Earth rod advice

[aptsys]

What sort of earth leakage protection is in place? A new type RCD with a trip current of 30mA perhaps? Or one of the old (and redundant) Voltage Operated ELCB devices?

If you think about it, the earth conductor is required to carry the earth fault current to trip the (whatever) device is there to provide safety to your daughter (and other family members?). If the fault trip current is limited to 30mA then a great big chunk of 10mm copper is going to be rather more than is needed.

How is it limited to 30mA?

 

[Risteard]

How is it limited to 30mA?

I wondered the same, as did Ohm's Law.

 

[JohnW2]

How is it limited to 30mA?

I didn't read TTC as suggesting that an RCD limited the fault current to 30mA (which is obviously not correct, and I think TTC knows that). I may be wrong, but I thought/think that when he wrote "If the fault trip current is limited to 30mA then ... ", he meant something like "If the fault trip current [i.e. IΔn] is no greater than 30mA then ... ".

Kind Regards, John

 

[Taylortwocities]

I didn't read TTC as suggesting that an RCD limited the fault current to 30mA (which is obviously not correct, and I think TTC knows that). I may be wrong, but I thought/think that when he wrote "If the fault trip current is limited to 30mA then ... ", he meant something like "If the fault trip current [i.e. IΔn] is no greater than 30mA then ... ".

Kind Regards, John

Thank you , John. That indeed what I had meant my post to say.

 

[JohnW2]

Thank you , John. That indeed what I had meant my post to say.

You're welcome

Kind Regards, John

 

[ericmark]

Since I have a full test set I have never tried using DIY test equipment, to test an earth rod one needs access to the ground for around 30 meters which is often not possible, however one can compare your earth with the DNO's earth using a loop impedance tester, the DIY tester the EZ150 has around 6 go/no go lights which show 1.7, 5, 10, 100, 200, and 500Ω likely with a ELCB-v it will exceed 500Ω to be sure it is stable we are told to look for better than 200Ω but in real life I would be looking for better than 100Ω. You could in theory fit a RCD in a box and use the ELCB-v as an isolator and test the loop impedance with the £40 plug in DIY loop tester. However what you can't do is test the tripping time for the RCD, the test button as I found worked A1 with a RCD but using the proper RCD tester it failed, a quick hunt and a RCD tester seems to start at around £125 and for that amount of money it could end up cheaper getting a scheme member electricians to do the work and get all the test certificates that go with the work.

 

[JohnW2]

.... to test an earth rod one needs access to the ground for around 30 meters which is often not possible, however one can compare your earth with the DNO's earth using a loop impedance tester, ...

Most people who have TT earths have them because they do not have any DNO-supplied earth.

Even if there were a DNO-supplied earth, I'm not sure what you mean by, or what point there would be in, "comparing it" with a TT earth rod - we know that one would be very much higher impedance than the other.

Kind Regards, John

 

[ericmark]

Most people who have TT earths have them because they do not have any DNO-supplied earth.

Even if there were a DNO-supplied earth, I'm not sure what you mean by, or what point there would be in, "comparing it" with a TT earth rod - we know that one would be very much higher impedance than the other.

Kind Regards, John

To test an earth rod we normally use a special meter and two long leads with spikes, I have done this hundreds of times. But where there is a supply to the house we can measure the impedance between the suppliers earth rod your own earth rod and the supply cables, this reading will be higher than the true earth reading as it includes the resistance between the suppliers earth and ground as well as your own earth rod and ground.

I did not use the correct wording your not comparing, sorry for that, with an earth rod of around 60Ω the resistance of the DNO's earth rod is hardly worth noting, however when installing an earth pit then it is possible the earth impedance is lower than the DNO's. Since it is the ability of the combined earth loop impedance to operate the automatic disconnection of the supply then to measure where possible the combined earth loop impedance is really the way to go.

I seem to remember being told we should allow for the DNO's earth to be 15Ω when testing earth rods and setting up a system before the DNO's supply is connected. We should also allow for a TN-S supply to have 0.8Ω and a TN-C-S supply to have 0.35Ω within the DNO supply. However this is for a 100 amp domestic supply, where the supply is to commercial premises then we can expect lower DNO readings, specially where their transformer is actually on the companies premises.

It has been a point of argument, if the incomer to a house is 0.20Ω in the days when for a ring final the limit was 1.44Ω should the recorded limit be 1.29Ω to allow for the DNO to use alternative supplies in the future which are 0.35Ω? Today with RCD protection this is no longer an issue, it would not matter if the DNO's earth was 200Ω the RCD would still disconnect in the event of a fault, EEBADS has gone.

 

[JohnW2]

I did not use the correct wording your not comparing, sorry for that ....

That's what confused me, hence made me uncertain of what you were talking about. As you have now clarified, you were merely talking about the measured Ze of an installation with a local TT rod - which, as you say, is the combination of the impedances of the local rod, the DNO's rod (at the transformer) and the DNO's L & E conductors between transformer and the installation in question.

Kind Regards, John

 

[nicam49]

Since I have a full test set I have never tried using DIY test equipment, to test an earth rod one needs access to the ground for around 30 meters which is often not possible, however one can compare your earth with the DNO's earth using a loop impedance tester, the DIY tester the EZ150 has around 6 go/no go lights which show 1.7, 5, 10, 100, 200, and 500Ω likely with a ELCB-v it will exceed 500Ω to be sure it is stable we are told to look for better than 200Ω but in real life I would be looking for better than 100Ω. You could in theory fit a RCD in a box and use the ELCB-v as an isolator and test the loop impedance with the £40 plug in DIY loop tester. However what you can't do is test the tripping time for the RCD, the test button as I found worked A1 with a RCD but using the proper RCD tester it failed, a quick hunt and a RCD tester seems to start at around £125 and for that amount of money it could end up cheaper getting a scheme member electricians to do the work and get all the test certificates that go with the work.

Hi, thanks for that advice, I think I'll get me an EZ150 then. I've just installed the earth spike (eventually) and have only a limited way of knowing if it's any good...in the olden days it was OK to measure the resistance between earth and neutral with a dvm, or connect a light bulb between live and earth to see how much or how little it glowed. I've tried with a dvm and the reading is 1meg approx, using the same method measuring Ohms between water main and neutral I'm getting 40-50 Ohms. So am wondering what on earth (pun intended) to do next.