Prospective Fault Current

[noel2507]

Hi all having a problem understanding PFC I tested a CU today and the result I got was 19A. It was a TT system and the Ze was 52.5 also all MCB were rated 6K, main income fuse 1361 type 2 with a 230v supply. My question is which of these results I check the PFC with. Could you please explain the calculation and if possible give an example of a failed PFC. Thank very much for any help.

 

[lilman]

What was the L to N reading? I take it your result was L to E (always low on TT)

 

[NozSpark]

Your PFC is the highest of your PEFC (prospective earth fault current) and PSCC (prospective short circuit current) and it should be < the kA rating of the OCPD's

sometimes you PFC is higher than the kA rating of your OCPD, but this is ok so long as there is an upstream protective device with a higher kA rating.

HTH

 

[rob196]

Done the same readings myself today and the readings are L-N 0.59 kA and the L-E 5A.....This is a sub main board.....The main boards reads L-N 1.29 KA and L-E 18A......just wondered why the sub board was lower then the main board ??

 

[sparkytim]

Its further away from the supply

As the resistence of a circuit increases the Prospective Fault current decreases.

 

[Nicky Tesla]

yes agreed but 18A then 5A thats a very large resistance if these boards have an earth conductor between them. 230/18=12.7ohms 230/5=46ohms 46-12.7=33.3ohms

33ohms is far too high for an earthing conductor between the boards, but does the sub board have its own separate earth rod? This would explain it.

 

[kme]

Concur with Nicky`s point above - the two external earth loop readings (Ra for a TT; NOT Ze) are far too different to be sharing a rod, unless there is a problem.......

The PSC of a TT, almost without fail (every one I`ve ever seen, anyway), will be between phase and neutral, at the closest point to the origin of the supply.

HTH

KME

 

[rob196]

Main board has an Earth rod, And the sub board has it's own Earth rod, The cable thats supplying the sub board is a 6mm but cannot get a R1+r2 reading as the there is a bad earth........done a R1+Rn which is 0.20ohms.....The circuit is protected by a 30ma RCD in th main Board.

Ze at the main board is 33.1ohms and The sub board reading is 49.6ohms...

 

[sparkytim]

Probably segregated the CPC at the 2nd Dis board, so you will not get an R1+R2 reading. This is ok. To check continuity of CPC of the distribution cct you will need to locate the cut of CPC and test from that point.

 

[rob196]

Probably segregated the CPC at the 2nd Dis board, so you will not get an R1+R2 reading. This is ok. To check continuity of CPC of the distribution cct you will need to locate the cut of CPC and test from that point.

I can get a reading but it's 172ohms, It's a 6mm pvc/pvc cable at the main board but goes up into the the landing floor, cannot find where it come's out of the building but at garage it looks like SWA but goes into metal conduit underground and it's only the Live and Neutral that come's out of the conduit at ground level, can only get the 172ohms reading connecting to the conduit......

 

[Andy™]

I can get a reading but it's 172ohms, It's a 6mm pvc/pvc cable at the main board but goes up into the the landing floor, cannot find where it come's out of the building but at garage it looks like SWA but goes into metal conduit underground and it's only the Live and Neutral that come's out of the conduit at ground level, can only get the 172ohms reading connecting to the conduit......

nothing wrong with the setup - no different to TN at house, and TT at outbuilding. you should however have armour continuity from the first board, but not into the second board

 

[rob196]

Thought you need to have continuity of the cpc but not terminated in the Sub Board.........

 

[sparkytim]

Not so good then

sounds as if the CPC connection on to the conduit has gone high resistence.

Can only suggest you try and locate it and reconnect or replace with SWA .

 

[rob196]

Not an option as the Cable is buried under the garage floor, have done IR test and all ok even the Earth, this was taken at the board...........

 

[kme]

Possibly the reason the second rod was installed in the first place?

Not perfect, but the best solution available ??

KME

 

[rob196]

Possibly the reason the second rod was installed in the first place?Not perfect, but the best solution available ??

KME

Good job that I put the Rod in then :| Imaging this garage has been wired up for ages without any earth and no RCD protection...........