RCBO tripping after spur distribution board has already tripped

[Susie]

Rcds operate when there is imbalance of the Live and neutral (current flowing to earth through a fault may cause this imbalance

0.03 is the milli amp rating of the rcd for the earth leakage that will cause it to operate.
Any other rcd in series with this 30 milli amp rcd will also operate under a fault condition
There are rcds that permit a time lag that overcomes this issue, they are called time delay rcds and generally a 100 milli amp rcd, time delayed, will be used on the supply sid,e whereas the 30 milli amp will be used on the loaded side

You appear to have two 30 milli amp rcds in series and a fault in the garage is causing operation of both

Des that was really well explained, and a piece falling into the jigsaw, I think I might get it now- thank you!

 

[Sintra]

Another link that explains what des has already said http://greenbrook.co.uk/pdf/pbcatpg21-22.pdf

 

[not clever]

where in the uk are you? maybe one of the forum members can drop by for you.

 

[Susie]

Thank you, after Des's well worded explanation I get it now, and Sintra the nice little diagram in the last link you posted confirms this. I like to understand these things, and I do find wiring interesting. It is difficult to get hold of a good book on the subject, DIY books don't go much beyond changing a light switch, a course text book would be good but they are hard to get hold of, and even the basic part P course is too expensive to justify when I don't plan on actually doing any of this myself. But like in this instance I was curious about the earthing of the SVA cable, and you guys told me what I had suspected, it would be great to KNOW my stuff so I can understand what electricians are doing. I do really appreciate all your time in answering my questions. Thank you.

 

[Susie]

Hi, I'm in Oxford. I do have a regular electrician who is very good. It was a one off using this guy because he was a friend of a friend and had some job uncertainty, he is a decent guy though and I'm sure he will sort this out. I will be able to nudge him in the right direction now if it doesn't look like he can fix it, and I will ask him to earth both ends of the SVA cable. I have increased my understanding of electrics this evening, so thank you!

 

[Sintra]

Susie don't ask him to earth both ends just the end at the house. The other end should be completely insulated from earth. The only earth you should have at the outbuilding needs to be from your earth electrode (rod). THIS IS VERY IMPORTANT.

 

[Susie]

Susie don't ask him to earth both ends just the end at the house. The other end should be completely insulated from earth. The only earth you should have at the outbuilding needs to be from your earth electrode (rod). THIS IS VERY IMPORTANT.

Susie don't ask him to earth both ends just the end at the house. The other end should be completely insulated from earth. The only earth you should have at the outbuilding needs to be from your earth electrode (rod). THIS IS VERY IMPORTANT.

Oh dear I am coming across as an idiot this evening. So why then does it matter which end of the cable is earthed? Ok so let's see if I can work this out- at the risk of coming across even more blonde- The SVA can act as a back up earth if there is cable damage and the insulation round the core is damaged so electrons can leak to either the central earth or the steel armouring, so it needs earthed, I understand that.

There an earth cable in the insulated core of the armoured cable, and this will presumably be connected to BD1 and BD2 so why does is matter if the steel armouring is also earthed to both?

Circuit breakers activate if the leak of current exceeds 0.03millimamps (or whatever the rating of the circuit breaker) and they take this reading from the difference in currents between live and neutral (the loads should be equal). So surely the circuit breaker is only interested in the current loads in live and neutral (and not earth)?

Why can't an earth wire be earthed to two separate points? It's only purpose is to dissipate electrons to the ground. Do the electrons have to flow in just one direction, do two points 'confuse' them, that doesn't sounds right. Is there a risk that the earth wire acts as a bridge to bypass something else? I think I need to look at the links above and learn more about distribution board functions to see if I can work out an answer.

Ok it is late, I'm coming across as stupid. Don't tell me yet let me see if I can work it out tomorrow!

(And sleep well knowing I don't touch distribution boards or do wiring and I do have a fully qualified electrician who will come back and fix this, even if he apparently got the SVA cable earthed at the wrong end).

night

 

[Sintra]

Ok Susie you have a think about when you have had your beauty sleep. I will explain why later.

 

[Octopus]

Susie. Time to get a proper sparky in with the necessary tools and competence to sort it out.

 

[Des 56]

Susie don't ask him to earth both ends just the end at the house. The other end should be completely insulated from earth. The only earth you should have at the outbuilding needs to be from your earth electrode (rod). THIS IS VERY IMPORTANT.

Susie,the type of earthing arrangement at the supply should have determined the electricians choice of whether to utilise that earthing system or provide his own (TT)

The supply earthing system, if used to extend out of the household earthed equipotential zone,can become a potentially unsafe system for the user in certain circumstances, unless action is taken to ensure the extended earthing zone from the house has adequate earth bonding
Earthing the swa at both ends in this situation can extend the zone without adequate precautions having been implemented and may be potentially unsafe
I have not read anywhere that you stated whether the supply is PME or no,but would advise that you look up information on system earthing types, equipotential bonding and the dangers that can occur,thinking about leakage to earth of the swa and rcds is not the right approach

 

[Susie]

Susie. Time to get a proper sparky in with the necessary tools and competence to sort it out.

Hi Murdoch, do not fret, I do have a proper qualified sparky, I don't do electrics I just like to understand things. ( Although I'm not doing a great job on this occasion). I should point out that it looks like the proper sparky has made a mistake here, everybody makes mistakes, but if I know my stuff as well then there is less chance of a mistake slipping by.

 

[Susie]

Ok, so deep breath, here we go;

I now know that bonding and earthing are different. A TT earthing system is one where the user doesn't earth via the supply from the electrical company but provides his own earth in the form of an electron rod, aka copper rod aka earth rod.
[I have an earth rod at BD1 as well as BD2].
Then I looked at earthing and the standard method of tying the electrical supply system to earth is to make a direct connection between the two. "This is usually carried out at the supply transformer, where the neutral conductor (often the star point of a three-phase supply) is connected to earth using an earth electrode or the metal sheath and armouring of a buried cable".
So if the cable runs between BD1 and BD2 and at each board the earth and neutral are connected, then if the SVA armouring is also bonded to earth at each end, then it completes the loop and current can flow through the armouring?
But would this not be the same with the earth cable in the insulated core of the SVA cable? Ah no it wouldn't because it is not bonded, and it is the bonding that would complete teh loop from one earth rod to the other and complete a circuit. I think!

Ok Sintra and Des, how did I do, am I close?

Wait I still haven't answered why it should be bonded to earth at BD1 end and not BD2 end.

 

[Octopus]

Hello, sorry about the time lag I was off googling the bits I didn't know- telectrix nothing google has to tell me about overcurrent or earth leakage makes me happy! Not least because the management squabble that left this sparky not sure if he would have a job or not was quickly resolved so he is back in full time work and can only finish this at weekends, and the next time he is free to fix this is probably January.

I have to admit I mentioned the not earthing at the house end as I was curious about that, I will try and politely ask him about it when he comes back!

Get him back or pay some one else who has the skill, tools and competence. Thats if he really exists - and before you comment we get so many posts here eluding to people having sparkies then later it transpires they are doing it themselves.

 

[ses]

This is a recipe for disaster

 

[Susie]

Murdoch,
I genuinely do not do any electrics, but yes I am sure you do have people trying to get instructions as to how to do the work themselves, I'd hope it is clear from the content of the posts on this thread that I am not one of them, if you look I am not asking HOW to do stuff but I am asking WHY you do things in a certain way. This really is just my curiosity.

I have said earlier on that this electrician will be coming back to fix the problem. He is fully qualified and has been working for at least ten years. Through no fault of his own he was out of work for what turned out to only be a couple of weeks, but he was worried and as a friend of a friend I was asked if I had any work for him, and I did have this project on the back burner so I gave it to him. I've not tried to do anything on the cheap or use someone under qualified. It probably was a mistake not to use my normal electrician, but I have worked with him enough that I don't think he'd mind me giving one small job to someone else.

I know it must be frustrating with DIYers taking work from you as a qualified electrician, but I suspect you get plenty of work sorting out the mess they make.