3 phase domestic installation issues

[Karl Atkins]

Hi, I have been out to a property 3 times in the past week to a country pile with a 3 phase supply that keeps blowing it's 100amp distributor fuse.

The property has a pool house, a large garage with power, stables with power and a wooden annex type space also with power.

I have taken off every accessory in this huge place and can't find any obvious issues.

At the board I have measured a PFC of 1.19 kA and a PSCC of similar.

Nothing trips in the board when the dist fuse goes, I've not done a lot of work with 3 phase so unsure where to go from here.

Any advice much appreciated.

 

[Deleted member 105166]

Is there also a washing machine on the "Tumble Dryer" circuit? Just that 40A is large for a tumble dryer on its own, unless it is a commercial unit (which I suppose it could be, at a 6-bed equestrian property). Also investigate what appliances are connected to the 32A ring final on the yellow phase. In fact, run the tumble dryer, water heater and whatever appliances are connected to the RFC with a clamp on the yellow phase and I'd wager you'll find the answer.

 

[Lucien Nunes]

There are two 32A rings labelled 'laundry' on red phase so I expect the washing machines are on those. 40A is not unreasonable for a big dryer that will take horse blankets, but it looks like a later addition, as is another unlabelled 40A MCB that might be a distribution circuit also on yellow. Blue has the cookers and some water heating, red has more water heating, the laundry and two further rings although not the kitchen one.

I'm kind-of surprised it's eating 100A bullets, I've seen worse load assignment efforts.

 

[Karl Atkins]

Is it the Yellow Phase 100A fuse which ruptures? The tumble dryer mcb(40A) and water heater mcb(40A) are both on the yellow phase. If they are both on, it does not leave much margin for everything else on this phase. Time to get out your trusty ampclamp.

No it's the red phase

 

[telectrix]

then the clamp meter comes to the rescue. get it in there.

 

[Deleted member 105166]

Just a thought - in the OP, stables and pool house are mentioned, yet there is no mention of these on the visible circuit labelling, although there are some blanks...
Have you confirmed that the three phases come straight into the board pictured and there aren't Henley blocks upstream splitting the red phase to another single phase board you are unaware of, powering outbuildings? When you said it was the red phase burning fuses, this got me thinking, as more often than not if someone also wanted a single phase supply, by default they'd use L1. You can see this at events venues regularly, particularly older installations, where you have a 63/3 with a 63/1 or 32/1 on L1 adjacent.

 

[Lucien Nunes]

Indeed - it's hard to see how there's enough load on L1 in that board to cause trouble unless all the rings are maxed out. I was thinking the unlabelled 40A on yellow could be a submain to an outbuilding db carrying pool load etc, but red, not so. Only one unlabelled 20A.

 

[marconi]

Using these diversity figures for a hotel I found quickly at:
Calculating Maximum Demand | Voltimum UK - https://www.voltimum.co.uk/content/calculating-maximum-demand-0

Lighting @ diversity of 75%
Heating & Ventilation @ diversity of 80%
Water Heating no diversity
Socket Outlets five 30 A ring circuits @ F.L. 1st circuit & 50% of other circuits

For red phase - maximum demand estimate

64 + 32 + 8 + 1 + 16 + 20 = 141A

3 rfc - 2 laundry and grd floor n02 3 x 32A = 96A
32 + 16 + 16 = 64A

7 lights 6A 7 x 6A= 42A
42 x 0.75 = 32A

1 lights 10A 1 x 10A= 10A
10 x 0.75 = 8A

1 fan convector/shower pump 6A 1 x 6A=6A
6 x 0.1 = 1A

IH 16A 1 x 16A=16A
16 x 1 = 16A

MID con panel boilers 20A 1 x 20A=20A
20 x 1 = 20A

 

[Midwest]

You might wanna help this chap;
SWA For meter tails? - https://www.electriciansforums.co.uk/threads/swa-for-meter-tails.141517/page-4#post-1375026

 

[Karl Atkins]

Thanks to everyone for the replies I've arranged to revisit this Friday to check the loads with clamp meter, I'll update with the findings.

 

[dmxtothemax]

Your best option is too analyze the load balancing,
You may be able to better spread the load over the three phases.
It may look like a big board ?
But 100A three phase should be adequate.

 

[Toneyz]

Data loggeron the mains supply, take current readings with a decent clamp meter, IR testing, Infra Red Temperature Measurement at critical points on the installation (main fuses, Tails, etc)

What he said is one of the meter tails warm by touch?

 

[Karl Atkins]

Hi,

I returned to the property today, was greeted by the gardener who told me that a couple of weeks ago he found a cable whilst laying some slabs that had tape wrapped around it. obviously I asked him to show me. Tape looks just a couple of weeks old, after disconnecting the tails at the DB in the pool house control room and running a continuity test to prove this was indeed the cable with the mysterious tape around it I then performed a IR and low and behold got a reading of zero between red phase and earth & red phase and neutral.

 

[westward10]

Interesting what was the circuit protection for this cable.

 

[Karl Atkins]

100A, plan to downgrade on customers say so

 

[westward10]

This cable was under protection by the fuse which kept rupturing and nothing else.

 

[mattg4321]

100A, plan to downgrade on customers say so

Seems odd that a cable buried underground and taped up would be protected by a 100A fuse only? I was expecting it to be an old outside light/socket some diy bodger had done away with, on a <32A MCB

 

[westward10]

Is the cable actually a dead short because the fuse, if that was all that protected the cable would have ruptured immediately assuming the impedance was low enough which it must have been at some point it must have been.

 

[Deleted member 105166]

On your photo of the main board, earlier in the topic, which circuit supplies the pool house DB?

 

[marconi]

The OP reports he measured the resistances using an IR meter. 'Dead short' could then be many Ohms. I wonder if the main current flow at this fault is Lred to terra firma or locally through terra firma to N and E.

 

[mattg4321]

OP gone awol.

Maybe he's unscrewing all the accessories again just to make sure he hasn't missed anything

 

[Karl Atkins]

None of them supply the pool house, the pool house is supplied from a separate isolator in a shed which is situated in the garden behind where the supply comes into the property.

 

[Karl Atkins]

None of them supply the pool house, the pool house is supplied from a separate isolator in a shed which is situated in the garden behind where the supply comes into the property.

 

[Deleted member 105166]

None of them supply the pool house, the pool house is supplied from a separate isolator in a shed which is situated in the garden behind where the supply comes into the property.

Is this an entirely independent single phase supply, with its own DNO incomer, or is it as I described in post #30?

 

[Karl Atkins]

All 3 phases split off to 4 different isolators, one is for the house, another is a 3 phase board supplying the stables, a workshop and the shed they are in, another to the pool heater and finally one to the pool house which has another 3 phase board.

Is this an entirely independent single phase supply, with its own DNO incomer, or is it as I described in post #30?

 

[SC0USESPARK]

main fuse a cartridge fuse? if so have you split it?? break it open.. if the element has melted, usually pieces still attached to the caps its an overload... if its disintegrated then short circuit....