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

[telectrix]

i will contine to enter the BS7671 values until someone higher up the food chain tells me i'm wrong.

 

[IQ Electrical]

i will contine to enter the BS7671 values until someone higher up the food chain tells me i'm wrong.

It's the ECA opinion too that 1667 is the value to be entered, if anyone has their (excellent) guide to the wiring regulations, it's mentioned in there.

I know they don't 'write' the regulations but they sit on the committee.

 

[malcolmsanford]

All I can really say is that everyone in our industry uses the tables in the BS 7671-2008 for the relevant Zs values for protection devices.

As in regulation 411.4.9 where we are using an RCD to satisfy the requiresments of the disconnection times in regulation 411.3.2.2 and in table 41.1 it tell us to take the Zs value from table 41.5 and that for a 30mA RCD is 1667, and that is the value that IMO should be entered onto your schedule of results.

As IQ and others say and I agree with, it is really only a technical point of the value entered on the certificate, what is important is that if you had a high R1 + R2 value as a competent person you should investigate why, and not just leave it. This is why tests are done and results are interpreted, and why IMO a lot of electricians that are not trained correctly do not understand why we do these tests, or how to interpret them.

 

[telectrix]

I know they don't 'write' the regulations but they sit on the committee.[/QUOTE]


and just remember that the camel is a horse designed by a committee

 

[giio]

as a little aside to the rights and wrongs of what to enter


RCDs and Indirect Contact Shock Protection
Indirect contact protection by fuses or circuit-breakers is dependent on the earth loop
impedance being within the parameters laid down by BS 7671. Where this cannot be
achieved or where there is some doubt about the consistency, then an alternative method
is required. It is in this situation that the residual current device can offer distinct
advantages over conventional overcurrent protection for indirect contact shock
protection.
The basis of RCD protection in this situation is to ensure that any voltage, due to earth
fault currents, that exceeds 50V is immediately disconnected. This is achieved by choosing
an appropriate residual current rating and calculating the maximum earth loop impedance
that would allow a fault voltage of 50V. This is calculated by using a simple formula given in
BS 7671 Regulation 413-02-16.
Zs x IΔn ≤ 50
Where Zs is the earth fault loop impedance (ohms)
IΔn is the rated residual operating current of the RCD (amps)


therefore a simple calc to arrive at the max zs is

zs = 50 (volts) / rated residual of rcd say for this example 30 ma

therefore max zs for a 30ma rcd (not time delayed) is max zs = 50 / 0.030 = 1666.67 ohms

although as some have said I would not be walking away if the zs reading was anywhere near that, nobody should

 

[malcolmsanford]

as a little aside to the rights and wrongs of what to enter


RCDs and Indirect Contact Shock Protection
Indirect contact protection by fuses or circuit-breakers is dependent on the earth loop
impedance being within the parameters laid down by BS 7671. Where this cannot be
achieved or where there is some doubt about the consistency, then an alternative method
is required. It is in this situation that the residual current device can offer distinct
advantages over conventional overcurrent protection for indirect contact shock
protection.
The basis of RCD protection in this situation is to ensure that any voltage, due to earth
fault currents, that exceeds 50V is immediately disconnected. This is achieved by choosing
an appropriate residual current rating and calculating the maximum earth loop impedance
that would allow a fault voltage of 50V. This is calculated by using a simple formula given in
BS 7671 Regulation 413-02-16.
Zs x IΔn ≤ 50
Where Zs is the earth fault loop impedance (ohms)
IΔn is the rated residual operating current of the RCD (amps)


therefore a simple calc to arrive at the max zs is

zs = 50 (volts) / rated residual of rcd say for this example 30 ma

therefore max zs for a 30ma rcd (not time delayed) is max zs = 50 / 0.030 = 1666.67 ohms

although as some have said I would not be walking away if the zs reading was anywhere near that, nobody should

Agree to what you have written but not sure why you are giving the 16th edition reference?? when it should be

 

[giio]

Agree to what you have written but not sure why you are giving the 16th edition reference?? when it should be

I just plucked the top section off the internet, it's the calc I was trying to get over, ha ha it might keep someone busy looking for the references

 

[baldsparkies]

i will contine to enter the BS7671 values until someone higher up the food chain tells me i'm wrong.

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.

 

[Deleted member 9648]

I use the max 1667 figure on my certs because that is the maximum figure and that is what you are recording. That said it is just a figure. If I was recording test results on a TN system I would not accept a very high Zs. If a reading above the maximum for the overcurrent device was obtained it would be necessary to have a close look at the Ze, R1R2,circuit design etc to establish why an unexpectedly high reading was obtained....experienced testers know what sort of figures to expect for a circuit and will not accept a high Zs just because an RCD raises the threshold.
Where an RCD comes in useful is in a situation on a circuit designed for a type B.(for example)..and it is desired to change to a type C for whatever reason. The measured Zs may well be slightly higher than the max for a C...but if there is an RCD there is no problem.
I had just such an instance in a school recently. A number of computers were running on a single ring via a master switch....the load when they were running was well within the 32a type C mcb,but when the teacher fired them up via the master switch in the morning sometimes the starting surge tripped the mcb.....changing to a type D was only possible because of an RCD on the circuit,and the problem was solved at minimal cost.
Once again...the 1667 ohms figure is just a theoretical figure.....nobody with any sense is actually accepting that kind of reading.

 

[Deleted member 9648]

double post removed

 

[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.