propective short circuit current

[alansteyert1]

Can anybody please tell me how to calculate the prospective short circuit current into me property

 

[Taylortwocities]

Use a calibrated line and loop test meter.

Do you have one?


Why do you want to know this information?

 

[riveralt]

Can anybody please tell me how to calculate the prospective short circuit current into me property

But you are a qualified electrician - how come you don't know how to do this?
Since you are working on the electrics in your loft I would have thought you would have the appropriate testing equipment - after all its yours and your families safety at stake here.

A quick search on google would have got you to here:
http://www.tlc-direct.co.uk/Book/3.7.2.htm

 

[ericmark]

To measure it's a special meter. Normally it will read as either loop impedance or prospective short circuit current at the flick of a switch as at 230 volt the two are related. I have never opened up the meter but I know it gets hot so I would assume it puts a resistance across the supply and measures volt drop and internally calculates from that. When one takes the readings a few times there is always a slight difference in each reading. Your looking at around the £200 mark for a cheap one. However for the incoming supply you can find it by inquiry. In theroy all you need to do is ring your supplier. In practice likely they will quote 0.35 ohms impedance without measuring it. 230/0.35 = 660A which is not really what you want.

Main reason for measuring is to select the protective devices. There is the actual value measured and the let through value of the fuse. Can't off hand remember the let through value of a 100A fuse but can't see even 4700A normally quoted on RCBO's being a problem with let through value of a fuse.

So why do you need to know?

 

[ericmark]

Just read other post. Your a qualified electrician of 40 years experience what are you doing testing us? Come on no electrician with that experience needs telling this.

 

[alansteyert1]

Yes i am a qualified electrician for 40 years and authorised to work on networks upto 132kv The only reply i have is to use a loop tester this will only measure the neutral earth fault back to the distribution transformer .As on distribution mains the neutral conductor has a smaller csa than the phase conductor therefore the readinge will be incorrect

 

[ricicle]

Yes i am a qualified electrician for 40 years and authorised to work on networks upto 132kv The only reply i have is to use a loop tester this will only measure the neutral earth fault back to the distribution transformer .As on distribution mains the neutral conductor has a smaller csa than the phase conductor therefore the readinge will be incorrect

Oh dear. I take it you've spent more time on HV networks then than LV ones.

A loop tester will measure the L to E impedance or the L to N impedance.

 

[Taylortwocities]

It's a worry..

Testing 101

1.PSCC is fault current between phase and neutral
2.PFC is fault current between phase and earth

The higher of the 2 figures is whats recorded on your EIC, but you only need to measure this at your protective devices to make sure they can clear faults safely and not potentially blow up in someones hand whilst being reset after tripping (assuming mcb) .

So typical example: single phase, domestic install type B 6Ka breakers fitted in consumer unit

measured voltage 242.5v

Measure Ze at consumer unit
ph to e=0.34 ohms pfc fault current=Uo/Ze 713amps

ph to N= 0.29ohms pscc= Uo/Ze = 836amps

So you now record the higher figure of 836amps as your fault current and can be satisfied that your protective devices will easily clear any fault currrents as they can cope with upto 6000 amps!

So you need first to determine Line to Neutral Impedance (your calibrated test equipment will do this no problem - you do have one of those?)
then
measure your supply voltage. Again your test meter will do this
then either use Ohmies Law to determine PSCC or (if you have a decent meter) the meter will tell you teh PSCC figure.

 

[ericmark]

Although worked on HV very limited and it seems you must have different equipment.

The tester used with LV measures between line and either Earth or Neutral it does not measure Neutral to Earth. It uses the supply to take these measurements so it has to be a live test. Clearly one would not do that with HV.

Depending on the make and model but often it requires if not using wandering leads for us to make adaptors and often the provided lead will only measure line - earth.

Because of the current involved there is a high chance of ionisation if the leads are not making good contact so one should wear the appropriate PPE.

I nearly got the sack over this as I had removed my high vis vest before doing the test as they are made with man made fibres and can melt with ionisation. Lucky the safety officer did understand.