Urgent advice!!!

[deanglossop]

I no theres a seperate feed for outbuilding surplies but need some urgent advice on wiring a d.b. and circuits in a newly built large shed, being used as an outbuilding.

The garage will have three circuits. One lighting wired in 1mm twin an earth on a 6a type b circuit breaker. One power in 2.5/1.5mm twin an earth on a 20a type b circuit breaker. One circuit powering a 3kw water heater in 2.5/1.5mm on a 16a mcb.

I was goin to run 2 core, 10mm swa cable from the main consumer unit to the garage d.b. part clipped direct to the external house wall and part burried and use the swa as main earth. The d.b. will have fault protection by the 80a rcd in the house consumer unit. I will terminate the swa in two metal boxes attached flush with the c.u. and d.b. using usual banjo method.

I was going to use a 42a type b in the house comsumer unit feeding the swa. The house earth arrangement is tn-s and has a Ze of 0.12 and PFC of 2.2 Ka.

My main query is do i need an rcd in the garage d.b. (if so what size) and whether anyone would wire this job differently. I have done the calculations using design current etc and as far as i can tell it is all above board, but would appreciate someone else checking or giving useful feedback.

Also please dont feel the need to remind be about part p etc

thanks

 

[ian.settle1]

Is ther a RCD in the main building supplying the garage? if you fit a second one down stream then there will have to be discrimination between them

And there is no such thing as a 42A type B mcb.

 

[ezzzekiel]

there is no need for rcd as there is one upstream, although it would be better to have the rcd at garage end so as not to interfere with house electrics

 

[GStueyXR]

Just run DB circuit from an unprotected way and fit RCBO....

 

[pushrod]

Are you using 10mm SWA because it is a long run and you are worried about volt drop, because if not 6mm would cope.

 

[deanglossop]

haha sory, meant 40a. Volt drop is fine but I was going to use 10a to ensure sufficient enough power for any additional circuits in the future. So basically i can either

a) have no rcd at the outbuilding, but in the event of a fault the house rcd will trip.
b) have a second rcd or rcbo at the outbuilding and will have to consider discrimination by posibly putting it on an unprotected way.

Allthough I prefer option a) as it is cheaper and tripping is not a frequent event my main concern is how well this will conform with bs7671?

 

[pushrod]

ok i'll mention it - the other thing to consider is earthing and if you don't export it whether the armouring is up to the fault current.

 

[ezzzekiel]

Is ther a RCD in the main building supplying the garage? if you fit a second one down stream then there will have to be discrimination between them

And there is no such thing as a 42A type B mcb.

sorry, your post beat mine

 

[nickblake]

I i do feel the SWA will be to Small for use as CPC may be an idea to install an additional 16mm earth to , one reccomendation though you could install a 100ma RCD at the main house and then use a main switch30 ma RCD in the shed this way you are protecting the cable from the house to the shed , and if the 30 ma RCD in the shed trips it wont cause the RCD in the house to trip , unless you have a fault drawing more than 100 ma

 

[Guest123]

A PVC SWA (70 degree C), 10mm 2 core armouring is more than sufficient to be utilised as the CPC.

An XLPE SWA (90 degree C) 10mm 2 core armouring is not.

I agree with the 100mA RCD at the supply end, only It should be time delayed as there still wouldn't be any discrimination between that and the 30mA one at the load end otherwise.


Also bear in mind these are our own personal preferences.

 

[pushrod]

A PVC SWA (70 degree C), 10mm 2 core armouring is more than sufficient to be utilised as the CPC.

An XLPE SWA (90 degree C) 10mm 2 core armouring is not.

I agree with the 100mA RCD at the supply end, only It should be time delayed as there still wouldn't be any discrimination between that and the 30mA one at the load end otherwise.

Also bear in mind these are our own personal preferences.

Hi Lenny, that reminds me of a conversation i was having other day, are you saying that - a 30mA rcd down stream of a 100mA one does not provide discrimination in the event of a normal large earth fault, only if it was just say a 50mA fault. But a time delayed 30mA could provide discrimination to a theoretically identical 30 mA one ? I know two 30 mA rcds are unlikely to trip at exactly the same fault current so you would really want the time delayed one to be larger.

cheers

 

[Guest123]

There's no such beast as a 30mA time delayed RCD. If we think about it, the 30mA RCD is there as additional protection and is rated as such to operate before our heart stops (technically), to time delay it would only serve to cause us more harm and defeat it's purpose.......

The 100mA would need to be time delayed for discrimination although as you say theoretically it shouldn't operate below 50mA (0.5 x I delta N).

A fault above that level on a final circuit in the garage and there's no telling which one or indeed if both would operate, hence the need for the time delay.

 

[pushrod]

A fault above that level on a final circuit in the garage and there's no telling which one or indeed if both would operate, hence the need for the time delay.

Cheers that's is what i was saying, but i wasn't sure because if for example you had a 5amp fuse downstream of a 30 amp and you had a 200amp fault current the 5amp would blow but the 30amp wouldn't (no practical experience of it - just what i've read/been taught) wasn't certain if the same principle applied to rcds

 

[Guest123]

I get you.

Overload and earth fault.....different beasts.