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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.
 
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.
 
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
 
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.
 
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.
 

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