Discuss Power to a Garage, with RCD at Garage and in the House? in the Electrical Forum area at ElectriciansForums.net

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Oli_100

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Hello,

I'm no Electrician, but I think I have a understanding of the basics.

Few years ago We had our Garage supplied with power by and "electrician". This was done by my Wife's family friend, and he never finished it in my eyes.

I've attached a diagram of what's going on:


Power to a Garage, with RCD at Garage and in the House? elec1.PNG - EletriciansForums.net


Does there need to be 2 RCD's in series for the Garage? The options I've seen are non-RCD supply from HOUSE to the Garage, and a RCD at Garage. Or RCD just at the House and not one at the Garage.

Should the Earth at the Garage be TT?

32A would indicate a ring circuit for the sockets in the Garage. Is it better to get a 20A to put in a Radial instead?

I Know 16A MCB in Garage for lights is too high, should be 6A I think.

I won't be doing any of the fixes myself if needed, I just want to understand what's been done wrong.

Id really appreciate your thoughts. Thanks.
 
Last edited:
Depending on how the lights have been wired, the installation is likely to be compliant.
there my be better ways to do it but only a site visit will explain all.
 
You can't non-RCD the supply from the house to the garage as on TT it won't disconnect if there is an internal cable fault. To avoid two RCDs in series tripping at once (or a garage fault taking out the house circuits) your options really come down to a 100mA delay-RCD up front for that sort of situation in place of the main switch. How practical that sort of a change is depends on the make/age of the house CU as to the availability of a suitable delay RCD and options for feeding the 40A MCB from that instead of its current 30mA instant RCD.

Certainly I would prefer the lights on a 6A MCB as most switches and accessories are only rated to 10A, though the 16A MCB may well be able to provide adequate fault protection, if not overload protection, which is acceptable if the set up has no realistic way of adding a high load to it.

You could also drop the sockets to 20A as sufficient, but if it is wired as a ring then 32A is fine. Generally you won't get good selectivity between MCBs anyway, even 40A & 20A on a fault as once you hit the "instant" magnetic trip point of the upstream MCB is will be committed to opening before the smaller downstream one has managed to open its contacts.
 
You can't non-RCD the supply from the house to the garage as on TT it won't disconnect if there is an internal cable fault.
Just to explain that statement, if you have a L-E short in the cable, say a nail penetrates it or it is crushed by a rock under ground, you need the 40A MCB to trip in under 1s for a TT system's sub-main so typically need at least 5*40A = 200A to flow (assuming it is the most common B-curve MCB).

Even ignoring the 0.95 factor for lowest voltage, and 0.8 for a hot cable, you are looking at below 1.15 ohms (0.87 done properly) and you are very very unlikely to have an earth rod that good.

So you need RCD protection for the sub-main (where it will trip with less than 1A) otherwise the MCB won't disconnect and your whole house CPC (earth) will sit more or less at 230V which is a very dangerous situation!
 
Agree with above. I was just writing that on a TT system, a phase to earth fault won't disconnect quickly enough (or probably at all) without an RCD the house end, so removing the RCD at the house is certainly not an option. @pc1966 explains what can be done.
It doesn't look too tragic from the information given, assuming the SWA size is sized correctly for the 40A breaker and volt drop has been considered.
The main questions are regarding the final circuits the garage end, again as explained above.
 
Thanks for the replies. Really appreciate it.

So can the Garage take the earth from the House that is TT or does the garage need its own independent TT(grounding rod attached to the garage)??

Also, Is the value of the 63RCB in the garage too high considering the MCB in the house its attached too is 40??

These may be simple questions and apologies If I'm sounding daft.
 
Thanks for the replies. Really appreciate it.

So can the Garage take the earth from the House that is TT or does the garage need its own independent TT(grounding rod attached to the garage)??
You can do either, but more rods are usually a good idea. I would put in a rod and use the house system as well.

However....think and check carefully where you put a rod in, you don't want to hit buried services like water/gas/electric/telecoms. If you can "hand dig" down around 1m using one of the fence post style of tools then you know if is clear for practically anything, so a 1.2m (4') rod can go in and be back-filled with the soil that came out.
Also, Is the value of the 63MCB in the garage too high considering the MCB in the house its attached too is 40??

These may be simple questions and apologies If I'm sounding daft.
That is the RCD rating. They do not limit current, only deal with leakage, so they need to be sized at or above the maximum possible load.
  • In the garage case the feed is limited to 40A, so any RCD of 40A or above is acceptable.
  • In the house they are fine. Technically the left-side is marginally high at 66A total but in reality lights never run at anything like max MCB capacity so that RCD will not be overloaded.
  • For the main switch the supply fuse is usually 60-100A so again it cannot have any sustained overload.
So basically that is all good.
 
Thanks for the replies. Really appreciate it.

So can the Garage take the earth from the House that is TT or does the garage need its own independent TT(grounding rod attached to the garage)??

Also, Is the value of the 63RCB in the garage too high considering the MCB in the house its attached too is 40??

These may be simple questions and apologies If I'm sounding daft.
The RCD is fine, it needs to be equal or greater current rating (A) than the device that supplies it, in this case a 40A mcb.
it could just as easily be a 100A isolator (switch)

unless the resistance to earth of the rod at the house is poor then there is no need for an extra rod at the shed end.
 

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