Tripping RCD

Hi, I'd like some advice please and my apologies for teaching anyone how to suck eggs or giving to much info or for waffling on far too much!

I'm Part way thru a complete rewire, including new consumer unit (CU) of my house and have encountered a problem.

The CU is a 16 way MCB split load, comprising of 100A Isolator, 6*6A non RCD protected, 7*32A + 3*40A RCD protected, which is rated at 100A 30mA. Once the whole job is completed - still a long way off mind - it was intended to have spare 1*6A, 1*32A and 1*40A.

Thus far I have run in 1 new ring main and 3 new lighting circuits and 1 shower main and transfered all of the old wiring to the new consumer unit and it has been stable without the RCD tripping.

That was until I have just connected another new ring main (for the kitchen) and the RCD now keeps tripping it's head off! The RCD has tripped in the following circumstances; i) There is a partial load on the circuit for a short while and then the RCD trips, ii) There is a partial load on the circuit and when trying to add more load - say plugging in a vacuum cleaner - the RCD trips or iii) There is a partial load on the circuit and turn on the kettle or shower, both on seperate circuits and the RCD trips.

The partial load includes Underfloor heating rated at a total of 11.6A, which is run via a 13A fused connection unit, large American style Fridge/Freezer, Dishwasher - although only on standby and the central heating combi boiler.

Anything that would help would be fantastic as I would like to get in the kitchen for my Christmas dinner and without the underfloor heating the room is like an ice block!

Thanks and regards Jules

your going to need someone with proper test equipment and the knowlage to use it. preferablly someone who can deal with both fixed installation testing and portable appliance testing (pat testing).

and i hope you haven't tiled that underfloor heating in or anything because i think its quite likely thats where the fault is.

Hi Plugwash

Thanks for the reply.

The kitchen is basically complete all bar the fiddly bits, so if it's the underfloor heating I may have some problems ahead!

Are you able to give me an idea of why you think there may be a problem with the underfloor heating?

Prior to tiling I test run the system and it all worked and it seems to be running fine now, at least when there is not too much load on the circuit it is wired to.

Also I set up a tempory kitchen and moved all the old appliances to the room and prior to the new ring main being installed none of my appliances caused the RCD to trip, and I have loaded up the RCD side of the CU with quite a load, i.e. washing machine, tumble dryer, kettle and old cooker all going at full tilt.

Thanks for your advice

Regards

Jules

I suspect your 'test run' consisting of powering it up, this is not recommended prior to insulation testing. Chances are you have live to eath leakage and thats what causing the tripping, not the loading of the circuit.
N.B. RCD's do not trip because of overloading the circuit.

Check the neutrals in your cu. Split units have two neutral bars. Got the right one?

Sorry if that's a stupid question.

Other than that you need to test the new circuit for faults.

julesg007 said:

Prior to tiling I test run the system and it all worked and it seems to be running fine now, at least when there is not too much load on the circuit it is wired to.

Click to expand...

Do you mean to say that you've embarked on a complete rewire and CU replacement without acquiring a continuity tester, insulation tester, loop tester & RCD tester, and the knowledge of how and when to use them?

Or without being actively engaged with an electrician to do testing for you?

I guess we can take it as read that you're in contravention of the Building Regulations - are you happy that that won't bite you in the future?


PS - Har_vey - mixing up the neutral connections would cause instant tripping.

you would think so, but thought I would ask.

Tripping levels that are load dependent, but no particular appliance, often indicate a neutral- earth low resistance - the earth path appears as a partial short of the neutral coil of the RCD current balance, and by diverting small percentage of the load current, this causes the RCD to think there is imbalance proportional to neutral current and trip at some threshold total depending on the diverted current fraction. Isolating circuits for testing by single pole MCB or fuse, can be misleading, as the neutral and earth wires are not disconnected - the fault may be on an unrelated circuit on the RCD side of the CU, that appears to be 'off'
Its time to dig out your meter, and measure L-E and N-E resistance on each circuit in turn. With all loads switched off expect tens or hundreds of megohms, depending on circuit length.

mapj1 said:

Tripping levels that are load dependent, but no particular appliance, often indicate a neutral- earth low resistance - the earth path appears as a partial short of the neutral coil of the RCD current balance, and by diverting small percentage of the load current, this causes the RCD to think there is imbalance proportional to neutral current and trip at some threshold total depending on the diverted current fraction. Isolating circuits for testing by single pole MCB or fuse, can be misleading, as the neutral and earth wires are not disconnected - the fault may be on an unrelated circuit on the RCD side of the CU, that appears to be 'off'
Its time to dig out your meter, and measure L-E and N-E resistance on each circuit in turn. With all loads switched off expect tens or hundreds of megohms, depending on circuit length.

Click to expand...

Yep - test each circuit on the RCD side of the CU. Turn all MCB's off, remove all neutral connections, LEAVE EARTHS IN.

I wonder how jules is getting on with his rewire?

may have burn't house down, (hope not but it happens) or perhaps the fault was his pc, so he threw it away