Breaking capacity

[saveloy]

ok so the installation is left in its normal state energised, the bit that confuses me is 2 readings are taken and the highest recorded, what terminals are the two readings taken from.

Phase to Neutral followed by phase to earth and the larger of the two readings recorded,

P-N = PFC Readings taken between Phase - Neutral on incoming terminals of main switch
P-E= PEFC Readings taken between Phase terminal of main switch and earth bar in the cu.

On a TNS and TT you will get two different values of which the highest should be recorded.

On a TNCS both readings should normally be the same or thereabouts.

 

[Richard3009]

Yep got that the test is clear now, what is the state of the main CU isolator during the test

 

[saveloy]

Main switch on cu should be on.

 

[Richard3009]

what about lamps and neons in the house

 

[zupos40]

Yep got that the test is clear now, what is the state of the main CU isolator during the test

I switch off all cbs or remove fuses then switch off the double pole main switch and lock it off, before I remove the main earth to measure Ze, then reconnect earth measure Pefc (L-E) and then finally PSCC (L-N) only then do I remove lock switch on main switch, then each cb/fuse.

 

[Richard3009]

hang on you are both saying different things one has an open DP switch the other has it closed

 

[electricAl]

I switch off all cbs or remove fuses then switch off the double pole main switch and lock it off, before I remove the main earth to measure Ze, then reconnect earth measure Pefc (L-E) and then finally PSCC (L-N) only then do I remove lock switch on main switch, then each cb/fuse.

That would be the textbook way.

Not saying i always lock off when i`m stood at the board, but no-one could fault this approach.

But equally it can be done with all live - just frowned upon by EAWR

Very bad Saveloy

 

[Richard3009]

Its nothing to do with risk Im wondering about the low readings through lamps

 

[zupos40]

hang on you are both saying different things one has an open DP switch the other has it closed

Different person do it different ways, some inspectors just switch off c.b or remove the fuses and connect 3 lead tester to the bar live busbar, neutral busbar and earth/cpc busbar, I do not, I prefer to switch the main switch off as previously stated.

 

[electricAl]

Its nothing to do with risk Im wondering about the low readings through lamps

Richard. Lamps don`t come into it. You are measuring the loop impedance of the L to N & the L to E fault paths external to the installation, then the meter calculates for you what fault current could be produced. Internal loads connected have no bearing on that whatsoever.

I think you need to re-read what has been written & have a deeper think about it.

 

[Richard3009]

surely the impedance between L and N is effected by a load like a lamp in the circuit when taking a measurement.

 

[electricAl]

Then just how would you be able to take a PSCC with the install isolated? That is the approved procedure.

I don`t think you could have read my last post mate. I explicitly said the loops being measured was external to the installation - that is, from point of measurement (supply side of incomer) to supply transformer & back again

Please.

 

[Richard3009]

Then just how would you be able to take a PSCC with the install isolated? That is the approved procedure.

By isolated I dont mean with the main incoming fuse removed im talking about the DP isolater in the CU, there is no hard and fast rule as to the state of this switch during the test?

page 57 fig 8 of GN3 shows load open circuit

 

[zupos40]

surely the impedance between L and N is effected by a load like a lamp in the circuit when taking a measurement.

If you have switched off all cb or removed all fuses and switched off the main switch, then no loads are connected.

 

[Richard3009]

Yes I realise that, my question is if the switch was closed would it impact the reading, sorry if Im being padantic but I like to FULLY understand things