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bethelec

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can someone please explain why you multiply the answer by 2 on a 3 phase supply and do you record the highest or lowest reading and why. thanks
 
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P

Plonker 3

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  • #2
Your record the highest reading of either the PFC or PSCC current after multiplying by 2, although it is technically the square roof of 3. But the lowest of the Ze or Zdb values.

It is to allow for a as the name suggest a Prospective Short Circuit Current or Prospective Fault Current which could be between 2 phases rather than Live and either Neutral/Earth.
 
1

1shortcircuit

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  • #3
can someone please explain why you multiply the answer by 2 on a 3 phase supply and do you record the highest or lowest reading and why. thanks
I don't see 3 phase in the domestic that I do but I imagine you test all three phases and find the highest and then x2 because you have 2 other phases?

Hope I have that right otherwise I'll look a right prat lol

You record the highest reading between pscc and pefc
 

Strima

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Arms
Esteemed
As Dillb has said. The rule of thumb is to double it but if calculations are getting tight try multiplying by Sqrt of 3 (1.723) which can give you a little leeway.
 

spark 68

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Arms
Esteemed
In addition to the other posters Which are correct, it is because not all testers are capable of measuring across phases (Phase to Phase).

So you measure each of the phases in turn to Neutral and then double the highest reading, which errs on the side of caution giving you the max PSSC reading.

Your PEFC reading is just each phase in turn to Earth, again you take the highest reading.

You then take the highest reading of either the PEFC or your now doubled PSSC reading, and record that as the PFC.
 

widdler

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Mentor
Arms
Following on from Dillb, when you take a measurement of PFC on a loop impedance tester, the instrument is simply utilising the loop test and then applying ohms law to display the result in Amperes instead.

So what is the difference with a test between two phases? Well the resistance of the loop under test is typically the same, however the voltage that would occur in a fault between two lines will be 400v instead of 230v. So instead of applying the 230v in the ohms law equation we should use 400v.

But as our tester was already using the 230v source we have to convert it from 230v to 400v. As Dillb stated, this is more accuratley done by multiplying the value by the square root of 3.

However, as the purpose of the fault current measurement is for verification of selected protective devices breaking capacities, simply doubling the measurement as a rule of thumb provides a slight over estimation - which results in 'over compliance' consideration.
 
D

Deleted member 26818

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  • #7
With 3 phase, the PFC will always be the PSCC.
 
S

SirKit Breaker

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  • #9
Following on from Dillb, when you take a measurement of PFC on a loop impedance tester, the instrument is simply utilising the loop test and then applying ohms law to display the result in Amperes instead.

So what is the difference with a test between two phases? Well the resistance of the loop under test is typically the same, however the voltage that would occur in a fault between two lines will be 400v instead of 230v. So instead of applying the 230v in the ohms law equation we should use 400v.

But as our tester was already using the 230v source we have to convert it from 230v to 400v. As Dillb stated, this is more accuratley done by multiplying the value by the square root of 3.

However, as the purpose of the fault current measurement is for verification of selected protective devices breaking capacities, simply doubling the measurement as a rule of thumb provides a slight over estimation - which results in 'over compliance' consideration.
I wish i could explain it that well!

Cheers.......Howard
 
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