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

 

[timhoward]

If you change to C3 you are saying it isn't potentially dangerous that automatic disconnection of supply won't happen within required disconnection times. Unless RCD protected I wouldn't even consider C3 for this.

 

[Sparky78]

If you change to C3 you are saying it isn't potentially dangerous that automatic disconnection of supply won't happen within required disconnection times. Unless RCD protected I wouldn't even consider C3 for this.

This is my thoughts. There is no RCD protection currently either.
One work around that the NIC helpline came up with was if I could prove that there was no extraneous metalwork that could be touched together that were unbonded then it didn’t warrant a code. The building is massive and there is no way I’m putting that on myself to prove.

 

[telectrix]

manufacturers advice for the MCCBs could be ablet out if they can confirm disconnection times at the current Zs valures.however, dependingon the purposeof the circuts in question, the lack of RCD could give rise to a C2 anyway.

 

[pc1966]

I guess the first thing to check is are Zs values at the sub-DBs used the 5s disconnect times for a sub-main?

Certainly the Best Practice Guide #4 has C2 coding for:

Earth fault loop impedance value greater than that required for operation of the protective device within the time prescribed in the version of BS 7671/IEE/IET Wiring Regulations or manufacturers’ published data current at the time of installation.

Which is basically saying if it is not meeting the design requirements for adequate ADS then it is not considered safe. Yes, the BPG#4 is targeted more at domestic work but it is still sensible guidance and one that all of the major organisations had inputs to.

I would have to agree that on some large building it would be impractical to assert that no extraneous parts are accessible or bonded!

 

[pc1966]

The 80% or 100% limit is more flexible, after all if you have a long sub-main that has much higher CSA to cope with voltage drop then it is never going to be sweating away at 70C in normal operations, so having a Zs limit closer to the 100% value is quite reasonable.

Also are the MCCB supplying the sub-boards adjustable? If so then it might be an option to look at the adjustment range and the required operating currents to see what Zs could be met.

 

[timhoward]

Also are the MCCB supplying the sub-boards adjustable? If so then it might be an option to look at the adjustment range and the required operating currents to see what Zs could be met.

This is the best solution, if viable. In particular are they C curve and do they need to be?
In extreme cases arranging to supply with BS88 fuses can buy you a lot of Zs.

 

[pc1966]

Another thought, is the supply at the nominal "old" 240V range and was that used at the time of design?

It might be that if you re-evaluate the Zs values needed for the Umin that applied at the time of design you will have a value that is a good few percent higher.

Finally, have you any idea of how much of a change in Zs would be needed to meet the disconnection specifications? For example, is it partly limited by the SWA armour R2 so running a parallel 16mm or 25mm copper CPC would be sufficient to meet the desired end of sub-main Zs?

 

[Sparky78]

This is the best solution, if viable. In particular are they C curve and do they need to be?
In extreme cases arranging to supply with BS88 fuses can buy you a lot of Zs.

These are Proteus 60947-2 F21 100a MCCBs, when I contacted Proteus for their Max Zs figures of these and outlined the issue I was having the helpline said that altering the settings wouldn’t do anything in this instance? The figures they gave were 0.23ohms as a max Zs which works out at 0.18ohms at 80% with the ze of the building being 0.16ohms. You can see my predicament here as the sub main supplies are on different levels throughout the building, leaving only a margin of 0.02ohms for the sub main circuits Zs value.

 

[timhoward]

These are Proteus 60947-2 F21 100a MCCBs, when I contacted Proteus for their Max Zs figures of these and outlined the issue I was having the helpline said that altering the settings wouldn’t do anything in this instance? The figures they gave were 0.23ohms as a max Zs which works out at 0.18ohms at 80% with the ze of the building being 0.16ohms. You can see my predicament here as the sub main supplies are on different levels throughout the building, leaving only a margin of 0.02ohms for the sub main circuits Zs value.

I actually think the 0.23 IS the 80% figure.
that breaker says


230/800 = 0.2875
*0.8 = 0.23

@pc1966 had better sanity check this thinking!

 

[timhoward]

I also note that the modern equivalent:

https://04646a9cf351cc0d3888-b8b406d15fe93f790abb5bf0e9ab7ab3.ssl.cf3.rackcdn.com/downloads/product/proteus_mc016t-mc250t_datasheet_af8ab4406838d044b28d3c6d3f6dc054.pdf?1551263982

States 0.30 ohms for 5 second thermally adjusted max Zs. But at £140 + vat each that doesn't help much!

It's worth speaking to them again and double checking whether they gave you a thermally adjusted value, and whether it was a 5s value.

 

[Sparky78]

I also note that the modern equivalent:

https://04646a9cf351cc0d3888-b8b406d15fe93f790abb5bf0e9ab7ab3.ssl.cf3.rackcdn.com/downloads/product/proteus_mc016t-mc250t_datasheet_af8ab4406838d044b28d3c6d3f6dc054.pdf?1551263982

States 0.30 ohms for 5 second thermally adjusted max Zs. But at £140 + vat each that doesn't help much!

It's worth speaking to them again and double checking whether they gave you a thermally adjusted value, and whether it was a 5s value.

Hi, on the helpline I definitely went through all this with them so I’m sure they said that the 0.23 value was the 100%, it was also for 5 second disconnection. I might actually ring them back and double check this but I’m positive I asked for the 5sec Zs value at 100%

 

[pc1966]

Looking at the data sheet for the newer one than 5s is very close to the point when the "instant" magnetic trip would go, so base on 800A for that on you model I would agree with @timhoward you would have your hot (100%) Zs as no worse than (230V * 0.95) / 800A = 0.273 ohms at Umin, and then your 80% for hot cables measured cold as 0.22 ohm.

In a case like this where it is tight you might also want to check how hot the cables where when you measured them (might have been done live for Zs with no cool-down) and what the realistic operating temperature is going to be based on the cable & loads for the final circuits.

 

[timhoward]

I'd pull out the clamp meter and do some max demand head scratching.
Assessing whether 100A is actually needed would help.

I know that convincing people to spend money on things that 'work fine' is tough, but maybe the steel armour is degrading, and the Zs is higher than it was when installed. I don't suppose IR test results support this theory do they?

 

[Sparky78]

I'd pull out the clamp meter and do some max demand head scratching.
Assessing whether 100A is actually needed would help.

I know that convincing people to spend money on things that 'work fine' is tough, but maybe the steel armour is degrading, and the Zs is higher than it was when installed. I don't suppose IR test results support this theory do

I'd pull out the clamp meter and do some max demand head scratching.
Assessing whether 100A is actually needed would help.

I know that convincing people to spend money on things that 'work fine' is tough, but maybe the steel armour is degrading, and the Zs is higher than it was when installed. I don't suppose IR test results support this theory do they?

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.

 

[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!