# Thread: Earth fault loop impedance test above 100A

1. ## Earth fault loop impedance test above 100A

Hi guys, would you guys agree with not using your loop tester to test at mains when its above 100A supply ? Cause ive heard that the earth fault loop impedance and PSSC test should only be carried out on installations with a main circuit breaker of 100A or less. For larger installations the value should be calculated. What if you don't have the values of the transformer etc to do the calculation?

2. As far as I know its only PSC that must be calculated above 100A, earth loop impedance can still be tested above 100A.

I have tested a 160A supply PSC with a meter, I had calculated the PSC and just wanted to check my calculations. I found that the meter overheated and the thermal cutout came in after testing the second phase. I would think on higher amp ratings you could damage the meter or worse have it catch fire or blow up in your hands while doing a PSC test at over 100A.

Lionel.

3. How would you go about calculating it?

4. Reg 8.5

5. I hoped there's another way cause I never have access to get the transformer vaulues needed for the calculation

6. On most supplies its impossible to get to the transformer. I usually test after the first small breaker if I don't feel it safe to test directly at mains position.

My meter, fluke 1653B, can test from 0 - 25kA. Its designed to handle such tests.

Its pretty standard practice in most countries to test with a meter (well i can speak for the UK, Australia and New Zealand)

If you are working with stuff with fault currents over 25kA then that is next level stuff. You must be practically working in the substation to get that kind of fault current.

Fluke 1653B
Computation: PFC or PSC determined by dividing measured mains voltage by measured loop (L-PE) resistance or line (L-N) resistance
Range: 0 - 25 kA
Resolution (Ik < 1000 A): 1 A
Resolution (Ik >= 1000 A): 0.1 kA
Accuracy: Determined by accuracy of loop resistance and mains voltage measurements.

7. All live tests can be dangerous and should only be done in conjunction with a risk assessment. That said, impedance testers are designed to accommodate overheating, it's expected and there's built in protection to prevent damage from occurring so I doubt the 100A live-test limit in the regs is motivated by the protecting the instrument.

I still use my old Robin loop tester for determining PSC, it uses a 25A test current rather than the somewhat limp wristed 12A test the new fandangled Flukes use . If there's three phases to test it often gives the overheating warning on the third test if I do them in quick succession. No big drama, no explosions just the warning light comes on and it won't perform further tests until it's cooled down.

If you can't calculate the PSC because of not having the transformer details you should be able to take the Ics rating of the suppliers MCB as an indication of the maximum possible PFC.

8. Originally Posted by Pieter00
I hoped there's another way cause I never have access to get the transformer vaulues needed for the calculation
Do your LOOP impedance test and use the formula to calculate PSC if your tester is not capable of conducting such test.
PSCC=V/Zloop 1PH
PSCC=V/SQ3.Zloop 3PH

V In both formulas is measured incoming voltage:
V- 1Ph (P - N)
V- 3Ph (P - P)

Regards
TDMI

9. ## Thanks given for this post:

Lionels (09-Mar-17)

10. Originally Posted by W-TDMI
Do your LOOP impedance test and use the formula to calculate PSC if your tester is not capable of conducting such test.
The earth loop impedance test?