High PFC.

[GBDamo]

I doubt there's any problem at all, but best to check to make sure. Do you have the part number of the RCBO's or a picture?

I used to run a 10mm earth alongside a 16mm T&E submain on the rare occasions I installed one. It helps keep Zs as low as possible, allows bonding to connect to the sub DB if needed, and a 6mm cpc just feels too small.

On the van in my notes, my legs have given up for the day but will update tomorrow.

 

[pc1966]

Is that PSSC measured between phases or from phase to neutral (replace phase with line if youre feeling modern)

That was measured L-N but I did not have enough time to be more systematic as access is difficult just now.

The building is a former sawmill, so our feed off the 800A bus-bar chamber fed by the dedicated substation they used to have. Now the building has been partitioned off and sections rented out, but for some odd reason they never made the switchgear a separate area so I have to get access via a slightly prickly guy who rents that section and is rarely there!

I don't know the details but if you're too close (electrically/conductor length, not physical distance) to the transformer then something about the transformer interferes with the measurement and leads to a false reading.
But I've only seen that when theres a few metres of big conductor between the tester and transformer.

I did a job last year where the Zs I measured at our new panel board (fed by 4 metres of 2x 150mm from a board in the room below which was fed from the DNO transformer in a room next door) using a high resolution loop tester was higher (slightly) than at the end of the outgoing circuits from it.

I know you tend to have Z dominated by L at the transformer, and there are other issues (that @Julie. knows far more about) relating to the impact of zero-sequence currents in a delta-star transformer action that can modify the observed PFC, etc. But as it seemed quite far out and I am beginning to mistrust my MFT as the sparky who did most of this original work was seeing 20% more than me at the other end with his MFT.

 

[davesparks]

That was measured L-N but I did not have enough time to be more systematic as access is difficult just now.


so I have to get access via a slightly prickly guy who rents that section and is rarely there!

OK, so PSCC L-L is going to be around 9kA (if my maths is right, I usually just measure it as my tester can) which is much closer to the quoted value from SSE, and if they are only giving one figure I suspect it will the L - L figure. If they quote two figures then it's usually PEFC and PSCC (L-L)



That's always the way, I think it's a fundamental law of nature that in these situations the tenant who rents the part of the building where the intake is will always be the grumpy awkward arse who is only there between 06:00 and 06:03 on the third Monday of months with 31 days.

 

[pc1966]

OK, so PSCC L-L is going to be around 9kA (if my maths is right, I usually just measure it as my tester can) which is much closer to the quoted value from SSE, and if they are only giving one figure I suspect it will the L - L figure. If they quote two figures then it's usually PEFC and PSCC (L-L)

It took me many weeks and attempts to get a figure out of SSE so getting clarification would be like pulling teeth without aesthetic.

My 5kA figure would translate to around 10kA for a "bolted three-phase fault" and that could be what they quoted. My MFT can also do L-L testing but time did not permit it.

That's always the way, I think it's a fundamental law of nature that in these situations the tenant who rents the part of the building where the intake is will always be the grumpy awkward arse who is only there between 06:00 and 06:03 on the third Monday of months with 31 days.

Indeed

The landlord has talked about making the switchgear in to a separate area which would be good. Also moving the plumbing nightmare (or at least partitioning it off) that was added less than 1m from the back of the electrics might be a good idea as well. But we will have to see if any of that actually happens.

 

[pc1966]

I don't know the details but if you're too close (electrically/conductor length, not physical distance) to the transformer then something about the transformer interferes with the measurement and leads to a false reading.

Thinking about that and looking at, say, the diagram in the IET design guide here:

In the event of a "three phase fault" as point (a) with a symmetric/balanced system the centre of the three line short would be a virtual neutral point due to all the currents balancing out, hence the PEN would have zero current and so the PEN impedance would have zero effect.

So the true worst-case PFC is computed from the assumption of a zero-impedance link between, say, L1 at point (a) in the diagram and the transformer star centre/ true Earth:

PFC0 = Uoc / |Zx + Zd|

However, the common thing to do is measure the PSSC L-N and double it, this would be:

PFC1 = 2 * Uoc / |Zx + Zd + Zpen|

Normally the assumption is that neutral and line conductors are the same impedance (i.e. Zpen = Zd) so we have:

PFC1 = Uoc / |0.5*Zx + Zd|

So PFC1 is under-estimating the effects of the transformer source impedance Zx in this simplified model of a three-phase transformer as 3 independent Thévenin equivalent sources and giving a higher fault current than the true PFC0 value.

But as you mentioned you can measure the line-line PFC and this is potentially more accurate as then you are measuring:

PFC2 = sqrt(3) * Uoc / (2 * |Zx + Zd|)

Rewriting this as PFC2 = (sqrt(3) / 2) * (Uoc / |Zx + Zd|) = 0.866 * PFC0

Hence PFC0 = 1.1547 * PFC2

In this case we don't under-estimate the transformer impedance, nor do we make the assumption that the PEN/neutral path is identical to the line conductors. As such this is probably the best way to measure the worst-case three phase fault current is to do line-line and multiply by 2/sqrt(3) = 1.1547

 

[pc1966]

Also just to point out that if |Zd| >> |Zx| (i.e. cable impedance much more than transformer impedance) which will almost certainly be the case any significant distance from the transformer then PFC1 is close to PFC0 (since 0.5*Zx can be ignored compared to Zd) and the "worst case = twice the L-N PSSC" is a perfectly OK method for computing the max break requirements for a three-phase supply.

What I am more uncertain about is the assumption above of 3 independent sources as a model of the star-connected transformer source.

 

[GBDamo]

Update,

Dodgy meter.

Retested today with 1721, the old dialog is being retired and in the market for a proper MFT.

These, are the double module RCBOs in the board,cant recall who was asking

Got the Electricity North West out monday to look at this dripping cutout.

This is nowhere near the worst I've come across but have been having to tidy it up just get to the earth before testing.

Out of interest how do you normally approach this?

Tidy it up during testing or quote to tidy then test? I'm ok on this as I'm on site on day rate.