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.

 

[Karl Atkins]

The logical approach of take is to think about what might cause a cutout fuse to blow without any mcb operating.
So either a fault between the cutout and the DB, am overload, or loose connections/overheating of the cutout.
Therefore randomly removing accessories is a waste of time and the customers money.

Why measure the PFC and PSCC? Did you carry out any other tests?

They weren't randomly removed, they were removed starting with according to the customer where they were and what they had been doing when the power was lost, then followed by exterior accessories that may have defects. I'm here looking for help, what other tests would you suggest, I haven't seen anything like this before as been used to new installation work on new builds until laid off recently and started self employed.

 

[Karl Atkins]

The logical approach of take is to think about what might cause a cutout fuse to blow without any mcb operating.
So either a fault between the cutout and the DB, am overload, or loose connections/overheating of the cutout.
Therefore randomly removing accessories is a waste of time and the customers money.

Why measure the PFC and PSCC? Did you carry out any other tests?

They weren't randomly removed, they were removed starting with according to the customer where they were and what they had been doing when the power was lost, then followed by exterior accessories that may have defects. I'm here looking for help, what other tests would you suggest, I haven't seen anything like this before as been used to new installation work on new builds until laid off recently and started self employed.

Taken off all accessories in a place like that ? That would take me a week I reckon. Wouldn't even consider that in the event of a phase loss.

Overload would be my first thought at that place and I would check the load balancing and take some measurements.

What bearing do old coloured cables have on this ?

Someone asked if anything new had been installed recently that's why the old colours were mentioned. I get that's it's likely an overload but why then do no mcb's/rcd's not go before DNO Fuse?

 

[Karl Atkins]

The logical approach of take is to think about what might cause a cutout fuse to blow without any mcb operating.
So either a fault between the cutout and the DB, am overload, or loose connections/overheating of the cutout.
Therefore randomly removing accessories is a waste of time and the customers money.

Why measure the PFC and PSCC? Did you carry out any other tests?

They weren't randomly removed, they were removed starting with according to the customer where they were and what they had been doing when the power was lost, then followed by exterior accessories that may have defects. I'm here looking for help, what other tests would you suggest, I haven't seen anything like this before as been used to new installation work on new builds until laid off recently and started self employed.

Taken off all accessories in a place like that ? That would take me a week I reckon. Wouldn't even consider that in the event of a phase loss.

Overload would be my first thought at that place and I would check the load balancing and take some measurements.

What bearing do old coloured cables have on this ?

Someone asked if anything new had been installed recently that's why the old colours were mentioned. I get that's it's likely an overload but why then do no mcb's/rcd's not go before DNO Fuse?

 

[Pete999]

The logical approach of take is to think about what might cause a cutout fuse to blow without any mcb operating.
So either a fault between the cutout and the DB, am overload, or loose connections/overheating of the cutout.
Therefore randomly removing accessories is a waste of time and the customers money.

Why measure the PFC and PSCC? Did you carry out any other tests?

What Dave said

 

[davesparks]

They weren't randomly removed, they were removed starting with according to the customer where they were and what they had been doing when the power was lost, then followed by exterior accessories that may have defects. I'm here looking for help, what other tests would you suggest, I haven't seen anything like this before as been used to new installation work on new builds until laid off recently and started self employed.

That still sounds pretty random to me, what was the logical reason for removing those accessories and how did you expect it to help you diagnose the problem?
What I’m asking is what fault you think is likely to occur in an accessory which could cause the cutout fuse to blow without causing an mcb to operate?

With regards testing I’d probably have measured the current in each phase to get an idea of the background load on the installation.

 

[Pete999]

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)

 

[Andy78]

Someone asked if anything new had been installed recently that's why the old colours were mentioned. I get that's it's likely an overload but why then do no mcb's/rcd's not go before DNO Fuse?

Ah, didn't see that comment about additions.
Why would you expect a single final circuit device to operate when the problem you may be looking for is accumulative overload on a phase ?

 

[westward10]

In normal operation the only thing to cause a fuse to rupture is excessive current. This will be caused by overcurrent or fault current. If you can eliminate any fault current issue between the fuse and the point of utilization that leaves overcurrent. The best answer came immediately in #2.

 

[Lucien Nunes]

Two likely causes - intermittent short-circuit between cutout and board or total load is too high. A short that has seen off three 100A BS1361s will have left some pretty clear scorch marks and metal splashes that will speak for themselves unless the tails go through conduit or underground.

That leaves overload. Have you measured the supply current on the affected phase with all normal equipment in use? If not, why not? I would do that within 1 minute of arrival at site...

BTW it doesn't matter that it's three phase when looking at an overload scenario, because all the load is single-phase. If the only symptom is that fuse blowing, the problem is confined to that phase. You will only be interested in the others if the affected phase turns out to be overloaded, in case there is capacity to spare on them that you can utilise. Being unbalanced at the moment will not of itself cause a problem.

 

[marconi]

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.

 

[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.