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

[Engineer54]

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

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!!!!

 

[Knobhead]

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.

 

[UKMeterman]

If you get desperate ABB do 25KA MCBs http://www05.abb.com/global/scot/scot209.nsf/veritydisplay/28adfb90da00e772c1257995004b7156/$file/2CCC413003C0202_low.pdf

 

[Engineer54]

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.

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!!

 

[chilliwilly]

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

 

[Engineer54]

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

Always!!

 

[chilliwilly]

Which instrument do you use and what readings do you record?

 

[Engineer54]

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]

Mft1730 can measure pfc between phases

 

[Engineer54]

Mft1730 can measure pfc between phases

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.

 

[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.