RCBO needed or not???

[matt.leung]

I am installing a garage CU which was a 40A 30mA RCD.

In the house it is a split load, protected and non-protected... I am having to install the new circuit on the non-protected due to lack of room and stuff can not be shifted round either..

the question is - so i need to install a RCBO in the main CU even thought there is a RCBO in the garage CU...

if so i would it have to be a 40A 30mA as they cost in the region of

 

[ianmacd]

It would be best not to RCD the house end to provide discrimination with the one in the garage but you will need to ensure that the distribution circuit does not require an RCD for other reasons (like concealed in a wall etc....)

 

[Lurch]

the question is - so i need to install a RCBO in the main CU even thought there is a RCBO in the garage CU...

Depends on the cable and where it is run etc... If it's buried internally at <50mm and\or is not mechanically protected etc... Basic stuff, take a look in the OSG or BRB, it tells you what to and what not to RCD protect.

if so i would it have to be a 40A 30mA as they cost in the region of

 

[ianmacd]

if there's a 30mA RCD on the other end of it then there should be some discrimination, so if it needed a 30mA RCBO at the main CU then you need a <30mA one in the garage

That will not provide discrimination. You would need a time delayed RCD to provide discrimination.

 

[matt.leung]

cable will be in mini trunking which is chased into the wall along the bottom of the kitchen walls. the cable will be 6mm T&E. this will then be terminated into a JB box and then swa outside into the garage CU.

it will be powering 2 sockets and about 4 lights (bulk heads)....

 

[Lurch]

cable will be in mini trunking which is chased into the wall

I see.

That will not provide discrimination. You would need a time delayed RCD to provide discrimination.

Well no, you'd need the opposite, wouldn;t you? 30mA 40mS in the main CU as mandated by the regs, so the downstream one would need to act quicker.

General gist is still the same, if there's an RCBO in the main CU then there's no need for any more.

 

[ianmacd]

I see.Well no, you'd need the opposite, wouldn;t you? 30mA 40mS in the main CU as mandated by the regs, so the downstream one would need to act quicker.

General gist is still the same, if there's an RCBO in the main CU then there's no need for any more.

Indeed. No need for one outside but still 2 RCD's in series without the upstream being time delayed will not satisfy the need to discriminate.

 

[batty]

cable will be in mini trunking which is chased into the wall along the bottom of the kitchen walls. the cable will be 6mm T&E. this will then be terminated into a JB box and then swa outside into the garage CU. it will be powering 2 sockets and about 4 lights (bulk heads)....

So is this cable going to be in a safe zone the way you have described it it won't be.

 

[tom1]

does 6mm 40 amps sound a bit over the top for 2 sockets and 4 lights? (20amp 2.5)

and does it make any odds if you dont discriminate? other than you dont know which will trip or they may both

 

[ianmacd]

And horizontal chases. Eeeeekk.

 

[ianmacd]

does 6mm 40 amps sound a bit over the top for 2 sockets and 4 lights? (20amp 2.5)and does it make any odds if you dont discriminate? other than you dont know which will trip or they may both

You will not have installed according to 7671 if you do not satisfy discrimination.

 

[Lurch]

does 6mm 40 amps sound a bit over the top for 2 sockets and 4 lights? (20amp 2.5)

Sorry, thought you were an electrician for some reason. It all depends what you're plugging into the sockets and what the lights are. Also depends on volt drop along the length of the submain and final circuits. Basically you need to work out what you need and see if what you have complies, or I could come round and have a look for you if you need someone to do that.

and does it make any odds if you dont discriminate? other than you dont know which will trip or they may both

Not sure why you'd need to put 2 RCD's in series anyway?

 

[ianmacd]

Sorry, thought you were an electrician for some reason. It all depends what you're plugging into the sockets and what the lights are. Also depends on volt drop along the length of the submain and final circuits. Basically you need to work out what you need and see if what you have complies, or I could come round and have a look for you if you need someone to do that.

Easy Lurchie....

 

[tom1]

harsh. There was me thinking I could ask a question Anyway I ain't going to write every detail in the post he has already said it 40 amps and 6mm so I would assume volt drop has been considered. And for 2 sockets and 4 lights 4o amps is am bit much is a lot no mater what your plugging in

 

[ianmacd]

Could be 32A sockets.

 

[spinlondon]

If you are not going to run the cable in a safe zone, then it will have to satisfy the requirements of the Regs. in some other way.

Such as having an earthed metalic sheath, or some form of adequate mechanical protection

It would then not require RCD protection.

 

[Lurch]

harsh.

Yeah, was a bit.

he has already said it 40 amps and 6mm so I would assume volt drop has been considered.

I didn;t assume anything, hence our differing conclusions.

 

[kme]

i.e. SWA all the way?

Unless other methods of avoiding RCD requirement are used, else you`d be looking for a 10mA RCD for the garage end, to provide adequate discrimination...

(Yes, I know the 3x requirement isn`t in the "new" book, but that doesn`t make it any less effective....)

KME

 

[ADS]

So, to sum up - 30mA in the house CU if the distribution cable requires protection - and none in the garage or...

MCB in the house CU and 30mA in the garage

You don't need two

 

[ianmacd]

i.e. SWA all the way?Unless other methods of avoiding RCD requirement are used, else you`d be looking for a 10mA RCD for the garage end, to provide adequate discrimination...

(Yes, I know the 3x requirement isn`t in the "new" book, but that doesn`t make it any less effective....)

KME

So whith a 31mA or larger imbalance which will trip?