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Dan007

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Please can people clarify what value they insert in the Max Zs field on certification.

I've always input 100%. Currently in disagreement with another person as they are specifying 80%.

Done my 2391 in 2008 and I was taught and told it's always 100% and it's down to the QS to interpret the results and ensure they fall with 80% allowing for correction factors.

Third argument is that in GN3 and OSG they specifically state 80% - however I always understood these to be quick reference pocket guides while on site.

In the mentioned publications and BBB the model forms show 100% in the compelte fields for examples. Secondly the small print at the bottom makes reference to the Publication and values in Chapter 41 aka BBB and 100% values.

Your thoughts please people
 
The purpose of a well designed form is to make sure you only have to think about one thing at a time.
Therefore when filling it in you should do the full calculation, and when measuring the zs all you do is measure and fill in. Then to determine compliance you just compare the two without any other thought.
If part of the calculation is mixed with the comparison things would get very hairy.
 
My 20p worth - I put in the values list for max Zs in the tables of Chapter 41. There's a little footnote at the bottom of Generic Schedule of Test Results in BBB p483 that sort of says to do this (in typical IET fashion). I pop the 80% value in the remarks column for JD's excellent reasons.
 
Do you guys think this is an ok reply?

Dear ****

Thank you for your comments.


I am still a little unsure why you feel the 80% corrected value should be listed on the certificate under the request for “Max Device Zs from BS7671”. The mentioned request for this information is a direct extraction from your own test certificate. Furthermore this is a replicated terminology which can be found on most model certificates which have been designed in line with the IET and BS7671 requirements?


Without trying to instigate a heated discussion – and only wanting to seek common ground, are you therefore insinuating the below excerpts I attached from BS7671 are actually incorrect? The model test certificate requires the same information as the HEA model and when referencing the Max Zs in BS7671 these values would be at 100%. It should also be noted this model form is extracted from GN3 where you make reference to corrected values.


Finally please again make reference to GN3 (as you extract the 80% values from this standard) where you will find the following statement: “Where the maximum permitted earth fault loop impedance value stated in column 8 is taken from a source other than the tabulated values given in chapter 41 (being BS7671 and listed at 100%) of this standard state the source of the data in the appropriate cell for the circuit in the remarks column (Column 25) of the schedule.


The reason I am seeking clarity on this matter is due to a recent meeting I held with another contractor. I believe you gave a NCR (Non compliance) for not using 80% when they indeed used 100%. On this occasion they themselves wanted to issue my team an NCR for not using 80% corrected values.


I respect the excerpt you have attached below – however this is relative to how we apply correction factors – not what we list on the certification.
 
We don’t half make our own trade look a bit disorganised with our constant quest to find new ways of complicating what is a simple question.

The certificate asks for the Max Zs.....that is quite clearly, the Max Zs characteristic for each device as publicised in BS1761.

It is such a simple question that it completely baffles me to why this is constantly debated.

80% means nothing in truth....because if your testing and the value you record is under the max value in the regs.....then it’s fine.

Give me strength.........
 
We don’t half make our own trade look a bit disorganised with our constant quest to find new ways of complicating what is a simple question.

The certificate asks for the Max Zs.....that is quite clearly, the Max Zs characteristic for each device as publicised in BS1761.

It is such a simple question that it completely baffles me to why this is constantly debated.

80% means nothing in truth....because if your testing and the value you record is under the max value in the regs.....then it’s fine.

Give me strength.........

Exactly. I can't believe I'm even questioning my own sanity and intelligence.
 
We don’t half make our own trade look a bit disorganised with our constant quest to find new ways of complicating what is a simple question.

The certificate asks for the Max Zs.....that is quite clearly, the Max Zs characteristic for each device as publicised in BS1761.

It is such a simple question that it completely baffles me to why this is constantly debated.

80% means nothing in truth....because if your testing and the value you record is under the max value in the regs.....then it’s fine.

Give me strength.........

Not quite correct. The reason when testing we do not want the measured Zs to exceed 80% of the max permitted is due to operating temperatures. So if a measured Zs is lower than the max permitted but higher than 80% then this would need looking into.
 
Essex.....really??? It would need looking at ??
And where does it say that, what code would you use and who is going to pay for your non-regulated investigation ???

Edited to add, I know exactly where the 80% comes from.
 
I check the results as i write them down following testing and the clever bods who set up the NICEIC online portal check them again as you enter them, its rare for a circuit in the domestic arena to be outside of the scope of the online software to check. Think I have only had one circuit that the NICEIC online system couldn't work out the Max Zs for. It saves arguments or debate come assessment time too,
I recall for a while the NICEIC had slightly different values to BYB/BBB and the adjusted values were different again, typical NICEIC making there own version up.
 
Not quite correct. The reason when testing we do not want the measured Zs to exceed 80% of the max permitted is due to operating temperatures. So if a measured Zs is lower than the max permitted but higher than 80% then this would need looking into.

Aka 0.004*50=20%
 
Essex.....really??? It would need looking at ??
And where does it say that, what code would you use and who is going to pay for your non-regulated investigation ???

Edited to add, I know exactly where the 80% comes from.

You can do whatever you like. Personally if there is a risk of an OCPD not operating correctly I would want to look at it.
 

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