Supply to a garage

[cmac91000]

I know there are many posts related to this but of the ones I have read, none of them appear to answer my questions specifically.

I have a dual 30mA RCD consumer unit in the house which is on a PME supply. I would like to supply the garage from one of the MCB's. I have a small CU in the garage which is also 30mA RCD protected and would like to put it on a TT system with an earth rod.

I have 2 questions: -

Am I right in thinking that the SWA has to be protected by the MCB and RCD in the house and not by the TT system - I know that the the earths have to be seperated from each other.

How do I achieve descrimination if the supply is through an MCB protected by the 30mA RCD in the house and the 30mA RCD in the garage.

Thanks in advance for any help.

 

[Spark123]

Does the consumer unit in the house have any non-RCD protected ways? The simple answer is you can't discriminate between the two RCDs.

The SWA needs to be earthed at source hence the house earth is right.

I take it you know about part p, this work is notifiable.

What is the reason for wanting a TT supply, do you have water / gas / metal framework in contact with earth in the garage?

 

[cmac91000]

Sparky, thanks for your quick reply.

The reason for wanting TT is two fold. The DNO will not approve of exporting the earth and yes there is a metal frame.

It is the way to achieve discrimination that I am having problems with as I know that nuisance tripping could occur.

Is the fact that a TT system should be protected by 100mA RCD a must or is infact 30mA acceptable as it is actually more sensitive?

 

[Spark123]

If you have an upstream 30mA and a downstream 100mA the upstream one is bound to trip in the event of a fault so isn't a viable option. Even the other way around you are not guarenteed discrimination.

30mA is required for socket outlets and any unprotected consealed cables in walls/partitions hence is probably going to be the correct one for the garage. SWA doesn't need to be RCD protected (unless for another reason such as the efli is too high).
You can use a 30mA RCD on a TT system, some systems have a time delayed 100mA on them but isn't really an option for you as the house consumer unit has been fitted with 30mA RCDs for a reason.

Do you have the correct test equipment to commision this?

 

[cmac91000]

I have recently requalified as a domestic installer as I left the industry 30 years ago, after doing an electrical apprenticeship 35 years ago.

As you can imagine things have changed a lot especially with additional protection. I have the right equipment with a multi tester and also an earthrod meter with which I would like to consider myself competent.

I have not started this project yet and would not until I am happy that whatever I do meets the regs and is absolutely safe. I strongly believe that the design / planning is crutial.

I am trying hard to find the correct way of maintaining the protection within the house, not to export the PME and to offer the correct protection in the garage where sockets are almost certainly going to be used for outside equipment such as mower etc.

Is discrimination that avoids nuisance tripping an absolute must or is it in fact just strongly advised? What would be the view if the lighting where the house CU is located was on the other RCD to the garage in the house?

 

[Spark123]

Discrimination is a good idea, especially if you get a fault in the garage causing your sockets inside the house to trip whilst the mrs is watching eastenders or the big brother final
There has got to be some way of feeding the SWA with a non-RCD protected supply such as using a "high integrity" way on the consumer unit or by installing an additional switch fuse.

You don't really need to use an electrode meter, a normal loop meter will tell you enough.

 

[JohnW2]

Is discrimination that avoids nuisance tripping an absolute must or is it in fact just strongly advised?

Although such discrimination is obviously highly advisable, I think that as far as the regs are concerned, it appears to be far from an 'absolute must'. I would think the most pertinent reg is 314.1(i), which says "Every installation shall be divided into circuits, as necessary, to: ... (i) avoid danger and minimise inconvenience in the event of a fault". That is clearly pretty vague and, in particular, "minimise inconvenience" is open to a range of subjective interpretations.

Kind Regards, John

 

[cmac91000]

Thanks Spark, John. It's good to see that there doesn't appear to be a black and white solution and that makes for an interesting conversation.

I don't think that feeding the SWA by a switched fuse would change much - I think the SWA still requires an RCD for protection?

I can see where John is coming from with the part that says 'minimise inconvenience' and that is why I suggested that the RCD used for the garage in the house CU should be a different one to that used for the lights in the area of the house CU - at least the missus wouldn't be grovelling around in the dark to reinstate the power whilst I was doing something silly in the garage! My point of view is that the belt and braces safety of two 30mA RCD's outweighs the inconvenience of 'nuisance' tripping which is really only likely if there is a fault anyway. Would this stand up as a reasonable 'risk assessment'

 

[JohnW2]

I don't think that feeding the SWA by a switched fuse would change much - I think the SWA still requires an RCD for protection?

No, SWA does not need RCD protection. Some of the circuits (e.g.sockets circuits) at the end of it would, but that could (and would best) be provided by an RCD in the garage CU. In many senses, the 'best' option is to have SWA fed from a switch-fuse (no RCD) and a single RCD at the garage end.

I can see where John is coming from with the part that says 'minimise inconvenience' and that is why I suggested that the RCD used for the garage in the house CU should be a different one to that used for the lights in the area of the house CU - at least the missus wouldn't be grovelling around in the dark to reinstate the power whilst I was doing something silly in the garage!

Indeed, I think you could, if you so wished, easily argue that you had taken 'reasonable steps to minimise inconvenience'.

My point of view is that the belt and braces safety of two 30mA RCD's outweighs the inconvenience of 'nuisance' tripping which is really only likely if there is a fault anyway. Would this stand up as a reasonable 'risk assessment'

As above, although it's obvioulsy a matter of personal opinion/judgement, I personally would not argue with such an assessment. However, I can't speak for Mr Jobsworth!

Kind Regards, John

 

[ban-all-sheds]

I have recently requalified as a domestic installer

Despite the fact that you don't know stuff like this.

 

[viewer]

If discrimination really is a problem then how about a henley after the isolation switch with mini-sub-main to a RCBO in a box? This would be similar in effect to our house where all are 30mA and we simply have two CUs. The one serving the outbuildings also contains the MCB for some lights in the house so acting as a call-for-action to help prevent a tripped RCD being un-noticed.

 

[xr4x4]

It's quite simple.. Run a swa to the garage, don't connect it to a rcd at the house end, mcb in house cu, or separate mini cu, or a switch fuse.

Earth the swa at the house end but not to the garage cu.

Put a 30ma rcd protected cu in garage, run earth to rod.

You only need a 100ma rcd before the 30ma if there is any exposed metal before the 30ma rcd on a TT earth.

So run the swa direct into a plastic cu or adaptable box.

Happy to be corrected on any of that.

 

[JohnW2]

It's quite simple.. Run a swa to the garage, don't connect it to a rcd at the house end, mcb in house cu, or separate mini cu, or a switch fuse. Earth the swa at the house end but not to the garage cu. Put a 30ma rcd protected cu in garage, run earth to rod. You only need a 100ma rcd before the 30ma if there is any exposed metal before the 30ma rcd on a TT earth. So run the swa direct into a plastic cu or adaptable box. Happy to be corrected on any of that.

I don't see that correction is required - that's certainly a reasonable way to do it. I think I understand what you're saying, but what do you mean by "exposed metal before the 30mA RCD"?

Kind Regards, John.

 

[xr4x4]

...that any metal has to have a minimum of 100ma rcd protection on a TT earth.

Not sure how to explain...

 

[JohnW2]

...that any metal has to have a minimum of 100ma rcd protection on a TT earth. Not sure how to explain...

I was really asking about what sort of 'metal' you had in mind - are you talking about the metal enclosure of something (an isolator or switchfuse, perhaps?) upstream of any other RCD protection?

Kind Regards, John