Theoretical earthing question.

[Graham3232]

Hi,

I did a Ze test on a TT installation and got 24 ohms and a PFC of 10 amps at the rod.

In theory if I removed the RCD and put a link from say a 32amp MCB to the earth bar in the CU with the bonding and other circuits disconnected, would a current of around 10 amps flow continuously to earth?


Graham.

 

[bernardgreen]

In theory yes. 10 amps would flow continuously.

In practise the rod to soil interface would be altered by the current, the most probable affect would be the heating and drying of the soil leading to higher impedance and lower current.

 

[Lectrician]

Yes, this is why an RCD is (usually) required in a TT installation, as not enough fault current can flow to operate a fuse or MCB.

I say usually required, but in practice on day to day jobs, it will always be required.

I have seen a TT with a faulty ELCB and a GU10 spot light connected from live to earth (incorrect connections at terminal block in the fitting). Customer thought the light was dim, but just assumed GU10 spots were crap!

 

[Spark123]

PFC will be more likely to be the PSCC in a TT system, PEFC can be fairly low as in your case (although a PEFC isn't measured in isolation where as Ze (or Ra) is

 

[Adam_151]

I have seen a TT with a faulty ELCB and a GU10 spot light connected from live to earth (incorrect connections at terminal block in the fitting). Customer thought the light was dim, but just assumed GU10 spots were crap!

Seen it with a extension lead powering the TV with neutral and earth crossed, faulty ELCB, no bonding and a poor connection to the electrode. Homeowner commented that they were getting shocks doing the washing up