase out halogenated polymers from ships and the offshore industry on grounds of cost effectiveness.
Turning to your second question then: yes, halogen free cables have a totally different reaction when involved in a fire. Halogenated compounds, as their name suggests, sup-press fires by shrouding the cable in non-flammable gases which prevent oxygen reaching the cable, thus
extinguish-ing the flames. These gases are still released after flamextinguish-ing has
ceased and thus act to prevent re-ignition.
Of course, with halogen free materials this mechanism is absent. With this type of compound the cablemaker must, for example, make use of active fillers that decompose to release
water, and thus remove heat from the fire and cause extinction
by this mechanism. The compound cannot contain too high a loading of these fillers or physical and electrical properties suffer, so the cablemaker must balance out the conflicting requirements of performance in a fire situation with those of the traditional virtues of ships and offshore cables.
A second route followed by cablemakers is the inclusion of materials which form or promote a layer of char, which
Fig 1 Aftermath of a20-30sflash fire in a steering gear flat with JIS EPR/PVC cable
forms a barrier between the flame and the underlying
com-pound.
Fortuitously, material suppliers are working upon
non-halogenated synergistic additives that improve
perform-ance in water for instperform-ance, yet also enhperform-ance performperform-ance in
a fire.
When one considers that we have been supplying CSP sheathed cables for nearly 30 years one cannot expect the
relative newcomer, OHLS, to have reached the same state of development and effectiveness.
Finally on this subject, for the record it should be noted that in general the insulation used in both halogenated and halogen free cables is the same, with only a modified flame retarded insulation being used in extreme cases.
With regard to the adoption of International Standards, unfortunately as we know to our cost, it is not a level playing
field out there and although the UK is quick to adopt
standards and open its markets, some of the competition is not. Certainly, by and large, technical consensus has been reached but until commercial or even nationalistic pressures are removed, universal adoption will elude us.
M G B Wannell (Trinity House) Is there any indication
how the more modern sheath materials stand up to UV deterioration when in exposed surface run installations,
over, say, 10 years?
If noticeable UV deterioration does take place can you
indicate how soon one would experience water absorption into the cable?
Are there any indications of cost factors for these new
materials against, say, EPR/CSP to BS 6883?
P Waterworth (Delta Crompton Cables Ltd) It is perhaps
fortunate that most of the traditional materials used to
sheath ships' and offshore cables are, in general, classed as UV resistant, examples being PVC, CSP/CPe.
Trans IMarE, Vol 107, Part 1, pp 27-45
Table I Relative volume costofmaterials
Elastomeric
Thermoplastic
It is also well documented that where cables are exposed
to solar radiation, compounds containing high levels of carbon black perform much better than lighter coloured
compounds.
As to the question of UV degradation and water uptake one does need to be more careful in the selection of some of
the modern zero halogen low smoke compounds. The
au-thor is not aware of any specific work in this field but
theoretically, dependent upon the wavelength of the light
incident upon the cable, excitation of side groups can take place. If severe enough, this can lead to disassociation and the formation of radicals and ion rich areas. Given water's attraction to charged areas this could result in an increase in moisture uptake.
As I stated in my paper, most simple zero halogen low
smoke compounds contain a higher ratio of filler to polymer, so the effect would be more pronounced and noticeable with
XLPe insulation 45
OHLS 110
PVC sheath 50
PEEK (thin wall) 2300
EPR insulation 60 C5P/CPe sheath 100 PCP sheath 90 OHLS typically EVA 120 OHLS typically EMA 180 MOON) sheath 300 Silicone insulation 350
P Waterworth
simple compounds of this nature. That said, provided the
old basic rules are followed, use black, high carbon loaded compounds on cable runs exposed to high solar radiation; then their performance should not be dissimilar to the more traditional materials.
As to cost factors these are covered in my response to Mr Williams.
H Rush (ASA Consulting Engineers Ltd) The paper
pro-vides a very comprehensive review of ships and offshore
cabling and current standards. It contains much useful infor-mation in a form which will, I feel, be readily referenced by many engineers.
In the review of naval cables at the start of the paper and in the conclusions, our appetites were whetted about their
properties. It is of interest to learn in which situations the
author would speculate naval cables to be cost effective. If we are to increase use of them, we need to know more about how they can help us.
P Waterworth (Delta Crompton Cables Ltd) By and large,
electrically and mechanically, the new generation of naval cables do not have anything significantly different to offer the Marine Engineer. The two areas that would be attractive are the area of weight/space saving and that of resistance to fluids.
Cables manufactured to NES525 /Def Stan 61-12 Pt 25 are
significantly smaller in diameter and hence mass than more
traditional types of marine cables. Typical examples are:
a 3 core 2.5 mm2 unarmoured cable 8.4 mm against 11.2 mm
(125 kg/km against 350 kg/km), and for a 7 pair 1 mrn2
individually screened cable 16.9 mm against 22.8 mm (420
kg/km against 580 kg/km).
With regard to resistance to fluids the outer sheathing
material complying with NES518 /Def Stan 61-12 Pt 31 has properties approaching those of the legendary 'Niplas 915'.
In areas where oil or chemical contamination has proved
problematic and costly due to frequent replacement of ca-bles, these new materials should be considered.
Work at the author's company laboratories involving
specific drilling MusD revealed acceptable retention of prop-erties for this material, when in the same series of compara-tive testing CS lost almost all of its physical properties and samples of PCP actually disintegrated and fell apart! J R Bennett (P&O/Princess Cruises) I would like to thank
the author for a most interesting paper.
My company have several large cruise vessels on order
from European shipyards. One shipyard has proposed a cable rated at U /UN (UM) = 3.3/6 (7.2) kV for a 6.6 kV
installation, instead of the more commonly used 6/10 (12)
kV cable. The system employs high resistance neutral
earthing, with earth fault alarm and tripping by manual intervention. Could the author please comment on this proposed cable, both in terms of the UN and U figures,
respectively.
Is painting cables likely to cause any premature ageing or
other problems, particularly with modern elastomeric ca-bles? Also, does the paint affect the flame retardancy, or
evolution of smoke, toxic and corrosive gases from OHLS cables in the event of a fire?
42
Lastly we have heard OHLS cables described as both
'zero halogen' and 'halogen free'. Is there any difference in these two terms?
P Waterworth (Delta Crompton Cables Ltd) Guidance on the selection of appropriate cable for a particular system can be found in the Supplement of the Sixth Edition of the IEE Ships Regulations (Blue Book).
The system proposed, automatic fault alarm but manual disconnection, would appear to be a Category C system, and
on this basis I would personally offer the more common 6/10(12) kV cable. Had the system been a Category B or even
Category A then the 3.3/6 (7.2) kV cable would suffice. This guidance and categorisation is derived from IEC502 and so should be known on the Continent.
With regard to painting, the general advice given is not to paint cables but we all know it goes on. Because there are
many different types of paint, specific advice cannot be
given but again, in general, most popular paints do not cause problems.
Certainly the thinners and solvents used in the
manufac-ture of paints, if used neat and in prolonged contact, are
more than capable of causing damage to cables, however in
practice they are not neat and are free to evaporate off, so
contact is not prolonged. Paints which could cause problems are the thick bitumastic types, where solvents do not evapo-rate so quickly and freely, thus causing degradation.
A further general statement is that usually rubbers and
cross-linked materials resist paints better than, say, simple
thermoplastics such as PVC. This is because the solvents, although having no effect upon the actual PVC resin, can leech out and extract the plasticisers used to change basic
PVC, a rigid material, into the flexible varieties used in the manufacture of electric cables.
As regards paints' effect on properties in the event of al fire, in theory it could assist propagation along a cable run
although the author has no personal experience of this.
Being dependent upon the actual formulation of the paint it could most definitely contribute to the generation of smoke,
toxic fumes and corrosive gases. Obviously chlorinated
paints will release, for instance, smoke and acidic gas when decomposed in a fire, the amount released depending upon the volume of paint present.
With regard to the terms 'zero halogen' and 'halogen
free', they are practically the same. Of the two the latter is the
more technically correct, mainly because 'free' does not mean absolutely none, rather none has been added
inten-tionally or knowingly as a deliberate part of the formulation. An analytical chemist could, with his sophisticated equip-ment, detect halogens down to the parts per million or even parts per hundred million level, so to claim 'zero halogen' to him is not strictly correct, hence 'halogen free'.
Prof I D Stewart (BP Exploration) The author is to be congratulated on an excellent paper charting the develop-ment and specifications of cables. As a point of interest If
would like to correct a minor error in his paper. On the topic of Fire Resistance and the references to the BP Engineering Standards 235 and 236 being current to this day, I would like to inform him that they were replaced in July 1993 by a new document, namely: S112-12 Requirements for Flame
