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

 

[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?

 

[Nigel]

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?

Dildo left on charge.

 

[SparkyChick]

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?

I hope it's not done any damage to your place Middy.

 

[Ian1981]

Midwest installed the socket it was plugged into
Run Midwest run!

 

[Midwest]

Midwest installed the socket it was plugged into
Run Midwest run!

Fortunately, I never did any work in the place

However, once the jungle drums started, it's amazing the amount of local trades persons in their vans were driving by.

 

[Midwest]

I hope it's not done any damage to your place Middy.

Nope, across the road. Their Xmas ball'sd up though.

 

[SparkyChick]

Nope, across the road. Their Xmas ball'sd up though.

That's good for you, was everyone in the house OK?

 

[Midwest]

That's good for you, was everyone in the house OK?

Yep. Luckily, family downstairs having tea, when smoke alarms went off.

 

[SparkyChick]

Yep. Luckily, family downstairs having tea, when smoke alarms went off.

That's good then... much better to have a disaster for Christmas than a tragedy.

 

[Marcus Vaughan]

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.

As others have said - it may not disconnect in time.
The reg group you want is 411.3.2 - automatic disconnection in case of a fault.
If you need more references from the BBB then under each of the 'Max Zs tables' there is "if the conductors are at different temperature when tested, the reading should be adjusted accordingly. See appendix 3."

If you read appendix 3 - it's all there - including the 'rule of thumb' for temperature correction.

 

[Generator]

The column for 'max Zs' should surely contain the limiting value being worked to, as a reference for the column containing the actual reading obtained. If the conductors are at full load working temperature, the table values would apply. If at ambient, then you are referred to appendix 14, for methods of correction of the maz Zs value.
The 80% factor is applied NOT to the table value, but to the calculated value of V0 / Ia which is the fault current which causes a trip within the requisite time. Funnily enough, for a 32A Type B breaker, the above calc at 230V, 160A is 1.15 ohm, and 0.8 x the table value of 1.44 is also 1.15 ohm ( 1.152).
Does that work for all of the tables and values?
Anyway, I still think the 'max' column should contain the Actual limiting value being worked to.