EuroWire November 2015

Technical article

November 2015

www.read-eurowire.com

This paper does not dive into statistics

to investigate the probability of failure

nor into economic sciences to quantify

financial impacts of any damages. The

focus here is the fire protection strategy

concerning in-house cabling.

3.3 Cable Fire Performance

Cables are important elements in fire

protection concepts of buildings. There

are different kinds of ingredients which

increase the fire performance of cable

compounds. Halogenated polymers are

self-extinguishing by chemical reactions,

but in case of fire they generate toxic

gases. Halogens are elements of the 7

principal group: Cl, Fl, Br, J.

In the oxidation process they react to acid

radicals, which generate acids by reaction

with hydrogen. When halogens are

burning at low temperatures, dioxins are

generated. The consequences of personal

injury or damage of goods are described

above.

Halogen-free

flame

retardants,

for

example Mg(OH)

or Al(OH)

, prevent

fire propagation by catching oxygen.

The chemical reaction generates water

which gives an additional extinguishing

and cooling effect. These mineralic flame

retardants generate very little smoke when

burning, and the fumes are non-toxic and

contain no acids.

But this class of material is not the perfect

solution, either. To achieve a very good

fire performance one must use these

ingredients in higher concentration. This

reduces the mechanical performance

of that cable, causes embrittlements or

reduces the operating temperature range.

Several cable fire-testing procedures are

defined by national and international

standardisation bodies. Each of them

alone represents just one of the different

threats of fire. An overview is given in

Table 1

3.3.1 Self Ignition

Cables should be designed in a proper

way so that neither voltage peaks nor

high ampacity should lead to self-

ignition. Voltage and ampacity testing

determines the ability of a cable regarding

self-ignition. The parameter self-ignition is

connected to the probability of fire.

3.3.2 Flame Propagation

Cables are connecting devices. So cables

carry the threat that a fire may propagate

along the cable from one building part

into another. This is the effect of a fuse

cord. To determine the flame propagation

(or fuse cord) properties of cables, IEC

60332 defines test methods on several

levels (eg IEC 60332-1-2,

Figure 3

). The

common idea of all these tests is the

same: a burning cable in a defined

position shall extinguish before the flame

has propagated a specified distance.

The sample position may be horizontal

or vertical, and the sample may be a

single cable or a cable bundle. Flame

propagation parameters are related to

both fire avoidance and impact reduction.

3.3.3 Fire Resistance

Especially for cables used in fire protection

application

there

are

specific

fire

resistance requirements defined in IEC

60331. This means a cable in a fire shall

maintain its function at least for a defined

time. These cable types are used, as an

example, for fire exit lighting, alarm and

warning devices and similar purposes.

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 con-

tinuously 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.

Parameter

Avoidance

Impact reduction

Self-ignition

–

Flame propagation

Fire resistance

(X)

Smoke exhaustion

–

Halogen free

–

▼

Table 1

Fire performance parameters and their correlation to fire risk elements

▲

Figure 3

Test set-up flame propagation