Max Zs readings where circuits are protected by 30ma RCD's

[baldsparkies]

I have noticed, a lot of max Zs readings on test certs (Where RCD's are applicable) are being blanket recorded at 1667 ohms.
On a personnel note I consider this to be poor practice, and prefer to see max Z's taken from tables 41.2 41.3 and 41.4 of chapter 41 of the good old guide.
My reasons are that 1667 is an unrealistic figure in most cases. and doesn't promote a true solid reliable earth fault return path (Like in the good old days before RCD reliance)
We all know that RCD's offer suplementary shock protection, and thats fair enough but chucking 1667 around like conffetti detracts from what a proper earth fault path is really all about.
Maybe I'm getting padantic in my old age, but its one of those things that bug me.
And if an RCD fails its nice to know the earth fault path will be low enough to take out the overcurrent device before touch voltages rise to a point that Zap's people.
RCD or no RCD. (TT aside that is)

(Rant over)

 

[telectrix]

100%agree. i only enter the 1667 when it's a TT system. hate reliance on the great god RCD. sometimes they're like car indicators. work-don't work-work-don't work.........

 

[Octopus]

I also agree, recently started using Formfill and that enters 1667 all the time, I'd say its bad practice.

 

[telectrix]

especially when your measured Zs is below the BS7671 figures. looks good on a cert.

 

[IQ Electrical]

especially when your measured Zs is below the BS7671 figures. looks good on a cert.

Our NICEIC inspector queried the Formfill 1667 values but I showed him the 'NICEIC Approved' label on the CD case and he shut up.

I don't really see a problem if the continuity of protective conductor test has been carried out as it should be, obviously if you had an R1+R2 or R2 value of 10 Ohms for example, you'd be rather negligent in not investigating the cause.

As always, as the competent person, you use your engineering judgement.

 

[ACEELEC]

with regards to the 1667 theoretical value on a 30mA rcd - a lot of conversation is now being given to any figure measured above 200 ohms should be attended to as it could prove unreliable (not stable) as the resistance could increase greatly due to soil and seasonal time of test and go beyond this figure meaning 50v touch voltage greatly increased.

Good practice will always be to add more electrodes or reposition if in excess of 200ohms (pouring water over the electrode is a no no i am afraid)

look at 411.5 Note 2 (and reg 542.2.2) - there was a lot of talk about this being highlighted in the latest upgrade to the regs (because of the 1667 ohm if on rcd trend)

 

[rfcses_com]

In My area where the old four open wire system has been replaced with ABC they have put PME lables on the polls, and a quick phone call to the supplier acknowledges that

 

[IQ Electrical]

with regards to the 1667 theoretical value on a 30mA rcd - a lot of conversation is now being given to any figure measured above 200 ohms should be attended to as it could prove unreliable (not stable) as the resistance could increase greatly due to soil and seasonal time of test and go beyond this figure meaning 50v touch voltage greatly increased.

Good practice will always be to add more electrodes or reposition if in excess of 200ohms (pouring water over the electrode is a no no i am afraid)

look at 411.5 Note 2 (and reg 542.2.2) - there was a lot of talk about this being highlighted in the latest upgrade to the regs (because of the 1667 ohm if on rcd trend)

I see your point but the OP was posting regarding Zs, not Ra on a TT system, there's a big difference there once continuity of protective conductors has been confirmed.

 

[rfcses_com]

Whhops sorry..

a Ze reading of less than 35Ohms the earth rod can be disconnected and the system changed from TT to TN-C-S.

Have you herd of this or what are your thoughts?

If you agree then you shouldn't get any readings ner the 1667.

 

[IQ Electrical]

Whhops sorry..

a Ze reading of less than 35Ohms the earth rod can be disconnected and the system changed from TT to TN-C-S.

Have you herd of this or what are your thoughts?

If you agree then you shouldn't get any readings ner the 1667.

Whoa there, there's more to a TN-C-S system than a L-N loop reading of less than 0.35 Ohms!!

 

[baldsparkies]

Agree with you on that IQ, You have just hit the nail on the head.
Today I checked out a property that had a dual rcd board.
The customer had a 5 branch brass light fitting in the dining room.
They had problems with the rcd intermittently tripping, and the chap said he was changing a bulb and thought he felt a tingle off the fitting prior to the power going off.
Customers can often pinpoint a problem without even knowing it. Just comes up in conversation so it always pays to listen to what they say.
Anyway, probelm was a chaffed flex entering one of the swan necks. The earth tag was missing leaving the fitting un earthed.
Now to the point. A new dual board had been fitted 18 months ago. On the cert the Zs for the 1st floor lighting was 0.74. for the ground floor it was 68.4 ohms. max Zs recorded as 1667 on all circuits. Earthing was PME.
Problem was an easy sort but leaves me in my rant mode, Sorry lads, I know it happens all the time but F F's (F's means Fathers) sake. They were in there 70s !!!!!!!

 

[rfcses_com]

I agree IQ,

However the electricity board have moved three meter positions in the last two months in Basingstoke, Tadley & Reading and on all three occasions they were TT with earth rods but after they were TN-C-S and when I asked the "Electrician" that's what he said.... I phoned the call center they said a quick call to them to check the post code etc will confirm if they can loose the earth rod....

 

[IQ Electrical]

I agree IQ,

However the electricity board have moved three meter positions in the last two months in Basingstoke, Tadley & Reading and on all three occasions they were TT with earth rods but after they were TN-C-S and when I asked the "Electrician" that's what he said.... I phoned the call center they said a quick call to them to check the post code etc will confirm if they can loose the earth rod....

I don't doubt that you've had some PME conversions but please don't believe that a L-N loop of less than 0.35 Ohms is any indication that the PM (protective multiple) of PME has been installed.

 

[IQ Electrical]

Agree with you on that IQ, You have just hit the nail on the head.
Today I checked out a property that had a dual rcd board.
The customer had a 5 branch brass light fitting in the dining room.
They had problems with the rcd intermittently tripping, and the chap said he was changing a bulb and thought he felt a tingle off the fitting prior to the power going off.
Customers can often pinpoint a problem without even knowing it. Just comes up in conversation so it always pays to listen to what they say.
Anyway, probelm was a chaffed flex entering one of the swan necks. The earth tag was missing leaving the fitting un earthed.
Now to the point. A new dual board had been fitted 18 months ago. On the cert the Zs for the 1st floor lighting was 0.74. for the ground floor it was 68.4 ohms. max Zs recorded as 1667 on all circuits. Earthing was PME.
Problem was an easy sort but leaves me in my rant mode, Sorry lads, I know it happens all the time but F F's (F's means Fathers) sake. They were in there 70s !!!!!!!

Well I don't care what anyone says, a qualified electrician could not walk away from any job with an R1+R2 value of that order without knowing that there was a latent defect.

 

[baldsparkies]

Well I don't care what anyone says, a qualified electrician could not walk away from any job with an R1+R2 value of that order without knowing that there was a latent defect.

Happens far to often sadly. We never stop learning, and we can all make mistakes but when I see things like this !!!! Sorry I rant yet again.