UK 100mA RCCB keeps tripping with certain items connected.

[Jackster]

Hi all

I am in an industrial unit and the 100mA RCCB keeps tripping.
They use it as the main switch inside of the unit's distribution board. The mainboard that feeds all the units has an MCB in it.

The RCCB keeps tripping when I plug-in AC to DC transformer loads such as laptops, speakers, monitors etc.
The 30mA RCBO that does the sockets (and my lights in my office) does not trip.
And I don't have all of these devices plugged in at the same time. Right now my laptop is plugged in with the other office equipment, but my monitors and speakers are not. If I plug those in, it trips. If I put any load on my constant current/voltage DC power supply, it trips. Talking less than 1w and it trips.

I have already replaced it with another RCCB of the same make and model just to see if the old one is faulty but it does the same thing.
When I swapped the RCCB over, it tripped as soon as I connected it back up to the board. The MCB was off, though it was only a 3 pole...

It also trips randomly when everything has been fine for hours. Not much if anything connected and it will just trip.
Every night it trips.

Any ideas on what I can do? What tests can I do?
I am thinking it is a natural leaking to earth somewhere? I should check between ground and neutral on the incoming supply and within my unit?

Thanks
Jack

(yes I am aware that I should get an electrical in, the site electrician was meant to be on site last week but never showed and I will be getting my personal electrical in next week or so when they can book me in)

 

[HappyHippyDad]

Hi Jackstar.
You just need to IR the consumer unit with a multi function tester, which may or may not show a fault. If it shows a fault you just track it down by breaking down the faulty circuit and IR testing.

If it doesn't show a fault then this points to natural leakage which you confirm with an earth leakage tester around the main tails. Your electrician should have those 2 tools. If its the latter then change the board for an RCBO board, so each separate breaker takes a smaller amount of earth leakage.

Ps.. I am purely domestic, so may have missed some other basic steps involved with industrial.

Pps.. of course it could just be one item of equipment causing the problem!

 

[timhoward]

I am thinking it is a natural leaking to earth somewhere? I should check between ground and neutral on the incoming supply and within my unit?

I agree it sounds like a N to E fault.
Don’t test the incoming supply - N is always joined to E somewhere, which might be inches away in your supply cut out or might be back at the substation transformer.

Does the test button work on your 30ma RCD , proving that works would be high on my list to be sure which side of it the real fault is.

In the case of N/E faults the connected load can be driving current through a fault on a completely different circuit so it’s almost impossible to make progress without an IR tester, an RCD tester and a ma clamp meter.

It sounds like the issue is on another circuit, upstream of the 30ma RCD but downstream of the 100ma RCD.
If you are struggling to get someone around you could mention what area you are in.

 

[pc1966]

I agree it sounds like a N to E fault.

That sounds likely, though the sensitivity to small things plugged in is odd.

Don’t test the incoming supply - N is always joined to E somewhere, which might be inches away in your supply cut out or might be back at the substation transformer.

If the incomer is a RCCB then it ought to cut the neutral as well, even if 3-phase, so IR testing L+N to E would show insulation faults.

Does the test button work on your 30ma RCD , proving that works would be high on my list to be sure which side of it the real fault is.

This!
Very important for the OP to check is if you have a fault on the sockets that the 30mA is simply not responding to then it would be the incomer that goes next.

In the case of N/E faults the connected load can be driving current through a fault on a completely different circuit so it’s almost impossible to make progress without an IR tester, an RCD tester and a ma clamp meter.

It sounds like the issue is on another circuit, upstream of the 30ma RCD but downstream of the 100ma RCD.
If you are struggling to get someone around you could mention what area you are in.

Hopefully they get someone soon to look at it!

 

[timhoward]

If the incomer is a RCCB then it ought to cut the neutral as well, even if 3-phase, so IR testing L+N to E would show insulation faults.

Yeah thing is I was a little concerned that he might start using non gs38 probes on the bottom of the incoming device - it was a little ambiguous what he meant by testing the incoming supply! Safety first and all that….

 

[pc1966]

Yeah thing is I was a little concerned that he might start using non g38 probes on the bottom of the incoming device - it was a little ambiguous what he meant by testing the incoming supply! Safety first and all that….

Yes, very good point! Getting a non-CAT-IV meter on the wrong range there, or accidentally touching a probe, is probably going to end very badly.

 

[Jackster]

Yeah thing is I was a little concerned that he might start using non gs38 probes on the bottom of the incoming device - it was a little ambiguous what he meant by testing the incoming supply! Safety first and all that….

Yes, very good point! Getting a non-CAT-IV meter on the wrong range there, or accidentally touching a probe, is probably going to end very badly.

My £20 CAT4 meter from eBay is very good. Doubt it will explode. It is Fluke yellow


Thanks for all the tips guys. I have a multi function meter arriving tomorrow on loan from Amazon and will do everything suggested here.

I just remembered that I removed the neutral fly lead from the socket RCBO the other day and it stopped tripping the RCCB.
Pretty sure I tested the RCBO button but I will test again later and report back.

 

[timhoward]

Thanks for all the tips guys. I have a multi function meter arriving tomorrow on loan from Amazon and will do everything suggested here.

For the avoidance of doubt, speaking for myself, I was not suggesting you start diving in with a tester!

 

[westward10]

It is wholly inappropriate for you to obtain a tester and then start testing the circuits. As competent people we understand what, how and why we are testing and in a safe manner, we can also interpret the results we are getting. I would recommend you wait until an electrician can attend your site.

 

[Jackster]

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

 

[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.