Prospective Fault Current

[WayneL]

Ze may be calculated by:
measurement
enquiry
calculation

It's all very well quoting guidance note 3, care to explain how you'd do it?

You'll have noticed GN3 doesn't tell you how.

 

[GaryM]

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.

 

[WayneL]

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

 

[pushrod]

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.

 

[WayneL]

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.

 

[pushrod]

Max Zs confirms your disconnection times, not minimum PFC.

.

Your max Zs will give you your min PFC though (I was thinking of similar things i have been asked at college and wondering if the OP had been asked the same). I know it is Zs figures that are quoted in appendix 2 of the OSG though not PFC's.

Thanks for the 3rd point though i hadn't thought of that

 

[WayneL]

To be honest, this discussion is a bit irrelevant as your PFC is most likely
to be your PSCC - (ie. L - N or L - L) and you're not gonna be calculating
that, so Ze/Zs/R1+R2 don't come into it.

 

[Jurasic Spark]

It's all very well quoting guidance note 3, care to explain how you'd do it?

You'll have noticed GN3 doesn't tell you how.

Ok then how would you answer this type of exam scenario or what about if it was real world scenario?

During a PIR on small comercial unit it is unpracticable to isolate the main switch, how can you ascertain a value of Ze? You have tried enquiry but no one will answer the phone?

 

[GaryM]

Either volunteer for out of hours or put it down as a limitation.

 

[Jurasic Spark]

Either volunteer for out of hours or put it down as a limitation.

There are no out of hours, the place runs 24/7.
Can't you elaborate on your limitation?

 

[andyb]

Ok then how would you answer this type of exam scenario or what about if it was real world scenario?

During a PIR on small comercial unit it is unpracticable to isolate the main switch, how can you ascertain a value of Ze? You have tried enquiry but no one will answer the phone?

Measure it.

The only limitation is that you can't remove the earth for a true reading, you put this down under limitations.

 

[Jurasic Spark]

Measure it.

The only limitation is that you can't remove the earth for a true reading, you put this down under limitations.

Or CALCULATE it using Uo and your measured Pefc and record this as a limitation.

 

[WayneL]

Ok then how would you answer this type of exam scenario or what about if it was real world scenario?

During a PIR on small comercial unit it is unpracticable to isolate the main switch, how can you ascertain a value of Ze? You have tried enquiry but no one will answer the phone?

Keep trying the phone - coz if they won't let you open the main isolator, you shouldn't have the cover off the board in
the first place

Failing that, measure it with the earthing conductor connected and mark as limitation.

Everyone is arguing the ---- but no-one has come up with an answer yet - How can you
possibly calculate it accurately?

Or CALCULATE it using Uo and your measured Pefc and record this as a limitation.

If you've measured Pefc, you might as well have measured Ze while your there and mark Lim

 

[Jurasic Spark]

Keep trying the phone (and what do you think they'd say if they answered it?)- coz if they won't let you open the main isolator, you shouldn't have the cover off the board in
the first place

Well if your engaged in this sort of work lets assume you have the level of competence to do it. So not a problem. Or else how you going to measure Pfc??

Failing that, measure it with the earthing conductor connected and mark as limitation.

Everyone is arguing the ---- but no-one has come up with an answer yet - How can you
possibly calculate it accurately?
How accurate does it need to be??


If you've measured Pefc, you might as well have measured Ze while your there and mark Lim

Well you can re-decorate while your there as well but why do things you don't need to ??

 

[SirKit Breaker]

My point was that you can't calculate Ze.

I thought the same, as it says on the Installation Cert "by measurement or enquiry".