pfc test

[m4fiaafg]

basically im working on a three phase installation. ive done a pfc test and im getting some abnormal readings. my ze is 0.02 ohms pfc is 19.9ka so times this by two will give me 48.8 ka im assuming that this installation is sitting on a substation hence the low ze, is this reading normal, the main fuse is a bs1361 type 2 please anyone

cheers in advance

 

[DurhamSparky]

0.02 Ze indicated you are VERY close to the sub station!! what was the PFC readings from all phases???

1361 type 2 has a breaking capacity of 33kA

 

[MarkieSparkie]

With a Ze of 0.02 the sub-station is not far from your intake, look out the upstairs windows you'll probably be able to see it, if not walk round the area it's got to be close, even allowing for the larger city cables sizes it's definitely going to be in that street or an adjacent one.

 

[redface]

are threre any em lights on circuit
sounds odd but if that circuit is on it makes a difference to pfc i think

 

[R1CH-G]

Probably a redundant question, but you have nulled your leads?

 

[redface]

what difference will nulling leads and how on pfc would u do that its the batts in emlighting i have come accross this and it was always the dbs with em lighting had high pfc but i was not aloud to turn lights out to check but out of 33 dbs it was always the lightin dbs which were high all lighting boards had emlighting aswell.

 

[R1CH-G]

what difference will nulling leads and how on pfc would u do that

Your PFC reading will be different.

Follow the thread

If your leads are giving you a really high reading and you have an auto nul, the respective pfc will be seriously out of kilter as this high reading will reduce the ACTUAL Ze reading.

 

[redface]

i Can auto null on continuity (dead testing) but on pfc ( live test) I can't do it I didn't think you couldi

 

[Corky 62]

This is a question, that I really never got a satisfactory answer to, until I spoke to a ECA auditor. With low Ze reading you get high Ka ratings. Although your overcurrent devices at your main intake DB may not meet PFC values, the overcurrent devices at the service head will. 1361's are rated at 33Ka. In other words in a short circuit situation, and worst case scenario, (at the main DB) the upstream protection will protect the installation. This is accepted as an industry standard, I hope this helps.

 

[Guest 004]

I am not going to say anything until IQ has had his input.

 

[IQ Electrical]

This is a question, that I really never got a satisfactory answer to, until I spoke to a ECA auditor. With low Ze reading you get high Ka ratings. Although your overcurrent devices at your main intake DB may not meet PFC values, the overcurrent devices at the service head will. 1361's are rated at 33Ka. In other words in a short circuit situation, and worst case scenario, (at the main DB) the upstream protection will protect the installation. This is accepted as an industry standard, I hope this helps.

Perfectly true but only on a single phase 100 A supplies protected by a BS 1361 type II with switchgear to BS 60439-3.

The problem is your meter has abysmal accuracy at values of less than 0.5 Ohms.

By the time you add the transformer winding and the distribution circuit impedance to your supply, it is almost impossible to achieve a genuine Ze of 0.02 Ohms.

We had a similar case on a job at a factory and ended up inserting a resistance into the circuit to lift the meter into it's accuracy 'sweet spot'.

The original measured Ze was 0.03 Ohms, the actual value was closer to 0.06 Ohms!

This looks like an insignificant difference until you look at the effect on fault levels.

 

[johnboy6083]

Can you use A normal resistor for that, or did you use a length of cable?

 

[IQ Electrical]

Can you use A normal resistor for that, or did you use a length of cable?

We used an R2 lead, I wrote a brief guide to the procedure for when we need to use it again (quite often now).

I'll search it out and post it up, in the middle of moving house at the moment so most of my technical stuff has been boxed up by 'er indoors'!

 

[johnboy6083]

Cheers. I had a similar problem a week ago. Measured value of <0.01 on firms megger but 0.06 on my fluke. Maybe fluke compensates for low value? I'd like to try your method though. What's your take on PSCC? On the samejob, I measured phase to phase PSCC less than live to neutral PSCC and PEFC Should I have just multiplied PEFC by 2? It will comply regardless as protected by bs88-2

 

[Guest 004]

We used an R2 lead, I wrote a brief guide to the procedure for when we need to use it again (quite often now).

I'll search it out and post it up, in the middle of moving house at the moment so most of my technical stuff has been boxed up by 'er indoors'!

I read with interest......