Prospective Fault Current

[Revved Up Sparky]

Hi all, hope you are well.

I did a PIR in a domestic dwelling today.

With power off I recorded Ze (TNS) as 0.08 ohms (transformer quite close).

The voltage measured on the incoming tails was 232V AC.

Yet when I measured PFC I kept getting 19.9KA.

Puzzled with the very high reading, I thought I would calculate it to check the reading so I divided the Ze by the voltage, ie 232/0.08.

The result was 2.9KA.

Have I done something wrong or do you think my multitester is playing up?.

If the PFC is indeed 19.9KA then what should I do with the MCB's as they are rated at 6KA. The main fuse is a BS1361 60A..... so PFC of that is about 16KA I think..........

Any thoughts greatly appreciated

Best Regards

RUS

 

[Andy™]

whats earthing type? if TNS, then L-N loop could be much lower and much higher PFC.

 

[Revved Up Sparky]

Cheers for that Andy, I just edited it in.... yes TNS. Why is that ? is it because of the sheath being the earth ?

 

[rugbymacc]

did you disconnect all the other parallel earth's ?

gas . water ?

 

[Andy™]

CSA of earth may be less than that of neutral. but either way, i wouldnt expect a PSCC of almost 20KA if L-E PEFC is 3KA

did you measure L-N loop?

 

[seajayess]

Dont forget that PFC is the current that would flow through the L/N and not L/Earth path which is the Ze you have recorded, when you do your PFC again see what the resistance is. 19KA is high though dont think you will get a MCB with that breaking capacity CJS

 

[Revved Up Sparky]

I used the three pronged lead so I was probing L,N & E rather than L & E.

Rugby.... yes... : O)

 

[rugbymacc]

might be worth going back and take another measurement,

Another day "an" all that

 

[Revved Up Sparky]

might be worth going back and take another measurement,Another day "an" all that

Yes it's an empty house and I've got the key so I will go back there on Monday and do a few more checks.... : O)

 

[Lurch]

19KA is high though dont think you will get a MCB with that breaking capacity CJS

No, but as long as something upstream will break the fault current you can pass it. Never been convinced though myself, would depend on the installation as to whether you'd want to or not.

 

[tom1]

No, but as long as something upstream will break the fault current you can pass it. Never been convinced though myself, would depend on the installation as to whether you'd want to or not.

off memory i think main fuse would need a type II for that i think the type I are 16.5 ka

 

[muffin]

what tester are you using, my fluke one has given a reading of 10ka on my part p inspection. i knew it was wrong as i did the job the day before and got a reading of 1.8ka. the nic guy tried telling me he would expect to see it. it beg to differ not seen over 3ka in a small town even right next to the sub station.

the three prong test leads use the neutral lead to make the bit of kit work, and the earth is the test lead. to fully test max ka you should put L-L N-N E-E record reading then again L-L N AND E of test lead onto the neutral. record the highest reading on the form.

 

[Andy™]

or use a megger with 2 lead loop test...

 

[muffin]

hmmm that would be good, bloody err 4

appears everytime the neutral slips on the termainal as you wait endlessly for the reading to come up. bring back the old robin ones that were instant. ok they knocked out rcds but you cant have it all

 

[sparkytim]

Have had similar readings in past when VERY close to supply transformer

My MFT 1553 has shown > 19.9 KA before, however DNO insists real value will NEVER excede 16 KA

BS 1361 TYPE 2 FUSE max 33 KA so will provide back up protection.

 

[Revved Up Sparky]

what tester are you using, my fluke one has given a reading of 10ka on my part p inspection. i knew it was wrong as i did the job the day before and got a reading of 1.8ka. the nic guy tried telling me he would expect to see it. it beg to differ not seen over 3ka in a small town even right next to the sub station. the three prong test leads use the neutral lead to make the bit of kit work, and the earth is the test lead. to fully test max ka you should put L-L N-N E-E record reading then again L-L N AND E of test lead onto the neutral. record the highest reading on the form.

It's a Megger multitester.... not sure of the model number right now.

Thanks for the comments- much appreciated

 

[tom1]

if its the 1552 then its a two lead test terminals L1 and Lo/L2