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GBDamo

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The company is in the process of dropping a gable end which requires many circuits being cut back and then reinstated.

I advised we do an EICR, although not due and for info only, to get a baseline on the installation before we start ripping it to pieces.

It was approved and despite the age and some iffy spurs it's in remarkable condition.

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.

Those in DB1 will almost certainly be under rated.

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.

The incomer is TP TN-S with only one phase in use.

Is it likely that originally the Ze was much higher and external DNO works have brought this down, hence the additional 10mm earth?



Obviously I have no access to any previous results.
 
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pc1966

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Off hand I don't know the CPC size for 16mm T&E but it might be under the 10mm or so needed for bonding on a TN-C-S supply, so that is one possible reason for the additional conductor.
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Same for the adiabatic limit depending on the supply fuse, etc.
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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.
 
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GBDamo

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Cheers but its TN-S and all bonding is direct to the MET and confirmed.
There may be some unknown unknowns lurking but there is nowt obvious.
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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.
[/QUOTE]

This, I'm too doubting my MFT and will be confirming with a Megger tomorrow.
 
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Ian1981

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Consumer units complying to BSEN 61439-3 have a declared Ka rating of 16Ka and if it is a domestic installation then you are not even required to measure it.
See appendix 14 of bs7671.
Also see the requirements for back up protection being provided by the DNO’s cut out fuse.
As long as the let through energy of the DNO fuse is equal to or lower than the upstream devices Ka , then back up protection is afforded.
If I’m wrong about this part then I’m sure someone can correct me.
But like I said it’s not required in a domestic installation where the CU has a 16Ka rating.
 
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pc1966

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Getting too high a PFC would mean lower cable R than the MFT has assumed, which would be unusual unless you had 3rd party probes.

Getting too low a PFC on the other hand just needs a bit of dirt/tarnishing/lack of tight banana plug fit, etc.

Of course suspect calibration could account for errors in both directions! My MFT is only 6 months old and hardly used so I'm expecting it to be in-cal but can only do simple checks on it using the low ohm measurement and some calibration resistors.
 

GBDamo

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Consumer units complying to BSEN 61439-3 have a declared Ka rating of 16Ka and if it is a domestic installation then you are not even required to measure it.
See appendix 14 of bs7671.
Also see the requirements for back up protection being provided by the DNO’s cut out fuse.
As long as the let through energy of the DNO fuse is equal to or lower than the upstream devices Ka , then back up protection is afforded.
But like I said it’s not required in a domestic installation where the CU has a 16Ka rating.
So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?
 

Wilko

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Sadly I’ve seen low Ze due to my measurement error a few times. Parallel earth paths not removed, tester lead calibration review and adding a long lead into the test usually resolved it.
Out of interest, can you check PFC on the outgoing L terminal of an MCB?
 

GBDamo

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Sadly I’ve seen low Ze due to my measurement error a few times. Parallel earth paths not removed, tester lead calibration review and adding a long lead into the test usually resolved it.
Out of interest, can you check PFC on the outgoing L terminal of an MCB?
Will be looking to do a few things tomorrow.

Use a new MFT and insert a length of single into the test lead, test and deduct then calculate the PFC rather than reading

I definitely removed all parallel paths at source. The 2 sub boards maybe not.
 

Ian1981

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So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?
Have a read of this
C8BAD6CE-81FC-4C03-AA9E-649C968D5FFA.png532D48D7-5C1A-4BCD-A8F8-5F5B45A99971.png3287EBA4-558F-4BC9-A97B-292F46C736F7.pngC8BAD6CE-81FC-4C03-AA9E-649C968D5FFA.png532D48D7-5C1A-4BCD-A8F8-5F5B45A99971.png3287EBA4-558F-4BC9-A97B-292F46C736F7.png
 

Julie.

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So, as long as the CU the RCBOs are housed in are matched then the Breaking Capacity becomes that of the housing not that of the RCBO?

This is not domestic.

I'll have a look at appendix 14.

Ta

Isn't a 1361 holder rated at 16.5 KA?
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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Will be looking to do a few things tomorrow.

Use a new MFT and insert a length of single into the test lead, test and deduct then calculate the PFC rather than reading

I definitely removed all parallel paths at source. The 2 sub boards maybe not.

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.
 
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GBDamo

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Julie.

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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?
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

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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.

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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

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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

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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

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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.
 
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GBDamo

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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

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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

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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.
 
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GBDamo

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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.
On the van in my notes, my legs have given up for the day but will update tomorrow.
 

pc1966

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Is that PSSC measured between phases or from phase to neutral (replace phase with line if youre feeling modern)
That was measured L-N but I did not have enough time to be more systematic as access is difficult just now.

The building is a former sawmill, so our feed off the 800A bus-bar chamber fed by the dedicated substation they used to have. Now the building has been partitioned off and sections rented out, but for some odd reason they never made the switchgear a separate area so I have to get access via a slightly prickly guy who rents that section and is rarely there!

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.
I know you tend to have Z dominated by L at the transformer, and there are other issues (that @Julie. knows far more about) relating to the impact of zero-sequence currents in a delta-star transformer action that can modify the observed PFC, etc. But as it seemed quite far out and I am beginning to mistrust my MFT as the sparky who did most of this original work was seeing 20% more than me at the other end with his MFT.
 

davesparks

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That was measured L-N but I did not have enough time to be more systematic as access is difficult just now.


so I have to get access via a slightly prickly guy who rents that section and is rarely there!
OK, so PSCC L-L is going to be around 9kA (if my maths is right, I usually just measure it as my tester can) which is much closer to the quoted value from SSE, and if they are only giving one figure I suspect it will the L - L figure. If they quote two figures then it's usually PEFC and PSCC (L-L)



That's always the way, I think it's a fundamental law of nature that in these situations the tenant who rents the part of the building where the intake is will always be the grumpy awkward arse who is only there between 06:00 and 06:03 on the third Monday of months with 31 days.
 

pc1966

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OK, so PSCC L-L is going to be around 9kA (if my maths is right, I usually just measure it as my tester can) which is much closer to the quoted value from SSE, and if they are only giving one figure I suspect it will the L - L figure. If they quote two figures then it's usually PEFC and PSCC (L-L)
It took me many weeks and attempts to get a figure out of SSE so getting clarification would be like pulling teeth without aesthetic.

My 5kA figure would translate to around 10kA for a "bolted three-phase fault" and that could be what they quoted. My MFT can also do L-L testing but time did not permit it.

That's always the way, I think it's a fundamental law of nature that in these situations the tenant who rents the part of the building where the intake is will always be the grumpy awkward arse who is only there between 06:00 and 06:03 on the third Monday of months with 31 days.
Indeed :(

The landlord has talked about making the switchgear in to a separate area which would be good. Also moving the plumbing nightmare (or at least partitioning it off) that was added less than 1m from the back of the electrics might be a good idea as well. But we will have to see if any of that actually happens.
 

pc1966

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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.
Thinking about that and looking at, say, the diagram in the IET design guide here:
IMG_5120_modified.JPG
In the event of a "three phase fault" as point (a) with a symmetric/balanced system the centre of the three line short would be a virtual neutral point due to all the currents balancing out, hence the PEN would have zero current and so the PEN impedance would have zero effect.

So the true worst-case PFC is computed from the assumption of a zero-impedance link between, say, L1 at point (a) in the diagram and the transformer star centre/ true Earth:

PFC0 = Uoc / |Zx + Zd|

However, the common thing to do is measure the PSSC L-N and double it, this would be:

PFC1 = 2 * Uoc / |Zx + Zd + Zpen|

Normally the assumption is that neutral and line conductors are the same impedance (i.e. Zpen = Zd) so we have:

PFC1 = Uoc / |0.5*Zx + Zd|

So PFC1 is under-estimating the effects of the transformer source impedance Zx in this simplified model of a three-phase transformer as 3 independent Thévenin equivalent sources and giving a higher fault current than the true PFC0 value.

But as you mentioned you can measure the line-line PFC and this is potentially more accurate as then you are measuring:

PFC2 = sqrt(3) * Uoc / (2 * |Zx + Zd|)

Rewriting this as PFC2 = (sqrt(3) / 2) * (Uoc / |Zx + Zd|) = 0.866 * PFC0

Hence PFC0 = 1.1547 * PFC2

In this case we don't under-estimate the transformer impedance, nor do we make the assumption that the PEN/neutral path is identical to the line conductors. As such this is probably the best way to measure the worst-case three phase fault current is to do line-line and multiply by 2/sqrt(3) = 1.1547
 
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pc1966

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Also just to point out that if |Zd| >> |Zx| (i.e. cable impedance much more than transformer impedance) which will almost certainly be the case any significant distance from the transformer then PFC1 is close to PFC0 (since 0.5*Zx can be ignored compared to Zd) and the "worst case = twice the L-N PSSC" is a perfectly OK method for computing the max break requirements for a three-phase supply.

What I am more uncertain about is the assumption above of 3 independent sources as a model of the star-connected transformer source.
 

GBDamo

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Update,

Dodgy meter.

Retested today with 1721, the old dialog is being retired and in the market for a proper MFT.

These, are the double module RCBOs in the board,cant recall who was asking :rolleyes:

Got the Electricity North West out monday to look at this dripping cutout.

This is nowhere near the worst I've come across but have been having to tidy it up just get to the earth before testing.

Out of interest how do you normally approach this?

Tidy it up during testing or quote to tidy then test? I'm ok on this as I'm on site on day rate.
 

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