WCA January 2016

64 / 72

62 Wire & Cable ASIA – January/February 2016
Parameter Avoidance Impact
reduction
Self-ignition X –
Flame propagation X X
Fire resistance (X) X
Smoke exhaustion – X
Halogen free – X
❍
❍ Table 1: Fire performance parameters and their correlation to
fire risk elements
❍
❍ Figure 3: Test set-up flame propagation
If the probability is reduced down to 4 and the impact
down to 5, a risk level of 20 as a product of the factors 4*5
is achieved. If the impact remains on its high level of 10,
the probability must be reduced down to 2 to achieve the
same risk level. Keeping in mind the Pareto principle, it will
be clear that the effort to achieve this extremely low level
of one factor will exceed the effort to keep both factors on
a medium level.
The advantage to distribute the efforts of risk reduction to
both factors (avoidance and impact reduction) is shown
in Figure 2. The dashed line shows the risk depending
on reduction efforts if all efforts are invested into threat
avoidance. The continuous line shows the risk if the
reduction efforts are distributed to both avoidance and
impact reduction in the same quantity.
Some simplifying assumptions are done in this approach
to show the basic principle easily. It can be clearly seen
that just in the medium part of the range, the distribution of
efforts to both factors brings clear advantages.
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 7th 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)2 or
Al(OH)3, 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.