High PFC.

[GBDamo]

Have a read of this
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Wonderful, cheers.

If, and this needs confirming, the PFC is indeed as low as I think (4-6KA) then it doesn't matter as all the CUs, if compliant with BS60947-3, will have a type default rating of 16KA as long as they are installed following MI.

Am I reading that correctly?

 

[Julie.]

Wonderful, cheers.

If, and this needs confirming, the PFC is indeed as low as I think (4-6KA) then it doesn't matter as all the CUs, if compliant with BS60947-3, will have a type default rating of 16KA as long as they are installed following MI.

Am I reading that correctly?

Yes, but it isn't a widespread yes - each manufacturer states which mcbs etc make this confirmation.

So if they state 4kA stuff is compliant with this, then you still have to do the calculations for other ratings

 

[GBDamo]

Unfortunately with 100A or 80A incoming fuses, the let-through from 6kA perspective is still going to be in the order of 5kA or more, so relying on the upstream fuses isn't likely to be possible here, but you would need to know the actual fault current and fuse size/type to really check.
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For fault levels, you shouldn't be removing the parallel paths.

To determine the maximum Zs or Zdb the parallel paths need to be removed, this gives the worse case for making sure the protection will operate with minimum current.

However for fault level confirmation of equipment then you need to find out what the maximum is, not the minimum for operation.

It would be no use removing all parallel paths and saying the equivalent is suitable for the fault level- say 3kA then put parallel paths back in and see the actual fault current now be beyond the capacity of the equipment installed.

Not saying you're wrong but how does that sit with the argument that some parallel paths, not part of the electrical installation, may change, i.e. gas and water bonding.

I was taught to measure Ze with all parallel paths removed and the meter will automatically calculate PSSC and PFC.?

 

[Julie.]

Not saying you're wrong but how does that sit with the argument that some parallel paths, not part of the electrical installation, may change, i.e. gas and water bonding.

I was taught to measure Ze with all parallel paths removed and the meter will automatically calculate PSSC and PFC.?

All you can do is measure what you can at the time, if something changes after you can't account for it.

If you remove the parallel paths then this presents the highest Zs and providing the protection will operate (ie still less than min Zs for the mcb/rcbo) this would be the worst case, so when the parallel paths are in, or someone adds further earthing etc, this would increase the fault current/make Zs still be below the minimum required.

However to make sure the equipment has a sufficient rating, you ought to re-test the pfc with everything back in parallel- I will check, but I think it is explained in gn3

 

[davesparks]

I was taught to measure Ze with all parallel paths removed and the meter will automatically calculate PSSC and PFC.?

Yes the meter will calculate a PFC based on that reading, but it would be incorrect to take that as being the measured PEFC for the installation.
To establish the PEFC for the installation you need the loop impedance with all bonding and other parallell earth paths connected.
To establish PSCC you need the L - N or L - L loop impedance, bonding etc shouldn't affect this but I guess for TNCS it maybe might do.

 

[GBDamo]

All you can do is measure what you can at the time, if something changes after you can't account for it.

If you remove the parallel paths then this presents the highest Zs and providing the protection will operate (ie still less than min Zs for the mcb/rcbo) this would be the worst case, so when the parallel paths are in, or someone adds further earthing etc, this would increase the fault current/make Zs still be below the minimum required.

However to make sure the equipment has a sufficient rating, you ought to re-test the pfc with everything back in parallel- I will check, but I think it is explained in gn3

Actually it made perfect sense after re-reading your earlier post but thanks for clarifying.

 

[davesparks]

One thing that stood out was at the main DB and two sub boards the the ZDBs were very low giving PFCs in the range of 3-6KA

All the boards are populated with Hager two module RCBO which have a breaking capacity of 4000A.

The other two DBs though have an additional 10mm earth run back to DB1 alongside the 16mm T&E sub main. Removing this brings the ZDBs up to sensible levels.

It's probably quite close to the substation, thats not an unreasonable PFC for a domestic supply, just not very common.

A 4kA breaking capacity is unusual, are these older units?

Don't go removing protective conductors to deliberately increase impedance, that's plain daft.
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Oddly enough this is opposite to my current project where I am seeing around 5kA measured at our origin switched-fuse but expected around double that. Not sure I really trust my MFT in that region though as 0.01 or so of test cable impedance difference makes a massive difference to the resulting PFC/PSSC.

What makes you think it should be double that? Is the 5kA PEFC or PSCC?

I apologise if I'm teaching you to suck eggs but if you are close to the substation transformer your meter will likely not measure the impedance correctly due to the effects of the transformer. Being close to the transformer will usually give you higher impedance values and lower PFC values than are actually there.

 

[GBDamo]

It's probably quite close to the substation, thats not an unreasonable PFC for a domestic supply, just not very common.

A 4kA breaking capacity is unusual, are these older units?

Don't go removing protective conductors to deliberately increase impedance, that's plain daft.

I need to get a few ducks in a line before offering a solution, it may turn out there isn't even a major problem.

Firstly I need to speak with Hager to ascertain if the components as assembled conform to BS609473 and therefore carry a 16KA+ type tested rating.

There are a few mixed manufacturer MCBs that need replacing, again Hager need contacting for type compliant alternatives.

I find the additional earths curious but would not dream of removing them.

 

[pc1966]

What makes you think it should be double that? Is the 5kA PEFC or PSCC?

I apologise if I'm teaching you to suck eggs but if you are close to the substation transformer your meter will likely not measure the impedance correctly due to the effects of the transformer. Being close to the transformer will usually give you higher impedance values and lower PFC values than are actually there.

Yes, it is around 25m from the 500kVA substation and SSE told me to expect around 11kA PFC/PSSC values. I'm seeing 4kA-ish PFC and 5kA-ish PSSC but not terribly stable

I'm not sure what you mean by "close to the transformer will usually give you higher impedance values and lower PFC values than are actually there" though.

However, I fully agree that measurements of this sort are hard to do and I'm too much of a cheapskate to hire a Kelvin lead supply impedance analyser (the minimum 1 week hire is roughly what I paid for my MFT...)

 

[davesparks]

I need to get a few ducks in a line before offering a solution, it may turn out there isn't even a major problem.

I find the additional earths curious but would not dream of removing them.

I doubt there's any problem at all, but best to check to make sure. Do you have the part number of the RCBO's or a picture?

I used to run a 10mm earth alongside a 16mm T&E submain on the rare occasions I installed one. It helps keep Zs as low as possible, allows bonding to connect to the sub DB if needed, and a 6mm cpc just feels too small.
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Yes, it is around 25m from the 500kVA substation and SSE told me to expect around 11kA PFC/PSSC values. I'm seeing 4kA-ish PFC and 5kA-ish PSSC but not terribly stable

I'm not sure what you mean by "close to the transformer will usually give you higher impedance values and lower PFC values than are actually there" though.

However, I fully agree that measurements of this sort are hard to do and I'm too much of a cheapskate to hire a Kelvin lead supply impedance analyser (the minimum 1 week hire is roughly what I paid for my MFT...)

Is that PSSC measured between phases or from phase to neutral (replace phase withline if youre feeling modern)

I don't know the details but if you're too close (electrically/conductor length, not physical distance) to the transformer then something about the transformer interferes with the measurement and leads to a false reading.
But I've only seen that when theres a few metres of big conductor between the tester and transformer.

I did a job last year where the Zs I measured at our new panel board (fed by 4 metres of 2x 150mm from a board in the room below which was fed from the DNO transformer in a room next door) using a high resolution loop tester was higher (slightly) than at the end of the outgoing circuits from it.