Substation High Earth resistance

weston · Feb 18, 2012

Hi All,

I have a theory, that in a small village with 2 substations, one substation has a good earth connection but the other has a high earth connection.
Some of the houses in the village use TT systems, the houses connected to substation 1 have a Ze of approx 6 ohms. The houses connected to substation 2 have a Ze of approx 600 ohm.
The distance between the substations is approx 500-600 metres.
Why worry I hear you say, most supplies could be changed to TN-C-S, this has been the case for most of the houses in the village, so change the rest to TN-C-S and hey presto, everything is happy. No!
There is a caravan park (touring caravans) connected to substation 2 (the bad earth).
Would you agree that the substation earth connection is at fault?

malcolmsanford · Feb 18, 2012

Well even at 600ohms it can be argued that as long as it is stable and with additional protection by RCD, it would still meet disconnection times, though the regs state a maximum of 200ohms and the BS 7430 state 100ohms. But that is another discussion.

I would imagine that the 6ohm values are being obtained because the rods there are being tied up through the mass of earth with the DNO PME rods. The 600 ohms one due to perhaps soil condition, location of the DNO PME rods are not.

Now I would be pretty sure if you called the DNO and told them what you told us that you have an Ra of 600 ohms on a TT, they will be telling you that it is home owners responsibility to improve it not them.

The DNO are required by law to maintain at their transformers a earth point not exceeding 21 ohms, and in most cases they obtain far smaller values, that is where that magical figure is quoted in the OSG. If they are not maintaining that, then you would get high PME readings over the stipulated 0.35 ohms and they would investigate

Do I think there is a sub station fault, possibly but I would be more inclined to think as I stated above, that is happening.

pvconnections · Feb 18, 2012

Hi Weston,

Sounds to me that everyone should be TT. Contact your local DNO before assuming you can TNCS it. They may not be linked up for it. I might be wrong but shouldn't a touring caravan site be TT anyway ?

Knobhead · Feb 18, 2012

With the substations being only 600M apart the neutrals and earths are most probably linked.
You should be talking to the DNO about this.

Engineer54 · Feb 18, 2012

weston said:
Hi All,

I have a theory, that in a small village with 2 substations, one substation has a good earth connection but the other has a high earth connection.
Some of the houses in the village use TT systems, the houses connected to substation 1 have a Ze of approx 6 ohms. The houses connected to substation 2 have a Ze of approx 600 ohm.
The distance between the substations is approx 500-600 metres.
Why worry I hear you say, most supplies could be changed to TN-C-S, this has been the case for most of the houses in the village, so change the rest to TN-C-S and hey presto, everything is happy. No!
There is a caravan park (touring caravans) connected to substation 2 (the bad earth).
Would you agree that the substation earth connection is at fault?

As a matter of interest how do you personally know that these are the values at these 2 sub-stations??

As far as i know the DNO have a legal obligation to maintain a maximum Ze level at their sub-station locations of 20 ohms, and as Malcolm states they are normally much much lower than that value. Maybe a phone call to the supervising authority (whatever it's called) and ask them the question, why your sub-station with the 600 ohm value, is not being maintained at 20ohms?? lol!!

As for the caravan park, they are not allowed to be connected to a PME/TNC-S earthing system. It's either TN-S or TT only, for these caravan park installations....

malcolmsanford · Feb 18, 2012

Sorry tony and E54 I may have missed this, but doesn't the OP state that he is getting as stated Ze (Ra??) values on TT systems of 6 ohms and 600 ohms, both these values would leave me to believe that the low ones are tieing themselves into the DNO spikes, and the high ones are not and are just poor system, possibly ground conditions or even poorly installed system, there's a novelty these days

He states that the majority of the houses have been converted to TNC-S, but does not mention any issues with them, and if the sub had an at source earth electrode problem, you would be getting plenty of problems on those

spark 68 · Feb 18, 2012

He states that the majority of the houses have been converted to TNC-S, but does not mention any issues with them, and if the sub had an at source earth electrode problem, you would be getting plenty of problems on those

Why would he have problems with TNC-S systems Malcom ?, surely it would not matter if the sub station earth rod/mat was 6 or 60 ohms on such a system, as your Ze on such a system would be going directly to the star point of the transformer, via the DNO's PEN conductor, regardless of their true earthing system (their Earth rodding matting etc.).

Iam just asking out of interest Malcom, and appreciate that such a situation may cause other problems.

malcolmsanford · Feb 18, 2012

If your getting a high ohmeric reading at a sub station there is a very good chance you have not got a stable earth. If that is a case on a PME system you could have the "floating" neutral at the consumers installation.

I'm by no means a transformer specialist like tony or E54, that's their bag,and I maybe over simplifying things, and ready to get shot down, but I do know that an unstable earth at it is not what your looking for, which is why the DNO by law have to maintain certain values.

The 0.8 and 0.35 values are not the law, they can be higher, but they have to be stable, and the way they judge stability is by using set values.

Engineer54 · Feb 18, 2012

I think me rather than you Malcolm, i misread...lol!!

If he is stating that he is getting a 6 ohm and 600ohm Ze at the house locations, then something must be very wrong!! If these supplies of 6 ohms from sub-station 1 are PME/TNC-S and 600ohms for a TN-S supply from substation 2, the DNO are well out of order, they are failing to meet their legal obligations BIG time!!!

Can't see how any of those houses can't have issue's, the Ze is way over what has been laid down as maximum permissible values. A blatant case of the DNO failing to take obligational measures to maintain there provided earthing point to the acceptable levels...

Even more of a case to put to the overseeing authority, they will have far more clout than the odd customer or local electrician complaining...lol!!

EDIT... The 0.35ohm and 0.80ohm may not be law, and yes they can be higher, but not to anything like these levels...

What part of the UK, are we talking about here??