TT Supplied garage circuit

[sw-electrical]

Went to look at a job this morning - additional socket in free standing garage.
The supply is fed from FCU in kitchen via a 2 core Hituff cable. There is a separate earth (10mm) that appears to be
TT'd at the garage, but without digging it up - i can't verify the termination. The Zs is 19.5 ohms and is protected by
a MK LN5760 DP 30mA RCD.

The customer has asked me to upgrade the cu to 17th edition and is prepared to change the supply to the garage, but it
would only be replacing the hituff with 3 core to provide suitable earthing.
As a TT supply the 19.5 ohms is acceptable, but is the supply allowed to be taken from a FCU on an existing circuit? I
would prefer it to be an independant circuit.
Thoughts please, as i don't want to charge customer if it doesn't need doing.
Thanks

 

[Engineer54]

It's acceptable depending on the power requirements of the garage, and if the customer accepts that a fault in the garage can take out the FRC in the house. The RCD in the garage will not discriminate with the RCD in the proposed CU!!


So Yes a separate garage circuit would be the ideal, but that's up to you to give the pro's and con's to the customer to convince him, to go that route!! lol!!

 

[Geoffsd]

What will the Zs be when you have installed the 3core? Do you mean swa?

Are there any extraneous parts in the garage?

Dedicated circuit would be better for convenience regarding tripping but that's up to the customer.

 

[sw-electrical]

what will the zs be when you have installed the 3core? Do you mean swa?

I could and would use swa if i need to replace. I would expect to get the zs below 1.44 ohms, preferably less than 1 ohm.

Are there any extraneous parts in the garage?

No

dedicated circuit would be better for convenience regarding tripping but that's up to the customer.

i agree

 

[Engineer54]

What will the Zs be when you have installed the 3core? Do you mean swa?

Are there any extraneous parts in the garage?

Dedicated circuit would be better for convenience regarding tripping but that's up to the customer.

Why would that matter on a TT earthing system??

Yes i meant to ask if the OP meant SWA too, lol!! If a dedicated circuit was to be used, the 3 core SWA cable from the CU could be used to link the the garage earth electrode with the main earth electrode for the house at the MET, which may very well improve the overall Ra value etc!!....

 

[sw-electrical]

It's acceptable depending on the power requirements of the garage, and if the customer accepts that a fault in the garage can take out the FRC in the house. The RCD in the garage will not discriminate with the RCD in the proposed CU!!

I realise that.


So Yes a separate garage circuit would be the ideal, but that's up to you to give the pro's and con's to the customer to convince him, to go that route!! lol!!

I just wanted to be sure that it complies with requirements of TT supply. My working Regualtion knowledge of TT is limited
as i don't get any work regarding this type of supply.

I'm assumming that this is TT'd as i cant see the earth cable entering the house, where the the hituff cable does.

I was fairly confident it was safe, just needed clarification. I wouldn't have done it this way if i had fitted it.

Thanks

 

[Engineer54]

I just wanted to be sure that it complies with requirements of TT supply. My working Regualtion knowledge of TT is limited
as i don't get any work regarding this type of supply.

I'm assumming that this is TT'd as i cant see the earth cable entering the house, where the the hituff cable does.

I was fairly confident it was safe, just needed clarification. I wouldn't have done it this way if i had fitted it.

Thanks

Are you saying, that the house isn't on a TT earthing system?? As soon as you do a Zs test you will know!! lol!!

 

[daverogers]

Hey 54 I didn't see the OP say anything about TT installation at the house or am I being think. Saying that if a 3core swa is to be used and its a TN system I would just banjo the swa and earth this to the MET. Then run a TT at the garage as to not extend DNO (earthing zone). Of course you are then left with a unused core

o btw earthing zone is that even the correct term for the DNO

 

[Engineer54]

Hey 54 I didn't see the OP say anything about TT installation at the house or am I being think. Saying that if a 3core swa is to be used and its a TN system I would just banjo the swa and earth this to the MET. Then run a TT at the garage as to not extend DNO (earthing zone). Of course you are then left with a unused core

o btw earthing zone is that even the correct term for the DNO

That was me assuming again...lol!!

I'd like to know your reasons behind this statement if it's a TN-S earthing arrangement?? You would be correct if TNC-S. But it would need a very good reason for me to leave a superior earthing system behind in favour of an earth rod!! lol!!

It's called ''Equipotential Zone''

 

[sw-electrical]

Maybe i should have been a bit clearer.

The house is about 25 years old, with a relatively untouched TNCS original installation, albeit with an old trip switch type board.

The garage is an extra, via a FCU into an RCD main switch and (appears) earthed by TT.
The sockets in the garage are for Fridge and lighting - not much else.

 

[Engineer54]

Maybe i should have been a bit clearer.

The house is about 25 years old, with a relatively untouched TNCS original installation, albeit with an old trip switch type board.

The garage is an extra, via a FCU into an RCD main switch and (appears) earthed by TT.
The sockets in the garage are for Fridge and lighting - not much else.

Yes, it would have been relevant useful information...lol!!

Now it depends on the distance to the garage from the house, and if any extraneous services etc, are present, whether i would extend the PME earthing or not. ...Over to you OP??

 

[Geoffsd]

That's what I meant.

If the garage is nearby new cable may give a low enough Zs.

 

[shocking_eg]

whats the regs now on extending the main supplys PME to outbuildings ? i thought it was a no no ?

 

[Deleted member 26818]

The Regs still allow PME supplies to outbuildings.

 

[daverogers]

That was me assuming again...lol!!

I'd like to know your reasons behind this statement if it's a TN-S earthing arrangement?? You would be correct if TNC-S. But it would need a very good reason for me to leave a superior earthing system behind in favour of an earth rod!! lol!!

It's called ''Equipotential Zone''

Equiptential zone that's the one thanks mate.

I stated Tn systems because it is illegal to extend outside this zone. Which would be the home not the garage if not attached to the building.

To cut a tag or not cut a tag that is the questions lol jokin

I suppose it's up to you but it is not allowed.