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Discuss Max Zs on certification in the Electrical Forum area at ElectriciansForums.net

Dan007

EF Member
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
 

westward10

In echoed steps I walk across an empty dream.
Staff member
Moderator
Hello Dan welcome, I have edited your post as you said you could not do it in #2, deleted.
 

SparkyChick

Making a banana smoothy for my fave gorilla
Staff member
Moderator
Personally I put down the adjusted value, because that for me is the real (if you like) value to which the circuit must conform.
 

Strima

Electrician's Arms
I always put in as required by BS7671, no arguments then or confusion.
 

johnduffell

Regular EF Member
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.
 

Dan007

EF Member
Typically most seem to think the same as myself. Always insert the maximum as the certificate requests.

Again the foot note does indeed require values from Chapter 41.
 

Dan007

EF Member
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.
 

Rodders099

Active EF Member
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.........
 

Dan007

EF Member
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.
 

essex

Forum Mentor
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.
 

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