Garages, Welders and RCD's

[stuchunk]

Just a quick question,

I am in the process or putting a 6mm SWA supply to my garage to run a 20A welder with its own supply, 32A socket ring and 6A lighting circuit from a new small CU on the garage wall

The garage is detached from the house, where is it legally required to fit an RCD, on the dedicated CU supply in the house or on the CU in the garage or on both, I am led to believe that the RCD should be at the point of origin of the circuit which would be in the house.

In my CU in the house (8 way Hager MCB only) I was going to fit a 40A RCBO and put a 4way non RCB 63A isolator CU with 2x32A MCB and 1x6A MCB in the garage would this be the best way to go??

Cheers

 

[RF Lighting]

There is no requirement to have RCD protection at the CU in the house at the start of the supply.
You are required to have RCD protection on all circiuts likely to supply equipment for use outdoors.

The best setup arrangement would be:

40A MCB in the house CU supplying the new CU in the garage.
MCBs supplying all the circuits in the garage except the sockets.
An RCBO to supply the garage socket circuit.

Do you know about MCB tripping curves?
Welders will often trip a type 'B' MCBs when they are switched on, due to their high inrush current requirements, and are often supplied by types 'C' and 'D' MCBs

Are you likely to ever upgrade to a higer powered welder in the future?
It will be far easier to put a bigger cable in now, than have to replace it in a few years time.

 

[stuchunk]

Thanks for the reply

The reason I was going to put an RCBO somewhere on the combined circuit was because at some point I will need to use the welder outside the garage possibly so I would also need RCD protection on the welder outlet as well as the socket outlet so I preferably wanted to put an RCd protected CU in the garage to give combined protection for the welder outlet and socket outlets but at the same time allow for resetting in the garage in the event of a trip without having to go back to the house with the boots on!!!, from the regulations this is perfectly acceptable.

A spark I have used in the past wanted to put RCBO feed at the CU in the house and RCD protected CU in the garage, he said this gives protection for the buried cable as well but I suspect two RCD's on the same circuit will give nuisance trips
Perhaps you could confirm or deny this


Thanks again

 

[Steve]

A spark I have used in the past wanted to put RCBO feed at the CU in the house and RCD protected CU in the garage, he said this gives protection for the buried cable as well but I suspect two RCD's on the same circuit will give nuisance trips
Perhaps you could confirm or deny this


Thanks again

he's talking rubbish. The cable is adequately protected by the MCB. When someone slices through it with a metal implement, the inner live conductor will have a perfect short to the earthed metal sheath. So an MCB is plenty adequate. One RCD is fine. 2 RCDs should never be installed in a line - they wont discriminate and in the event of a fault either could trip.

 

[nozspark]

How's about a 100mA S type RCD in the house if he's on A TT supply, then you have a requirement...

 

[stuchunk]

Lost on me now, how do I know if Im on a TT supply or not??
Think ill leave the tail ends at the house CU for someone who does this for a living to connect but Ill do the rest myself and get it checked before connection into the house CU.

Thanks

 

[nozspark]

Lost on me now, how do I know if Im on a TT supply or not??

TT is a method of earthing for the installation which uses an local earth rod instead of relying on a connection to the incomming supply for it's earth.

You will find descriptions of earthing methods in the FAQ's / sticky's at the top of the forum listings.

 

[Steve]

Lost on me now, how do I know if Im on a TT supply or not??

TT is a method of earthing for the installation which uses an local earth rod instead of relying on a connection to the incomming supply for it's earth.

You will find descriptions of earthing methods in the FAQ's / sticky's at the top of the forum listings.

Also in the Wiki.

 

[nozspark]

Following on, however if you don't have a TT supply then a 30mA RCD at the house CU and a 30mA RCD at the garage is against the regs. In this case, as crafty says the right way to go is with a MCB at the house with the RCD in the garage.

 

[stuchunk]

Thanks for the replies again.

Can someone just clarify the following for me:

I plan to install a 40A breaker in the house to supply the garage. My main board (non rcd) will then have the following:

3x40A MCB's for shower, cooker and garage
3x32A MCB's for downstairs ring (2off) and upstairs ring
2x6A lighting circuits (up and down)

The main fuse is 100A

Can someone clarify if this loading is ok on 100A single phase incomer fuse , I believe there is a calculation which takes into account loading of breakers against mains fuse size etc.

In the garage I propose to fit 40A RCD enclosure (fed from the house 6mm SWA) with 2x32A MCB's (one for welder to commando plug and one for socket ring) and 1x6A mcb for lighting.

Can anyone see any obvious errors here before I go ahead and do the garage although I plan to leave the tie in at the house CU to someone who can test and inspect it prior to connection.

Thanks

 

[Steve]

For little extra cost, 10mm² would be a better choice, given the nature of work you will be doing in the garage - sounds like its going to be a workshop of some sort.

The load on the house CU sounds fine, i wouldn't worry about it.

As someone mentioned before, it would be better to have the welder NOT on an RCD - so feed the welder with a 40A MCB, and the socket ring with a RCBO. Only the sockets need to be protected. Just use a normal main switch as the incomer for the shed CU.

http://www.tlc-direct.co.uk/Products/MK6936.html <<an RCBO