Earth loop impedance and prospective short circuit (PSC)

[mikilianis]

I am a bit confused I was going through some old notes of mine and came across earth-loop impedance test:main switch with a value .74 then Prospective shot circuit with a value .297 is there a formula that is used I have forgotten.Then all the referances I have are for single phase are there any calcs for three phases

[AndyD]

I'm not fully understanding your question but with PSCC it's a live to neutral fault. EFLI would obviously dictate the current in a live to earth fault.

[Dave A]

Then all the referances I have are for single phase are there any calcs for three phases

The formulas needed to calculate PSCC are set out in section 8.5 of SANS 10142-1.

[bergie]

the oldest formula in the book. R= V/I use 297 amps not 0.297kA

[MullerR]

Question:
When I calculate the PSCC (v/r), I get 120A (228v/1.90ohm), but when measured with tester, it reads 674.5A. Why such a big difference? The Measurement for Earth loop (Ze) is 0.36ohm which is fine, cause it is lower that the 1.90ohm. Im just concerned why the high PSCC reading in comparison with PSCC Calculation.. It is still lower that the kA rating of mainswitch though.

[Dave A]

I'm guessing you measured the 1.9 ohm resistance...
If yes, you're likely getting inductive resistance from the coil distorting your reading.

Assuming for a moment that the actual line resistance for live to neutral is similar to your Earth loop reading, 228v / 0.36 ohms = 633.33 Amps.

[AndyD]

.......Assuming for a moment that the actual line resistance for live to neutral is similar to your Earth loop reading, 228v / 0.36 ohms = 633.33 Amps.

If it's a supply with a TN-C-S earthing arrangement the Ze and Zn should be near as dammit identical so PSCC should be the same as PEFC.

I'd suspect there's an inaccuracy with the 1.9 ohms you've used in your calculation, it does sound very high. How did you get this figure? If this is a 60A domestic supply the calculated PSCC of 120A would be too low to make required disconnect times if it's a c-curve 60A main breaker.

[MullerR]

My voltage is 228v. The main switch is 60A, thus to calculate Earth loop, it is v/2x1 -> 228/120=1.90ohm. The actual Earth loop reading with earth loop and PSCC tester is 0.36ohm which is correct, as it is lower than the 1.90ohm. Now, when I calculate the PSCC (v/r) must i use the calculated Resistance or the Measured Resistance? When I use the calculated resistance I get 228/1.90 = 120A. When I use the measured resistance I get 228/0.36 = 633.33A, which is closer to the actual PSCC reading of 674.5A. Please bare with me here, Ive done my training in 2010 and ever did coc's till now, so im still a bit rusted. Hope you dont mind me asking lot of 'stupid' questions.

[Dave A]

My voltage is 228v. The main switch is 60A, thus to calculate Earth loop, it is v/2x1 -> 228/120=1.90ohm.

That's your upper limit threshold allowed - not the actual earth loop value (be that calculated or measured).

[MullerR]

Yes that im aware of. The measured value must not be more than the calculated value, as the calculated value is the upper limit threshold as you mentioned. My question now, when i calculate the value of the pscc, must i use the earth loop value calculated (1.90ohm) or the measured value (0.36ohm) to get the pscc? Because when i use the measured value of 0.36ohm, I get 633A which is closer to the pscc measurement of 674A.

[Dave A]

Yes that im aware of. The measured value must not be more than the calculated value, as the calculated value is the upper limit threshold as you mentioned. My question now, when i calculate the value of the pscc, must i use the earth loop value calculated (1.90ohm) or the measured value (0.36ohm) to get the pscc? Because when i use the measured value of 0.36ohm, I get 633A which is closer to the pscc measurement of 674A.

You'd use the measured value. The objective of establishing the actual PSCC is to ensure the protection equipment is adequately rated to handle a short-circuit situation for that particular installation, so you need to be using the actual values of the environment.

As an aside, when you have a moment read Section 8.5 of SANS 10142-1 to get where I'm coming from on what the term "calculated PSCC" means.

[MullerR]

Now i'm getting closer to everything. Still a bit rusted, but between the books and guys like you who can answer me when I ask something, I feel more comfortable now.

Thank you Dave and Andy