27-02-2017, 09:36 AM
If theearth leakage is greater than the trip current of the highest rated RCD it's pot luck which one in the circuit will trip first. The only way to ensure discrimination is to use time delay RCDs for the higher rated ones.
I've forgotten most of my 3 phase theory but I think the following are true:
1: Equal loads (both resistance and reactance) - neutral current is zero. This is because the peaks on each phase never coincide. The posh way to work it out is to integrate the 3 sinusoids over a cycle period and find that the answer is zero.
2: Unequal resistive loads - neutral current can approach the phase current. Limiting case is one phase fully loaded, others unloaded.
3: Unequal reactive loads - it gets complicated
but I think in extreme cases neutral current can exceed a single phase current.
Upstream of the LV distribution transformers all the circuits are delta wired with no neutral at all.
I've forgotten most of my 3 phase theory but I think the following are true:
1: Equal loads (both resistance and reactance) - neutral current is zero. This is because the peaks on each phase never coincide. The posh way to work it out is to integrate the 3 sinusoids over a cycle period and find that the answer is zero.
2: Unequal resistive loads - neutral current can approach the phase current. Limiting case is one phase fully loaded, others unloaded.
3: Unequal reactive loads - it gets complicated
but I think in extreme cases neutral current can exceed a single phase current.Upstream of the LV distribution transformers all the circuits are delta wired with no neutral at all.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
