Backup Generator Questions...

[Spark123]

From meter you take lead to rod rather than DNO's earth. As already said it will read a little on high side but anything under 200 ohms is a pass when using an RCD anyway so good enough.

No, 200 ohms isn't a pass. As a means of earthing the rod needs to be less than 21 ohms with respect to mother earth (figure from the front of the on site guide.)
We are not talking TT supplies here, we are talking means of earthing for a Terra system (TN-S).
GN3 allows us to use a normal EFLI tester for a TT system. For a means of earthing for a Terra system the 3 or 4 lead tester needs to be used.

 

[ericmark]

From meter you take lead to rod rather than DNO's earth. As already said it will read a little on high side but anything under 200 ohms is a pass when using an RCD anyway so good enough.

No, 200 ohms isn't a pass. As a means of earthing the rod needs to be less than 21 ohms with respect to mother earth (figure from the front of the on site guide.)
We are not talking TT supplies here, we are talking means of earthing for a Terra system (TN-S).
GN3 allows us to use a normal EFLI tester for a TT system. For a means of earthing for a Terra system the 3 or 4 lead tester needs to be used.

Sorry but I thought when using the generator it would be a TT system. Of course it is not it is really a TN-S system and of course since the earth is bonded to the neutral at the generator you will get a reading of a very low ohm when you measure the ELI with the generator running.

However to use the mains supply to work out how well the earth rod is connected to mother earth is something completely different. What one is checking for is if one was to use a lawn mower from the generator will it trip the RCD if a fault should arise. And the red book considered over 200 ohms as being un-reliable. Hence 200 ohm figure.

But of course at the generator the ELI will be the resistance of the windings of the generator and nothing to do with the earth rod. Having used a tester many times and needing in the gas terminal I was working at max of 8 ohms I know all too well how hard it is to get a low reading and it would be very unlikely to get a figure of 21 ohms with a single earth rod. To get the 8 ohms I required, I required between 3 and 6 times 1.2 meter earth rods in a salty ground and most rods fitted for a TT system will be around the 50 ohms mark, and I would suspect the tester had failed to remove the bonding conductors to water and gas if I saw a reading of below 21 ohms.

The reading of 21 ohms refers to the boards earth at their transformer not the earth at the house and this will be for a minimum of a 100A supply. Come on have some though with a maximum of 2KVA or 8A do you really think it needs same earth as a 25KVA transformer?

What it says is when you measure using a three or four lead tester you can expect the reading to be up to 21 ohms less than when measuring same rod using an earth loop impedance tester.

Note:- We expect the Earth loop Impedance to be 21Ω or better outside the consumers installation.

 

[electronicsuk]

Sorry but I thought when using the generator it would be a TT system. Of course it is not it is really a TN-S system and of course since the earth is bonded to the neutral at the generator you will get a reading of a very low ohm when you measure the ELI with the generator running.

I'd say it is a TN-S system - neutral and earth are linked and tied to true earth at the generating equipment, with separate L/N/E distribution wiring from thereon.

 

[electronicsuk]

Sorry but I thought when using the generator it would be a TT system. Of course it is not it is really a TN-S system and of course since the earth is bonded to the neutral at the generator you will get a reading of a very low ohm when you measure the ELI with the generator running.

I'd say it is a TN-S system - neutral and earth are linked and tied to true earth at the generating equipment, with separate L/N/E distribution wiring from thereon.

The reading of 21 ohms refers to the boards earth at their transformer not the earth at the house and this will be for a minimum of a 100A supply. Come on have some though with a maximum of 2KVA or 8A do you really think it needs same earth as a 25KVA transformer?

It doesn't make any difference, 21 Ohms is still required. Your above statement re 25kVA vs 2kVA is pretty illogical bearing in mind 230v / 21 Ohms is 10A, which excluding parallel paths is the maximum current that could flow in true earth even if the transformer in your example were 2MVA. Regardless of the fault current, it's still important to have a low impedance earth path to ensure correct operation of protective devices.

 

[ericmark]

As you correctly pointed out it is not a TT supply and the only reason for an earth rod is so if you go outside a live to earth fault will open the RCD. In the house everything is bonded and any fault current to earth is a direct path line to neutral/earth and the earth rod does nothing. The earth loop impedance is dependent on the internal resistance of the generators windings.

Without the earth rod in the house there would be no problem but any item outside the bonded area would rely on the earth rod to trip the RCD.

The 21 ohms refers to a TT system and is the maximum resistance of the boards earth but it's not a TT system so the earth rod is not really that important as long as under 200 ohms so a fault will open RCD.

Forget the book look at the circuit formed with an earth fault and tell me what earth fault will cause any current to flow through earth rod?

 

[electronicsuk]

Forget the book look at the circuit formed with an earth fault and tell me what earth fault will cause any current to flow through earth rod?

Eric, while I agree that there will be many cases when little or no fault current will flow through the mass of earth, I don't think that's a valid reason for choosing not to follow the red book. Mind you, I'm only going on memory and haven't actually checked the book myself on this one, so am probably leaving myself wide open to being proven wrong.