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Cables with "improved behaviour in the case of a fire" are much more expensive, but not so much better than ordinary PVC cables. This presentation first gives an attractive alternative and then points out a serious gap in the standards.
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Which cable is really fire resistant?Stefan FassbinderDeutsches KupferinstitutAm Bonneshof 5D-40474 DüsseldorfTel.: +49 211 4796-323Fax: +49 211 [email protected]@eurocopper.orgwww.kupferinstitut.dewww.leonardo-energy.org
The German Copper Institute, DKI, is the central information and advisory service dealing with all uses of copper and copper alloys.We offer our services to:
Commercial companies The skilled trades Industry R & D institutes Universities Artists and craftsmen Students Private individuals
We can be contacted by: post phone fax e-mail internet online database, or personally
Areas with special safety requirements also requirea special electrical installation
This includes the cables used, which have to comply with specific tests and standards:• Fire retardant insulants,• Limited fuel energy per unit building space,• Low fumes / low toxicity (no halogens),• and prior to all sustained functionality for a defined
period of time
so that accidents like the fire at Düsseldorf Airport on 11 April 1996 that left 17 people dead shall have the minimum possible impact – if they cannot be totally avoided.
There are two gaps in the standards:
• Which really are the fire conditions, and how realistic are the defined test methods to simulate these?
• What about the resistivity of copper under fire temperature?
Problem No. 1:
»They all do burn«,fire protection experts say.
Cables and leads »with improved properties under fire conditions« are only »somewhat« better than ordinary PVC cables.
But they are many times more expensive than ordinary PVC cables!
Copper conductor,
melting point:
1083°C
Mineral insulated cables could be an alternative!
They consist exclusively of metals and minerals
Magnesia oxide,
melting point:
2800°C
Outer copper sheath,
melting point:
1083°C
Mineral insulated cables could be an alternative!
• They are absolutely uncombustible,
• they are extremely sturdy and robust,
• they even survive a fire rather than just to remain functional for a limited period of time,
• they even come at a lower price
• and on top of this save some space!
Mineral insulated cable 2*1.5mm2 with copper sheath acting as protective conductor
optional outer sheath made of LSF plastic material (low smoke and fume)
Cable diameter7.2 mm
Typical fire retardant cable 3*1.5mm2 with protective conductor
Cable diameter13.2 mm
Saving space:One conductor less
Mechanically extremely robust and sturdy
• Commonplace »fireproof« cables »are not appropriate for installation underground or in water«
• Mineral insulated cable »shall be im-mersed into water for at least one hour before the test«!
Many variants available
Installation is nothingmore difficult, it is only very different
Special cable glands will be needed,
to be fixed with somespecialized tools and requiring some special skills
Very pliable
but replacing elastic with plastic deformation.
Just don‘t bend too often:Work hardening!
Beyond this, just one more disadvantage to cope with:
Marking of conductors is missing
Further opportunityfor application: Hisorically relevant sites
Where is the cable?
There it is!
Where is the cable?
There it is!
But now it becomes really difficult:
There it is!
Problem No. 2:The conductivity of copper istemperature dependent.
This rouses two concerns:
• Under fire conditions the voltage drop increases.
• Under fire conditions the cable over-temperature above ambient increases.
The former issue is taken into account in the standards.
The latter is not!
Side issue 2: Calculation of the copper temperature riseExcluding the b coefficient:
Including the b coefficient:
According to EN 60742:
x = 234.5, α = 3.9*10-3 and β = 0.6*10-6 for copper
Wiedemann-Franz Law:
))20()20(1( 220 CtCtRR t
))20(1(20 CtRRt
)()( 1211
12 tttxR
RRt
161
2020
.
T T
TRR
The difference is insignificant in the range of normal operating temperatures
0,9
1,0
1,1
1,2
1,3
1,4
1,5
1,6
1,7
0°C 20°C 40°C 60°C 80°C 100°C120°C140°C160°C180°C
R(T
)/R
(20
°C)
T
calculated according to EN 60742calculated including square componentcalculated using exponential formula
The difference is very significant in the range of ambient temperatures of a fire!
0,5
1,0
1,5
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
0°C 200°C 400°C 600°C 800°C 1000°C
R(T
)/R
(20
°C)
T
calculated according to EN 60742calculated including square componentcalculated using exponential formula
Main issue 2:Conductor temperature rise!With the ambient temperature rising
• from 30°C in normal operation
• to 950°C in the case of a fire,
• the resistivity of copper rises by a factor of 5!
Standards take the resulting increase of the voltage drop into consideration – but only for a fraction of the cable length.
This is justified because only a fraction of the entire cable run will be exposed to a fire…
Main issue 2:Conductor temperature rise!…but no standard requires a test under load!
Only one incandescent lamp is used(in order to identify the instance of failure).
However:
• With the resistivity rising by a factor of 5,
the cable over-temperature above ambient at full load rises
• from 40 K
• to 200 K!
Morals: The conductor melts!Regardless of the share of length exposed to the fire• Conductor temperature at full load: 1150°C.• Melting point of copper: 1083°C.• (Of aluminium not even to speak: 650°C).All relevant standards dealing with this require an upgrade!In theory all »fireproof« or »fire retardant« safety cables would need to be designed with 5 times the cross section a standard cable has for the same current rating.In practice a compromise needs to be found.After all, mineral insulated cables would be the better choice in any case!
