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!