Intermittent RCD Tripping - opposite side of board

[Hollinbusk]

Cant get my head round this

I understand N-E faults cause RCD's to trip, but I cant understand why on these 2 occasions, googled allover but no real answer. I have 2 intermittent faults on the go, one Dual RCD board, one split load, 1 RCD.

Dual 2 RCD
Occasionally the left RCD trips when load is used on right, not a new install, no alterations and no crossed neutrals as far as I'm aware (assume this would have caused issues on board install if crossed neutrals day 1). Both RCD tests done and working fine .

Split load 1 RCD
Intermittent RCD tripping, seems to be when turning oven off (oven fed from non RCD side of board)

Any explanations appreciated.

 

[pc1966]

(1) If you have a N-E fault on one side then anything that causes the N-E voltage to increase will prompt the RCD to trip as it causes a current imbalance by the flow N-E round the RCD via the fault. For TN-C-S that can be quite a bit of current as the impedance from the CU's N to the N-E linking point is very low. For TN-S that link is much further back in the distribution system so far less current needed to prompt a fault. Often high surge items like hoovers, power tools, etc, are seen as the problem but they are just making the underlying fault more obvious.

Always start with a global IR test with incoming supply isolated! The norm for single-phase CU just by switching off. If it fails deal with that problem before worrying about anything else (though bear in mind some RCD sockets/FCU can show 25k path N-E).

(2) RCBO/RCD tripping when something like the oven is turned off is not uncommon, my own ~30 year old NEF oven does that. I think it is due to the N-path opening just before the L-path so the capacitance and/or leakage resistance of the heater element goes to full 230V for long enough to imbalance the RCD and trip it.

Again, try an IR test on the oven L+N to E and see if it looks poor (under 0.5M), and also check with a clamp ammeter for the total leakage on that RCD in case it is siting close to tripping anyway due to the accumulation of many other loads' leakage currents.

If you are close to tripping (so 9mA or above under "normal" conditions) you ought to be changing to RCBOs or shuffling circuits round to keep the leakage on any given RCD/RCBO down. If nothing else, RCBOs at least minimise the inconvenience of a trip on others!

If oven is leaky then a new element might help, or at least running rarely used elements to max temperature for a few minutes to drive out moisture.

 

[Hollinbusk]

(1) If you have a N-E fault on one side then anything that causes the N-E voltage to increase will prompt the RCD to trip as it causes a current imbalance by the flow N-E round the RCD via the fault. For TN-C-S that can be quite a bit of current as the impedance from the CU's N to the N-E linking point is very low. For TN-S that link is much further back in the distribution system so far less current needed to prompt a fault. Often high surge items like hoovers, power tools, etc, are seen as the problem but they are just making the underlying fault more obvious.

Always start with a global IR test with incoming supply isolated! The norm for single-phase CU just by switching off. If it fails deal with that problem before worrying about anything else (though bear in mind some RCD sockets/FCU can show 25k path N-E).

(2) RCBO/RCD tripping when something like the oven is turned off is not uncommon, my own ~30 year old NEF oven does that. I think it is due to the N-path opening just before the L-path so the capacitance and/or leakage resistance of the heater element goes to full 230V for long enough to imbalance the RCD and trip it.

Again, try an IR test on the oven L+N to E and see if it looks poor (under 0.5M), and also check with a clamp ammeter for the total leakage on that RCD in case it is siting close to tripping anyway due to the accumulation of many other loads' leakage currents.

If you are close to tripping (so 9mA or above under "normal" conditions) you ought to be changing to RCBOs or shuffling circuits round to keep the leakage on any given RCD/RCBO down. If nothing else, RCBOs at least minimise the inconvenience of a trip on others!

If oven is leaky then a new element might help, or at least running rarely used elements to max temperature for a few minutes to drive out moisture.

Many thanks for taking the time to reply to my post.

Whilst I accept everything you're saying and appreciate I likely have a neutral / earth fault causing leakage, my question was how does that affect the opposite RCD on the other side of the board from the side using the load (Dual RCD issue) and why does the RCD trip via the oven when the oven isn't RCD protected ? (issue with split load only 1 RCD)

This is what Im struggling to understand

 

[Chivers]

Can you Check all the Neutrals terminated into the board are in the right place & tight?

 

[pc1966]

Whilst I accept everything you're saying and appreciate I likely have a neutral / earth fault causing leakage, my question was how does that affect the opposite RCD on the other side of the board from the side using the load (Dual RCD issue)

Both share the same supply neutral. So any current on RCD #2 circuits also pushes up the N-E voltage on RCD #1 circuits, leading to an imbalance if RCD #1 has a N-E fault on the load side, etc.

and why does the RCD trip via the oven when the oven isn't RCD protected ? (issue with split load only 1 RCD)

I missed the bit about the oven not being RCD protected. In that case most likely it is the voltage disturbance / arcing as it goes off that is causing current to flow in the load-side capacitance (as current I = C*dV/dt so fast-changing voltage leads to more current to charge/discharge the capacitance).

Again, check if the RCD is close to tripping anyway. Also same advice that going to RCBOs will reduce the problem as well, since each has less load capacitance.

 

[Hollinbusk]

Can you Check all the Neutrals terminated into the board are in the right place & tight?

HI, Yes I can check this but assumed that you wouldn't be able to get an RCD to stay in, in the first place, if a neutral was crossed on installation - nothings been changed ?

 

[Hollinbusk]

Both share the same supply neutral. So any current on RCD #2 circuits also pushes up the N-E voltage on RCD #1 circuits, leading to an imbalance if RCD #1 has a N-E fault on the load side, etc.

I missed the bit about the oven not being RCD protected. In that case most likely it is the voltage disturbance / arcing as it goes off that is causing current to flow in the load-side capacitance (as current I = C*dV/dt so fast-changing voltage leads to more current to charge/discharge the capacitance).

Again, check if the RCD is close to tripping anyway. Also same advice that going to RCBOs will reduce the problem as well, since each has less load capacitance.

Hi again. Right, so Dual RCD option, whilst I get that the common element on both sides of the board is earth and leakage from side A or B will travel to the shared earth, the relevant RCD is only measuring its own loss ? (RCD's measure an imbalance - they don't measure actual leakage on the earth do they ? - earth isn't actually measured ? - inst it just assumed that's where the leakage is going ?)

So how can leakage voltage (N-E) be pushed up on the opposite RCD ? Surely the overall amount of earth leakage is only cumulative on the earth ?
e.g. - 20ma leak on both RCD (measured with leakage meter on each RCD) would be 40ma combined leakage on the earth ?

And back to the oven tripping the when not on the RCD side - isn't this the same ?

 

[Chivers]

Does seem an odd situation.

 

[Hollinbusk]

Does seem an odd situation.

Yes I just can’t fathom if no crossed nutrals, how one side can affect the other

 

[Chivers]

I think it's got to be a bit of trial and error here. Could even be something happening that's purely coincidental. test circuits for IR where possible.

 

[Hollinbusk]

I think it's got to be a bit of trial and error here. Could even be something happening that's purely coincidental. test circuits for IR where possible.

Yes I agree, will be doing IR’s on next visit, just wanted to try and make sense of how it can happen ….. for my own sanity lol.

 

[Chivers]

My Wife's friend had an RCD keeping tripping. Turned out to be a Fridge

 

[Brains]

Cant get my head round this

I understand N-E faults cause RCD's to trip, but I cant understand why on these 2 occasions, googled allover but no real answer. I have 2 intermittent faults on the go, one Dual RCD board, one split load, 1 RCD.

Dual 2 RCD
Occasionally the left RCD trips when load is used on right, not a new install, no alterations and no crossed neutrals as far as I'm aware (assume this would have caused issues on board install if crossed neutrals day 1). Both RCD tests done and working fine .

Split load 1 RCD
Intermittent RCD tripping, seems to be when turning oven off (oven fed from non RCD side of board)

Any explanations appreciated.

Suggest you check both for excessive leakage current by carrying out differential current test using a leakage current clamp meter such as Megger DCM305E.
Also check for insufficient insulation resistance.
Check load profile vs RCD type

 

[Hollinbusk]

Suggest you check both for excessive leakage current by carrying out differential current test using a leakage current clamp meter such as Megger DCM305E.
Also check for insufficient insulation resistance.
Check load profile vs RCD type

Thanks for your reply, yes I will be returning with a leakage clamp - can you explain why the RCD's are tripping the opposite side ? especially when the single board RCD is tripping with a circuit on the non RCD side ? - cant seem to get an answer off anyone