Prospective Fault Current

[Ashley Williams]

Hi Guys

How do you calculate Prospective Fault Current??

Wouldi be correct in thinking it is nominal voltage divided by Zs + R1 + R2 ???

So

Nominal Voltage
Zs + R1 + R2

A

 

[solo2771]

Hi Guys

How do you calculate Prospective Fault Current??

Wouldi be correct in thinking it is nominal voltage divided by Zs + R1 + R2 ???

So

Nominal Voltage
Zs + R1 + R2

A

Try 230/Ze

 

[Graeme Harrold]

You are looking a little confused.

Ze = external earth loop
Zs = Ze + (R1+R2)

Your highest PFC will be at the origin of your installation thus PFC = Uo(Nominal Voltage)/Ze


(off the top of my head and hopefully correct )

 

[solo2771]

If you not got Ze

Ze =Zs-(R1+R2)

 

[Ashley Williams]

Ok

Well i think ive calculated it right. measured Earth loop Impedance is 1.08. my r1 + r2 = 0.05. So ze = 1.03 so..

230/1.03= 223.3 ka..

Yup Pretty poor earth.

But this is a college test fortunatly and main protective conductor is only small as the circuit isnt used to power anything simply for testing purposes.

Cheers Guys

A

 

[GaryM]

If you not got Ze

Ze =Zs-(R1+R2)

Not necessarily as parallel earth paths (water or gas bonding for example) can bring the overall Zs measurement down thus hiding a potentially higher than acceptable external impedance.

By all means, calculate Zs but never back calculate Ze.

To the OP:

Also, remember that PFC is the highest figure of PEFC (prospective earth fault current - Line to Earth) or PSSC (prospective short circuit current - Line to Neutral).

 

[Deleted member 9648]

Ok

Well i think ive calculated it right. measured Earth loop Impedance is 1.08. my r1 + r2 = 0.05. So ze = 1.03 so..

230/1.03= 223.3 ka..

Yup Pretty poor earth.

But this is a college test fortunatly and main protective conductor is only small as the circuit isnt used to power anything simply for testing purposes.

Cheers Guys

A

Not 223.3 ka but 223.3 amps or 0.223 ka

 

[Ashley Williams]

Cheers Guys

Im a N00b!

A

 

[roukel01]

Not necessarily as parallel earth paths (water or gas bonding for example) can bring the overall Zs measurement down thus hiding a potentially higher than acceptable external impedance.

By all means, calculate Zs but never back calculate Ze.

To the OP:

Also, remember that PFC is the highest figure of PEFC (prospective earth fault current - Line to Earth) or PSSC (prospective short circuit current - Line to Neutral).

When measuring Ze you must do so with the main earth disconnected so that no prarallel paths interfere with the reading, however, measuring PEFC you do with all Earths connected to include Parallel paths because you are looking for the lowest Earth Impedance reading and hence the Largest PEFC reading.

 

[GaryM]

My point was that you can't calculate Ze.

 

[roukel01]

My point was that you can't calculate Ze, not the ins and outs of measuring PFC of which I'm fully conversant with.

Your post was a little confusing. I was just trying to clarify for people who aren't as 'Conversant'

 

[Jurasic Spark]

My point was that you can't calculate Ze.

Not strictly true, GN3 p49.
But you are correct that you can't calc' it by Zs -( R1+R2).
You would have to know the Pefc then use Ohm's Law the other way.

 

[GaryM]

What if the incoming earth has a fault and PEFC is high due to parallel earth paths from the water and gas supplies?
Zs may well be fine but an underlying problem could go amiss.

 

[WayneL]

GaryM is correct - you can't accurately calculate Ze using other measurements
(eg Zs, PEFC) as these measurements are taken with the main earthing
conductor connected (parallel paths), giving a lower Ze than it would
actually be.
As stated earlier, Ze is measured with the main earthing conductor
disconnected and is pretty much impossible to calculate accurately

 

[ezzzekiel]

Ze may be calculated by:
measurement
enquiry
calculation