Max PFC gone through the roof....Ideas anyone?

Discuss Max PFC gone through the roof....Ideas anyone? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Eggster71

Eggster71

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I've been doing Periodics/ EICRs on some flats for a fella for the last 6 or 7 years, all ok, with the usual small faults, switchline IDs etc.

This time though, following some "repairs" that were done in the road a few months ago the max measured PFC has jumped from between 1.7 and 2.8 right up between 8.6 and >19.9kA. Verified this with my spare tester and then borrowed one off a mate, Zs is down to <0.01 ohms for one flat.

So.... What the hell do I do to sort this fella's flat's out.

Supply cut outs are 1361 type2 60A so if I'm correct rated at 16kA and the breakers are retrofit wylex 60898s all rated at 3kA so obviously they're no good.

Ok, so I could fit 63A double pole 10kA MCBs to protect the CCUs on 2 flats no worries (no discrimination obviously...) but what about the >19.9 kA one, supplier's fuse won't guarantee disconnection, a 20kA mcb will obviously not be adequate as the PFC is somewhere in excess of this.

Am I really expected to tell the landlord he's got to pay through the nose to have me fit an MCCB and associated enclosure???? Oh, plus invent the space in the intake cupboard for it?

Cheers fellas, I'll be on the phone to NICEIC technical and a couple of consultants I know on monday, just wondered if anyone had come up with a solution......
 
I think the BS1361 will be rated to 33kA.
If the CU is to BSEN60439-3 then the whole assembly is rated for a 16kA breaking capacity as per annex ZA. In this case the CPDs can be standard devices.

For the 19.9kA sorry can't think of a reasonable solution to that one.
 
Firstly you are stressing yaself for no reason.
The ka value of a 1361 iib fuse is 33ka which is above and way beyond your recorded value.
Secondly the ka rating of your board retrofits although being less than the recorded value are irrelevant in this instance as your upstream main fuse is sufficiently rated. You would only have a problem here if your cut out was an old 3030 fuse wire pertaining to a 60's install which you clearly haven't got.
 
Just a thought you mention you are carrying out EICR and you have old retrofits. Are these boards housing any RCd protection of some description?If they are not then this would be a good time to uprate the consumer units to modern type mcb/Rcbo with higher Ka capacity.
 
rattlehead85 said:
Just a thought you mention you are carrying out EICR and you have old retrofits. Are these boards housing any RCd protection of some description?If they are not then this would be a good time to uprate the consumer units to modern type mcb/Rcbo with higher Ka capacity.
Click to expand...

Cool, cheers for the replys fellas. Rattlehead, external 30 mA DP rcds protecting each board, not ideal I know..same comment every year about potential nuisance tripping... the problem is there's physically not enough room to fit 17th ed boards to replace the old 6 ways, or for that matter any modern board which rules out rcbos, at least without extending the existing cables....
 
Eggster71 said:
Cool, cheers for the replys fellas. Rattlehead, external 30 mA DP rcds protecting each board, not ideal I know..same comment every year about potential nuisance tripping... the problem is there's physically not enough room to fit 17th ed boards to replace the old 6 ways, or for that matter any modern board which rules out rcbos, at least without extending the existing cables....
Click to expand...


Hows that then, an RCBO CU will be no bigger than the board it's replacing!! Extending existing circuit tails within a new CU is standard practice, so no excuse there to hide behind!! lol!!
 
The reason you readings are inconsistent is your probably nearly sat on top of the local substation and you have approached the resolution limits of your meters hence the differing readings.... you can get around this issue by putting a known impedance into the testing then deducting it after.
 
I'm not entirely convinced by that method.
Reason being, it will drag you out of the % error but you're still going to get the digit error, the measurement is rattling around at the instrument's bottom end.
 
I only agree if the result could be an issue with the installation but even at its possible max PFC it would be covered by the breaking capacity of the DNO fuse, so on that line of thought its pointless for the once in a lue moon when the higher res' meter (expensive) would be needed. Now if it was for more specialised jobs where such accuracy would be needed then yes I would invest in a new meter.
 
Eggster71 said:
Cool, cheers for the replys fellas. Rattlehead, external 30 mA DP rcds protecting each board, not ideal I know..same comment every year about potential nuisance tripping... the problem is there's physically not enough room to fit 17th ed boards to replace the old 6 ways, or for that matter any modern board which rules out rcbos, at least without extending the existing cables....
Click to expand...

I've found the opposite when fitting RCBO's. As the Line and neutral cables are only going to the RCBO itself, and they are higher up than the original MCB, there is usually excess available.
If there is literally no more room beneath the existing board for a modern type, then I see your dilemma.
 
Another option that you might consider is splitting the meter tails via henly blocks, then feeding 3 or more switched fuses with bs 1361 II 40 amp fuses, that in turn feed 4 way boards via 10 mm tails, the smaller feeders to the boards will lower the psc to a value more in spec with protection with bs/en 60898.

It might look like something out of the late 70s with 3 or more switched fuses and boards, but when economy favours over appearance, then compliance is paramount.

Just out of interest what psc are you getting at the first outlet on any circuit when you perform a loop test?
 
Engineer54 said:
Hows that then, an RCBO CU will be no bigger than the board it's replacing!! Extending existing circuit tails within a new CU is standard practice, so no excuse there to hide behind!! lol!!
Click to expand...
Width wise, yes, height wise, problems. Not impossible but as far as the L/L is concerned the circuits have RCD protection and although not ideal he gets a "satisfactory" cert back so why spend another 4-500 if he dosen't need to...His choice, not mine...
darkwood said:
The reason you readings are inconsistent is your probably nearly sat on top of the local substation and you have approached the resolution limits of your meters hence the differing readings....
Click to expand...
I've tested this property for quite a few years and never had these readings, previously all below 3kA, some "repairs" have been done in the road since last time I tested it. Agreed the >19.9 result is over-range for my tester the other 2, on a seperate phase are both a great deal lower. Same results recorded, give or take a bit on 2 different meters.
 
chilliwilly said:
Another option that you might consider is splitting the meter tails via henly blocks, then feeding 3 or more switched fuses with bs 1361 II 40 amp fuses, that in turn feed 4 way boards via 10 mm tails, the smaller feeders to the boards will lower the psc to a value more in spec with protection with bs/en 60898.

It might look like something out of the late 70s with 3 or more switched fuses and boards, but when economy favours over appearance, then compliance is paramount.

Just out of interest what psc are you getting at the first outlet on any circuit when you perform a loop test?
Click to expand...

Definately no room to fit all that stuff in mate! TBH, never did a test at first point, closest final CCT loop result was cooker outlet at one flat which came back at approximately 0.35 ohms I think so no real concern there.
 
Eggster71 said:
Definately no room to fit all that stuff in mate! TBH, never did a test at first point, closest final CCT loop result was cooker outlet at one flat which came back at approximately 0.35 ohms I think so no real concern there.
Click to expand...

I know its a periodic that you are doing and you've ticked satisfactory, but if it ever arises during an addition or alteration. You could consider using the existing consumer unit as a junction box, and barrel crimp final circuit tails to extend them, then mount a fire rated board in front of it and bring the cables through it to new switch gear.
 
We have this from time to time when transformer is close by or on site more an issue of the meter. Meggar mft 1500s do it. Readings are not a concern The cut out fuse provides adequate short circuit protection 3ka mcb devices are fine. We have used a long lead to measure the psc and ze and then deduct lead measurement. All to with having low impedance and no resistance metres don't like it
 
Norwich said:
We have this from time to time when transformer is close by or on site more an issue of the meter. Meggar mft 1500s do it. Readings are not a concern The cut out fuse provides adequate short circuit protection 3ka mcb devices are fine. We have used a long lead to measure the psc and ze and then deduct lead measurement. All to with having low impedance and no resistance metres don't like it
Click to expand...

Do they?? ...lol!!

Yep, these Companies only manufacture 6/ 10/ 22 KA MCB's just for show, or maybe just for the hell of it!! Try using that philosophy for a final circuit DB in a sub-station building. ...lol!!!!
 
Depending on the size of the transformer the fault level at the terminals would be 29KA for a “DNO standard” 1000KVA transformer. That doesn’t take network impedance in to account, which will drop the fault level considerably. Add the supply cable impedance and you’re down to respectable levels.

From E54’s comment about substation supplies, we never installed MCB’s from the busbars without an intermediate fuse or MCCB.
 
Tony said:
Depending on the size of the transformer the fault level at the terminals would be 29KA for a “DNO standard” 1000KVA transformer. That doesn’t take network impedance in to account, which will drop the fault level considerably. Add the supply cable impedance and you’re down to respectable levels.

From E54’s comment about substation supplies, we never installed MCB’s from the busbars without an intermediate fuse or MCCB.
Click to expand...


Of course the substations DB would be fed from an MCCB on the switchboard!! lol!! But even up-stream fuses and MCCB's will have a ''let through'' element. If correctly designed, by the time you get to the final circuit DB, you'll be at or around the 20 KA mark. Several manufactures can supply 20 to 25 KA breakers MG, ABB, Siemens etc... They do tend to be much fatter than your standard MCB's mind!! lol!! One thing is for dammed sure, i wouldn't be installing 3 KA MCB's in a sub-stations local lighting and small power DB!! lol!!
 
Just re-read your post, no i don't ''record'' all those tests, only the highest values!! Not sure what instrument we'll be using on this project, but on previous projects it was either a Megger or Metrel high resolution ELI test kits.

In the past, you had to place a special order for the Megger HR loop tester, but i believe they now direct market such a unit.
 
Tim said:
Mft1730 can measure pfc between phases
Click to expand...

It probably does, but it's resolution will still be a problem when conducting loop tests near/close to Distribution transformers, which is what were talking about here. Hence the need to use IQs test method or use a HR loop tester.
 
chilliwilly said:
Has any body actually recorded the pfc/psc between phases? And not just the usual phase to earth/neutral pfc/psc?
Click to expand...

Where relevant I do, as already said by others, I'll record it if it's the highest value. MFT 1552 but as also mentioned, the range maxes out at 19.9kA.

Am awaiting a reply from Wylex as we speak, had to email them as the phone technical didn't have information as to how a fault's likely to affect them...

Oh, and no, I've not given a satisfactory cert back yet!!!! At the moment, as far as I'm concerned it's potentially not satisfactory at all!! lol lol.
 
darkwood said:
The reason you readings are inconsistent is your probably nearly sat on top of the local substation and you have approached the resolution limits of your meters hence the differing readings.... you can get around this issue by putting a known impedance into the testing then deducting it after.
Click to expand...
That's the wierd thing...I'm not aware of a substation or similar locally, and only last year all the PFCs were way way lower...
Maybe they decided to replace that bit of twin and earth they'd mended the cable in the road with last time..... lol
 
Did you just perform one test, or perform several within a short space of time? I onced performed a psc on a switch cubicle 3 meters away from an 1000KA sub. And got 21 KA ph-n at 05:30 in a morning, then did another at approx 10:00 with the factory running, and it came down to 16 - 19 KA after several tests within a minute of each other.

About 80% of the phases were balanced, the remainder imbalance was due to varying single phase loads switching, more than likely causing different test results. It the end they agreed to install an airblast CB to compenstate for the breaking capacity of the main switched fuse.
 
chilliwilly said:
Did you just perform one test, or perform several within a short space of time? I onced performed a psc on a switch cubicle 3 meters away from an 1000KA sub. And got 21 KA ph-n at 05:30 in a morning, then did another at approx 10:00 with the factory running, and it came down to 16 - 19 KA after several tests within a minute of each other.

About 80% of the phases were balanced, the remainder imbalance was due to varying single phase loads switching, more than likely causing different test results. It the end they agreed to install an airblast CB to compenstate for the breaking capacity of the main switched fuse.
Click to expand...

Hmm, first tests, on my 1552 were 12.30 then with an old "spare" an uncalibrated one... at about half one, then my mate's new version of the same at about half 2. Last round of testing was with all 3 together in a short space of time, also did a few end of lines to make sure all the readings were in the same ballpark....maybe I'll just whack in a bit of 4mm singles as tails, should get the readings down a bit! :wink:

Also, bearing in mind it was a saturday I'd have thought overall usage in the area would've been quite high which presumably would have been the reason you got your lower readings at a busier time of day...

To be honest, I'll kick back and see what the wylex boys come up with and assess it then, might even go proper old skool and buy him some nice cartridge fuse carriers!! lol
 
chilliwilly said:
Did you just perform one test, or perform several within a short space of time? I onced performed a psc on a switch cubicle 3 meters away from an 1000KA sub. And got 21 KA ph-n at 05:30 in a morning, then did another at approx 10:00 with the factory running, and it came down to 16 - 19 KA after several tests within a minute of each other.

About 80% of the phases were balanced, the remainder imbalance was due to varying single phase loads switching, more than likely causing different test results. It the end they agreed to install an airblast CB to compenstate for the breaking capacity of the main switched fuse.
Click to expand...

Generally a series of tests over about 15 minutes or so.

The reason you were getting varying readings, is that you are probably at the bottom end of meters resolution, and at the top end of your maximum meters range (generally 19.9 KA) Which is why you need to either use IQ's method, or better still a HR loop tester, with a max range up to 40 KA whenever your testing installations close to distribution transformers of say 750 KVA and above!!

I prefer ACB's every time after the TX (we actually use GCBs), or striker Fuse, fuse switch as an alternative. Most MCCB's never look meaty enough, or look as if they have enough air separation within, to be able to contain a bolted short situation... Just my preference, i hasten to state!! lol!!
 
This was such a meter, that I borrowed from my brother who was an ex senior lines engineer for the CEGB. Then worked as a tech at Drax power station after being made redundant, then moved to the US to do a similar job. He had forgotten to tell me that I could've used it to test between phases, so I just did ph-n and doubled up the reading on my EIC. I knew the meters that I normally use wouldn't have been up to the job.

The meter was a heavy unit in a bakerlite case, inside a leather satchell case with an analogue display, long before digital displays. And you could feel the tension go through it when you operated it, probably due to the high wattage shunting resistor. I think it was a voltex or megger, to be honest I can't remember but there were flash mittens on the probes and a face guard in the same satchell. I didn't bother with the flash apron and gauntlets for obvious reasons.
 
On the subject of high pssc s I just looked at a cu change 2 doors from a sub station, did a Ze, checked pssc, 10 kA. BS 88 fuse. What would happen to mcbs under fault conditions when rated at 3kA .
Cheers chaps.
 
They could fail to stop the fault as they could be destroyed and left in closed circuit but due to the DNO fuse been rated for the fault this would take over and rupture which satisfies regulation in your standard domestic house. If you are quoting for a CU change allow for 10k rated mcb's as a matter of good practice.
 
RoB2 said:
If suggest higher than 10ka rated mcbs if the pfc was measured at 10ka.
Click to expand...
Was a minimum suggestion as he said 10k not higher.... as its domestic most common brands are 6k or 10k ratings ....if he had said 11k 12k etc then yes i would obviously have suggested mcb's for such but its only a recommendation as he's not required to fit them anyway.
 
Darkwood I know you know that, I was just pointed it out to those who are still green.

E54 think the company paid £400 is for it maybe a little less. It's an alright peice of kit, pfc in our subs are 15~35ka dependent on the tx feeding them obviously.
 
RoB2 said:
Darkwood I know you know that, I was just pointed it out to those who are still green.

E54 think the company paid £400 is for it maybe a little less. It's an alright peice of kit, pfc in our subs are 15~35ka dependent on the tx feeding them obviously.
Click to expand...


That's a pretty good price, i'm sure on the last project, a comparable tester was over the £500 mark!!
 
Most DNO's, if asked (by enquiry) would say 16kA is the max PFC available on a single phase 100A or less supply.

I don't for one minute believe the Op's supply is any where near the levels of PFC his meter's are indicating, due to inaccuracies and method of measurement, as many posters here have already pointed out.

Typically EFLI meters measure the volt drop in mV at a measurement of around 20-25A load, and at close to the TX is at the lower end of the instruments capabilities (check the accuracy figures quoted in the user manual).
Under actual kA fault levels it is doubtful the TX could even supply the installation at that level of fault current before the cut fuse failed

The OP would either need a more accurate meter, or use IQ's method of measurement, as any slight discrepancy in the loop reading has a massive bearing on the PFC reading.

When you consider the DNO's supply cable has some resistance, as well as the cut out fuse and the meter itself as well the tails, then the calculations don't really add up, as the fuse, the Dno's cable, the meter and the tails would need a total resistance of 0.001 ohms for a PFC of 23kA single phase calculated @ 230V
 

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