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

[Eggster71]

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......

 

[Richard Burns]

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.

 

[rattlehead85]

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.

 

[spark 68]

You don't need to do anything mate, Annexe Za allows the CU as a whole to be classed as 16kA, for supplies rated at a 100A or less.

 

[rattlehead85]

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.

 

[Eggster71]

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.

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....

 

[Engineer54]

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....

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!!

 

[Darkwood]

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.

 

[Silly Sausage]

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.

 

[Darkwood]

This subject has come up a few times and yes I see what your concerns are but using such method gives a repeatable reliable result and its not as if the PFC is too low and a concern here. Here's a link to the method that was posted on the forum some time ago.... http://www.electriciansforums.net/i...racy-low-ze-test-procedure.html#ixzz2IuBbE752

 

[Silly Sausage]

Aye, I remember that thread.
I still think if you want to make measurements at that level, use a higher (or is it finer?) resolution instrument

 

[Darkwood]

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.

 

[Guitarist]

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....

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.

 

[Knobhead]

Just how close to the transformer is this installation? Must be damn near on top of it.

 

[chilliwilly]

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?

 

[Eggster71]

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!!

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...

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....

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.

 

[Eggster71]

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?

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]

Higher-accuracy low Ze test procedure

Cunning!

- - - Updated - - -

Higher-accuracy low Ze test procedure

Cunning!

 

[chilliwilly]

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.

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.

 

[Norwich]

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