Measuring the Earth Electroded resistance

[Podooser]

The earth rod will be used to earth a stable 50 m from the house. The stable CU is suppied from split tails, 60amp fuses via 16mm armoured cable. Is it correct that the measurement for earth electrode resistance is taken between the phase conductor at the stable end of the SWA and the earth electrode?

 

[ColJack]

no..

you need a specialist piece of test equipment to test earth electrodes..
you disconnect it from the MET, and then use probes supplied with the tester to sink 2 spikes about 10m from the earth electrode and measure between these and the electrode..

or something along those lines.. it's in the OSG and GN3.. don't have them to hand right now.

 

[plugwash]

Yes if you have mains power availible you can obtain a perfectly acceptable measurement (it will actually be an overestimate as it will count the resistance of the suppliers earth as well) of the earth rod by using a loop tester between the mains live and the rod.

In situations where you don't have mains power availible then you have to use a 4 terminal resistance meter with a pair of temporary electrodes a suitable distance away.

 

[Podooser]

Yes there is a mains supply. Can you explain why it will count the resistance of the suppliers earth.

 

[ColJack]

that's just measuring the earth loop impedance... or the Ze of this sub circuit...

not the earth electrode resistance as the OP asked for..

 

[plugwash]

it will also count the impedance of the supply transformer but that is negligable. Basically it measures the impedance of the complete loop.

tester-live supply conductor-thier transformer-thier earth rod-your earth rod-tester

 

[Podooser]

The reading was 133 ohms. I am using a 30mA rcd which by my calculation gives me a result below the 50V required.

 

[plugwash]

indeed but it doesn't meet NICEIC reccomendations (which say that earth rod resistance should be below 100 ohms iirc).

 

[Podooser]

Would that be so that all eventualities are covered ie building sites etc

 

[RF Lighting]

Did I read in the 17th ed. DPC that the maximum allowed impedence is to be lowered to 100 ohms?

 

[securespark]

Can't find it, but did find that if you cannot protect cables in walls by mechanical means, you protect them with a 30mA RCD instead...

 

[plugwash]

that's just measuring the earth loop impedance... or the Ze of this sub circuit...

Generally you just need to know the rod resistance is below a certain value, the exact value isn't generally a major concern. The loop impedance will be slightly greater than the rod resistance so its perfectly acceptable to use it for such calculations.

 

[Podooser]

By the way, this is the best forum I have come across.

 

[Spark123]

Can't find it, but did find that if you cannot protect cables in walls by mechanical means, you protect them with a 30mA RCD instead...

Yeah, I have seen this in the 17th edition DPC too (522.6.7) .

Currently it is written in GN5 that for reliability the external loop impedance of a TT system should not exceed 200 ohms for reliability though as has been pointed out, this may be lowered next year to 100 ohms.

 

[nozspark]

The reading was 133 ohms. I am using a 30mA rcd which by my calculation gives me a result below the 50V required.

Isn't the max touch voltage <25v for a stable/agricultural environment??