UK 100mA RCCB keeps tripping with certain items connected.

[pc1966]

Just pressed the test button on the sockets RCBO and it tripped. The main RCCB did not trip.

That suggests the fault is not actually on the circuit supplied by that RCBO. However, as many RCBO don't isolate the neutral (unlike RCCB that are normally all-pole switching) then it could still be a N-E fault downstream of the RCBO.

 

[HappyHippyDad]

That suggests the fault is not actually on the circuit supplied by that RCBO. However, as many RCBO don't isolate the neutral (unlike RCCB that are normally all-pole switching) then it could still be a N-E fault downstream of the RCBO.

That's narrowed it down then

 

[pc1966]

That's narrowed it down then

Yes, it is not really that helpful unless the details of the RCD device are known, if it is N-switching or not.

But at least it identifies the fault is not on the L-side!

 

[Jackster]

Yes, it is not really that helpful unless the details of the RCD device are known, if it is N-switching or not.

But at least it identifies the fault is not on the L-side!

When I was swapping out the RCCB, I also disconnected that RCBO's neutral and it did not trigger the RCCB. But as soon as I touched the lead to the neutral bar, it tripped.
What is odd about it all though. I can have nothing plugged in and come back the next day to it being tripped.

 

[pc1966]

When I was swapping out the RCCB, I also disconnected that RCBO's neutral and it did not trigger the RCCB. But as soon as I touched the lead to the neutral bar, it tripped.
What is odd about it all though. I can have nothing plugged in and come back the next day to it being tripped.

N-E faults are like that. Often there is very little difference in N & E potential (that is almost the definition of neutral, as it is linked to earth somewhere) so no trip.

But then something draws a lot of current (and it could be external to the installation) thus causing N to change due to the volt drop and that allows enough stray to E for the RCCB to trip!

 

[pc1966]

Actually thinking a bit more - if there is a N-E fault downstream of the 30mA RCBO it should also trip as it would see the imbalance as well. That assumes the trip speeds are selective, is the 100mA one a delay RCCB (i.e. "type S" for selective)?

 

[Jackster]

Actually thinking a bit more - if there is a N-E fault downstream of the 30mA RCBO it should also trip as it would see the imbalance as well. That assumes the trip speeds are selective, is the 100mA one a delay RCCB (i.e. "type S" for selective)?

I guess we shall find out when my "incompetent" --- gets a multi-function tester on Monday

 

[timhoward]

I guess we shall find out when my "incompetent" --- gets a multi-function tester on Monday

Don’t take it personally! We have to assume that unknown people can’t test safely. In colleges testing is taught after installation and after experience has been built up with how things are typically wired.
And electrical testing can be a dangerous business and doesn’t always give clear answers - there are regular questions on here from qualified sparks about interpreting results. Some tests are dead and some are live and it’s very easy to make a mistake.
We just care about safety.

 

[Jackster]

Well, I am still alive.

So I managed to find my clamp meter and it is showing 0.15A to 0.5A on the 20A setting depending on if I clamp near the RCBO or the main tails where the 4mm cable is accessible.
Don't think one can trust it... Ideally need something that measures in the 2A range. Will be worth it in my electronics lab so I will grab one tomorrow.


Got my hands on the multifunction tester. A pretty good bit of kit to have a play with. Found a fault on an old extension cord (not been used in years) so that has gone in the bin so already paying itself off.

I did the RCD tests on both AC/AC+DC settings. The 30mA RCBO for the sockets was fine as was tripping within spec.
The 100mA RCCB tripped at a max of 105mA on AC and at 450mA on AC+DC. Not 100% sure about those results.


I did an IR test on the board between N+E and L+E * 3. All fine.

Having been messing around with all the plugs and sockets (mostly extension cables in the office area), I have reduced the apparently N to E leakage down from the average reading on the socket circuit of 0.4-0.5A to 0.1-0.2A.
Put a few new 8 way extensions in while I was down under the desks which might have helped?

I did do an IR test on all the existing multi-way extension cables and all were fine on 500v.

The RCCB has yet to trip under normal conditions but as soon as I put a load on my benchtop AC to DC power supply, it trips.
Could be a fault with that bit of kit but that kit is not normally plugged in. I tested it at home and the RCD did not trip there.

 

[pc1966]

Well, I am still alive.

A good start to the day!

So I managed to find my clamp meter and it is showing 0.15A to 0.5A on the 20A setting depending on if I clamp near the RCBO or the main tails where the 4mm cable is accessible.
Don't think one can trust it... Ideally need something that measures in the 2A range. Will be worth it in my electronics lab so I will grab one tomorrow.

Something like this is needed:

https://cpc.farnell.com/multicomp-pro/mp780050/mini-leakage-current-tester/dp/IN08364

Also you need to get the L and matching N in to the clamp to see the residual. I would strongly advise against moving cables in an energised DB! Try and get anything to measure in-place and accessible with all power off.

Got my hands on the multifunction tester. A pretty good bit of kit to have a play with. Found a fault on an old extension cord (not been used in years) so that has gone in the bin so already paying itself off.

I did the RCD tests on both AC/AC+DC settings. The 30mA RCBO for the sockets was fine as was tripping within spec.

Good to hear.

The 100mA RCCB tripped at a max of 105mA on AC and at 450mA on AC+DC. Not 100% sure about those results.

If it is type AC and tested on a type A waveform with DC component then it can be desensitised.

If 100mA rating it ought to trip between 50-100mA. You need to have all loads off so you are just testing it alone, and not having system currents also present.

I did an IR test on the board between N+E and L+E * 3. All fine.

Having been messing around with all the plugs and sockets (mostly extension cables in the office area), I have reduced the apparently N to E leakage down from the average reading on the socket circuit of 0.4-0.5A to 0.1-0.2A.
Put a few new 8 way extensions in while I was down under the desks which might have helped?

Those are massive currents! Is that measured with L&N in the clamp, or are you (incorrectly) clamping the CPC? If so then any metalwork anything is in contact with could be causing odd currents to circulate.

I did do an IR test on all the existing multi-way extension cables and all were fine on 500v.

OK.

The RCCB has yet to trip under normal conditions but as soon as I put a load on my benchtop AC to DC power supply, it trips.
Could be a fault with that bit of kit but that kit is not normally plugged in. I tested it at home and the RCD did not trip there.

Have you IR tested the power supply? I.e. the sort of basic PAT testing part (along with CPC bond resistance, etc)

Are you sure the power supply is going in to the RCBO's supply? There is something inconsistent with being able to trip the 100mA incomer and not trip the 30mA socket feed, unless the installation happens to be sitting only a few mA leackage below the trip point normally.