Nussance tripping rcds

[dabube]

Hi guys

Wonder if you can help cause im baffled got call out to a house that i did some work at in kitchen a few weeks ago as rcd keeps tripping when plugging in the hoover
split load 17th cu think it is basics make ring main is on the right hand rcd however it is taking the left hand rcd out so my immediate thought was crossed or borrowed neutral however Ir and all readiings are above 500 m ohm tested between all neutrauls on circuits so decided that the hoover may have been to blame however customer tryied to use new induction hob today which is fed from a diffrent cu and i installed circuit which is around 5 meters long so definatley no borrowed neutral however still tripping the left hand rcd on first board did a ramp test left hand rcd comes out at 24ma and right hand is around 27 ma but from previous experiences this dosent strike me as overly sensative but either way cant see how using the hob on seperate cu

Any ideas would be greatly appricated as i said im kinda out of ideas

 

[WayneL]

Where is the second CU powered from?

You haven't mentioned how you've proven that there isn't a borrowed neutral - how are you so certain there isn't one?

 

[dabube]

second cu is fed from same origin via double pole isco block however there is 2 phases in the property but second phase soley feds a 3rd cu
new hob wiring i installed direct to cu 2 must admit with prob need to go a little more into its just it dosent do it every time ran my hoover an angle grinder and a washing machine at same time while the fitters were there today and it didnt trip

 

[pushrod]

Thought about a cumulative trip ie small amounts of leakage from several sources. Checking circuits with an EL Clamp Meter would show it.

 

[dabube]

yeah going back tommorrow another sparks i know said about doin that im just strugglin to see why it dosent take out its relavent rcd with the origianl problem u turned the hoover on and the rcd would take out the lighting in that room but the ring rcd stayed on an carried on working and now the same with the hob yet if you turn the rcd back on with the hoover runnng it all powers up and stays on then u can turn hoover on and of as u please then radomly it will do it again

 

[WayneL]

I'd say a neutral/earth fault on one of the circuits connected to the tripping RCD.
This would only trip the RCD randomly, as the amount of leaking current would be dependant on the loads - the hoover 'starting up' could have been enough to trip it, due to the higher current drawn on start up.

Are you sure your IR readings (N-E) are all OK?

 

[laney06]

Have you considered a loose neutral? I've seen this before, where the incoming neutral was loose in the supply meter supply side. May be worth checking tightness of all neutrals, from meter onwards.
Plus if there's 2 phases in the property, do they need to be balanced?