RCD a pain!

Bruthal

M cebee wrote: »

i may not have thought the n-e fault situation through:)

anyhow on socket circuits the 30ma rcd is for personal protection-that is it's purpose

Yes thats its entire design purpose, although it has some very good side effects like when faults develop in elements etc and they do reduce fire risk as you mentioned yourself a while back, one of best inventions in electrical i think.

Bruthal

We often hear that electricity takes the easiest path, but that statement is not really right, it usually takes every path possible, if it only took the easiest path then when you turn on a cooker the lights would go off, so current flowing back to the MCB board earth bar has the neutralising conductor to take back to the transformer, but it also has the path along the earth wire to where its touching the neutral in a fault, and back to the neutral that way which is back through the RCD, this route will only divert a tiny current compared to the easy route back, but some small amount will take it just the same,

The same if a shower is now turned on (or any device but shower is a high load), its neutral path back is to its neutral bar, but another route when there is a neutral earth short in the sockets is from the main neutral bar through the socket RCD onto the sockets neutral bar and down to the neutral earth short and back to the earth bar then onto the neutralising point, this is how a shower can trip the sockets RCD which we seen in another thread, again a tiny percentage of the showers current would go this way and in many cases not enough to trip the sockets RCD, but 30ma out of 35 amps is less than 0.1% so it can happen and does. When a house is neutralised this reduces the chance of this happening as the main neutral bar and earth bar are linked with very low impedence so the diverted current percentage is much smaller.

dingding

robbie7730 wrote: »

We often hear that electricity takes the easiest path, but that statement is not really right, it usually takes every path possible, if it only took the easiest path then when you turn on a cooker the lights would go off, so current flowing back to the MCB board earth bar has the neutralising conductor to take back to the transformer, but it also has the path along the earth wire to where its touching the neutral in a fault, and back to the neutral that way which is back through the RCD, this route will only divert a tiny current compared to the easy route back, but some small amount will take it just the same,

The same if a shower is now turned on (or any device but shower is a high load), its neutral path back is to its neutral bar, but another route when there is a neutral earth short in the sockets is from the main neutral bar through the socket RCD onto the sockets neutral bar and down to the neutral earth short and back to the earth bar then onto the neutralising point, this is how a shower can trip the sockets RCD which we seen in another thread, again a tiny percentage of the showers current would go this way and in many cases not enough to trip the sockets RCD, but 30ma out of 35 amps is less than 0.1% so it can happen and does. When a house is neutralised this reduces the chance of this happening as the main neutral bar and earth bar are linked with very low impedence so the diverted current percentage is much smaller.

Correct, they are essentially Resistors in parallel with some high resistance / impedance in the circuit.