For the big boys...

[il78]

Bit of help for me, if ya’ would……

Getting a few dodgy readings from a rod..

Ze = 488Ohms (gas / water disconnected)
Zs = 5.4 Ohms up to 9.3 Ohms various circuits (Avo LCB2000)

R1 + R2 Readings to circuits are 0.21 up to 0.54 (aint got paperwork in front of me)

Now I thought Zs = Ze + (R1+R2), not the case…….

The wiring methods were to be desired, but what would cause these readings? I cant see it picking up earth paths through the brickwork etc, gas & water bond was disconnected at time of test.

Also 100mA protecting circuits tripping around 43ms x1, so other calculation (just confirming)

Ra x Ia therefore 488 x 0.10 = 48.8V (less than 50V)

kA calculated, 230 / 488 = 0.47A (0.00047kA)

I dunno if its me, missing something obvious or the lads paperwork, or the tester is FUBAR……

Regards in advanced

Ian

 

[Big_Spark]

Something is very wrong there.

If your reading for Ze, as measured, is 488 Ohms, then you have a serious problem.

Either the Earth for the installation is cr*p, the tester is cr*pp*d or the Guy using it has no done so properly.

However, as you have other readings it is a little confusing, and I can certainly see why your having a headache with this.

Ze should be no more than about 1.0 Ohm, if it is much more than this then you will have compliance problems with circuits. I personally have never come across a Ze exceeding 0.3Ohms, the average I would say is about 0.22 Ohms.

There is the possibility that the Earth Electrode was tested BEFORE some water was poured around it for the other tests. Don't laugh, I have seen this done, and was not impressed one bit. It may be the electrode is in soil unsuitable for the task.

I would suggest that you use a different test meter and go back and test the installation yourself. This will allow you to identify the cause of the headache.

One last thing...could the Ze reading actually be 0.488 Ohms? Perhaps in haste your Guy wrote it as .488, and the point is not clear?

Might be an idea to speak to him and test the electrode again.

 

[il78]

I was actually in the area when he called me, so I called in. I had a Robin EFLI meter with but unfortunely without "D" lock so I could only test the Ze (without bypassing the RCD) and the reading was 493 Ohms on the Robin, the earth rod was not installed properly so that explained the high Ze, but when trying to obtain the Zs's with the Avo....nightmare

The Avo was set to L-PE xtra (wont trip the RCD) but usually the readings match up, in some way........

The wiring methods were bad considering it had been recently rewired. I've also asked the NIC technical department so that will be interesting.

Maybe I'm missing something on the meter, I'll speak to Avo Technical aswell me thinks..

 

[Big_Spark]

It certainly does not sound right. I am curious as to why it has an Earth Rod. What type of supply does it have from the grid?

Normally you only use an Earth rod if you have an Overhead supply with no integral Earth Bond, it is a temporary Installation or these are simply part of the lightning protection system.

Maybe we're spoilt in the South as well, All the supplies I come across down here are TN-S or TN-C-S.

I even have a TN-S at my cottage and that has overhead lines!

Have you considered sinking a second rod in a different location and then testing that?

Also, have you tested the resistance of the Earth Rod where it sits in the ground with bonding cable disconnected?

You can do this with a digital multimeter or a digital IR meter for an accurate reading.

 

[il78]

It is a TT system....

I could go further and do what you suggested about installing extra rods, but we only carried out a Periodic Inspection Report. The client was a bit concerned with the condition of there rewire....and got us in to test to give them the proof of there suspicions....

But something aint 100% I cant help but find out...

 

[Big_Spark]

The problem you have here is that you have completed a Periodic and found the installation wanting.

I would suggest you right up a detailed report, include a copy of the certificate with it, and hand this to the client. Let them make a decision as to how to deal with it.

The more time you spend on it, the less money you make from the Periodic, and there must be a cut-off point.

Suggest to the client that the installation requires further investigation, but due to the problem you are unable to give them a quote as you have no idea how long it will take to resolve the issue..it could take half a day, or half a week!

 

[il78]

Cheers Matey....

 

[Big_Spark]

You say it is a TT system, overhead cables, yet it has no Earth conductor coming into the premises?

I know that years ago they did not supply one with overhead supplies, making the system an IT, but this was banned sometime ago, now they MUST supply an Earth conductor, making it the TT system....in effect it is a TN-S, although there is no sheath on the cables, it is a seperate conductor hence the differentiation is nomenclature.

If there is an Earth coming into the premises with the supply cables, then the stake/rod is not required.

It might be worth a quick check to see if the incomer earth has been disconnected, check outside to see if two or three cables actually enter the building. Unless it is TPN then you should have 5!

 

[il78]

Phase & neutral run along all the houses (about 7 houses) loop in & out, no earth....

 

[Big_Spark]

Well as yours is a domestic premises, I think you will find that the supply company MUST supply you with an adequate Earth as per the Supply Regulations.