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

[Deleted member 9648]

And that's an informed common sense approach that I will also continue to adopt.
This 1667 as a maximum permissable Ze is IMO a very dangerous approach. Sure the calculations stack up in theory, but in adopting these figures you are going down the slippery road of becoming totally reliant on the reliability of the RCD.
A possible way around could be to put more emphasis on the importance of the R1 + R2 readings rather than max Ze.
But when you have maximum Ze's permissable as 1667 in black and white, there really is nothing to stop the less informed from pointing a finger at that statement and saying , sorry but its within the value stated. End of.
Someone is going to die as a result of a failed RCD one day, and suddenly you will see 1667 disappear and the whole situation/regulations re-written as a result.

A lot of truth in what you say....however it rather ignores TT systems in which you are reliaint on RCD's for earth fault protection in practice....unless you go down the road of multiple linked electrodes to achieve a low enough Ra to operate an overcurrent device....IMO not practical in the majority of TT installations,and reliant on consistant ground conditions after initial verification. IMO the way to address this is more than one RCD ,ensuring discrimination.

Re the highlighted section of the quote....IMO as far as TN systems go,nobody is actually adopting the 1667 figure,it is a purely theoretical figure and technically the correct figure.....It could be argued that filling in the max figure for the overcurrent device on the cert is incorrect,as the 1667 is the maximum permitted.

 

[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)

Wirepuller, I did bracket TT in my very first post/rant. And yes, what you say is very true.
But with TT there are fewer options than other earthing systems.
I still feel that the 1667 max on TN-S or TNC-S systems, is all but condoning very high earth loops between R1+R2. Its all very well saying a competant spark would spot a potential problem, but the fact is 1667 as you rightly say is on the certs, technicaly correct, and being widely accepted. If my family had a choice between using a shower with a good solid earth, on a pme system with a Zs of 0.39. AND NO RCD fitted. Or the same shower with a far higher Zs, but with an RCD.
I would opt for them using the non RCD set up. If an earth fault should develop, I would not want to risk reliance on the RCD with the people I love and care for.
Of course combining the best of both worlds is the better option, But a Zs of say 22 ohms, and saying all is good because you have an RCD and therefore max permissable is 1667.
Not in my world no way, the whole things going down the wrong roads. IMO.

Sorry for waffling, Lets make this simple.
Does anyone think a max Zs of 1667 ohms is acceptable ???? Then why the hell are we putting it on our certificates ???

 

[IQ Electrical]

The only important figure is the measured Zs of the circuit, wirepuller is absolutely correct in what he posted.

If I see a PIR with say a lighting circuit protected by a BS60898 B 6 with a measured Zs of 6 Ohms, immediately alarm bells are ringing and I just know something isn't right, even though the max Zs for the device is over 6 Ohms!

This is competency and conversely, when I see a circuit protected by say a BS61009 C 32 and the max Zs has been entered as 0.71 Ohms then I immediately become a little less confident in the quality of the PIR because that is NOT the maximum Zs for that device.

 

[baldsparkies]

The only important figure is the measured Zs of the circuit, wirepuller is absolutely correct in what he posted.

If I see a PIR with say a lighting circuit protected by a BS60898 B 6 with a measured Zs of 6 Ohms, immediately alarm bells are ringing and I just know something isn't right, even though the max Zs for the device is over 6 Ohms!

This is competency and conversely, when I see a circuit protected by say a BS61009 C 32 and the max Zs has been entered as 0.71 Ohms then I immediately become a little less confident in the quality of the PIR because that is NOT the maximum Zs for that device.

I agree with all you say, and yes wirepuller is correct in what he says.
But none of this detracts from the fact, that an accepted written statement is being implied that 1667 is a permmissable Zs.
There is no mention of a competant inspector changing the goal posts.
Its stated as a maximum permissable figure, and I really feel thats not the case, because no competant sparks would consider it so.
Using TT as an example we are looking at 200 ohms, down to 100 by the niceic, 21 ohms aside. Thats a Ze figure I appreciate, but its the statement itself that 1667 is permmisable that bugs me.
I can see your viewpoint completely, just hope you can see mine.
As you say IQ it's about spotting problems through competance at the end of the day.
But the lighting issue I sorted had a problem staring the inspector in the face, he even recorded it on his certs, and I gaurantee, he looked at the max permissable and considered all was fine.
Thats the danger, the interpretation is all wrong.

 

[IQ Electrical]

But we have to assume competency on the part of the installer or the entire content of BS7671:2008 is pointless.

The 200 Ohm figure for earth electrode Ra is a figure that allows for degradation of soil resistivity from freezing/drying etc. but as i've said many times, I'd take an Ra of 500 Ohms on a deeply driven rod rather than 200 Ohms on a 1.5 Metre rod because on the first good drought or frost, my deeply driven rod will be almost immune.

I accept that there are installers that are not competent, I read their PIR's every day but we can't alter the regulations to suit the lowest common denominator.

 

[baldsparkies]

But we have to assume competency on the part of the installer or the entire content of BS7671:2008 is pointless.

The 200 Ohm figure for earth electrode Ra is a figure that allows for degradation of soil resistivity from freezing/drying etc. but as i've said many times, I'd take an Ra of 500 Ohms on a deeply driven rod rather than 200 Ohms on a 1.5 Metre rod because on the first good drought or frost, my deeply driven rod will be almost immune.

I accept that there are installers that are not competent, I read their PIR's every day but we can't alter the regulations to suit the lowest common denominator.

Been talking to a couple of old friends on the subject,
Respected and very competant electrical engineers I might add.
Enjoyed our pint, and one said.
If you were driving down a road where the speed limit was 60 mph. You wouldnt just keep to that speed because it says so.
You would use your own judgement as a competant driver to negotiate bends in the roads ect at much lower speeds than stated, and for reasons of safety.
When put like that, who am I to argue.

 

[IQ Electrical]

Been talking to a couple of old friends on the subject,
Respected and very competant electrical engineers I might add.
Enjoyed our pint, and one said.
If you were driving down a road where the speed limit was 60 mph. You wouldnt just keep to that speed because it says so.
You would use your own judgement as a competant driver to negotiate bends in the roads ect at much lower speeds than stated, and for reasons of safety.
When put like that, who am I to argue.

If I'd have been more 'skilled with the pen' I'd have used that analogy, perfect!

 

[telectrix]

wish my van could do 60.

 

[giio]

if I were to be pedantic, as a lot of people on here are, by saying they put 1667 because that is what it says ,then they are wrong the actual figure is 1666.67 so they should be filling in 1666 if they are rounding it off ( round down not up, it's the pessimistic nature of all regulations especially IEE ones)

50 (volts) / 30 (milliamps) = 1666.67

 

[IQ Electrical]

While I can't dispute your maths, the 1667 Ohm value is taken from Table 41.5 in BS7671:2008

 

[telectrix]

no mate. roundeedd off it's 1667 to the nearest whole number. if i remember correctly the last significant figure of 1666 was the great fire of london. now that was a disaster, but at least it killed off the plague. let's hope this figure related to our work don't kill off anyone.

 

[giio]

no mate. roundeedd off it's 1667 to the nearest whole number. if i remember correctly the last significant figure of 1666 was the great fire of london. now that was a disaster, but at least it killed off the plague. let's hope this figure related to our work don't kill off anyone.

a few ohms were lost in that one

 

[IQ Electrical]

a few ohms were lost in that one

13200 Ohms were lost, apparently telectrix picked up a fair old few rewires from it.

 

[baldsparkies]

13200 Ohms were lost, apparently telectrix picked up a fair old few rewires from it.

Yep, But Telectrix was just a young apprentice in those days. lol

 

[Des 56]

Yep, But Telectrix was just a young apprentice in those days. lol

I will have you know,he was well qualified and had "countless" years of experience behind him by then

 

[IQ Electrical]

I only started this because of his Arsenal trophy joke this morning!

 

[Guest123]

Some say that he wrote the 1st edition on the back of his JPS packet.......and that if it wasnt for him the RCD would be a myth.......all we know is he's called.......what ever we feel like at the time!!!!

 

[IQ Electrical]

Some say that he wrote the 1st edition on the back of his JPS packet.......and that if it wasnt for him the RCD would be a myth.......all we know is he's called.......what ever we feel like at the time!!!!

And he's the oldest spark (ten second Clarkson pause) in the world.

 

[Des 56]

And he's the oldest spark (ten second Clarkson pause) in the world.

and
His rewiring times are legendary

 

[baldsparkies]

and
His rewiring times are legendary

With 13200 homes/Ohms to cover, they need to be. lol