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stev999

Calculating Prospective Fault Current

Question

Hi everyone. I have just received a Newlec NL111 loop impedance tester, it is quite old but its useful for me to learn with. It doesn't have a d lock so it does trip RCDs, so to get Ze i have to disconnect main earth with the board isolated & test between this and incoming live phase. What i need to know is can the prospective fault current be calculated from Ze. I notice on test sheets your required to note down the PFC, but not short circuit or earth fault current, although its a requirement of BS7671 Reg 612.11. Basically does this mean this tester isn't suitable for modern requirements. Any advice would be greatly appreciated.

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14 answers to this question

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You could just calculate the PFC you know!

Volts/ohms=?????

Your PFC is the higher of PSSC and PEFC (both taken with earth and bonds connected)

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It is fine, but you would have to do more leg work at obtain all of the results that you require. Has the tester and the leads been calibrated recently?

AndyGuinness

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You are allowed to use calculations to get PFC.

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And can I just add that the only way for ANYONE to measure Ze is the way you describe - if earthing conductor is connected to MET , you`ve effectively measured Zs(DB) - which isn`t a recognised measurement in 7671; but is widely used.

KME

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ID: 6   Posted (edited)

You are allowed to use calculations to get PFC. Ifc = Uo/Ze

Edited by johninlondon1
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I wish i knew how to do it

Think ill stick to Grape Testing as thats easier......

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And you`re DAMN good at it, too....

You`d make a good living from it if you could do more in a day, but alas! Its so tiring. Makes you want to just curl up in a baZzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

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Is that Newlec tester the one that's basically a Megger LT5 ? I still have one in the cupboard. Would still be using it but not practical with having no dlok.

I regretted selling my other Megger separates good quality and reliable is how I'd describe them.

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Would still be using it but not practical with having no dik.

I regretted selling my other dik.

Can you say things like that on here??? :slap]:)

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WHY are you concerned with tripping RCD's when testing Ze?

You test Ze right at the incomer, BEFORE any RCD's, so your tester will work just exactly the way it was designed to work. If you are having trouble tripping RCD's you are testing in the wrong place.

Granted your tester won't be able to measure Zs on a final circuit with an RCD, so you will have to bypass the RCD or calculate that, but you were talking about measuring Ze

Anyway back to the original question, that tester only gives the impedance, so PSSC and PEFC HAVE to be calculated using ohms law.

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You can always add PEFC and PSCC to your test sheets, I have them on there as well as Ipf

Example:

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Remember the old 2wire testers with the graph on the back?

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ID: 14   Posted (edited)

OK Tip of the day

How to test Zs on a RCD protected radial circuit.

Turn OFF RCD, short L to E at the end of line of the circuit (Furthest point from supply)

Connect one lead of your tester and connect to OUT GOING line terminal of the RCD ( RCD SWITCHED OFF)

CAREFULLY Connect the other lead to the INCOMING Line terminal and take reading.

Your Meter bypasses the RCD so it will not trip and you get a reading using the full 25 Amp test current.

Remove leads, remove short at end of line and you have it, accurate direct reading

Ps

I had to draw it out to prove to myself it would be a true reading when shown this Method.

Try it and see for yourself

Edited by sparkytim
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And can I just add that the only way for ANYONE to measure Ze is the way you describe - if earthing conductor is connected to MET , you`ve effectively measured Zs(DB) - which isn`t a recognised measurement in 7671; but is widely used.

KME

Ze is measured with the isolator off and the earth disconnected BUT the PEFC is measured with the earth connected and all bonding in place.

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