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]

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.

 

[Nigel]

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?

I am not entirely sure what mechanical protection has to do with an OCPDs mac Zs but I am willing to learn. Can you please explain?

If your NICEIC inspector would bum you into orbit over using an RCD for fault protection then best not show him a TT installation at your next inspection.

 

[Midwest]

What if you measure the ambient temperature where said cables are loaded. If its 20', jobs a good'en.

I used these values/chart provided by Electrical Safety First, cloak in the dark NIC bods. Their preamble does give some descriptive guidance; not saying you lot are dummies, and they are genius, just sort of.....well;

Damn wrong chart. I'll report back later

 

[Diddy]

Dear Sir
My measured values say they comply with the tabulated values for the maximum Zs noted in tabular form in BS7671 - But since we are testing the circuit without it's day to day usual load, then we must apply what we call the 80% correction rule to simulate true operating conditions. I'm afraid after applying such correction to my measured values then under normal operating conditions the circuit/s identified on my report would in fact NOT meet the conditions to satisfy maximum Zs - Failure to further investigate and remedy this issue would mean that your installation would not comply with BS7671.

Or If my customer was in any way "normal" - Unsatisfactory - FI Code 2. They are paying for me to test...not to explain the regulations inside out it's, in reality, satisfactory/unsatisfactory...Usually, with the latter, you will get a "Fix it please" order, but if not I'm sure, the next professional that they got in to deal with the noncompliance would know the issue and quote to fix the non-compliance in the manner he/she saw fit.

 

[Leesparkykent]

I haven’t got book by me either but what do the notes under table 41.3 say?

 

[Strima]

Appendix A of GN3 gives guidance on when to apply 1.25 correction factor taking into account testing temperatures, assumed initial temperature and maximum permitted operating temperature. This is an assumed conductor operating temperature of 70c.

If the circuit doesn't comply then operating temperature may exceed the stated requirements and under fault conditions the CPC temperature may exceed what is calculated using the adiabatic equation.

 

[Dan007]

I am not entirely sure what mechanical protection has to do with an OCPDs mac Zs but I am willing to learn. Can you please explain?

If your NICEIC inspector would bum you into orbit over using an RCD for fault protection then best not show him a TT installation at your next inspection.

Just for my sanity... Are you saying that if the Max Zs value for the respective circuit exceeds that of 80% we should instead input 1667 instead to allow for the Max value for a 0.03mA RCD? In the Max Zs field?

 

[Leesparkykent]

Just for my sanity... Are you saying that if the Max Zs value for the respective circuit exceeds that of 80% we should instead input 1667 instead to allow for the Max value for a 0.03mA RCD? In the Max Zs field?

Yes if you are relying on the RCD for earth fault protection rather than the OCPD.

 

[Dan007]

Yes if you are relying on the RCD for earth fault protection rather than the OCPD.

Hummm. Horses and courses n all that. I'd complete the field as required and complete the 30mA section so the certificate detailed the correct OCPD and associated Max Zs and/or RCD type.

Can anyone shed any light on this? Have they had this discussion with their area engineer?

 

[MFS Electrical]

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.........

The reason we use the Rule of thumb 80% of the value stated is because of the temperature differences measured values are usually taken at around 20 degrees whereas the maximum allowable operating temperature of a cable is 70 degrees increase in temp = increase in resistance so if it’s anywhere near the max value in BS7671 it’s failed Zs because the values in BS7671 are for conductors at 70degrees.
The values in GN3 however are at 20degrees so we don’t have to adjust the value unless we take the measurement at a different temperature

 

[MFS Electrical]

Yes if you are relying on the RCD for earth fault protection rather than the OCPD.

Which ideally you shouldn’t be if it’s for a newly designed install that would be bad design. ideally an RCD should only be used for additional protection or to help mitigate the risk of fire.