earth loop impedence values

[neilyboy1961]

morning everyone a small problem here I'm hoping someone can clear up.
I am in the process of selling my house and got a major player in the gas industry to come and check the boiler and he has also tested the electrics as well.

Upshot is he plugged a meter into a ring circuit a couple of lights flashed and he has told me that the 200ohm reading is not acceptable and has now condemmed the system and put an at risk notice on it.

The test he did was a fault loop impedence test.I have tried searching on the forums for answer but no luck.

For your info I have 6mm earth cables at my consumer unit 6mm going to my gas and water pipes and all the hot and cold taps are connected together with 6mm.the main hydoo cable coming in has an earth strap on which in turn is connected via a terminal block to my consumer unit.
The consumer unit although approx 15 yrs old does have circuit breakers in it and all the wiring in the house is pvc.

Can anyone help me as why it is at risk.I have a martingdale tester and went round all the sockets and everything is fine according to the martingdale.

Thanks

 

[EdwardCurrent]

Hello Neilyboy1961.
I believe that the high earth loop path resistance may mean that your disconnection times may not be sufficient.
Do you have the test results handy ?
If so, did the engineer do a Ze test ? This is your 'external' earth fault impedance. Can you tell us your supply type, and the Ze reading from the test results maybe ? Just trying to sort your Ze from your Zs. Did he test other circuits Zs values ?
Regards
Ed.

 

[neilyboy1961]

Thanks for he reply Ed.

Im not at home at the moment but he did'nt carry out any other tests other than plug his tester into some other sockets in the house.
The only values I have are the piece of paper with the at risk notice on it and the 200 ohm reading.
By supply do you mean the hydro cable coming in ? if so it looks like neutral and live coming in with something akin to lead at the collar where the earth strap is attached. Does that make sense ?

 

[ericmark]

There are three standard earthing arrangements for a house. If TN-S or TN-C-S then the reading shows a very bad connection somewhere the reading should be less than 5 ohm it varies according to the trip size on the circuit which is why you find it hard to find on internet.

If the earthing arrangement is TT then a value exceeding 200 ohms may not be stable so that is normally considered as maximum but the value can still pass when a lot higher. The size of the earth leakage trip is what fixes the maximum value.

On the paper work he gave you it should have the earthing system stated.

What has happened with many old houses is the water and gas were used as earths and now the street pipes are plastic it no longer works. If a TT system you should have an earth rod and an earth leakage trip.

The supplier is duty bound to tell you what type of supply you have. Often they do not know so they have to send some one to the house to check and often a chat over a cup of tea and they will install a TN-C-S where they can.

I think it is very bad that anyone does a PIR without explaining what is wrong. He should have told you what needed doing to correct the fault.
What did the paperwork say? Maybe you can scan so we can see?

 

[neilyboy1961]

thanks for the reply eric,
I should maybe have mentioned that the tester was a gas engineer who came to check the boiler and in his own words "we only get the basics on electrical testing"
He also said the boiler was at risk as well as the gap between the louvres on the doors which the boiler is hidden behind was too big.
I have seen an electrical test cert before and remember the section stating which earthing system was used but the paper work from this guy is a general document with blank spaces which he fills in his comments and that is where he wrote that the 200 ohm reading was unacceptable

another question whats a PIR >

 

[mbga9pgf]

There are three standard earthing arrangements for a house. If TN-S or TN-C-S then the reading shows a very bad connection somewhere the reading should be less than 5 ohm it varies according to the trip size on the circuit which is why you find it hard to find on internet.

If the earthing arrangement is TT then a value exceeding 200 ohms may not be stable so that is normally considered as maximum but the value can still pass when a lot higher. The size of the earth leakage trip is what fixes the maximum value.

On the paper work he gave you it should have the earthing system stated.

What has happened with many old houses is the water and gas were used as earths and now the street pipes are plastic it no longer works. If a TT system you should have an earth rod and an earth leakage trip.

The supplier is duty bound to tell you what type of supply you have. Often they do not know so they have to send some one to the house to check and often a chat over a cup of tea and they will install a TN-C-S where they can.

I think it is very bad that anyone does a PIR without explaining what is wrong. He should have told you what needed doing to correct the fault.
What did the paperwork say? Maybe you can scan so we can see?

With a TT system (you will have an earth spike), Doesnt the CU usually have a split box with time-delayed RCD? 200 is high, but as long as the RCD Op current rating isnt above 100, then the ELI is well within limits for TT (460 max). I agree that 200 is on the high side and on the limit though and you might want to get another spike knocked in or have your current one deepened.

Are you sure the guy didnt mean that its ABOVE 200? Bit of a fluke that your ELI was 200.

 

[neilyboy1961]

the consumer unit is old and does not have an rcd but it is fitted with MCB's

 

[davelx]

.the main hydoo cable coming in has an earth strap on which in turn is connected via a terminal block to my consumer unit.

That description sounds like TN-S - ie the Main Earth Terminal is connected to the sheath of the supplier's incomer. In which case 200 Ohm Zs is a sign of a problem. A Ze test is needed to confirm if the problem is the supplier's earth or your wiring. If the former then you can call them in to fix it.

Can you post a single photo showing fuseboard, meter, main fuse (cutout) and the incomer and earth connections - please make sure it is in focus!

 

[neilyboy1961]

Thanks for all the replys lads, I will be home lunchtime ish and will get more details for you,I'm a bit of a techno philistine an so will be unable to post a picture of the installation thats way above my level

 

[ricicle]

then the ELI is well within limits for TT (460 max).

It is actually 500 Ohms max. if you take Uo as 230v.

But either way with anything above 200 it must be ascertained that this is a stable reading and not likely to increase as ground conditions change.

To the op:-

I suggest you seek the services of a competant Electrician

 

[ban-all-sheds]

I am in the process of selling my house and got a major player in the gas industry to come and check the boiler

Phil Bentley??

 

[nozspark]

TBH it sounds like a typical British Gas job..

They come and carry out a gas safety check and as part of that plug in their "Socket & see" polarity/loop indicator.. and when it indicates >200ohms they don't understand the reading..

If I were you I'd get in a recommended electrician to do a Periodic Inspection... It is as important to know that your electrics are safe as it is to know your gas is safe[/url]

 

[ericmark]

TBH it sounds like a typical British Gas job..

They come and carry out a gas safety check and as part of that plug in their "Socket & see" polarity/loop indicator.. and when it indicates >200ohms they don't understand the reading..

If I were you I'd get in a recommended electrician to do a Periodic Inspection... It is as important to know that your electrics are safe as it is to know your gas is safe

Now things are making more sense. If you are right than maybe there is little wrong.

thanks for the reply eric,
I should maybe have mentioned that the tester was a gas engineer who came to check the boiler and in his own words "we only get the basics on electrical testing"
He also said the boiler was at risk as well as the gap between the louvres on the doors which the boiler is hidden behind was too big.
I have seen an electrical test cert before and remember the section stating which earthing system was used but the paper work from this guy is a general document with blank spaces which he fills in his comments and that is where he wrote that the 200 ohm reading was unacceptable

another question whats a PIR >

The PIR stands for periodic inspection report

the consumer unit is old and does not have an rcd but it is fitted with MCB's

A reading of more than 200 ohms without an RCD is a problem and although the idea of testing with the device linked to seems a little pointless in some ways it has highlighted you do have a problem. However to work out where the problem lies will need a proper test set and at around £100 to hire it is not worth considering DIY. You just need to get an electrician in to test it.

It could be something really silly. As to gas tests my Dad was a little frightened about gas as many are so he removed the bung on the gas supply to fire and screwed in the stop so isolating them.

Had the gas man around to test and was give a clean bill of health so he opened up the stop tap again.

Can't see how gas man tested the fire without a gas supply? Maybe they can do non live tests like we do with electric but not sure how they measure CO2 without lighting fire? So when my dad suggested I also have it tested I didn't bother. I don't trust the gas man with gas so I am not going to trust him with electricity.

 

[neilyboy1961]

Thanks for all the replys lads, I will be home lunchtime ish and will get more details for you,I'm a bit of a techno philistine an so will be unable to post a picture of the installation thats way above my level

I have been home and got some more details,

the incoming supply comes in through an iron pipe with some form of sleeve at the top it has 2 phase wires and a neutral with only one of the phase wires being used.
the top of the the metal pipe has a screwed bolt into the pipe and the main earth connecting to the consumer unit is bolted on to it.
the consumer unit is wylex and the previous rewirable fuses were replaced with mcb's which fitted into the existing fuse holders
the mcb's are type b

the tests were done under the gas boards home care scheme and the paperwork he left was an Advice note.

 

[neilyboy1961]

.the main hydoo cable coming in has an earth strap on which in turn is connected via a terminal block to my consumer unit.

That description sounds like TN-S - ie the Main Earth Terminal is connected to the sheath of the supplier's incomer. In which case 200 Ohm Zs is a sign of a problem. A Ze test is needed to confirm if the problem is the supplier's earth or your wiring. If the former then you can call them in to fix it.

Can you post a single photo showing fuseboard, meter, main fuse (cutout) and the incomer and earth connections - please make sure it is in focus!

How do you test for Ze ? and what type of meter is required