ABB CI853-1 Analog module industrial system controller

Description

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As described in the previous chapters, the Standards give indications about
the maximum disconnecting time for the protective devices, in order to avoid
pathophysiological effects for people touching live parts.
For the protection against indirect contact, it shall be verified that the circuitbreaker trips within a time lower than the maximum time stated by the Standard;
this verification is carried out by comparing the minimum short-circuit current
of the exposed conductive part to be protected with the operating current
corresponding to the time stated by the Standard.
The minimum short-circuit current occurs when there is a short-circuit between
the phase and the protective conductors at the farthest point on the protected
conductor.
For the calculation of the minimum short-circuit current, an approximate method
can be used, assuming that:
– a 50 % increasing of the conductors resistance, with respect to the 20 °C
value, is accepted, due to the overheating caused by the short-circuit
current;
– a 80 % reduction of the supply voltage is considered as effect
of the short-circuit current;
– the conductor reactance is considered only for cross sections
larger than 95 mm2.
The formula below is obtained by applying Ohm’s law between the protective
device and the fault point.
Legend of the symbols and constants of the formula:
– 0.8 is the coefficient representing the reduction of the voltage;
– 1.5 is the coefficient representing the increasing in the resistance;
– Ur is the rated voltage between phases;
– U0 is the rated voltage between phase and ground;
– S is the phase conductor cross section;
– SN is the neutral conductor cross section;
– SPE is the protection conductor cross section;
– ρ is the conductor resistivity at 20 °C;
– L is the length of the cable;
–
SPE
S n
m=
⋅
is the ratio between the total phase conductor cross section
(single phase conductor cross section S multiplied by n, number of
conductors in parallel) and the protective conductor cross section SPE
assuming they are made of the same conductor material;
–
PE
N
S
S n
m1=
⋅
is the ratio between the total neutral conductor cross section
(single neutral conductor cross section SN multiplied by n, number of
conductors in parallel) and the protective conductor cross section SPE
assuming they are made of the same conductor material;