Certainly possible - for single phase it's just a matter of a better PE than N (e.g. due to parallel paths on the PE side - e.g. bonding to extraneous-conductive-parts shared with installations closer to the substation for instance).

3-phase SSC would of course be higher, so there would need to be a more dramatic difference - but still possible in a few cases.

Of course it might be useful to show that the PEFC is lower (e.g. because RCDs don't have the same breaking capacity).

   - Andy,

Andy,

I struggle to believe that a bolt 3 phase short circuit between all phases could be smaller in magnitude than a fault between any phase and earth

Thats why I refer to a 3 phase DB and not a 1 phase DB where I know that there are cases where the fault between L and E is higher than the fault between L and N

Cheers

But you're taking L-N and doubling it - so if there's a reduced N in the supply somewhere (reduced N cables were quite common a few decades back - I guess there's still rather a lot still in service) - L might have rather less than half the impedance of your assumptions, so a small PE impedance might still bring the total within the same ballpark.

   - Andy.

In some industrial installations the parallel paths from various bits of metalwork: structural steel, pipe work, containment etc provides a much lower impedance path than the actual conductors.

There will be cases when the earth path is a lot lower than the neutral loop - indeed there are places where supply neutral does not arrive at all, and any 230V supply is derived from L-L by local transformer.

And the other way about - if the earth impedance is unusually high compared to the neutral loop, then there may be a need for interlocked earth fault trips.
In effect PEFC is a Zs test revisited.
Large TT 3 phase supplies are far from unknown once you get out of sub-urbia.

Mike.

I know we measure the prospective short circuit current between L1,L2,L3 and N and then double the highest reading

Or, more accurately, establish Ipf between lines and divide by 0.87. Mind you, from my experience, its all fruit machine numbers anyway. As I understand it, the standard MFT instrument simply divides the measured resistance (rather than impedance) into 230. If you are out even by a small margin in the measurement of resistance, the resulting difference in KA can be substantial.

A candidate on the 2391 last week measured 0.33 ohms and 700A on his brand new Megger X, (the new tester that has little electronic diagrams to tell the operator how to conduct each test, heaven help us!) The new 1741 owned by the centre showed 0.26 ohms with an Ipf of nearly 900A. The impedance difference was only 0.07 ohms but the resulting current difference was substantial. The test rig is well into the installation, so you would expect a reasonable degree of accuracy at the higher resistance values than you would at the real intake position with a 1MVA tx in the next field. 

Why should we waste time with prospective earth fault current?

The Ipef should be taken with the earthing system connected so should be greatest at intake. So as you move to a single-phase board downstream and you know the PN and PE KA values at intake, you may choose to set aside duplicate re-testing at the SP board providing the Icn values of the OCPDs are grater that the values obtained at intake..

lyledunn said:

The Ipef should be taken with the earthing system connected so should be greatest at intake. So as you move to a single-phase board downstream and you know the PN and PE KA values at intake, you may choose to set aside duplicate re-testing at the SP board providing the Icn values of the OCPDs are grater that the values obtained at intake

Important to note, though, that unlike Ze, in most installations you can't calculate the prospective earth fault current at a distribution board in the same way, because of parallel earth paths.

Any calculation would have to ignore R2 (or Z2 if CSA of cpc > 16 sq mm, or SWA cables are used) to give you "worst-case conditions".

The resulting calculation will give you a reading higher than the actual prospective earth fault current ... sometimes much higher.

The alternative is measurement, but that's not accurate either !

0.33 ohms and 700A on his brand new Megger X, .. snip...

1741 owned by the centre showed

0.26 ohms with an Ipf of nearly 900A.

That is not a million miles out from identical ;-) 

230/0.33 is 696

230/0.26 is 885

0.33 and 0.26 are  both 0.295 +/ 0.035

i.e. within 12%.

Actually at least for the few MFT designs I have seen inside, because really they look at voltage droop during application of a test load to deduce supply impedance, it is closer to a reading of absolute magnitude of total Z rather than resistive part - measuring that requires accurate I/V phase comparison and that is not so easy.. A

The situation is not helped,  because the test currents are  set small enough not to trouble the lightest ADS (or RCD) it is trying to deduce things by multiplying up from very small changes in the measured voltage numbers, so if other loads are also making the supply voltage fluctuate, the readings can be quite amusing.

A mechanical equivalent is something like trying to open the door from the hinge side, where a very small change in fulcrum position alters the forces massively because the lever length is too short.

And then there is the problem of extra contact resistance in the test loop - quite often a scale and polish of the 'nana' plugs and the probe ends can find a few tens of milliohms.

'Fruit machine' can indeed be an accurate description. In some ways a test lamp and an 'is it earthed at all' test finds  the most serious errors just as reliably, it's just not a very safe test to perform.

Mike

The certificate states that the Nature of Supply Parameters can be obtained by measurement or enquiry, National Grid guidance says:

Maximum prospective short circuit current

The maximum prospective short circuit current is used to determine the short circuit rating requirement of electrical equipment.

Low voltage connections (up to 1000V)

The following values can be assumed at low voltage connections:

• Single phase connection; 19.6kA*
• Two phase, split phase or three phase connection; 25.9kA

https://connections.nationalgrid.co.uk/information-for-electrical-installers/

So you can write 25.9 kA on the certificate without testing, but that may be problematic.

I struggle to believe that a bolt 3 phase short circuit between all phases could be smaller in magnitude than a fault between any phase and earth

I think I know where you're coming from. If you have a bolted 3-phase fault it should in theory be perfectly balanced - so no N (or PE) current. From the point of view of each phase it's like having a L-N fault onto an artificial N point right at the point of the fault - so while the L conductor contributes some impedance to the loop, the N (or PE) contributes nothing at all - so it's like a L-N fault with a perfect zero-Ohms N (or PE) conductor back to the source. Any number of parallel paths still isn't going to reduce R2 entirely to zero.

On the other hand say we had a reduced N in the supply - so let's say R1 = 0.8Ω and Rn = 1.2Ω (round numbers for ease of calculation rather than being realistic) - you might measure L-N on each phase as 2Ω/115A and double it to 230A. But in reality (unbeknown to you) the fault current on each phase for a bolted fault would be closer to 230V/0.8Ω = 287.5A and so R2 could be anything up to 0.2Ω and still give a higher fault current.

As has been said, "establish Ipf between lines and divide by 0.87" would be better but that does require an instrument that suitable for 400V use (or 460V if it happens to be a splt-phase supply) - which not all instruments are.

   - Andy.