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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
 
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 :eek: )
 
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
 
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).
 
Last edited by a moderator:
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
 
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.
 
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.
 
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:)
 
It's all very well quoting guidance note 3, care to explain how you'd do it?:)

Whilst proving you have a reliable earth coming into the property which hasn't been supplemented by a gas, water or any other extraneous conductive part which has been bonded to the main earthing terminal.
 
Whilst proving you have a reliable earth coming into the property which hasn't been supplemented by a gas, water or any other extraneous conductive part which has been bonded to the main earthing terminal.

Very Good:D
 
Highest PFC will be voltage divided by Ze, but isn't it the lowest PFC that you are actually interested in - to to know if the minimum disconnection times for the protective device are met. If this is the case, do you not want voltage divided by Zs (taken from the end of the circuit) ie Ze +(R1+R2).

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

If you did this at college on a rig Your 1.08 ohms figure is too high but that is probably because it was not a true Ze taken at the distribution board but just taken from a plug socket that your rig was connected to. ie more of a Zs figure.
And be careful that you always change amps to kA as that is what is usually asked for on the certificate.
 
Last edited by a moderator:
Highest PFC will be voltage divided by Ze, but isn't it the lowest PFC that you are actually interested in - to to know if the minimum disconnection times for the protective device are met. If this is the case, do you not want voltage divided by Zs (taken from the end of the circuit) ie Ze +(R1+R2).

.

Max Zs confirms your disconnection times, not minimum PFC.

You need to know the maximum fault current that could flow to ensure
that switchgear and breakers etc can withstand the bang:) - this is found
at the origin.

You can also measure PFC at remote boards if you wanted to downgrade
the breaking capacity of your MCBs at that board.
 

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