RCD TROUBLE!

[SparkyJack]

Hey guys im new at this forum stuff so bare with me.
just installed a new dis board in my house, replacing the old fuses.
house still has some split conduit and cotton covered cable.

but my problem is with the RCDs they work, but they will only trip unless theirs a decent load on them. eg. my lighting circuit is a 10A RCD/MCB and it will only if most lights in the house are on. i am testing in at the Dis board just touch in a jumper cable from RCD N out to the earth bar.

any ideas?

 

[ColJack]

and?

it's supposed to do that..

a N-E fault will trip an RCD, but only if there is enough current flowing from N-E..
for this to happen, a current must be flowing in the rest of the circuit..

after all , what's 0 - 0?

it's hard to explain.. we worked it all out in one post.. I'll see if I can dig it up..

 

[ColJack]

ok. dug it out.. here's the thread..

figures show that for a given circuit as described at the end of the thread, you'd need about a 6A load to trip a 30mA RCD...

 

[ericmark]

First I don't know your system in the UK we have three TN-S TN-C-S and TT there is also a IT but we can't use it.

With our TN-C-S to trip a RCD on earth neutral fault is not easy at the head they are the same voltage as bonded together as we move out into the house the cables will have a volt drop due to load the max allowed on lights is 3% in UK and in most of our houses due to the ratings of the ceiling rose the max current is 6A. I will use those figures.

Assuming equal drop on line and neutral the voltage between neutral and earth will be 3.45 volts for that to push 30ma you need less than 115 ohms as the amps drop to say 1 amp then so will the ohms so 19 ohms now needed get closer to source and it goes down again at consumer unit it is zero i.e. it will not trip.

With a TT system we could start at 200 ohms to earth above that it is considered as unstable.

The whole aim is to stop the supply before you get a 50 volts shock so there is no problem as it will do that.