Design Practice - Protection and Control

1. Switchgear

The operating time of short-circuit protection shall be as short as possible and shall in any event not exceed the through-fault withstand duration of the switch board. For HV and LV switch boards, the rated short circuit withstand duration corresponding to rated withstand current shall normally by 1s. The maximum permissible operating time of circuit protection shall not therefore exceed 1s.

2. Transformers

Distribution transformers shall normally be connected Dyn. The protection may consist of phase short-circuit and earth-fault by means of two-stage overcurrent relays. Stage 1 shall be IDMT and set to detect secondary side faults. Stage 2 shall be instantaneous in operation and set to detect primary side faults only. For primary voltages up to and including 6.6 kV, fused contactors may be used.

3. Capacitors

HV capacitor banks shall comprise individually fused capacitor units. For large capacitor banks exceeding 1000 kVar, the capacitors shall be connected in double-star with unbalance protection monitoring the star-point voltages. Capacitor failure shall trip and bank and produce an alarm. Individual capacitors shall be controlled by contactors, circuit breakers, or, for LV applications, fused switch units. The switching devices shall be rated for at least 1.5 x In and must be able to withstand transient inrush up to 100 x In.

4. Motors

Overload protection of electric motors that are required to remain operational in the event of fire shall be set at a minimum of 300% of the motor full load current in accordance with NFPA 20. Such motors shall be clearly identified.

5. Power and convenience outlets

Each LV power and convenience outlet circuit shall be protected by a phase short circuit protective device and by an ELCB/RCCB. The ELCB operating current shall be 30 mA for circuits less than 125 A and 300 mA for circuits equal to or greater than 125 A.