Measuring PFC on 3 phase supply

[londonboy]

Hi guys, have been talked into sitting the AM2 exam next week and have a query with PFC.
Do you just test each phase to earth, then each phase to neutral?
I seem to remember having to double a figure, but I can't remember to be honest.

Cheers for you help

 

[Spark123]

You are basically after the absolute maximum fault which is going to flow if there was a fault in the fuse board.

Firstly measure each phase to earth - record those values
Measure and record each phase to neutral.
If - and ONLY IF your tester is sufficiently rated to do so - measure each phase to phase fault current.
Record the highest figure from all of the above.
If your tester is not rated to do phase to phase measurements then you may take the maximum figure obtained from the P-N or P-E test, double it and record that.

 

[londonboy]

Thanks thats a great answer thats made it much clearer,
cheers

 

[RF Lighting]

Be careful though.

You can get some pretty high readings

 

[londonboy]

Haha where did you find that? Up town next to a sub station?!

 

[Spark123]

It's a metrel - what do you expect (sorry - I'll get my coat)

 

[londonboy]

0.005 ohms? If my maths is correct

 

[Adam_151]

... Assuming of couse that he is measuring phase-> neutral... IIRC the metrel is capable of doing a 415v loop reading (but I never use that, just double the highest SP value... its the way I was taught... saves you blowing up someone elses test equipment!)

A value that low is going have quite a bit of error/uncertaintly... perhaps if rob presses the test probes on better he might see a higher fault level , chances are he will if he pulls some overtime and tests when network loading is low ,if you are thinking about what the tester is doing... its applying a load of 23A, recording the volt drop and applying ohms law to give an impedance, you start to see that it becomes very difficult to measure low impedance fault paths accuratly (same again with high values of insulation resistance... but luckily, the differece between 800meg and 1000meg is largely irrelevant!)

Not that a fault of that magnatude will over occur IRL, in real life faults don't involve a silver dagger 10mm along the submain from where it leaves the panelboard ,plus the inductance of the supply transformer means that currents of that manatude don't just instantly start flowing... there it'll take a a small fraction of a second to happen, and the fuse will have opened by then (and even before it does, the reistance will have increased)

 

[RF Lighting]

It was a new TP feed we had put in recently.

The reading is between 2 phases U = 420V

It was near the sub, and our cutout was at max three meters from the ring main cable, which looked fairly new to me.

The job was well out in the sticks so loading would have been light.

 

[Adam_151]

You use a BS88 DB on that job then?

 

[RF Lighting]

The cutout feeds straight into a BS88 TP switchfuse.

The DB is maybe 30m or so away.