Testing

[gpwatts]

Could somebody please tell me how to test resistance of an earth rod in a domestic installation using megger MFT1553?
Thanks

 

[scotsparky]

ok here we go.

1 First of all totally isolate the installation
2 disconnect the main earth conductor from the main earth terminal.
3 with the tester set to the ZE setting clamp onto the main earth conductor and put your probe into the main incomming phaswe conductor.

you will get a Ze reading. this is your earth rod reading.

remember to re terminate the Main earth conductor before energising the system.

If you look at GN 3 earth rod testing method 2 its a bit clearer.

 

[gpwatts]

Many thanks for your reply Scotsparky.

 

[electricAl]

Just to add to what Scotsparky has already said, technically the reading you obtain is referred to as an Ra - as in the equation...

Ra (resistance of earth rod) x I delta n (residual op` current of RCD) must be no greater than 50v (considered safe touch voltage)

It`s thru this formula that we arrive at max permissible value of 1667 ohms for a 30mA protected TT install (altho` we have to aim for something less than 200 ohms)

 

[gpwatts]

Thanks for your reply.