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63

www.read-wca.com

Wire & Cable ASIA – January/February 2016

Material

Flame

retardant

Low

smoke

Halogen

free

PVC

X

No

Polyolefin

– –

X

Yes

Polyurethane

– –

X

?

TPE

X

?

FRNC

+

X

Yes

Fluorpolymer

+ +

No!

Table 2

:

Typical fire performance properties of cable compounds

The test purpose is to prove that no short circuit appears

when a defined burner flame works on the cable for the

required time. Fire resistance is more an aspect of impact

reduction.

3.3.4 Smoke Exhaustion

The quantity of smoke and fume is an important indicator.

Smoke reduces the sight of escaping people and rescue

teams, thus the translucence of smoke emissions is tested

according to IEC 61034. A high quantity of exhaustion as

well as very dense fumes reduces the light transfer. The

reduction of fire impacts is clearly the purpose of smoke

exhaustion parameters.

3.3.5 Absence of Halogens

There are different test methods described in IEC 60754:

to detect the acidity of smoke the quantity of halogen

carboxylic acid is determined (IEC 60754-1). The electrical

conductivity of smoke indicates the quantity of acid

radicals. This is to be tested according to IEC 60754-2.

Another test procedure in this standard is the toxicity of

smoke, measured by the pH value which indicates the acid

content of the smoke in a liquid solution.

All the tests and parameters to prove a cable to be halogen

free are aspects of impact reduction, too.

3.4 Cable Construction Aspects

In cable construction a lot of parameters affect the cable

fire performance. The selection of materials is of main

importance. Thus

Table 2

gives an overview for some

common compounds for cable insulation and jacketing

regarding the fire characteristics. This deals with the basic

material.

Of course compound engineering improves continuously

and by use of specific additives there are materials of the

same family available with far better fire performance.

Nevertheless we should remain realistic and keep in mind

that there will never be the perfect material. The addition

of mineralic fire retardants keeps the material halogen

free and reduces flame propagation, but it also reduces

mechanical properties such as elongation and elasticity.

But not only material affects the fire performance of

cables. A lot of detailed construction parameters are

important. So for example the tightness of a jacket should

be taken into account.

Interstice filling jackets provide more combustive material

to a fire, but they prevent the air flow inside the interstices

and reduce the oxygen available to the flame. A jacket

extruded as a tube has a similar effect as a funnel when

the cable is burning, especially in vertical fire tests.

4 Fire Protection Strategies

Fire protection is not only a cabling issue. There must be a

general fire protection concept regarding all construction

elements of a building. This need is taken into account in

the European Constructive Products Regulation

[6]

.

We have seen there are two aspects of fire protection:

fire avoidance and reduction of fire impacts. How do

these aspects correlate to the regional differences in fire

protection strategies?

4.1 Fire Avoidance

The common fire protection philosophy in America is to

prevent fire at any cost. There are high amounts invested

into research and investigation and the results are very

challenging regulations regarding the fire performance of

indoor cabling in terms of self ignition, flame propagation

and fire resistance.

To meet these requirements there is no other way than to

use halogens as flame retardants in cable compounds, as

well as in building materials.

This approach risks the threat of personal injury by acid

fumes and of emergency exits being hidden by dense

smoke.

4.2 Reduction of Fire Impacts

It seems to be complete nonsense to reduce potential

fire impact but to do nothing to avoid fire. And it really is,

because there are so many and various impacts of fire.

There may be very specific situations where

such a scenario makes sense, but such an exotic

application shall not be discussed in this paper.

Nevertheless we do not know any regulations which just

support this approach.

Reduction of fire impact cannot be a fire protection

strategy itself but it should be an important part of a

combined strategy, as it is European standard.

4.3 Diverse Redundancy

It is said that Europeans and especially Germans have a

preference for multiple safety. We are happy to know there

is a second protection instrument if the first protection

instrument should fail.

Combined safety strategy is well-known in many

technologies relevant to safety. So in safety discussions

regarding nuclear power plants the idea of diverse

redundancy is a basic approach.

This means there must be an additional safety procedure

which works completely independently from the first one in

case the basic safety procedure does not work.

So in Europe it is the consensus to avoid fire as much

as possible but at the same time to keep low the

consequences on health or goods if a fire happens.

Due to physical reasons a better reduction of the effects

is achieved by reduced fire avoidance. But in total the risk

according to

Equation (1)

is significantly lower. This is also

shown in

Figure 2

.