RCD Tripping

[Ash1991]

Hi
Hoping someone has an idea of what is causing my RCD to trip before I have to get an electrician out.
If I have my kitchen sockets on they work fine and the RCD doesn't trip but if the MCB for the kitchen sockets is on and the boiler fires up the RCD protecting the kitchen MCB trips out, the boiler will keep working as it's on a seperate MCB and RCD.
With the kitchen MCB turned off the boiler fires up and the RCD doesn't trip out

Confused as to how something on a seperate circuit is causing the RCD to trip when the boiler fires up.

Anybody have an idea why??

 

[ericmark]

Each circuit will have some back ground leakage, you can never get zero with an AC system, as always some capacitive and inductive leaking, but we can also have leakage we don't want which can add up.

So as an electrician I start by measuring

here showing the total leakage for my house, minus boiler and freezers which are on an uninterruptible supply. This reading is just within limits for a single RCD, we are allowed 30% of the RCD rating as back ground leakage, so 9 mA is max. Mine is actually split into 14 with all RCBO's so I am well within limits, but before the RCBO came in often electricians did not really test leakage, as there was little they could do even if too high, it was cross ones fingers and hope.

So likely two items are leaking more then they should, one you have identified as central heating, the other some thing in the kitchen, 3.5 mA it the limit for a single plugged in item, so step one, unplug all you can, and see if it still trips.

I mean unplug, not just switch off, as we can have leakage on neutral as well as line (both called live) so any item which is class I that means it has a three core cable and an earth connected, can be causing the problem, however in the main it is mineral insulated heating elements which cause the problems, so kettle, washing machine, dish washer, oven, old non induction hobs, and freezers, the latter is the nasty one, as only with defrost cycle is the heater used, so it seems rather random when the frost free freezer causes the trip. The only way is test neutral to earth with a different tester but that is when simple unplugging has not identified the fault. The 2 testers shown both cost £35 each, so likely cheaper and easier to get an electrician if unplugging does not identify the fault.

The earth neutral fault can be miss leading, a bit of wet bread stuck in a toaster may be the earth neutral fault, but the RCD trips when you use the kettle, as the kettle is a high load, so increases the voltage between earth and neutral so more current flows.

 

[Murdochcat]

Probably caused by cumulative earth leakage.

 

[Deleted member 267285]

Are you sure they’re separate? Can you provide evidence of this, eg the consumer unit?

 

[Harry Bloomfield]

Anybody have an idea why??

Look up, and investigate 'borrowed neutral'..

 

[ericmark]

Look up, and investigate 'borrowed neutral'..

I would not think so in this case, borrowed neutral faults appear after work has been done, like new consumer unit, likely been there for years, but not many people have the 10 year EICR done, and even if done, it needs the electrician to use the clamp on I have shown and load each circuit to find them, so often when doing an EICR the circuits are not loaded, so it can still be missed.

I have looked at expensive clamp on meters, aimed at the trade, which only measure in 10 mA increments, and do wonder who would buy them?

 

[Jurassicspark]

I have looked at expensive clamp on meters, aimed at the trade, which only measure in 10 mA increments, and do wonder who would buy them?

A Diyer buying one of them would be very rare.

Mostly expensive equipment is given by companies (who you work with) and are expected to be returned back.

 

[Harry Bloomfield]

I would not think so in this case, borrowed neutral faults appear after work has been done, like new consumer unit, likely been there for years, but not many people have the 10 year EICR done, and even if done, it needs the electrician to use the clamp on I have shown and load each circuit to find them, so often when doing an EICR the circuits are not loaded, so it can still be missed.

The OP's post seemed to read as if the two items were on separate RCD's..

Perhaps the OP could add more detail to their post, to improve our chances of diagnosing?

 

[ebee]

My initial thoughts were akin to borrowed N with perhaps 2 RCDs or an RCD and an RCBO the N being derived from a differing one or tother via the L but the fact that the boiler still works on its own circuit irrespective of the kitchen circuit being tripped threw me out of this one. prhaps a simplified line diagram of basic in out between relevant parts might help us see it (or a clear pic of the set up)

What Im thinking is two distinct RCCBs (RCDs) and then the boiler circuit MCB or RCBO fed L from the main bus or from one RCCB and the N fed from the other RCCB.

If one of those RCCBs only breaks L but not N (ie a SP RCD) then that might explain it?

 

[Murdochcat]

Earth leakage from one side of a dual board can trip the other side. Seen it, fixed it, been there

 

[ebee]

Well that`s taken as a given!

 

[Murdochcat]

So the obvious question is has anything changed electrically recently?

Can the OP post a photo of the board?

 

[ericmark]

I have to admit, old house, two RCD's which seemed prone to spikes on the supply, OK old maybe 1992, but reset one and often other would trip, and standard way to reset was turn off all MCB's then reset, then turn on MCB's one at a time.

It would go maybe two years without tripping, then trip 4 times in 2 weeks, only had an insulation tester, never did find a fault, always assumed some one welding on the estate.

It should have not behaved like that, but it did. My old clamp on, bought in Hong Kong before take over, left seen here with new one right, old one would measure frequency which new one will not, but new one also measures DC amps, and resolution 1mA rather than 10 mA with old one, and also has non contact voltage detection in 4 stages. 30 years between them old one 300 HK$ (approx £25) new one £35 things have moved on.

The old one never showed a fault with old house, but since the pass level is 9 mA and it did not register less than 10 mA not surprising.

New one so easy, as shown post #2 on incomers to consumer unit to read the leakage, and questions like borrowed neutrals so easy answered, if trips and not showing leakage on clamp on, clearly some wiring fault. But my failure to find any fault with old house, makes me realise an insulation tester great for finding fault when tripped, but not so good working out if there is a build up of leakage due to filters etc. As interference suppression capacitors and inductances will not show up with DC testing.

From the first time I started to work with RCD's on returning from the Falklands and working on the building of Sizewell 'B' I have said the big problem is we often don't switch the neutral. So to work out what is causing the problem we need to unplug, or turn off isolators. So cooker isolator clearly cookers don't unplug, boiler switch on the fused connection unit, drawing the fuse no good, same with immersion heater, and even then not all switched FCU switch the neutral.

This picture

posted many times on this forum not sure if @bernardgreen own image? Seem to remember the late Wesley use to post this image, but it does show the problem with neutral earth faults, which is not a borrowed neutral fault.

 

[ebee]

problem with neutral earth faults, which is not a borrowed neutral fault.

yes agreed

if we take L & N from differing sides (input/output) of an RCD to an MCB or an RCBO are we having a borrowed N day? perhaps or perhaps not,

 

[EFLImpudence]

...but that is not the same as the OP stating that another different RCD is tripping.