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

[topquark]

Can't help but think we're placing a little toooo much reliance on the good ole RCD for protection.

 

[Octopus]

I questioned Castline systems about their default 1667 ohms and they say its OK. I disagree and update each line to show the "correct" values.

 

[topquark]

So when they get it checked by their scheme operator, they'll be able to show 1667Ohms, I think NOT!

 

[Octopus]

Its approved by NICEIC, says it all really. Due my annual review with Elecsa soon so will see what my assessor says, me thinks he and I will agree.

Maybe Castline will read this and understand the views of the professionals, rather that those of the corner cutters.

 

[telectrix]

so.it's about 8-1 in favour of BS7671 values. bit like an arsenal defeat.

 

[topquark]

Its approved by NICEIC, says it all really. Due my annual review with Elecsa soon so will see what my assessor says, me thinks he and I will agree.

Maybe Castline will read this and understand the views of the professionals, rather that those of the corner cutters.

Have we verified this with them, or is that what Castline are saying?

 

[topquark]

so.it's about 8-1 in favour of BS7671 values. bit like an arsenal defeat.

You beat ManU didn't you!

 

[Octopus]

Have we verified this with them, or is that what Castline are saying?

Look at their home page!

 

[topquark]

Look at their home page!

Have we verified with NICEIC, that it is acceptable ... not that Castline are members?

 

[topquark]

I see what you mean, as it's in the software and the software's approved, it's implied.

 

[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