high pfc domestic property

[Deleted member 38070]

hi, fitted new consumer in a domestic house.

testing.

ze 0.16
zs at origin 0.04
pfc 6.2ka

the pfc is my problem, ohms law would make the pfc work out around 5.7Ka (using 0.04 as your resistance) (with all paths in place)
using megger 1553 two lead test, high current, pefc and pscc are the same as its pme

im starting to think my meter is faulty, cant think of any other reason for the reading...

i did read if its downstream from 100A bs1361 cutout then effectively if 16Ka breaking capacity, that doesnt really help with mcbs rated at 6Ka

what are your thoughts as ive drawn a blank...?

the pfc just seems too high, its a new (to me) meter, with calibration cert done in jan

thanks for any help

 

[Anthony S]

Calculate your pfc

 

[telectrix]

240/0.04 =6kA. are you using the mythical 230V? can't see a problem. you expect your measured PFC to be a tad higher due to parallel paths.

 

[Deleted member 38070]

yeah was using the mythical 230.. ! morning :-0 thanks for help guys

 

[Anthony S]

You could measure the voltage then work the pfc to see how close it gets to your meter reading.

 

[Deleted member 38070]

247/0.04 6.17ka...... yep :-/ cheers... the only reason i queried it, is because its the highest pfc ive ever had in a domestic property.... i asked a couple of other sparks and they said it seemed high, never mind...... move on! :-D

 

[Anthony S]

Good stuff mate, you keeping the new meter now lol

 

[Deleted member 38070]

lol... yes, may start drinking more coffee :-D

(just found out the transformer is about 10 meters away!!!!)

 

[Engineer54]

You could always use this method of measuring Ze as proposed by IQ in the sticky section..... Based on the fact that most MFT's and Loop Testers resolution limits cannot accuratly measure the very low Ze values etc...
There are specialised Loop Testers out there that are capable of giving accurate values at or near substations but are beyond the pockets of most sole traders and even some small companies...

Higher-accuracy low Ze test procedure

This is the method that we use to ensure a higher accuracy on very low Ze readings.

At first glance, a difference of say 0.09 Ohms does not seem much on a Ze reading but when this value is used (by the instrument or by calculation) to determine PFC then the difference can be massive.

For example: 230/0.10 = 2.3KA 230/0.01 = 23KA

It goes without saying that any connections must be made safely and live testing carried out with equipment in accordance with GS 38.



1 Find a suitable impedance to be used-this should be around about 0.5 Ohms, R2 leads are good for connectivity reasons.

2 Find an outlet on the installation (fused connection unit, socket outlet etc.) that is likely to have a Zs of around 0.20 Ohms.

3 Measure that Zs with the instrument test leads (not the BS 1363 plug lead) and note the value.

4 Measure the Zs again but with the selected impedance (SAFELY) in series with the earth test lead.

5 Subtract the value measured in step 3 from that measured in step 4 and you now have a value for your test impedance (only for that particular installation).

6 Now return to the origin and measure Ze with the test impedance in series with the earth lead.

7 Subtract the value obtained in step 5 and you now have a far more accurate figure for Ze.

Obviously ambient temperature, harmonics, transformer noise etc. all affect readings but unless you have the transformer impedance and the details of the distribution circuit length and CSA etc. then this is about as accurate a reading as you can hope to measure!​

 

[GB.Kayak]

247/0.04 6.17ka...... yep :-/ cheers... the only reason i queried it, is because its the highest pfc ive ever had in a domestic property.... i asked a couple of other sparks and they said it seemed high, never mind...... move on! :-D

Smiffy, be aware of the limitations imposed by the measuring and display accuracy of your meter together with the limitations involved with the 'maths'. Assuming that the meter operates according to the normal mathematical rules, 247 V could be between 246.5 V and 247.4999999999999 V ... near enough 247.5 V and the Zs could be between 0.035 Ohms and ~ 0.045 Ohms. Taking the relevant extremes of each measurement, to give the maximum range of PFC, gives 7.07 kA to as low as 5.48 kA!

We tend to forget that all measurement is only ever an approximation with limits and the digital age somehow fools many into believing that the value 247 V and 0.04 Ohms are 'exact'. With an analogue meter most of our 'elders' will have been forced to make some kind of approximation in the reading of the value that a needle pointed to on a scale thereby eliminating the 'exactitude' of the digital age!

 

[Deleted member 38070]

i wonder if anyone can point me in the direction of where the underlined comes from, i cant seem to find it anywhere...
cheers

[h=2]Re: high pfc reading[/h]

Well there's you answer, it's that chap Ohms again!

Your Zs is probably so low as a result of a low Ze plus parallel paths from protective bonding conductors.

If your consumer unit is to BS60439-3 or BS5486-13 and you have a BS1361 fuse at the DNO supply then you can use 16kA as the short circuit rating of your protective devices.
​

 

[Tuttle]

i wonder if anyone can point me in the direction of where the underlined comes from, i cant seem to find it anywhere...
cheers

Re: high pfc reading

Well there's you answer, it's that chap Ohms again!

Your Zs is probably so low as a result of a low Ze plus parallel paths from protective bonding conductors.

If your consumer unit is to BS60439-3 or BS5486-13 and you have a BS1361 fuse at the DNO supply then you can use 16kA as the short circuit rating of your protective devices.
​

That was mentioned in the 17th Edition red OSG, in section 7 I think, but I can't find it in the AM1 green OSG, it looks like it has been removed (and I don't have the red one with me).

It is also in this ECA document (above the table on page 6): http://www.google.co.uk/url?sa=t&rc...zqaJX10ldGVLc8VoJ0lZKYA&bvm=bv.85761416,d.ZGU

(It's also in my GN1, but I only have the original 17th Edition version, prior to AM1)

 

[IQ Electrical]

It's from BS 60439-3 UK Annex ZA

http://www.google.co.uk/url?sa=t&rc...MAMpd4PXGux0nEQ&bvm=bv.85761416,d.ZGU&cad=rja

 

[Farmelectrics]

must admit i allways thought the max value was 16ka so why are you worried about the reading of 6.2ka or am i wrong

 

[IQ Electrical]

must admit i allways thought the max value was 16ka so why are you worried about the reading of 6.2ka or am i wrong

Remember though that the conditional 16ka value is only valid when the conditions regarding upstream device (only BS88-3, BS1361 100A single phase) and distribution board BS referenced on page 4 of the annex document are met.

 

[spark 68]

In another thread on the same subject I pointed out that the OCPDs themselves can have significant impedances at the kA fault level (current limiting), as well as the relatively smaller size conductors on the outgoing ways, for instance what are the chances of say obtaining 16kA fault current levels at the outgoing way of a 6A breaker connected via a 1mm conductor ?, the further away from the origin you get the PFC rapidly diminishes. A small rise in impedance (both outgoing conductor and OCPD impedance) has a much larger corresponding effect on the PFC.

BS7671 even has a provision for this, as it does say this in parts further down in 435.1, "it may be assumed that the requirements of this section are satisfied as regards fault protection of the conductors on the load side of that point."

I read this (the bottom of paragraph of 435.1) to mean if there are doubts in respect of PFC levels that a further PFC measurement at the load side of that OCPD may well confirm that the OCPD is suitable, or not as the case may be., there are of course other exemptions, such as the 3m rule, annexe Za etc.

 

[Anthony S]

Also see page 62/63 GN3