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

 

[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 ....