Disconnection times on EPS circuits

[biaelectric]

When there is a fault on an EPS circuit, how are we meeting the disconnecting times? Earth fault is easy - just use a 30mA B-type RCD, but for the overload, are MCB good enough? On a 3.6kW supply, the voltage will drop so quickly, will there be enough energy to trip conventional MCBs or do we need to look towards wired fuses?

 

[pc1966]

When there is a fault on an EPS circuit, how are we meeting the disconnecting times? Earth fault is easy - just use a 30mA B-type RCD,

Probably.

You need to ensure that the neutral is still reference to Earth when it is operating in emergency power mode. If it is floating (an IT supply) you have all sorts of other issues to consider.

If you have a MFT then check Zs normally and then switch off the supply isolator (so mains N is open) and check Zs again. It should still be low, in fact it might even be lower if the inverter has very good regulation.

If you don't have a MFT then any decent socket tester with an RCD check button would allow a quick verification that the RCD would trip on an earth fault (different test to the 'Test' button that simply diverts around the RCD/RCBO sense coil).

but for the overload, are MCB good enough? On a 3.6kW supply, the voltage will drop so quickly, will there be enough energy to trip conventional MCBs or do we need to look towards wired fuses?

You don't need to.

The MCB still protect the cables, etc, from overload and catastrophic fault currents. The fact the inverter cannot deliver enough power to trip one means it is not a threat to safety. Assuming, of course, the inverter is well design and has good output protection of its own, but usually they are.