TT and Ze certificate query

[pcboffinuk]

Today's conundrum,

Supply is TT, new 100ma time delayed RCD, then split via henley blocks to two CU in the kitchen, and one in the garage/office. The two kitchen CU have their main earth in the MET along with the cable to the earth rod. The cable to the garage DB has its earth terminated in one of the kitchen CU earth bar.

I've Measured Ze in the two CU in the kitchen from the phase to the rod cable (disconnected from the MET).

In the garage, am I right in thinking that I should be taking Ze as the measurement from phase in that CU, direct to the rod via the earth in the distribution cable.

i.e. connect the earth from the garage DB direct to the rod cable for the test?

Out of interest, if the garage CU was run from a breaker on one of the kitchen CU, would Ze at the garage CU become the Zs of that circuit, with any parallel paths included.

The reason I ask is because the supposed NICEIC approved contractor who certified the new kitchen CU in 2005 put Ze down as ~16ohm and while that is what I get at the MET, disconnecting and using just the earth rod for the test gives me 185ohm, whereas the mains water gives ~18 and Zs on the final circuits is about 21 ohms.

None of the texts I've read seem to offer much advice on completing the cert in these sort of circumstances, so hope wiser heads on here will confirm I'm on the right lines?

Thanks

Gavin

p.s. Hope the above makes sense, but if not in my defence its late!

 

[HVMAN]

The garage db would be a zdb reading if it is a submain from the kitchen distribution board

zdb = ze + ( r1+r2 )

with water and gas bonds disconnected test the ze from live, to the cable connecting to the earth rod.

 

[securespark]

If the NIC guy tested the Ze with MEB's in place, that will give a false reading through utility pipework.

Ze should be measured from incoming tails (with incomer switched OFF) and the MEC with MEB's disconn'd.

If the true Ze is indeed 185 Ohms, then he should have flagged this up, as although the OSG says a maximum of 200 Ohms, the NIC cut-off is 100. I would give him a call to discuss this.

Ze at garage submain I would call Zdb. What is the Zs like at the destination of circuits in the garage?

You say you have a S type RCD here. Is that on the tails?

Do you have 30mA protection? If not, I would say this installation is dangerous.

 

[pcboffinuk]

Thanks for the replies,

From what I can see, the NIC guy must have tested Ze with MEB in place, because the mains water is the only link with a low ohmic value.
The fact that he gave test values for the ELCB 'protecting' the system, which was clearly not working when I started looking at this job before Christmas, and the fact that the cert says the earth rod is at the front of the building when it is actually at the rear makes me suspicious of the rest of it.

Apparently they (possibly another contractor, the clients have rather got lost) spent a long time installing this new earth rod, but doesn't seem they did a great job. The trouble is that different parts have been installed by various electricians, handymen, and morons over the years so I'm gradually bringing it all up to spec. and will be certifying everything.

I tested Ze from phase on the incomer of each CU to the cable connecting to the earth rod.

Zs in the garage circuits(and at all final circuits in the house) is ~21 due to the parallel path through the mains water pipe.

The garage is built onto the side of the house, but not accessible except from outside. The Garage DB is not a sub main of the kitchen CU. It is split via henley from the tails to one of the kitchen CU, with its earth connected to the CU earth bar. I have been considering if it should be made a submain, since its run in 16mm T&E.

Securespark, the S type RCD is on the tails in its own insulated enclosure. One of the kitchen CU is split load with 30ma RCD for sockets, jacuzzi spur and outside lights. The other board currently had no rcds but I will be replacing the main switch with an 80A 30ma RCD to ensure safety.

The Garage db is already running entirely on a 30ma RCD.

Assuming Ze is 185, is this something that has to be reduced, or just investigated and noted? Obviously I'm aware that the earth via water pipes may not last for ever. Given that they apparently dug up half the garden to install this one, I will investigate further and check the wiring.

Cheers

Gavin

 

[Adam_151]

Thanks for the replies,

Assuming Ze is 185, is this something that has to be reduced, or just investigated and noted?
Gavin

It sounds ok to me, 0.1Ax185ohms = 18.5v, and its below 200 (which is the value the iee say it ought to be under if its going to be stable*)

You can get special soil additives, or even special mixes to bury it in which are designed to get ze lower, and I'm told the less reputable contractors sometimes **** on it to get ze down (but that fails as a long term measure!)

Also consider carefully your recommended PIR peroid, might be wise to give less than the full 10 years, maybe about 5?



*nicieic recommend below 100 though

 

[festa]

As from BS7671 2008......
IEE are also lowering their 200ohm max to the 100ohm mark

I know it doesnt make any difference as to what your doing now but just thought id tell ya.

 

[ColJack]

whack in another earth rod in a damper location and connect this to the first one.. or add a few more sections to the first one and whack it deeper...


the earthing conductor of the garage circuit should be back to the MET, not through the kitchen CU.. unless the garage was fed as a sub main from the kitchen CU..