RCBO tripping after spur distribution board has already tripped

[Susie]

Hello All,
So to be clear I'm not an electrician, I just like to know how how things work. I will run cables so it's all ready for my sparky to make the connections but I don't do any electrical work. Normally I'd ask my sparky if there is something I don't understand, but I can't on this occasion as I'm using someone different* and I've always found trade forums a brilliant place to learn stuff so here goes;

[*A friend of a friend is doing the work, he is fully qualified and I know he will come back and sort out this problem, as I said this is just my curiosity]

So we're wiring from the main house to a workshop at the bottom of the garden.The workshop has it's own distribution board run from a dedicated RCBO on the main house distribution board. When we switched a light on in the workshop it lit for about 2 seconds then tripped the workshop board, AND the RCBO in the main house distribution board. This is what is foxing me, surely if the workshop board trips the main house RCBO shouldn't?

So the details;
Main house distribution board (fitted 2012 meets 17[SUP]th[/SUP] ed) – let's call in BD1

→ 16amp RCBO to outside circuit. (NB I know this needs to be a 32 amp but the electrical retailer gave me the wrong shape RCBO so we had to use an existing one, it will get swapped when the sparky returns. )

→ cable goes out through external wall to waterproof junction box, from here a spur to a waterproof socket AND

→ 10mm SVA cable runs for approx 38metres underground to workshop building

→ workshop building has it's own distribution board , let's call it BD2 ( this BD I've recycled from an older project but still in good condition so I don't think there is a problem here, can give more info if you think could be relevant)

NB the SVA cable is not earthed at the house end of the cable but BD2 has an earth spike fitted to it, so sparky said it didn't need earthing at the BD1/ house end

→ the workshop has one lighting ring circuit and a single radial run, ( he may have wired these to one MCB, I didn't look to see what he did on BD2). And one power ring circuit (8 double sockets).

(Oh I just remembered that when I ran the wire for the lighting radial circuit I accidentally used 3 core and earth instead of 2 core, but I told him this and he so he just didn't use the third wire. I don't think this could be the problem, it isn't the lights I switched on anyway. The single radial run is in the correct wire, as is everything else)


So stick for accidentally picking up 3 core instead of 2 core aside, ( I bet most of you have done it at least once!) Can anyone explain to me why the RCBO on BD 1 is tripping when BD2 trips?

 

[Sintra]

Has BD2 got an RCD protecting the circuits or are the circuits on BD2 protected by RCBO.

 

[telectrix]

possible a lamp blew and took out the RCBO/s on overcurrent as opposed to earth leakage.

 

[Susie]

I'd think so- I'm pretty sure BD2 had an RCB on it, so I don't know why he wouldn't have used it. But the only way to be sure is to take the cover off the board and see what he's wired to where, and right now it is cold outside, dark in the workshop, and as I don't know what is live I'd rather not go look! I will just go and check I'm right that there was an RCB on the board though.

 

[Sintra]

Sounds like you have a fault on a circuit on BD2. This fault will trip 2 RCD's / RCBO's of the same rating.
Seems like you have 2 x 30mA devices in series plus a fault.

 

[telectrix]

and just to point out from your OP, the armour of the SWA cable must be earthed at the supply end, even if you are not using it for earthing the workshop.

 

[Susie]

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!

 

[Sintra]

Yes Tel is correct the supply cable should be earthed at the house end and not connected to earth at the outbuilding.

 

[Susie]

Hi Sintra, thank you for this.
As I'm not an electrician can I just clarify- the RCB on BD2 and BD1 are probably the same value, i.e 16amps, so that is why they are tripping at the same time, and if the RCB on BD2 was of a lower ampage, or the RCBO on BD1 the 32amps that I think it should be, then only BD2 would have tripped?
I have to admit I find this really interesting that they work in unison because they are of the same rating, my basic understanding of electrics would still have lead me to think that BD2 would trip first as it was 'the first line of defence', but I do understand what you are saying, and as current 'flows quickly' it acts on both equally rated circuit breakers at the same time. Thank you.
I'm going to have to brave the cold and see if the RCB on BD2 is 16amps now.

 

[Sintra]

It's not the 16A overcurrent part of it that is relevant it is the earth leakage rating of 30mA that will cause the problem.

 

[Sintra]

My suggestion would be this

1. Find and repair fault at outbuilding
2. Change outdoor socket to one incorporating a 30mA RCD
3. Supply cable from house to outdoors with a standard MCB
4. Connect supply cable to earth at house end
5. Have a 30mA RCD in BD2 protecting its outgoing circuit(s)

 

[Susie]

Hi Sintra, thank you I will find a way of giving him that crib sheet when he returns. For now I will need to go and read and learn a bit more to fully understand the problem. But thank you for your patience and thanks for your answers. (There was nothing on the BD2 RCB that made sense to me either- 63 and 0.03 neither number looked like an amp rating to me). Hmm any recommendations for books on the subject?!

 

[Sintra]

Hi Sintra, thank you I will find a way of giving him that crib sheet when he returns. For now I will need to go and read and learn a bit more to fully understand the problem. But thank you for your patience and thanks for your answers. (There was nothing on the BD2 RCB that made sense to me either- 63 and 0.03 neither number looked like an amp rating to me). Hmm any recommendations for books on the subject?!

Hi Susie the 63 refers to the amp rating for overcurrent ie 63 Amps and the 0.03 is the amp rating for earth leakage ie 30 milli amps.

Let me see if I can find some info for you on the subject.

 

[Des 56]

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

 

[Sintra]

Susie have a look at this link it might help explain the different type of devices. http://www.mkelectric.com/Documents...l Specifications/Circuit Protection/RCBOs.pdf

 

[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.

 

[ses]

You said he ran a 10mm swa with a spur to a socket what size was the spur

 

[Octopus]

(Oh I just remembered that when I ran the wire for the lighting radial circuit I accidentally used 3 core and earth instead of 2 core, but I told him this and he so he just didn't use the third wire. I don't think this could be the problem, it isn't the lights I switched on anyway.

I thought you stated you didn't do any of the work. Make up your mind and stop telling stories.

This reminds me of a thread a while bacak where the OP stated he was checking what the sparky was doing, etc, etc then a couple of months later posted he was getting his part P registration.

 

[Susie]

Murdoch you have PM. In it I have explained my background to you further as I do understand your scepticism, but I hope that you will give me the benefit of the doubt on this occasion. Putting a bit of cable in a ceiling before it gets plastered is very different from touching any kind of live installation, I simply am not so stupid as to try that. I like to know how things work, and I have really appreciated the time other members have taken to help me increase my understanding. Again I will say I am not askign HOW to do things but WHY. This is curiosity nothing more.

 

[Octopus]

Murdoch you have PM. In it I have explained my background to you further as I do understand your scepticism, but I hope that you will give me the benefit of the doubt on this occasion. Putting a bit of cable in a ceiling before it gets plastered is very different from touching any kind of live installation, I simply am not so stupid as to try that. I like to know how things work, and I have really appreciated the time other members have taken to help me increase my understanding. Again I will say I am not askign HOW to do things but WHY. This is curiosity nothing more.

And I've read it and was very tempted to copy and paste it into this thread!

 

[Des 56]

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

Well.even if you were telling porkies and Murdoch is correct,at the least you are trying to understand why things are done a particular way and where the theory relates to these particular ways,instead of going in blind and installing willy nilly as many home owners are known to have done,"it works don't it"

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


Um Not yet,but keep at it and your understanding will improve

 

[Sintra]

Have a look at what you can find regarding extraneous conductive parts and bonding back to the main earth terminal.

By the way what is the construction of the outbuilding block, timber, steel etc. and does it have any other services like a water or gas supply?

 

[Susie]

Concrete raft foundation and clad timber frame. No gas or water.

Thank you for the reply, and thank you for the pointer I shall go and google!

 

[Susie]

Well.even if you were telling porkies and Murdoch is correct,at the least you are trying to understand why things are done a particular way and where the theory relates to these particular ways,instead of going in blind and installing willy nilly as many home owners are known to have done,"it works don't it"

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


Um Not yet,but keep at it and your understanding will improve

Argh, I feel like a school kid! Ok so I will keep looking. Thank you!

 

[not clever]

Do away with the rcbo's/rcd in the workshop & use MCB's. Keep the rcbo for the submain at the house end (will be a pain the the backside when it trips & you have to wander back to the house)this will provide 30mA protection for submain & all workshop circuits.
Connect swa to earth by using the gland banjo or a earthnut, no worries if its both ends or one end as you say the house supply is also TT.

 

[richy3333]

And I've read it and was very tempted to copy and paste it into this thread!

Am Ithe only one gagging to know what she said?