Max Zs on certification

[Dan007]

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]

Hello Dan welcome, I have edited your post as you said you could not do it in #2, deleted.

 

[SparkyChick]

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]

I always put in as required by BS7671, no arguments then or confusion.

 

[johnduffell]

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.

 

[Wilko]

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.

 

[Risteard]

You should not insert an adjusted value but the actual maximum value as per BS7671.

 

[Dan007]

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]

Anyone have anything from BS7671 minus the footnotes in the example forms?

 

[Nigel]

You should put in 100% value as that is what the true max Zs is and what the OCPD has been tested to.

 

[Dan007]

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.

 

[Dan007]

You should put in 100% value as that is what the true max Zs is and what the OCPD has been tested to.

Spot on mate

 

[Rodders099]

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]

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.

 

[Nigel]

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.

 

[Rodders099]

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.

 

[Gavin John Hyde]

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.

 

[Dan007]

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%

 

[Nigel]

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.

 

[Rodders099]

seriously though....your in a 5 bed house over 3 floors.
Max Zs for 32a ring is 1.37. 80% is 1.1
You record 1.20.
What exactly are you going to do about this ???
I’m going to do nothing.....record 1.20 on the EICR,
and carry on.

 

[Strima]

seriously though....your in a 5 bed house over 3 floors.
Max Zs for 32a ring is 1.37. 80% is 1.1
You record 1.20.
What exactly are you going to do about this ???
I’m going to do nothing.....record 1.20 on the EICR,
and carry on.

In a domestic situation I doubt a ring final would ever operate at a constant 32 amps. On an EICR I would judge the circuit on the particular installation, it's use and projected future loading and note accordingly as and where required.

However if a circuit was subjected to a constant load of 32 amps and I measured the R1+R2 and added the Ze and came up with 1.2o ohms then I would be investigating further as the conductors at the higher operating temperature would have a higher resistance therefore the required disconnection time may not be met.

 

[Rodders099]

I’m seriously confused here......you’d investigate what and whose orders ??
Your in for an EICR, the Cert asks for MAX Zs.
What code are you going to give a 1.20zs on a 32a ring like in my example above ??
Then, how are you going to explain to the person requesting the EICR that further time...possibly hours, is required for investigating.....even though your recorded results satisfy the cert ??
Finally, which regulation are you quoting to the customer....which justifies your advice for further investigation ??
I’m all ears guys.......I’d love the extra work I could self generate by going down this road.....

 

[Ian1981]

If your measured zs is too high for the adjusted site temperature at 80% of the max value stated in bs7671 then it stands to reason that at the cables maximum operating temperature at 70 degrees, the resistance will be higher than the required values given to operate the OCPD.

 

[Rodders099]

noones answering my questions.......how you gonna convince the customer, that although your results meet the certs requirements......you still need further investigation.
And which code and which reg are you gonna quote ??
This is the minimum info that would be required in order to carryout further investigation.

 

[Ian1981]

How does it meet the requirements?
If your zs is too high at the tabulated value it will be too high when he circuit is in use and operating at a higher temperature than the site testing temperature

 

[Dan007]

Is there an RCD or RCBO fitted to the circuit? Or are we talking old 3036 dinosaur wiring etc. I've been to plenty of new installs where the Zs is oitsido of Zs parameters but protected mechanically. Not much we can do here. Down to poor design really.

 

[Midwest]

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.

I used the same Certsure software; thought it gave the 100% figure when inputting ocpd type & rated current?

 

[Rodders099]

its not that I entirely dismiss the 80% thing or don’t see the vague point.....it’s more that I can’t see how I explain to a customer that although my tests say it’s ok and it’s below the value stated in the regs, I’m going to code it and recommend potentially costly further investigation......which I’d struggle to justify with a clear reg.

 

[Ian1981]

Ironically if the circuit has a 30mA rcd in circuit, then technically you have your fault protection right there.
Poor to design this way on a TN earthing arrangement but on an older install you may have no choice but to record the max zs as say 1667 ohms on the certificate

 

[Octopus]

An. Interesting debate so far ....

How about the certificate having tick boxes for 100% and 80%

tin hat on ....

 

[Rodders099]

An. Interesting debate so far ....

How about the certificate having tick boxes for 100% and 80%

tin hat on ....

Would never happen....because the iet decide on these things and don’t see it as something they need to introduce to the form.
The NICEIC love to make their own stuff up, to somehow seem regulatory.

If this issue was in any way important....it would have been changed to a mandatory requirement.

 

[westward10]

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 think you are misreading what @essex is quoting.

 

[Nigel]

seriously though....your in a 5 bed house over 3 floors.
Max Zs for 32a ring is 1.37. 80% is 1.1
You record 1.20.
What exactly are you going to do about this ???
I’m going to do nothing.....record 1.20 on the EICR,
and carry on.

That would then mean you are doing an EICR incorrectly.

It is debatable if it is a C2 or a FI but it is an ‘unsatisfactory’ all the same.

 

[Rodders099]

That would then mean you are doing an EICR incorrectly.

It is debatable if it is a C2 or a FI but it is an ‘unsatisfactory’ all the same.

And still I’ve had no reply to my question.....which BS1761 reg are you attaching to your C2 or FI decision ???

 

[Silly Sausage]

An. Interesting debate so far ....

How about the certificate having tick boxes for 100% and 80%

tin hat on ....

Just write 100% above any Zs columns, sorted

 

[Zdb]

Always the 100% values for me. The NICEIC inspector insists upon it.

 

[Nigel]

I’m seriously confused here......you’d investigate what and whose orders ??
Your in for an EICR, the Cert asks for MAX Zs.
What code are you going to give a 1.20zs on a 32a ring like in my example above ??
Then, how are you going to explain to the person requesting the EICR that further time...possibly hours, is required for investigating.....even though your recorded results satisfy the cert ??
Finally, which regulation are you quoting to the customer....which justifies your advice for further investigation ??
I’m all ears guys.......I’d love the extra work I could self generate by going down this road.....

I would not investigate anything without the permission of the person responsible for paying. What I would do is one of two things:

1. Mark it on the EICR as ‘FI’ and then submit the report if I felt loose connections could be the cause.

2. Mark it as a C2 as the OCPD would not operate in time which is a direct non-conformance to BS7671. I have not got the book in front of me. Maybe someone se can help.

We do not carry out periodic insepctions to ‘satisfy the cert’. We carry out periodic inspections to issue a report based on BS7671.

I would just use BS7671 to justify any issues I found.

 

[Nigel]

noones answering my questions.......how you gonna convince the customer, that although your results meet the certs requirements......you still need further investigation.
And which code and which reg are you gonna quote ??
This is the minimum info that would be required in order to carryout further investigation.

I do not need to convince anyone. My qualifications and 20 years experience should speak for themselves. Couple that with a quote from BS7671 and then it is up to the customer to decide what they want to do.

 

[Nigel]

Is there an RCD or RCBO fitted to the circuit? Or are we talking old 3036 dinosaur wiring etc. I've been to plenty of new installs where the Zs is oitsido of Zs parameters but protected mechanically. Not much we can do here. Down to poor design really.

In this instance I woukd record the max Zs permitted as 1667 if the RCD is 30mA.

 

[Nigel]

its not that I entirely dismiss the 80% thing or don’t see the vague point.....it’s more that I can’t see how I explain to a customer that although my tests say it’s ok and it’s below the value stated in the regs, I’m going to code it and recommend potentially costly further investigation......which I’d struggle to justify with a clear reg.

You need to get past thinking your test results say it is ok. They are not.

 

[Rodders099]

Fair enough Essex...I’m not actually saying your wrong or that your not experienced or criticising how you operate.
I was asking for reasoning as to how I would justify to a customer that FI was required at possibly considerable cost....for something which I couldn’t back up with a reg.

I strongly disagree that a measured figure below the tabulated Max Zs in the BS1761 would ever be a C2 though.

 

[Nigel]

An. Interesting debate so far ....

How about the certificate having tick boxes for 100% and 80%

tin hat on ....

Why?

 

[Nigel]

Fair enough Essex...I’m not actually saying your wrong or that your not experienced or criticising how you operate.
I was asking for reasoning as to how I would justify to a customer that FI was required at possibly considerable cost....for something which I couldn’t back up with a reg.

I strongly disagree that a measured figure below the tabulated Max Zs in the BS1761 would ever be a C2 though.

Are you suggesting an OCPD not operating on time is not against BS7671?

How long is it really going to take to check connections? I would actually only do this on a borderline case. If it was way over I would quote to install an RCD as fault protection.

What code would you recommend for an OCPD not operating in time?

 

[Nigel]

And still I’ve had no reply to my question.....which BS1761 reg are you attaching to your C2 or FI decision ???

I don’t have the book at the moment.

 

[Rodders099]

Are you suggesting an OCPD not operating on time is not against BS7671?

How long is it really going to take to check connections? I would actually only do this on a borderline case. If it was way over I would quote to install an RCD as fault protection.

What code would you recommend for an OCPD not operating in time?

But it is operating in time.....the measured Zs of my example of 1.2 on a 32A ring circuit....is below 1.37 Max Zs required to make it operate on time.

 

[westward10]

Just go around shortening all the cpcs and line conductors if it is borderline.

 

[Nigel]

But it is operating in time.....the measured Zs of my example of 1.2 on a 32A ring circuit....is below 1.37 Max Zs required to make it operate on time.

Yes in factory conditions. When you apply correction factors it does not.

It is like saying BS7671 says a 4mm cable will take 40 amps but seeing it clearly run in 100mm insulation and not applying the correct correction factors and saying it is fine as Bs7671 says the max is 40 amps.

Makes no sense and is lazy sparking.

 

[westward10]

But it is operating in time.....the measured Zs of my example of 1.2 on a 32A ring circuit....is below 1.37 Max Zs required to make it operate on time.

If your measured Zs without any adjustment falls in the bracket between 80 and 100% then fault protection is not likely to be provided.

 

[Dan007]

In this instance I woukd record the max Zs permitted as 1667 if the RCD is 30mA.

Why? It's the duty of the competent person to both interpret and detail the results and supporting mechanical protection? Inserting 1667 my NICEIC Area engineer would literally bum me in to orbit.

I think whilst its sensible to detail it - there is supporting boxes and columns for RCD's etc. That could lead people to question the Inspectors competence to understand their duties entirely?

 

[suffolkspark]

I insert the 80% values in the required column on the napit forms and theres a tab above the column to select 80% or 100%. We have a quick reference chart containing these values so its easy.