TT earth rod resistance

[who took my fluke?]

Any ideas how to test the value of the earth rod resistance, as of yet I do not have the 4 lead meter tht is required. Is there another way?? Was under the impression that if the whole house is under RCD protection (100ma time delay and 30ma split) there would no need to calculate this. Please clarify!! Thanks

 

[Softus]

Pay an electrician a fee to test it?

 

[plugwash]

if its on a mains supplied install just do a loop test on the rod (without anything else connected to it) and record that value (it will be a slight overestimate because it will also count the resistance of the suppliers rods but that shouldn't be an issue unless its marginal already).

you only really need a rod tester if you are installing a rod for a generator or transformers output.

 

[Spark123]

Agree with Plugwash,
The maximum external loop inpedance should be less than 200 ohms for reliability (or 100 ohms if you are NICIEC ) .

 

[securespark]

Seconded. You can get fancy testers for rods, but I just do a Ze on it. (Disgusting creature!)

 

[didthathurt]

I've just tested a TT system with a Ze of only 9 ohms I've never seen one with such a low Ze before - I even suspected the meter.

 

[plugwash]

you did make sure you disconnected everything from the rod and tested the rod alone right.

but even so it can happen if there is a lot of the right materials in the ground or the eath electrode is big.

 

[didthathurt]

Oh yes. It seems to be a combination of a large rod driven deep into wet clay within 100m of a sub-station with its own large earth spike. I still found it hard to believe and double checked it was properly disconnected before doing it again.

 

[Softus]

Just checking my understanding here - presumably the lower the TT Ze value is the safer the installation is?

 

[plugwash]

pretty much, if it gets very low there may be issues with high currents in the event of a fault though.

a very low Ze can also indicate your TTs earth may not be as seperate from other systems earths as you'd like.

 

[Softus]

Sp, is there an optimum value that lies between 0 and 100 Ohms?

Or does it depend upon the resistance of the rest of the earth fault circuit, i.e. between the point of failure and the earth spike?

 

[plugwash]

optimum is close to zero ohms (high fault currents may become an issue if its too close) to the general mass of earth.

thing is a low resistance to either the transformer neutral or to real earth may be the result of your rod being rather close to a rod on another system and that may be bad (e.g. if you are trying to avoid a PME earth for a caravan pitch supply you don't wan't your rod too close to metalwork that is in contact with the ground and connected to said PME earth).

 

[Softus]

Thanks!