Why convert TNS to TT???

Johnlondon · Jul 24, 2013

Got a small job to fit an outside RCD socket and when looking at the Cu, I noticed the original main earth from the lead had been disconnected (seemed just a few strands left) and the house had been converted to a TT. The householder explained a previous electrician did this some years ago as the 'lights kept tripping out'. Haven't tested the rod resistance yet as it was a flying visit so don't know if it's under 200 ohms. My question is why would a TNS be converted to a TT?

Octopus · Jul 24, 2013

Could it be that the TN-S wasn't actually providing an earth so the TT was used instead. Done this a couple of times.

shagbite · Jul 24, 2013

Johnlondon said:
Got a small job to fit an outside RCD socket and when looking at the Cu, I noticed the original main earth from the lead had been disconnected (seemed just a few strands left) and the house had been converted to a TT. The householder explained a previous electrician did this some years ago as the 'lights kept tripping out'. Haven't tested the rod resistance yet as it was a flying visit so

don't know if it's under 200 ohms.

Click to expand...

My question is why would a TNS be converted to a TT?

careful what you say here mate - '54's hovering about this thread!

Engineer54 · Jul 24, 2013

Johnlondon said:
Got a small job to fit an outside RCD socket and when looking at the Cu, I noticed the original main earth from the lead had been disconnected (seemed just a few strands left) and the house had been converted to a TT. The householder explained a previous electrician did this some years ago as the 'lights kept tripping out'. Haven't tested the rod resistance yet as it was a flying visit so don't know if it's under 200 ohms. My question is why would a TNS be converted to a TT?

Seeing as an earthing point had been provided by the DNO, no reason whatsoever!! If the earthing provision had failed then all the electrician had to do was inform the DNO and remind them of their duty to maintain that earthing provision.
I don't know how a failing Ze on the main cable is going to influence a lighting circuit to trip out mind!!

My guess is that we had a lazy sparks, that made a few bob on installing a TT system, i bet all he used was a thin twig too.

After going back and testing the Ze on the cable sheath of this installation, if it is outside the designated maximum of 0.80 ohms, inform the local DNO Duty Engineer of that fact and request an urgent time and date for repairs to be made.
I wouldn't inform them that a TT system has been installed, that will just give them an excuse to do nothing!! Maybe disguise the connection at the MET as a bonding connection... lol!!