Strange RCD behavior, anyone got any ideas.

[gwrigh15]

Hi all,

Got a call saying the RCD had tripped on a 16th edition board where all the sockets (2 rings) cooker and immersion heater were on 1 rcd.

Going through what was on at the time led me to the tumble dryer in the garage which proved to have a short neutral to earth.

My question is that when i was trying to find the fault all the circuits when turned on would trip the RCD, normal for a neutral to earth fault apart from the cooker circuit which would stay on????

Testing the cooker circuit the zs was 0.3 and it tripped out in the required time for the rcd test. Looked in the CU and all seemed well in there.

I dont understand why it would allow the cooker circuit to stay on?

Any ideas anyne?

Thanks for you help.

Gareth

 

[Sparky Ninja]

Going through what was on at the time led me to the tumble dryer in the garage which proved to have a short neutral to earth.

My question is that when i was trying to find the fault all the circuits when turned on would trip the RCD, normal for a neutral to earth fault apart from the cooker circuit which would stay on????

If the fault is within the appliance then all other circuits should stay on.



oh, and welcome

 

[gwrigh15]

Hi,

Thanks for the reply.

But with the circuits in banks (controlled by 1 rcd) even if the circuits are isolated by turning the mcb's off, if you touch the neutral to earth it will trip the rcd. It puts it out of balance.

Try it! I have done it many times working on dead circuits but knocking the rcd off by accidentally touching the neutral to earth.

Cheers
Gareth

 

[Des 56]

The neutral earth fault in an appliance that is left plugged in, can result in the RCD firstly staying in,because there is not enough leakage to earth to trip the rcd
When a load from a seperate circuit that shares the RCd is applied
The load current will use the neutral earth fault of the appliance, and may cause there to be enough leakage to then trip the rcd

 

[gwrigh15]

Ah ok yes,

So what your saying is there wasn't enough leakage current from the cooker circuit to trip the rcd but when the other circuits were turned on this was enough, i guess this is probably because the cooker circuit was wired in 6mm so it would be easier to use the cooker neutral rather than to find its return through the fault in the appliance.
Not sure if ive grasped this properly but i kind of get it.
Its a bit of a strange one i have never seen before and thought there must be some sort of explanation.
Have you seen this before des56?

Thanks very much for your help.

Gareth

 

[telectrix]

suppose the fault you found on the tumble dryer caused a leakage current of say 25mA, this in itself would not trip the RCD. Say the cooker had a leakage current of 5mA or a bit more, then there's your 30mA to cause the RCD to trip.

 

[Sparky Ninja]

Yes I know


I assumed you would have un-plugged it.

 

[gwrigh15]

Yes,
but this was happening whilst i was trying to find it, and once i did i was wondering why the cooker circuit would be on with the rcd on but all the other circuits would trip the rcd because of the neutral earth fault.
I think des56 probably hit the cause on the head, with the other circuits which share the rcd then puting a load current on the neutral sufficient enough too cause enough earth leakage to trip the rcd. But the cooker circuit not putting sufficient neutral current down to trip it. Im guessing that if i had used the cooker it probably would also have tripped the rcd.
Confusing!!!!
Thanks for all the answers.
Gareth

 

[Sparky Ninja]

Well it's likely that the cooker was the only circuit that was not drawing current, i.e. it was turned off?