New outbuilding/ house is on TT

Hi all,

I'm a relatively new electrician and gaining experience.  I have a simple CU installation for an outbuilding, with a few ways to add.  Question is of earthing.  The main house is on a TT, with a nice 10mm sub main to the outbuilding.  (As yet not tested, to be re-used form a previous installation).

Should I be installing a separate TT rod  to the outbuilding, or using the exported earth from the main house?  My inclination is to put a new one in.

Any advice gratefully received.

Many thanks

Either or..........assuming the submain is using a core as cpc & armour is glanded correctly

1. If supplying a cu I tend to 100mA s type the submain at source & drop a new rod in for the outbuildings with 30mA rcd/rcbo for outbuilding circuits.
   
2. If it's just for a couple of sockets & fcu for lights then 30mA at source & treat as a normal circuit supplied from the house.
   

If the supply is a 3c I’d sink a rod at the remote end and bond/link the two rods together. You’ll improve both the house and the outhouse earth nest by spreading it over a larger area.

Agree with Tony, bond the two rods together with a suitably sized conductor. Gains to be had all round. 

I'd think that if the 10mm sub main has an earth core I would be connecting it through to the garage , but TT being what it is , a rod at the garage too would improve the overall reading  .  Depends , in my poinion , of what the main earth rod reading is.    A rod isn't an expensive item so just wang one in . 

 The "Exporting  Earth"  becomes a  dilemma  only when the main supply is PME  AFAIK

Thanks everyone

For clarification (as this is my first rod install....) if I put a rod in locally at the outbuilding, and bond it to the earthing terminal in the new CU, presumably the E core from the sub main is also connected there, and by definition, also connected to the existing rod for the house?

Looks like I need to buy some more testing gear!

Is this something I should refer to DNO?

thanks again.

JACCD said:

For clarification (as this is my first rod install....) if I put a rod in locally at the outbuilding, and bond it to the earthing terminal in the new CU, presumably the E core from the sub main is also connected there, and by definition, also connected to the existing rod for the house?

Correct

Looks like I need to buy some more testing gear!

?

Is this something I should refer to DNO?

Nope

thanks again.

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Answered in red

I mean about the testing gear that I don't have an earth resistance tester yet.  Is it necessary, or can you get by with spikes and a regular ohm meter (I have Kewtech KT63). 

JACCD said:

I mean about the testing gear that I don't have an earth resistance tester yet.  Is it necessary, or can you get by with spikes and a regular ohm meter (I have Kewtech KT63). 

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I have a nice mft with Ra (earth rod resistance) capability, that is surplus to requirements,   :innocent

Really , how would that be then to an old poor pensioner,

PS . I have a Wheatsone Bridge gathering dust.

ruston said:

PS . I have a Wheatsone Bridge gathering dust.

Click to expand...

Is that alongside the galvanometer?

Lol.

You can have the WB if you want it .

I have an old chrome clip on ammeter hidden away somewhere too , but I am keeping that if I can remember where it is.

ruston said:

Lol.

You can have the WB if you want it .

I have an old chrome clip on ammeter hidden away somewhere too , but I am keeping that if I can remember where it is.

Click to expand...

I have a brand new still in the box ohm meter, the little green bakelite one, same as the brown one that was a megger, but without the winding handle. Dated from the 40's I believe, it even has the book on testing the installation, and not a mention of loop testing anywhere. lol

ruston said:

Lol.

You can have the WB if you want it .

I have an old chrome clip on ammeter hidden away somewhere too , but I am keeping that if I can remember where it is.

Click to expand...

When the foundry closed down I wish I’d “browed” the Ferranti clip on ammeters. A polished wood case with three sets of tongues and a dozen of the meters that clip on to the tongues. 

Ok....

Well, i shall just add my unqualified welders opinion....

I would keep them separate for the following reasons;

1, Rcd's are lettuced.. [I think all will agree with that]

2, Yes, by having two rods linked you may well improve Ra somewhat, but what will you REALLY gain?? Ra will be already be low enough with just the one rod to operate the RCD, but even with TWO rods, will not be low enough to operate an MCB.

So.... you have an earth fault in the one installation, [and the rcd decides not to work] and you have "exported" the resultant all exposed conductive parts live, plus anything bonded to the MET now also live, to the other installation too... and ADS will not work either..

Why would you want to risk that for no real gain??

john..

Not necessarily john, two separated rods are better than one single, especially if they are correctly interlinked.

Think local PME without all the risks of diverted N current...

They become an earth nest, a good concept.  I have seen one rod low enough to operate a B6, by the time you add bonding, probably a B16, add another rod and some more inadvertent earth paths, and you could be below 1 Ohm?..

RCD's are rubbish devices though, that I do agree with.