Downstream LV network protection

Priority circuit protection

Choice of breaking capacity

This must be systematically checked with the characteristics of the main source (MV/LV transformer).

Setting of the Short Time Delay (STD) tripping current

• Subdistribution boards

The ratings of the protection devices for the subdistribution and final distribution circuits are always lower than the generator rated current. Consequently, except in special cases, conditions are the same as with transformer supply.

• Main LV switchboard

  -  The sizing of the main feeder protection devices is normally similar to that of the generator set. Setting of the STD must allow for the
     short-circuit characteristic of the generator set (see “Short-circuit current protection” before)
  -  Discrimination of protection devices on the priority feeders must be provided in generator set operation (it can even be compulsory for 
     safety feeders). It is necessary to check proper staggering of STD setting of the protection devices of the main feeders with that of the
     subdistribution protection devices downstream (normally set for distribution circuits at 10 In).
Note: When operating on the generator set, use of a low sensitivity Residual Current Device enables management of the insulation fault and ensures very simple discrimination.

Safety of people

In the IT (2^nd fault) and TN grounding systems, protection of people against indirect contacts is provided by the STD protection of circuit-breakers. Their operation on a fault must be ensured, whether the installation is supplied by the main source (Transformer) or by the replacement source (generator set).

Calculating the insulation fault current

Zero-sequence reactance formulated as a% of Uo by the manufacturer x’o. The typical value is 8%.
The phase-to-neutral single-phase short-circuit current is given by:
[math]\displaystyle{ If=\frac{Un \sqrt 3}{2\ X^'d + X'o} }[/math]
The insulation fault current in the TN system is slightly greater than the three phase fault current. For example, in event of an insulation fault on the system in the previous example, the insulation fault current is equal to 3 kA.