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