Prospective fault current testing

[dunscombe]

hi, new to this forum and still training. During testing for prospective fault current at the point I get a bad reading, what ever that may be????, what possible situations would be causing and why? still trying to get my head round it...
thanks Adam

 

[SparkyChick]

Hi Adam and welcome to the forum...

I would suggest the only 'bad' reading you can get for prospective fault current is one caused by poor test connections. The prospective fault current is what it is, there isn't really a good or bad value for it as it will depend mainly on the distance from the supply source (most typically the substation supplying you).

That being said, it is possible to get weird readings.

Remembering that the PFC is derived (i.e. you use the highest value) from the Prospective Earth Fault Current (PEFC) and the Prospective Short Circuit Current (PSCC) it can reveal things about the installation which could be one interpretation of a 'bad' reading.

So consider a TNCS earthing arrangement on a single phase 2 wire supply (typical domestic arrangement). If you get a PSCC of say 0.865kA and a PEFC of say 0.125kA... what could that be telling you?

If you got similar readings from say a TNS earthing arrangement, would you be concerned? What about a TT earthing arrangement?

The only other thing that you could consider to be bad might be if you get a reading of say 6.5kA... your house is right next to the substation for the entire estate and you're on a TNCS earthing arrangement. Why might that be bad?

 

[James]

Firstly, I agree with everything above from sparkychick.

second I have had the problem listed at the end of her post about being right next to the substation on a couple of occasions where distribution is feet away from customers own transformer.

the only "bad reading" that i can think of was in a plastics factory that had so many variable speed drives with poor filtering that my fluke tester returned an error because of excessive harmonics, came back sat morning when all was off and tested ok.

 

[pc1966]

When you say "bad reading" I tend to read that as a measurement problem: maybe you get different readings on trying it twice due to dirty contacts or dodgy test leads (with breaking up conductors due to lots of flexing, etc). Or more likely if doing a "no trip" PFC test it varies a bit due to noise, harmonics, etc.

But in the context of determining the PEFC & PSCC at the supply point you have a few things to think about as above:

• Is the value very high?
• If so what are the implications for the connected distribution board and/or planned work?
• Is the value very low?
• If so what are the implications of that in terms of a possible supply fault?
• Or if it simply is the normal case for the supply type/location, what are the implications for the existing distribution board (e.g. on EICR work) or new design in terms of sub-main or final circuit protection?