RCD test button not working

[appspark]

Hi all, was testing some lighting circuits today(nice easy friday job), over a few different boards, ze, r1+r2, zs coming up normal. putting in the lamps, first one blew the bulb straight away, oh well i though stupid you for leaving the switch on(even though i'd done 4 flats previously with no probs), switch off, new bulb in, turn on and fine.
Move to bedroom, switch off this time, put in bulb, flick switch, rcd trips, pull up rcd again both lights on fine, turn bedroom switch on and off a few times and pop goes the rcd. hmm, so i open up the switch and rose again to make sure all is in order, seems good.
go back to board to reset rcd, think, oh il just test it incase the contacts are playing up and the test button doesn't work. its a split way board, 63a main switch with 2 30mA rcd, left side wont trip when test button is pushed but the right side seems fine, double checked all connections, can Zs of far side of both rcd's(same reading).
anyone any idea what would cause this or is it just a gubbed rcd?
Thanks for the help.

 

[telectrix]

posssible N-E fault. remove all load from RCD the try the test button again. then get mft on it and do a proper test.if it then works, you have a fault on a circuit.

 

[camerabloke]

Deleted

 

[Pete999]

Pedantic Pete's on the warpath Mate no such thing as r1+r2, what sort of tests did you carry out on both RCDs?

 

[appspark]

tel, tried test button with no load and no difference.
Have just thought though, in one the boards(most likely this one), there was a N-E short on a ring main(found during IR test), wasn't needing it on but noticed when i tried to save taking the earths and neutrals out of their terminals. this could be the problem then, thanks.
pete, on both lighting joined line and cpc at boards and went round each rose and took highest reading. then insulation resistance tested each individually), then zs at rose which had highest reading before. may have just solved it when i've though about tels reply above.

 

[telectrix]

and just to muddy the water, the fault could be on the other RCD as well.

 

[Engineer54]

and just to muddy the water, the fault could be on the other RCD as well.

I maybe having a brain fart here (always possible before my second morning coffee...lol!!) but what external influences are going to affect the operation of the internal test circuit button??

 

[Guitarist]

I maybe having a brain fart here (always possible before my second morning coffee...lol!!) but what external influences are going to affect the operation of the internal test circuit button??

My thoughts exactly. As long as you have a line and neutral correctly connected, then the test button is only there to test the mechanism by producing an internal imbalance. Faulty RCD seems likely here.

 

[telectrix]

other week i went to a house where both RCDs on a MK CU would not trip either on test or with the button. found an unused 2.5mm radial with a fault. once this had been dissed, both worked perfectly: buttons, x1, x5, and ramp test. don't ask me why or how, but that's what i found.

 

[Richard Burns]

If there is a neutral earth fault in a circuit then when the RCD test button is pressed the current that should flow from the test button, to out going neutral, though the coil and then via the incoming neutral, can flow instead along the final circuit neutral through the fault and to earth bypassing the coil in the RCD.
If the relative resistances of the two possible paths are right there then may not be enough current to trip the RCD.

 

[GLENNSPARK]

I maybe having a brain fart here (always possible before my second morning coffee...lol!!) but what external influences are going to affect the operation of the internal test circuit button??

its hard neutral-earth faults eng.....

causes some RCDs to not work at all....even on manual test..

 

[Lucien Nunes]

If the test button circuit goes from supply L to load N, a downstream N-E fault can stop it working by reducing the test current returning to the supply N through the sensing transformer. But if the test was from load L to supply N, the fraction of test current going to earth at the fault would pass through the transformer twice in the same direction, possibly making it test OK even if it had reduced sensitivity. This makes me think that all RCDs have the test circuit the first way round, but I can't say I've spent enough time looking at the schematic on the side to notice.

 

[PEG]

Nice explanation,Lucien. I have stripped a fair few for fun,and most standard items seem configured as you first suggest,with the test circuit coil,that simulates the imbalance,linked from supply line to load neutral. A N-E fault downstream would divert returning current from sensing coil and not trip. There's no way round it,my faith in test buttons has evaporated,and in future,i am amending my quarterly press of the button to include mandatory disconnecting of out going L and N. Some of my less able clients are going to struggle...

 

[Geoffsd]

If this is the case then wouldn't the RCD already have tripped in normal use?

 

[appspark]

Hey guys i've got an update for you. There was a ring main with a N-E fault on the same RCD. had to put in a new leg, guessing the door had a fixing through the cable, couldnt pull it at all, site agent wasn't too happy, flats were basically being commissioned. When i removed the ring from the neutral and earth bars the rcd would trip on test. The lighting circuit was functioning properly now aswell.
so in conclusion the N-E fault on the ring main was stopping the rcd from functioning properly.

 

[dazza1981]

Hey guys i've got an update for you. There was a ring main with a N-E fault on the same RCD. had to put in a new leg, guessing the door had a fixing through the cable, couldnt pull it at all, site agent wasn't too happy, flats were basically being commissioned. When i removed the ring from the neutral and earth bars the rcd would trip on test. The lighting circuit was functioning properly now aswell.
so in conclusion the N-E fault on the ring main was stopping the rcd from functioning properly.

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

 

[appspark]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

 

[dazza1981]

i probably spent a good 8 hours in total tracing the fault and getting my head around it, then another 2 to replace the faulty leg. definitely learned a few things along the way, i actually had the same fault on another board, loose earth at a socket shorting to neutral, much easier to rectify.

I know what you mean i had N/E fault the other week in a big cottage and it took me best part of a day to find it and rectify it...... i swear i must have done 1000 laps of that house lol

 

[jaypp]

Appspark i congratulate you on not only tracing the fault but also letting us know what the problem was. Not many do !!!

Indeed.

 

[Midwest]

posssible N-E fault. remove all load from RCD the try the test button again. then get mft on it and do a proper test.if it then works, you have a fault on a circuit.

I've had same situation, found neutral earth fault on radial on a rewire doing IR test. Put that circuit to one side to get the rest of the house powered up, except I mixed it up with another radial and actually connected it up! Went on to do remaining tests, but Tester would not do ZS nor RCD tests on that side of the CU. Test button also wouldn't work. Went back to the beginning and found my mistake.

 

[Negrita]

Hi guys I'm an amateur but I have the same problem. Can you explain what you mean by an N-E fault (an earth cable on the neutral and vice versa?) Also if I swich of each MCB one by one and try the test button would that identify which circuit the fault was on? I won't fix it myself but I like to understand what's happening and where.

 

[brianmoooore]

First of all, there is a very low resistance between the earth and neutral wires where they enter your property. If you have a TNC-S (PME) earthing system, they are actually connected directly together at the service head.
If there is a short circuit or bad leakage between the earth and neutral wires anywhere in your property, some of the current flowing normally in the neutral wire on an unrelated circuit anywhere else in the property will find its way through this N-E fault, back through the earth wire to the service head, and then flow into the incoming neutral, so bypassing the N side of the RCD, while the full current is passing through the L side.
Very difficult to diagnose without the correct equipment, but hopefully should become a problem of the past, as RCBOs which switch both N and L (far from all do at the moment) become more common.

 

[Negrita]

Thank you.
So can this be caused by an appliance that is plugged in and has a fault?

 

[Lucien Nunes]

Yes, appliances can be equally responsible.

 

[Negrita]

So unplugging all appliances and retrying the test button might be a good first step - or the house has 11 mcbs might turning them off one and trying the test button by one identify a duff (technical word) circuit?

 

[Lucien Nunes]

Turning off normal single-pole (SP) MCBs won't narrow it down, as they don't switch the neutrals. Hence Brian's comment that over time, with the increasing use of RCBOs with double-pole switching that do isolate the neutrals, troubleshooting will get easier.

 

[brianmoooore]

Thank you.
So can this be caused by an appliance that is plugged in and has a fault?

And turned off at the socket, in most cases. Some sockets have double pole switching, but they are rare.
This is something I've spent hours on the 'phone, trying to explain customers, while trying to avoid being dragged out on 'emergency' calls.

 

[Negrita]

So how common is it for the actual RCB0 to fail?

 

[brianmoooore]

Not unknown, but rare, as long as the test button is pressed a couple of times a year.

 

[Negrita]

Thank you - but what if it hasn't been pressed for say 3 years - I am just buying a socket tester with a "RCD test button" I will try this on a circuit to see if that trips the MCB (I assume that "safely" shorts the live and earth) I will explain the "3 Years later!"

 

[brianmoooore]

If a RCD isn't tested regularly, the hair trip mechanism can become stuck in place and the contacts of the test button can tarnish. If you can get it to trip, and then 'exercise' the mechanism, by tripping and resetting it several times, they can usually be brought back to life.
Note that anon working test button is a fail when an electrical condition report (EICR) is undertaken, and will result in the installation being declared 'unsatisfactory'.

 

[Negrita]

Thanks for all your advice I really appreciate it - so trying to trip the MCB via a socket tester would show if the problem was with the test button and if the RCD was working or not?
Now I will explain the three years - The Consumer unit is in Thailand! and was installed by a "professional" electrician (at least as professional as you will get in Thailand when we built the house - EICR in Thailand -non existent) 8 years ago.
I tested the RCD at least twice a year until - Covid struck and I was in the UK and my Thai partner was in Thailand we were apart for just over 2 years and asking her to check it would have been a useless exercise and I must admit when I got back in January I forgot to check it until now!

 

[brianmoooore]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

 

[Negrita]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

I understand what you are telling me and of course if I find I need to replace it I will get a "professional" to do that.
This is the tester I am buying - I'll let you know and thanks again

 

[Negrita]

Your socket tester is better than nothing, in that it will confirm that the RCD is kaput, if it is, but a full test involves checking that it trips within a certain range of leakage current, and measuring the time it takes to disconnect the power, which has an upper limit.

 

[Negrita]

Sorry to bother you again but I have just noticed that the RCD in my consumer unit has a variable trip can be set to 5, 10, 20, 30 (currently 20) and the socket tester is available in 30 Ma or 5 Ma RCD tester should I buy the 5 Ma socket tester not the 30 Ma tester as being more sensitive?

 

[westward10]

Can you show a picture of the RCD.

 

[Negrita]

Can you show a picture of the RCD.

Sorry for the delay - it's dinner time over here

Can you show a picture of the RCD.

 

[westward10]

The 30ma tester should trip it. Being set at 20ma anything above this which 30ma is should cause it to trip.

 

[Negrita]

The 30ma tester should trip it. Being set at 20ma anything above this which 30ma is should cause it to trip.

Thank you

 

[pc1966]

That is the first time I have seen an adjustable RCBO!

Generally if you get a 30mA tester it will show if any RCD/RCBO that is rated from 5-30mA is working OK or not. It is not as precise a test as an electrician can do with the appropriate test equipment to verify the trip-time (usual test at fixed 'fault' current at device's specification, e.g. 30mA) and the trip current (done by slowly ramping up the 'fault' current to see at what point it fires). Any external tester would confirm if it is the RCD that has failed/stuck stuck, or if it is the self-test button that has failed.

The use of 30mA is normal in the UK/Europe for most applications as that is the threshold for 'reversible effects' (i.e. it hurts but you are OK immediately afterwards):

Electrical injury - Wikipedia

en.wikipedia.org

What RCD/RCBO don't do is limit the shock current, what they do is limit the exposure time above a certain current which limits the physiological impact but it still depends on how conductive the event was (e.g. if dry skin contact or wet, etc ,etc). So RCDs are very useful in the prevention of death/injury but are always "additional protection" along with the basic protection of suitable insulation and earthing of metalwork, etc.

 

[Negrita]

That is the first time I have seen an adjustable RCBO!

Generally if you get a 30mA tester it will show if any RCD/RCBO that is rated from 5-30mA is working OK or not. It is not as precise a test as an electrician can do with the appropriate test equipment to verify the trip-time (usual test at fixed 'fault' current at device's specification, e.g. 30mA) and the trip current (done by slowly ramping up the 'fault' current to see at what point it fires). Any external tester would confirm if it is the RCD that has failed/stuck stuck, or if it is the self-test button that has failed.

The use of 30mA is normal in the UK/Europe for most applications as that is the threshold for 'reversible effects' (i.e. it hurts but you are OK immediately afterwards):

Electrical injury - Wikipedia

en.wikipedia.org

What RCD/RCBO don't do is limit the shock current, what they do is limit the exposure time above a certain current which limits the physiological impact but it still depends on how conductive the event was (e.g. if dry skin contact or wet, etc ,etc). So RCDs are very useful in the prevention of death/injury but are always "additional protection" along with the basic protection of suitable insulation and earthing of metalwork, etc.

 

[Negrita]

As promised feedack on this thread - socket tester with RCB test button fails to trip MCB on any circuit.
So I guess that means a new RCB.
Once again thanks to all those who gave me advice - and the seasons greetings.

 

[brianmoooore]

Just noticed a little switch, labelled on/off on the pic of this RCBO. Is this to switch off the RCD part, just leaving the MCB?
Before giving up on this device, I'd give it a good 'exercise', by switching it off and on again several times..

 

[Negrita]

Just noticed a little switch, labelled on/off on the pic of this RCBO. Is this to switch off the RCD part, just leaving the MCB?
Before giving up on this device, I'd give it a good 'exercise', by switching it off and on again several times..

Thank you for that - this is what happened - I switched of the little black switch and inserted the socket tester in a socket and pressed the test RCD button - One of the main switches tripped and the black button (which is labelled earth leakage indicator) came out (see pic) resetting the earth leakage black button allowed me to reset the the switch that had tripped.
Not really sure what that is telling me except that isolating the RCDO appears to provide some degree of protection (and maybe that the RCDO is Defunct).
Until I get an electrician to look at it do you think it would be better to leave the RCDO isolated?

 

[brianmoooore]

The switch that you have the red arrow pointing at IS the RCD, as far as I can make out from the pic, and tripping to the downwards position is exactly what it is supposed to do when earth leakage is detected. You introduced an imbalance between the live and neutral currents going through it, by using your socket tester, and the RCBO indicated earth leakage to be present (a feature not found on UK RCBOs) and then the big red switch turned itself off.
This is exactly what it should do, as far as I can tell from here.

 

[Negrita]

The switch that you have the red arrow pointing at IS the RCD, as far as I can make out from the pic, and tripping to the downwards position is exactly what it is supposed to do when earth leakage is detected. You introduced an imbalance between the live and neutral currents going through it, by using your socket tester, and the RCBO indicated earth leakage to be present (a feature not found on UK RCBOs) and then the big red switch turned itself off.
This is exactly what it should do, as far as I can tell from here.

Once again thank you Brian.

I must admit to being somewhat confused. The black switch that you told me to exercise was ON when the test button on the consumer unit would not work – after you told me to exercise the switch, I switched it OFF and then tried the socket tester and the trip worked!

I have now tried the test button on the consumer unit and it works correctly – this is with that little black switch set to the OFF position! So I’m unsure what that switch is switching ON and OFF.

Very confusing for me (but maybe not for you!).

Another thing I have now noticed is this: the picture I sent you was taken in May this year just before we came to the UK for 5 months (I added the arrows just before I sent it to you). In that picture you can see the little black switch is OFF and the trip value was set to the minimum 6mA.

I haven’t changed any of the settings since my return but the mA is now set to 30 and as I said above the little black switch was ON.

A Thai neighbour “took care” of the house whilst we were away and I am wondering if some “tinkering” took place for some reason.

Anyway, these are my problems and getting an admission from a Thai is impossible – I am happy that the RCBO protection is working. (not sure about the MCBs for tripping although all circuits appear to be working).

Finally, I worked in IT for 40 years (I’m 78) and I know how difficult it can be for a professional dealing with people who already have some knowledge, but limited, on a subject, so I say again I really do appreciate that you were prepared to offer your assistance with this issue.