Max Zs Coding Question 80/100%

[Sparky78]

Hi new member here looking for a bit of advice.
I recently undertook a condition report of a building where the measured Zs at the distribution boards (board Zs) were above 80% of the max Zs value of the MCCBs in the panel board supplying them. In some cases it’s also above the 100% value.
I’m being asked to change these to C3 faults.
What is your opinion on this please and what would you code a measured Zs that was above the 80% value but below the 100% value?

Thanks

 

[pc1966]

The readings and installation in general are pretty much perfect with no degradation that I can see. In any other instance you’d be jumping for joy at the readings I’m getting but as I said with a incoming ze of 0.16 and a max Zs of 0.18 I’m pretty much snookered.

I am sceptical that 0.22/0.23 is the hot Zs value. The current Proteus data sheet has 0.3 hot for 5s, and computing from the 800A magnetic trip for the model shown above looks like 0.27 ohms, so I would think 0.22 is the cold Zs value giving you 0.06 for R1+R2 which, while difficult, is a bit better.

For the sake of argument, if it is 4C 25mm SWA then R1 = 0.727 and R2 = 2.3 ohm/km so you would be OK for about 20m. If a 16mm CPC is added at 1.15 ohm/km then 32m, etc.

The other aspect is to get a more realistic value for hot operation to cold measurement. Was it all off for a long time before you measured Zs?

Did you measure Zs on the system under normal operating conditions? If so it would already be hot, OK maybe not "max load" hot, but depending on the usage maybe not far off it. If you know the cable rating/method and what the supply max is then you can also see if the max operation is noticeable less than the 70C assumed for typical use-case.

 

[ImpededLoop]

Another consideration is that our MFT's are not very accurate under 0.20 ohms. What are the Zdb values that you've measured?

 

[Sparky78]

Hi all,
Just off the phone with Proteus and they’ve told me there is only one Zs value for these MCCBs so they are advising that 0.23ohms is the 100% value and they didn’t think that altering the mag on it would effect this as it’s for overload.
The sub main readings I’m getting for the 4 dis boards are:
0.23
0.23
0.21
0.21
So they are bang on the max or just under the Zs rating of 0.23ohms.
Would you still code a Zs that breached the 80% value but not the 100% value as a C2? Could it be put through as a C3 with a recommendation that this is looked at again closely during the next EICR? The owner has agreed to push through upgrades for the panel board at the earliest opertunity.

Help!

 

[pc1966]

I would find it difficult to justify a C2 given they are measuring the 100% value where that seems not to quite agree with the figures given on the MCCB (800A magnetic trip) or the value for the replacement model.

Do you have any idea of how hot the cables were when you measured those values?

What is the nominal voltage there and is it on a site with its own substation transformer?

 

[Sparky78]

Ok guys, here’s an update.
I bit the bullet and phoned the NICEIC helpline and the nice guy on the phone let me know the following.
If the installation is in use whilst the EICR is being carried out then you use the 100% max Zs values of the protective devices.
If the installation is not currently in use or hasn’t been for some time that is when you use the 80% value of the Max Zs of the protective devices to account for there being no thermal effects in place for the circuits.

This came as a great surprise to me as I had always thought that EICRs used the 80% values at all times but the advisor told me that this was a myth and that he has this conversation a lot with contractors on any given week. He referenced the areas on the restricted areas of the NICEIC website where members can find this, it’s also on the front inside cover of the NIC Test Certs and also on Pocket Guide 18.

So with the information in hand I can pass that installation without any codes or notes… WHEW!

Every day is a school day as they say.
Thanks for all of your helpful input guys, much appreciated!

 

[pc1966]

This came as a great surprise to me as I had always thought that EICRs used the 80% values at all times but the advisor told me that this was a myth and that he has this conversation a lot with contractors on any given week.

Like many things in life, there is the quick practical answer, and then there is the real answer...

The 80% factor is for cold-measured circuits to allow for them running hot, and is also based on a roughly 50C rise in operating temperature as typically most cables have the CCC limit based on 70C PVC and most UK places are around 20C when at room temperature.

If your measured-to-operating temperatures are different then the 80% should differ as well. So if very high temp (e.g. MICC cabling in some steelworks style factory) you might need less then 80%, or if the cable is long so has to be larger CSA for VD than the required CCC would otherwise dictate, then you might above 80%.