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Section 10
9
Safety perimeters -
utilisation limits
Electrical
auxiliaries
High altitude temperature derating
Influence of altitude on the circuit breaker’s characteristics
The IEC 60947.2 construction standard stipulates the dielectric characteristics to be
respected. It follows that altitude has no effect on the characteristics of ID
circuit breakers up to 2000 m.
Any higher, and it is nécessary to take the fall in dielectric withstand and the air’s
cooling powers into account. The ID circuit breakers, designed to work in these
conditions, should be built or used in accordance with an agreement which should be
drawn up between the constructor and the user.
The table below shows corrections that are to be made depending on altitude. The ID
circuit breaker’s breaking capacity remains the same.
Altitude (m)
2000
3000
4000
Dielectric withstand (V)
2500
2200
1950
Maximum operating voltage (V)
440
440
440
Thermal rating
In
0.96 In
0.93 In
Derating in chemical atmospheres
Metal parts
n
Chlorine Cl
2
n
Nitrogen dioxide NO
2
n
Sulphurous hydrogen H
2
S
n
Sulphurous anhydride SO
2
Copper
n
The copper sulphur Cu
2
S layer’s thickness doubles in the presence of chlorine
compared with a normal environment
n
The same happens with nytrogen dioxide
Silver
The silver or silver-plated contacts go black and create an insulating layer of sulphur
in the presence of anhydride or sulphurous hydrogen. This insulating sulphur causes
excessive overheating of contacts and eventually leads to their destruction.
The presence of chlorine, for example in humid areas, mixed with sulphurous
hydrogen increases the thickness of the silver suplur by 7 and adding suphurous
hydrogen and nitrogen dioxide increases the thickness of sulphurous silver by 20.
Practical advice
n
Devices which are installed in oil refineries, steel works, paper mills, synthetic fibre
(nylon) factories and in general in sulphur chemical factories, will be exposed to
sulphurising which chemists refer to as oxidisation by analogy with the chemical
reaction which causes oxidisation by oxygen.
n
It must not be thought that the devices installed in “technical” areas are protected
from this oxidisation process. Air intakes, which keep the area’s pressure higher
than the atmospheric pressure, are “too short”; often the air drawn in is clearly less
poluted than the air near the ground, but it is polluted enough for the devices to go
black five or six years after their installation.
n
There is no cure for this oxidisation, just advice: a device’s nominal rating is
multiplied by 0.6 or a maximum of 0.8 if it is to be installed in a factory where
sulphurous gas may be released, the aim being to prevent overheating which
speeds up the oxidisation reaction.