RCBO not restting

[sparkside1]

Hi all

Have a problem with a Wylex 32A Type C RCBO that has tripped to a utility room and heating plant room to a west London mid terraced house.
It will not reset even with all loads removed. I tried swapping the line and neutral conductors to an adjacent same rated RCBO and the same happened. I tested the ring between L-N at the utility sockets and incoming spur to the plant room and was 0.42 ohms. I only had time for a quick look on xmas eve and will return on Monday to carry out further tests. But not having had many issues with RCBO's I need some advice on how to test these devices properly and what the possible causes could be. There is no history on this fault as the property is vacant and the problem miraculously appeared!

Cheers

 

[shanky887614]

Good luck when you get an insulation fault!

some dmm's can do ir as well

 

[Darkwood]

You have answered your own question there, the fault is present at 230v and is operating the RCBO, therefore higher than that is not required to trace the fault. I appreciate what 500VDC testing provides, but in the case of locating a fault that is tripping a 30ma RCD, 250VDC is adequate and with no risk.

Think about your response... the rcd is operating with 230v RMS but it stil goes through its waveform and peaks at 320v so if the leakage of a fault only occured at 300v plus then testing at 250v would be insufficient as the rcbo is only seeing the leakage at peak waveform but it will still react.

 

[TJ Anderson]

Think about your response... the rcd is operating with 230v RMS but it stil goes through its waveform and peaks at 320v so if the leakage of a fault only occured at 300v plus then testing at 250v would be insufficient as the rcbo is only seeing the leakage at peak waveform but it will still react.

As a theoretical point, what you are saying is electrical fact so I agree with what you are saying. However, what situation can you envisage (in a domestic situation such as the OP's) where there is an insulation fault that would only leak to earth at the peak of the sinewave (320V) enough to operate RCD, yet show a reading on an insulation resistance tester set on 250VDC that would cause no concern to an experienced electrician that there is an insulation issue. On the balance of probabilities, I would say that was a highly unlikely circumstance.

 

[TJ Anderson]

So do MFT's, they do it all. You said "id use my fluke dmm on continuity. it goes a lot lower than the continuity on a megger"

 

[shanky887614]

So do MFT's, they do it all. You said "id use my fluke dmm on continuity. it goes a lot lower than the continuity on a megger"

i prefer to run around using a dmm than an mft for dead tests on fault finding

 

[Darkwood]

As a theoretical point, what you are saying is electrical fact so I agree with what you are saying. However, what situation can you envisage (in a domestic situation such as the OP's) where there is an insulation fault that would only leak to earth at the peak of the sinewave (320V) enough to operate RCD, yet show a reading on an insulation resistance tester set on 250VDC that would cause no concern to an experienced electrician that there is an insulation issue. On the balance of probabilities, I would say that was a highly unlikely circumstance.

You challanged my post, I answered to why we use 500v and not the percentage of it occuring, that fact it can occur justifies my answer and again think about it, any contaminant that might leak on a joint may slowly cause a leakage path and as with most high impedance faults they tend to get progressively worse with time due to carbonisation of the leakage path, believe it or not the circumstances are very common and usually do involve moisture in one form or another.

Have you ever opened a water logged joint tripping an rcd?... you may find it wasn't a one off event that caused the water to build up but progressive until the leakage satified the RCD's operational tripping curve, its the contaminants in the water that create conductance and just splashing water over a L/E joint will not neccessarily trip even a 30mA rcd but leave it and contaminants dissolve into the water and create leakage and tracking and then it satifies the trip value of the RCD.

I design new equipment and machine controls for a living so its my job to understand further than most the circumstances of pollution on electrical joints as thats what you have to deal with in the industrial scene especially my area which is stone cutting, metal shaping and punching and lathes which have steel drums been shaved - all harsh environments for electrical equipment so voltages, IP ratings and knowledge of the contaminants is crucial... this give me an edge on simple domestic problems so Ill stand my ground and say you should be doing standard insulation tests in the OP's case at 500v and its not good advice to suggest 250v.

 

[TJ Anderson]

You challanged my post, I answered to why we use 500v and not the percentage of it occuring, that fact it can occur justifies my answer and again think about it, any contaminant that might leak on a joint may slowly cause a leakage path and as with most high impedance faults they tend to get progressively worse with time due to carbonisation of the leakage path, believe it or not the circumstances are very common and usually do involve moisture in one form or another.

Have you ever opened a water logged joint tripping an rcd?... you may find it wasn't a one off event that caused the water to build up but progressive until the leakage satified the RCD's operational tripping curve, its the contaminants in the water that create conductance and just splashing water over a L/E joint will not neccessarily trip even a 30mA rcd but leave it and contaminants dissolve into the water and create leakage and tracking and then it satifies the trip value of the RCD.

I design new equipment and machine controls for a living so its my job to understand further than most the circumstances of pollution on electrical joints as thats what you have to deal with in the industrial scene especially my area which is stone cutting, metal shaping and punching and lathes which have steel drums been shaved - all harsh environments for electrical equipment so voltages, IP ratings and knowledge of the contaminants is crucial... this give me an edge on simple domestic problems so Ill stand my ground and say you should be doing standard insulation tests in the OP's case at 500v and its not good advice to suggest 250v.

You have not answered my post at all and gone all round the houses about the cause of faults and your job experience. You stated (#63) that 250VDC IR was insufficient because the leakage sufficient to operate the RCD my only occur at the peak of the waveform voltage. I am saying that even at 320V with 20-30ma leakage, 250VDC I/R will still show a very low reading which an experienced electrician could determine as an insulation fault with subsequent leakage. I will add that I still use 500VDC where required by BS7671.

 

[Darkwood]

You have not answered my post at all and gone all round the houses about the cause of faults and your job experience. You stated (#63) that 250VDC IR was insufficient because the leakage sufficient to operate the RCD my only occur at the peak of the waveform voltage. I am saying that even at 320V with 20-30ma leakage, 250VDC I/R will still show a very low reading which an experienced electrician could determine as an insulation fault with subsequent leakage. I will add that I still use 500VDC where required by BS7671.

Yes most of the time even at 250v IR faults would show but your not appreciating dialectric properties of materials here, insulation breakdown is not linear therefore you cannot say that 250v IR test would expose a issues proportionally as that of a reading using 500v. It may be the case that the issue can only be detected at say 300v and remain undetected below that, this could lead to a false positive if testing on 250v, the value may show lower than expected but well within concerning values but at 500v you may see a significant breakdown in IR.

Even though many IR issues can clearly be identified at both test voltages due to the severity it remains the case that you can have a breakdown occuring at only close to peak voltages, now in the days before rcd's were thrown at every situation this would just be a matter of time before the breakdown got worse to cause noticable problems and a sparky called in, rcd's now have such sensitivity that adding general background leakage of devices its possible for the rcd to be sensitive to say 5-10mA of a fault where the rest is made up of electronics goods and other designed leakages, this in itself can make for a device able to pick up peak only issues that cannot be detected at a lower test voltage, also to note is the conductor temp' - because you are not really putting any current down the cable the conductors will not be subject to the same conditions as when loaded so the 500v also compensates for this too.

 

[BES]

Love this site the op has long gone now the usual suspects battle it out to see who has the biggest wang.

 

[Geordie Spark]

Love this site the op has long gone now the usual suspects battle it out to see who has the biggest wang.

I already know the answer to that and that's why I'm not bothering to get involved. :sailor:

 

[TJ Anderson]

Yes most of the time even at 250v IR faults would show but your not appreciating dialectric properties of materials here, insulation breakdown is not linear therefore you cannot say that 250v IR test would expose a issues proportionally as that of a reading using 500v. It may be the case that the issue can only be detected at say 300v and remain undetected below that, this could lead to a false positive if testing on 250v, the value may show lower than expected but well within concerning values but at 500v you may see a significant breakdown in IR.

Even though many IR issues can clearly be identified at both test voltages due to the severity it remains the case that you can have a breakdown occuring at only close to peak voltages, now in the days before rcd's were thrown at every situation this would just be a matter of time before the breakdown got worse to cause noticable problems and a sparky called in, rcd's now have such sensitivity that adding general background leakage of devices its possible for the rcd to be sensitive to say 5-10mA of a fault where the rest is made up of electronics goods and other designed leakages, this in itself can make for a device able to pick up peak only issues that cannot be detected at a lower test voltage, also to note is the conductor temp' - because you are not really putting any current down the cable the conductors will not be subject to the same conditions as when loaded so the 500v also compensates for this too.

That is not practical or indeed necessary. Why would you need to note conductor temperature and compensation for this particular fault in a home. The cable is not loaded, it is dead and won't reset. That is totally OTT and you don't need to for this one. You should appreciate that the fault impedance we are talking about here is the region of 7666 ohms. Initially, 250V range would pick this fault up, and you can identify faulty circuit and subsequently the fault itself far quicker and without the issue of isolating vulnerable equipment. You have even said yourself above "Yes most of the time even at 250v IR faults would show" so why even bother with all this, it is just overcomplicating and confusing for the OP, who I also appreciate is probably long gone now

 

[PEG]

I can see TJ's argument,for many occasions,in a domestic testing environment,but that does not alter the fact that Darkwood is correct. It boils down to absolute electrical fact,not supposition. There are many installations where ONLY being there,and therefore in possession of ALL information,is vital to knowing what results are acceptable,or suspect. We all know,for example,GN3 states minimum values for IR testing,that could be cause for concern on at one job,and perfectly fine at another......now shake hands,and have a nice game of scrabble...:icon12:

 

[TJ Anderson]

I can see TJ's argument,for many occasions,in a domestic testing environment,but that does not alter the fact that Darkwood is correct. It boils down to absolute electrical fact,not supposition. There are many installations where ONLY being there,and therefore in possession of ALL information,is vital to knowing what results are acceptable,or suspect. We all know,for example,GN3 states minimum values for IR testing,that could be cause for concern on at one job,and perfectly fine at another......now shake hands,and have a nice game of scrabble...:icon12:

We are only debating, no problem. We can all quote electrical theory, it is how you apply to a given situation that matters.

 

[Geordie Spark]

I can see TJ's argument,for many occasions,in a domestic testing environment,but that does not alter the fact that Darkwood is correct. It boils down to absolute electrical fact,not supposition. There are many installations where ONLY being there,and therefore in possession of ALL information,is vital to knowing what results are acceptable,or suspect. We all know,for example,GN3 states minimum values for IR testing,that could be cause for concern on at one job,and perfectly fine at another......now shake hands,and have a nice game of scrabble...:icon12:

Sod the Scrabble lark ...... shove them both ootside and let them beat the shyte oot of each other!! :45::boxing_smiley:

 

[Darkwood]

We are only debating, no problem. We can all quote electrical theory, it is how you apply to a given situation that matters.

Yes! very good point for all the readers, its debate and the fact we respond like we do is presidence to our experience and understanding and enjoy the debate, so don't get my responses as a challenge on your competence as its what this site is about but regarding this particular issue and the comment about 250v been sufficient Ill still stand ground as we went off the OP's particular query with the respective posts and that's why I got a bit deeper, and realise you also test at 500v and hence this is just a personal debate but with regard I'm trying to show the reason for 500v testing of IR.... If there exists even a 1% false positive testing at 250v then it justifies the use of 500v, most cable is rated 750v - 1000v so no reason at all as some posts do to suggest that 250v is acceptable ... I've laid my ground for my argument and in most you have technically agreed we me but still back up yourself for your own argument to which you now confuse me, yes as a percentage it may not be likely but the regulations for testing IR voltages exist because all what we debate has already been debate and agreed... PS drunk dot com.... apologise tommorow if this is BS lol ..