Outside Circuits (supply to Garage & Summerhouse)

[SparkJ]

I have to quote for a job to supply power from a house to a garage and a summer house. Neither the garage nor summerhouse has any extraneous metal. Garage is 15m from the house and the summer house is 10m from the garage.

There are two modern Wylex consumer units in the house - one is protected by one 30ma RCD covering all ways and the other does not have any RCD protection but is fitted out with RCBOs. There are spare ways in both consumer units.

Earthing is TT. Zs at a downstairs socket in the house is 67Ω.

My plan is to run 6mm2 3 core SWA connected to a plastic adaptable box with the cable earth core insulated from a spare way in the non RCD consumer unit from a 40A breaker

At the garage end I plan to install a metal consumer unit with a 30ma RCD covering all ways with its own earth spike and terminate the SWA in the consumer unit. The plan is to connect together the armouring and the earth core of the SWA at the garage consumer unit and connect these to the earth bar in the consumer unit.

I then plan to run a 2.5mm2 SWA from the garage consumer unit with a 20A breaker to the Summer House and terminate it in a metal box (again with the armouring and earth core connected together) and then run T&E cable to three sockets and two lights with an FCU to protect the lights.

I have assumed a maximum load for the garage and summer house as 2kW each.

I realise that there are issues with contention of RCDs and equipotential zones.

My questions are:-

Have I used the right approach with connecting the armouring and the earth core of the SWA?

Should be SWA be protected by an RCD at the house consumer unit end despite the contention problems that this would pose?

Is the earthing satisfactory in the Summer House?

 

[a1]

Hello Mate.

Why are you using 3 core?

You could use 2 Core Swa, because you are going to rod it, the garage end.

As it is TT, I do believe that Metal Clad CU's cannot be used and it has to be insulated PVC plastic CU.

I would connect a 2.5mm flying earth from MET in house CU to Banjo (house end) and I would not do the same thing the Garage end! This way the SWA is protected by the earth from the house to the garage.

I started a thread similar to this, I will find it and post a link here.

Are you going to use an RCD RCBO to supply the garage (house end)? If so, It was mentioned in my thread that you don't need one in the garage CU (to discrepancy issues).

The dearer method is to fit a "100ma Type S Time delayed RCD" and come off of a suitably rated MCB to supply the Garage - That way the Garage can have a 30ma RCD.

 

[a1]

Having said that - with the price difference being minimal between 2 core and 3 core SWA, It is my preferrence to instal a 3 core and keep the 3rd conductor as a "Spare".

 

[spinlondon]

Don't need to RCD the house end of the SWA, if the Zs are low enough.

 

[NozSpark]

Don't need to RCD the house end of the SWA, if the Zs are low enough.

Sorry, but that's not right..

All circuits require RCD protection (fire & livestock) in a TT install

If you are going to install a rod at the garage then you would always earth the armour at the supply end and drop the earth at the other

There are quite a few ways of doing what you want to and you'll have to way up cost against inconvenience (in the event of a fault).

 

[M107]

Well I'd go with 2 core swa using the armour as the circuit cpc, isolate cpc from the outbuilding rod/cu (for Zdb testing) .

A 100mA s type at the house end & insulated cu with 30mA rcd/rcbo's at the outbuildings & all will be tickety boo.

 

[ianmacd]

Well I'd go with 2 core swa using the armour as the circuit cpc, isolate cpc from the outbuilding rod/cu (for Zdb testing) .A 100mA s type at the house end & insulated cu with 30mA rcd/rcbo's at the outbuildings & all will be tickety boo.

+1

 

[sellers]

Hello Mate.As it is TT, I do believe that Metal Clad CU's cannot be used and it has to be insulated PVC plastic CU.

Is this true? Whats the reasoning?

 

[M107]

You could use a metal clad cu if the tails were 30mA protected at supply end (pain if rcd trips ...its a walk back to house to reset) or if a special insulating gland is used at point of entry to the metal clad cu (theese are solid rubber with 3x holes for tails & mec entry).

Think rcd unprotected tails on a TT supply & metal clad cu ..... exposed conductive part under fault conditions

 

[spinlondon]

Sorry, but that's not right..All circuits require RCD protection (fire & livestock) in a TT install

If you are going to install a rod at the garage then you would always earth the armour at the supply end and drop the earth at the other

There are quite a few ways of doing what you want to and you'll have to way up cost against inconvenience (in the event of a fault).

I don't suppose you could quote the Regulation?

 

[steptoe]

Well I'd go with 2 core swa using the armour as the circuit cpc, isolate cpc from the outbuilding rod/cu (for Zdb testing) .A 100mA s type at the house end & insulated cu with 30mA rcd/rcbo's at the outbuildings & all will be tickety boo.

+1 , and simply use the additional rod as an extra earth point

Sorry, but that's not right..All circuits require RCD protection (fire & livestock) in a TT install

If you are going to install a rod at the garage then you would always earth the armour at the supply end and drop the earth at the other

There are quite a few ways of doing what you want to and you'll have to way up cost against inconvenience (in the event of a fault).

correctomundo, except I would have the earth continuous throughout, its still TT, but if you were only to earth the supply cable at one end then it should be the source.

You could use a metal clad cu if the tails were 30mA protected at supply end (pain if rcd trips ...its a walk back to house to reset) or if a special insulating gland is used at point of entry to the metal clad cu (theese are solid rubber with 3x holes for tails & mec entry).Think rcd unprotected tails on a TT supply & metal clad cu ..... exposed conductive part under fault conditions

+1

I don't suppose you could quote the Regulation?

nope, look in the big book.

 

[batty]

I don't suppose you could quote the Regulation?

Don't no what regulation no. it is but you will be very hard pushed to get low enough loop impedance on a TT system unless you have a good earth of your water/gas pipe to trip any circuit breaker.