TT and RCD protection

[Nigel]

Just noticed that the requirement for RCD protection has been removed for TT installations.

 

[jamie-spark]

I would bed to differ. The last TT system we tested was 0.73 and that was only a single rod.

Although I do agree it is unlikely to be ever able to prove a permanently and reliable Zs on a TT system it does loosen the requirements a little and in this case a 500mA RCD would be more than enough.

Think the installer may have hit an earthed source . Done loads of TT systems, to get an RA of 0.73 ohms with one electrode I’d need to see it to belive it and then see it again

 

[Strima]

Think the installer may have hit an earthed source . Done loads of TT systems, to get an RA of 0.73 ohms with one electrode I’d need to see it to belive it and then see it again

Had one the other week and it's been bone dry for a while...

 

[123]

Had one the other week and it's been bone dry for a while...

View attachment 43254

Next to no chance that is off one rod though, unless they have nicked something on the way down

 

[Strima]

Next to no chance that is off one rod though, unless they have nicked something on the way down

That's what I’m thinking.

 

[123]

A few weeks ago managed to get 4 rods down (don't recall ever getting that far down before!), thought happy day's this will be nice and low. 60 ohms

Start of the year, same street, 2 rods down, 9 Ohms.

 

[jamie-spark]

That's what I’m thinking.

That’s what am saying

 

[Strima]

That’s what am saying

Unless we excavate the rod we’ll never know.

 

[ipf]

Essex, you're always (or rather 99.9% of the time) going to need an RCD for TT as you'll never be able to meet the Zs values unless the Ra<1Ω which (unless you are Trev) is nigh impossible.

I'm pretty sure that reg you have given was in the 17th, so nothing new. Would be very interesting to know if anyone on here has ever been able to get an Ra low enough to forgo an RCD on a TT though?

Are you talking domestic?
I've worked in industrial units, multi rodded, maybe nested with Ra of well under. You'd think they were TN, until closer examination.

 

[multimick]

Are you talking domestic?
I've worked in industrial units, multi rodded, maybe nested with Ra of well under. You'd think they were TN, until closer examination.

that must be one of the cleanest meggers i have seen ,just like mine

 

[PEG]

that must be one of the cleanest meggers i have seen ,just like mine

It'll be the fresh,sea breeze,blowing off the dust...oil-rigs are like that...

 

[EbolaSideRoom]

Last week on a new install we put 5 rods down into what seemed like sand to achieve an Ra of 132.4 ohms. We only stopped at 5 because we ran out of rods. This week we will be digging a new trench from the incoming supply building to an area of much firmer ground so that another rod will provide a more acceptable result.
Incidentally, 400v supply leaves the meter and hits a distributors 4 pole switch, then a 4 pole 100mA/100A Time delayed RCD. Splits 3 ways to 2x 3P+N switch fuses for distribution to remote buildings and a local 1P consumer unit for construction supply, treatment plant & local lighting.

 

[telectrix]

I would bed to differ. The last TT system we tested was 0.73 and that was only a single rod.

Although I do agree it is unlikely to be ever able to prove a permanently and reliable Zs on a TT system it does loosen the requirements a little and in this case a 500mA RCD would be more than enough.

must be all that fake tan lotion soaked into the ground.

 

[magnoliafan89]

Quick question guys im moving my board 5 metres. Currently on a tt system but within the next few months will have western power convert it to tncs. But in the mean time would it be ok to put up just a kmf switch or a time delayed rcd before the tails?

 

[telectrix]

all depends on whether the new tails require RCD protection.

 

[magnoliafan89]

They are going to be run under the floor so through the joists and then come down inyo the next room along cable tray