High PFC

[Paul Chick]

High guys

Happy Snowday! Im doing a periodic on an industrial unit and getting pretty high fault readings. This doesn't bother me, the transformer/sub station is behind it so they are going to be high. I just want to know what the best way is to get around mentioning the PFC in regards the breaking capacity of the MCB's on the condition report just to cover myself?

The PFC is 19.1KA The majority of the breakers are rated at 10KA with a couple 6KA's
I know they mention it being fine in regards to main cut and its rating but I haven't got a clue what fuse it is because I cant get into it.


Any help would be good as always : )

Cheers

Paul

 

[Deleted member 26818]

Sounds like it requires further investigation.
Whithout knowledge of the DNO's fuse, you cannot make any determination, on whether the rating of the MCBs is adequate.
You don't mention whether the DBs are type tested, although the PFC is higher than type tested DBs would allow for.
However it may have some bearing in your determination once you have the details for the DNO'd device.

 

[Darkwood]

Is there a front end device like hrc fuses or mccb as these will afford the pfc rating upstream of the downstream devices

 

[Paul Chick]

There is no mccb. I would imagine there are fuses within the enclosure. Its a large plastic box with delta wrote on it, not tried to get in there but thats where the meter tails head.

 

[Engineer54]

We don't take any chances of relying on upstream protection when talking about final distribution DB's/circuits that are very close to the supply distribution transformer(s). We use 25KA MCB's in such instances. The old solution was installing HRC final fuse boards (DB's)... which in fact, could still be used....

 

[Darkwood]

I know what you saying E54 but 17th isnt that strict but there is also the recommendation that if a fault current exceeding the fault current rating of the device occurs then the device should be changed but i have never seen this happen although ive happened upon many failed mcb's in my time after a fault... the way the 17th addresses it is a capable upstream device will still operate safely if a device with a under-rated breaking capacity downstream fails due to the high current.

 

[Paul Chick]

E54 From a design perspective you would take those sort of things in to account. But on a periodic where cost is an issue thats never going to happen. I just want a good way to note it down on the condition report so it doesnt bite me in the ---.

 

[capitolltd]

If you measured the fault level with a loop impedance meter, then the figure would be inaccurate, due to it being outside its accuracy capabilities (below 0.2 Ohms). If possible locate the TX V% and calculate the fault level.

 

[Engineer54]

If you measured the fault level with a loop impedance meter, then the figure would be inaccurate, due to it being outside its accuracy capabilities (below 0.2 Ohms). If possible locate the TX V% and calculate the fault level.

Or use IQ's method, as shown in the sticky section of this forum...

[h=2]Higher-accuracy low Ze test procedure[/h]

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! ​

Read more: http://www.electriciansforums.net/i...racy-low-ze-test-procedure.html#ixzz2IuBbE752​

 

[Heisenberg]

E54 From a design perspective you would take those sort of things in to account. But on a periodic where cost is an issue thats never going to happen. I just want a good way to note it down on the condition report so it doesnt bite me in the ---.

Surely for an EICR to be worth the paper it is written on, you would need to be transparent and completely honest in your appraisal. Needing to sugar coat things because you feel they will not be welcomed on the report, is a very slippery slope to start down. Not judging you, just saying.

 

[capitolltd]

The above method for trying to bring the meter into its calibration accuracy by adding resistance and subtracting, does not provide you with an accurate figure. This is due to the impedance at the source of supply being highly reactive. Test meters also have a field error tolerance of +/- 30%.
A test current of over 200A would be required to achieve any valid results.
Calculating the Ipf using V% is very accurate, especially if you can obtain the primary MVA values.
If you have a megger 1500 series meter, these are known to show exceptionally low values of impedance.

 

[Engineer54]

It'll give you a dam sight better accuracy than using a MFT without using the above method!! All our substation testing is conducted with high resolution test equipment, but that is way out of the means of most self employed journeyman electricians. Besides, any testing close to a Distribution TX that gives a value in excess of 10KA, and you'll be relying on the upstream protection anyway, unless your using HRC fuses in the final circuit DB's....

9 times out of 10 an electrician won't be allowed anywhere near the distribution TX, especially if it's provided by the DNO. So as a general rule of thumb you would use a 5% impedance, on a standard 1MVA TX if your wanting to go down the calculation route...