TT and RCD protection

[Nigel]

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

 

[Octopus]

RCD protection for what? The whole installation?

 

[Nigel]

Yeah. The requirement for RCD protection has been replaced with an option for RCD protection or on OCPD. For an OCPD to be used the Zs must be 'permanently and reliably assured'.

 

[Octopus]

Really?

That doesn't sound right .......

 

[Strima]

Which reg you looking at?

 

[DPG]

Come on - don't drip feed stuff to us, give us some proper detail.

 

[Risteard]

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

The 17th did that. RCD protection has not been mandatory for a TT system since the 16th Edition.

 

[Nigel]

411.5.2 -

'One or more of the following types of protective device shall be used, the former being preferred:

(i) An RCD
(ii) An overcurrent protective device.

NOTE 1: An appropriate overcurrent protective device may be used for fault protection provided a suitably low value of Zs is permanently and reliably assured.

NOTE 2: Where an RCD is used for fault protection the circuit should also incorporate an overcurrent protective device in accordance with Chapter 43.'

 

[Risteard]

Yes, but the 17th had the same requirements. The 16th Edition mandated RCDs for fault protection as I recall, but the 17th definitely didn't.

 

[HappyHippyDad]

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?

 

[Risteard]

I'm pretty sure that reg you have given was in the 17th, so nothing new.

Precisely. It's a decade new.

 

[sinewove]

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?

Once got an RA of 6 ohms on a steel farm building by drilling and tapping a girder for the lug.Would have been ok for a B6 on a lighting cct I think.
Regards S

 

[Nigel]

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?

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.

 

[Leesparkykent]

The trouble is even if you did achieve TN values then you would also have to be 100% confident that it would be consistent.

 

[Deleted member 26818]

Was never required.

 

[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.