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

[Eggster71]

Has any body actually recorded the pfc/psc between phases? And not just the usual phase to earth/neutral pfc/psc?

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.

 

[Eggster71]

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.

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

 

[chilliwilly]

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.

 

[Eggster71]

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.

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!

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

 

[Engineer54]

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.

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!!

 

[chilliwilly]

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.

 

[Bobster]

Megger - Megger LTW425

I use one of these.

 

[Tim]

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.

Then i stand corrected

 

[baggy61]

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.

 

[Darkwood]

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.

 

[Bobster]

If suggest higher than 10ka rated mcbs if the pfc was measured at 10ka.

 

[Engineer54]

Megger - Megger LTW425

I use one of these.

Just looked at that model, ...and yep, that'll do the trick!! ...lol!!

How much did the tester cost in the UK??

 

[Darkwood]

If suggest higher than 10ka rated mcbs if the pfc was measured at 10ka.

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.

 

[Bobster]

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.

 

[Engineer54]

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.

That's a pretty good price, i'm sure on the last project, a comparable tester was over the £500 mark!!

 

[Bobster]

Megger LTW425 Loop Impedance Tester

£379 that's without forum discount.

 

[spark 68]

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