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!