Select (Scottish) Certification Scheme Question on volt drop from 2391

[Ian1981]

That is doggy the 1.2 value is only really used for design unless the temperature is stated in the question.

again on 2391-051
and question 23 ignores sequence of tests I know you can have limitations on Periodic inspection but it says to use Earth fault loop impedance test to prove continuity of CPC.
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sorry OP thats not very helpful of me and paper 2391-52 isn't that bad, it's pretty good.

If it’s a periodic then yes you can use an earth fault loop impedance test as per gn3 to carry out earth continuity, R1+R2 testing is for initial verification but it’s not precluded for periodics if you wish to do it, tho this is nigh on impossible for say an installation which has a metallic Wiring system throughout, say like an industrial application.
The order of tests for initial verification is not applicable to periodic inspection and testing as the installation has already been in service for x amount of years.

 

[Mark Wright]

Hi, I've never seen that does it say in GN3 you can use Earth Fault Impedance for CPC continuity on Periodic are you also allowed to take you R1+R2 values from this? I thought this was bad practice?
I.e Zs-Ze=R1+R2
Thanks

 

[Ian1981]

Hi, I've never seen that does it say in GN3 you can use Earth Fault Impedance for CPC continuity on Periodic are you also allowed to take you R1+R2 values from this? I thought this was bad practice?
I.e Zs-Ze=R1+R2
Thanks

For initial verification continuity of the cpc must be performed as a dead test so you can use the R1+R2 method or the wander lead R2 method.
This must be confirmed before any live testing ie an earth fault loop impedance test.
For periodics the installation is already in service, by measuring just Zs we are confirming that the disconnection times for our protective devices are met for ADS.
Modern test instruments will detect if the touch voltage to earth rises above your meters set value of 25 or 50 volts by there being no cpc present and abort any live testing.
For periodics the inspector can use an array of tests to confirm and satisfy safety.
It is much easier to de-energise a small domestic installation and remove parallel paths to perform theR1+R2 tests than it is on a say commercial offices and industrial installations where often it impossible to isolate every circuit and complete every test if the building is required to be energised 24/7 for client operational use.
R1+R2 values must only be calculated at design stage.

 

[Mark Wright]

So you could do a Zs test on a periodic and then just Zs-Ze=R1+R2 and that would be acceptable?

 

[Ian1981]

So you could do a Zs test on a periodic and then just Zs-Ze=R1+R2 and that would be acceptable?

No,like I say it is not a requirement to measure it on a EICR

 

[Mark Wright]

No,like I say it is not a requirement to measure it on a EICR

Don't you still have to fill in a Generic Schedule of test results?
Even though you have agreed limitations with the client etc.

 

[Ian1981]

Don't you still have to fill in a Generic Schedule of test results?
Even though you have confirmed limitations with the client etc.

Yes you fill in test results, as far as ADS is concerned you need the measured Zs for the circuits.
It does not necessarily mean that each circuits cpc is continuous as you may pick up a cpc by parallel paths but there’s no requirement for a circuit to have its own cpc (take trunking systems , conduit where the containment is the cpc for multiple circuits. )
We are primarily interested that the circuits ocpd will operate in under the required times.

 

[Mark Wright]

sorry Im getting bit confused here wires crossed I think
You don't need a reading of R1+R2 for periodic inspection if you agreed that limitation with you client but there is a Schedule of test results ?

 

[Ian1981]

sorry Im getting bit confused here wires crossed I think
You don't need a reading of R1+R2 for periodic inspection if you agreed that limitation with you client but there is a Schedule of test results ?

It’s not a limitation, it’s not required full stop for a EICR but it’s not precluded either.
Record it if you wish and add it to the Ze if you do not want to directly measure zs.
It’s pointless in certain installations tho and unachievable.

 

[AJshep]

But the question has no information to state the cables will be running at 70C, nor the information (if any) to calculate what temperature it might be operating at.

What if it were XLPE SWA at 90C instead?

What if a long run of 16mm that would barely show any rise?

The examiners deserves a boll0cking for that.

C&G do this alot especially in multiply choice papers, although the wording they use is generally "maximum volt drop" I believe they just do it to check the candidates knows how to apply basic correction values, as they always give both answers.

Another one they like to check is that the candidates are applying 80% to the max Zs values in the BBB.

 

[Mark Wright]

Chris Kitcher does use 1.2 in his video and gives an explanation so yeah your right

View: https://m.youtube.com/watch?v=f1NNHgxHiBw

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so at a guess the mV/A/M values from OSG etc are actually at Max working temp already i.e 70°C

 

[Stalag71]

Chris Kitcher does use 1.2 in his video and gives an explanation so yeah your right

View: https://m.youtube.com/watch?v=f1NNHgxHiBw

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so at a guess the mV/A/M values from OSG etc are actually at Max working temp already i.e 70°C

It's my understanding that the values in the OSG are adjusted for temp which accounts for the difference you'll find in tables for max Zs etc....because the OSG is your go to while 'in the field' and BS7671 would be your go to when carrying out 'design'.