Garage job

[sprinter66]

I have just installed a small CU in a garage

It's feed from the house view 2.5 swa.

In the garage i have installed a small CU with a 30ma rcd and 20amp MCB with a
Earth rod in the garden.

When I started testing the RCD in the house trips on the RCD test but not the 1 in the garage.

The house RCD is an old wylex unit fitted around the end of the 1980's.

Any ideas how I can sort this with out changing the house CU

Thanks

 

[gaffer]

Is the RCD in the house only supplying the garage or the whole board ,if its the whole board you need to change your supply end or remove the RCD in the garage.If you have two RCDs in series you get nuisace tripping.

 

[Mark.W]

Reg 531.2.9 The downstream RCD may need to disconnect all live conductors.

 

[mark_the_spark]

If its an RCD, not an RCCB then disconnect the earth from the 2.5 t&e at the garage consumer side so the earths are seperate. That way the supply cable is protected by an RCD to the garage point then your garage RCD will protect from there, that is so long as you have an adequate earth with your TT system that you have just made. (your Zs should read below 50 ohms as a rough guide, but can be greater if you know what your doing)

This system will work with only certain setups and RCD's though, any chance you can give us the BS no. of the primary RCD??

1. Introduction
It can be necessary for an electrical installation to include residual current devices (RCDs) connected in series or ‘cascade’, particularly where the installation forms part of a TT system. Where such RCDs are used for fault protection or otherwise to prevent danger, and discrimination in their operation is necessary to prevent danger, Regulation 531.2.9 of BS 7671 requires that their characteristics are such that discrimination is achieved. Discrimination is explained in item 2 of this topic.
The need for discrimination is also implicit in Regulation 314.2, which requires that a separate circuit be provided for each part of an installation which needs to be separately controlled so that it remains energized in the event of failure of any other circuit of the installation.
Furthermore, discrimination between protective devices (including RCDs) connected in series is implicit in Regulation 314.1 which requires that every installation be divided into circuits, as necessary, to avoid hazards and minimize inconvenience in the event of a fault.
Even where it is not strictly necessary to provide discrimination between RCDs to avoid danger or to minimize inconvenience, it is considered good practice to do so.
Where two or more RCDs are connected in series, discrimination will generally be achieved by selecting time-delayed characteristic(s) for the upstream device(s).

 

[TPES]

Is the RCD in the house only supplying the garage or the whole board ,if its the whole board you need to change your supply end or remove the RCD in the garage.If you have two RCDs in series you get nuisace tripping.

If this was the case would you have an Main switch in the garage CU then into RCD, then on to circuits.. as to break up the 2 RCD's inline..??

 

[Spudnik]

You should not have 2 RCD's in line.

If there is one in the house then there should not be one in the garage.

 

[TPES]

Im with you.

I was thinking for when you have a garage supply feeding lighting/sockets, So if there was only the RCD supplying the garage CU from the main house CU, If there was a fault then the whole house would trip out also, instead of just trippin at the garage CU... Which would be a pain in the backside.

 

[Spudnik]

Every circuit should be on an RCD now so it doesn't matter.

Mind you, Part P and the regs are due for a severe pounding next year as they are looking at a few changes.

But for now, if the CU has a spare unprotected way, SWA should be used and an RCD in the garage.

 

[TPES]

What other requirments are to be met for the use of SWA and then RCD protection is not needed? is there any? trying to think back to my regs

 

[sprinter66]

To solve the problem of having 2 RCD's would you be able to install a new type 17th edition CU and supply the garage from the non RCD protected side of the CU and use SWA to the garage CU

 

[Had8Lives]

Another possible solution, if the customer won't agree to a consumer unit change, would be to split the meter tails after the meter and put a small (shower type) enclosure with a main switch and breaker to supply the garage seperate to the existing House RCD and consumer unit.

 

[sparks1234]

Another possible solution, if the customer won't agree to a consumer unit change, would be to split the meter tails after the meter and put a small (shower type) enclosure with a main switch and breaker to supply the garage seperate to the existing House RCD and consumer unit.

That gets you into the realms of not having 1 main switch in an installation

 

[Spudnik]

One could be installed in the tails prior to the henley block.

Then again, some argue that the single point of isolation is the service cut out.

 

[mark_the_spark]

What other requirments are to be met for the use of SWA and then RCD protection is not needed? is there any? trying to think back to my regs

If you know what you are doing then you can get away with no RCD protecting the SWA however if in doupt and working outside of the equipotential zone then RCD protect....