Max Zs on certification

[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.

 

[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.

 

[Nigel]

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?

On an EICR I would yes.

I would not on an EIC unless it was designed to be.

 

[Leesparkykent]

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?

I’m not sure why you would need the discussion with your area engineer? What value would you record on the schedule if it was TT....The max Zs value of the OCPD or the max Zs permitted for the RCD that’s providing the earth fault protection? It’s clear to me what you should be doing.

 

[Nigel]

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?

I do not need to speak to my NICEIC Engineer. I understand what is required.

How else would you meet disconnection times if the Zs is too high?

 

[Dan007]

Is there an excerpt from BS7671 that details this (Use of max RCD values) over the Max Zs of the OCPD when considering Zs values that exceed the 80% factor?

 

[Nigel]

Is there an excerpt from BS7671 that details this (Use of max RCD values) over the Max Zs of the OCPD when considering Zs values that exceed the 80% factor?

To be honest the use of an RCD to cover fault protection on a TN system is not ‘textbook’. But sometimes you need to think outside the box. There are otherways to combat this issue, which is only an issue due yo poor design but it is completly unrealistic to rewire a circuit due to high measured Zs when an RCD will provide protection and still meet 7671.

 

[Dan007]

To be honest the use of an RCD to cover fault protection on a TN system is not ‘textbook’. But sometimes you need to think outside the box. There are otherways to combat this issue, which is only an issue due yo poor design but it is completly unrealistic to rewire a circuit due to high measured Zs when an RCD will provide protection and still meet 7671.

Absolutely! Couldnt agree more. But is there an excerpt for stating one value over the other? Or is this just an understanding you have with your engineer. I'll definitely raise this with mine tomorrow if thats what you guys/girls are doing and its deemed acceptable.

 

[Deleted account]

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

Mine didn't. 80% for me.

 

[Nigel]

Absolutely! Couldnt agree more. But is there an excerpt for stating one value over the other? Or is this just an understanding you have with your engineer. I'll definitely raise this with mine tomorrow if thats what you guys/girls are doing and its deemed acceptable.

I don’t run anything by my engineer.

You used the value of whatever the OCPD is that you are using.

 

[Deleted account]

Why the disagree Essex.
My inspector didn't insist upon 100%. Fact!

Unless your not disagreeing with what I said, but with my inspector not insisting on 100%??

 

[Nigel]

Why the disagree Essex.
My inspector didn't insist upon 100%. Fact!

Unless your not disagreeing with what I said, but with my inspector not insisting on 100%??

In my opinion your inspector is wrong.

 

[SparkyChick]

What the foot note on the model form says is...

"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 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"

So it doesn't state you must record the 100% value, only that if it isn't from BS 7671 that you state where you got it from.

 

[Nigel]

What the foot note on the model form says is...

"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 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"

So it doesn't state you must record the 100% value, only that if it isn't from BS 7671 that you state where you got it from.

But must comply with 7671.

 

[SparkyChick]

But must comply with 7671.

Yeah, but I've just looked and I can't find anything in the regs that stipulates you record the 100% value. Just that footnote which stipulates if it's not from BS 7671 you state where it is from.

If you multiply a value from the tables by 80%, the source of the value is still the standard.

I said earlier I record 80%... I do because it's the default in NAPIT Desktop which I use for doing EICs and EICRs, you can change it, but the cert then includes markup to show whether it's the 100% value or the 80% value.

It's how I was taught when I did C+G 2932, 2394 and 2395 a couple of years ago.

 

[Strima]

Shall we throw 60947-2 into the mix just for fun?

 

[Nigel]

Shall we throw 60947-2 into the mix just for fun?

What would that do?

 

[Strima]

What would that do?

Get me some more funny post notifications?

 

[Silly Sausage]

Get me some more funny post notifications?

Is there a 'Whooosh' icon button thingy thing to click on?

 

[Midwest]

Well the little chart I chopped up & used for the 80% figures, now excludes the preamble I previously mentioned, but it does mention App 14. Which I've just read, and haven't the scooby what its on about.

But I guess its you can do some maths, or just use the table.

 

[Ian1981]

Well if you consider the formula for compliance is zs x Ia < Uo x cmin we must have to use the maximum zs value for that OCPD given in appendix 3 ie for a 32 amp type B 1.37 ohms

 

[Midwest]

Anyways, just as an aside, been called across to my vacant house up for sale. Neighbour has just had a small fire, FB in attendance. Water everywhere. Laptop in upstairs bedroom appears to be the culprit.

Too high Zs values, or dodgy lithium batteries?