Main fuse irregularly blows, why?

[Harry Bloomfield]

Is there still the potential that there is some kind of damage on the supply side which could be causing this?

Only excess voltage. You would need a recording voltmeter, or a peak recorder to detect that, but I would expect appliance damage.

 

[flameport]

in the past it was two different fuses.

The fact it's different fuses supplying different installations rules out problems with the cable(s) to the various different parts - if one of those was damaged, then it would be the same fuse every time.
That also eliminates any of the isolators for the various parts, or the consumer unit(s) for each installation. Final circuits within the installations are also unrelated.
The cutout is a relatively new one, grey GRP, so that almost certainly eliminates phenolic degradation as a problem.

Is there still the potential that there is some kind of damage on the supply side which could be causing this?

The only common item to all of the installations and the four fuses is the incoming cable itself - and the only plausible failure there is that the neutral is loose/failing, so installation current is returning via that nasty looking earth connection on the outer sheath.
This could be due to a loose neutral connection inside the cutout, or the cable itself is damaged somewhere along it's length, or at the junction point where it connects to another cable outside in the road.
If that earth connection and the neutral are both unreliable, this would cause overvoltage within the installations and therefore a significant overcurrent. Exactly how much and for how long would depend on the various loads connected at the particular time.
This would also explain the sparks/smoke from that earth connection as if the neutral was open at that point, all of the current would return via that earth connection instead.

Other symptoms of such a failure would be equipment damage due to overvoltage, but whether that occurs again depends on what combination of loads were connected when the neutral/earth goes open, and if the equipment was of a type which would be damaged by overvoltage. Generally electronic items would be the first to be destroyed, whereas items such as water heaters, ovens and the like would probably not be damaged at all.

 

[foxhole]

Had a random fault in a flat , pyro was used and a leak filled part of it with water, it passed over a steel under flimsy floor board ( poor design) when someone stood on one particular section, the floor board flexed , compressing the pyro just enough to push the standing water to a damaged area tripping fuse board. The bang cleared the water just enough until the next time.

 

[Murdochcat]

OP - what is your interest in this?

 

[ericmark]

It does raise an interesting question, will a smart meter auto report over or under voltage? My solar software does, and I assume EV charging software will also report it, but there is no reason why the smart meter should not report it, but does it?

 

[dishman]

OP - what is your interest in this?

I am a friend of the building owner. They have had this ongoing puzzling issue for 5 years.

This time they were away when it popped and I helped out by going there to let the DNO in to replace the fuse and witnessed it all.

So knowing about the knowledge base here I thought it would be a good place to muse what the possibilities were.

Thank you to all those that contributed.

The only thing that does not quite fit with the likely diagnosis @flameport made, is that there has never been any equipment damage each time this has happened. The "shop" supply has multiple computers which have been fine. The flat has all the usual stuff connected. From what I remember the mains fuses were all rated at 60A.

 

[ericmark]

If one lists the equipment and look at the amps, nearly every property can use well over the supply amps, I remember a problem with a caravan site, and we fitted an 80 amp moulded overload between the 100 amp fuse and the distribution board so it could be reset faster, but if I look at my own house there is the possibility to use well over the 60 amp supply fuse, but it has not happened, we have 5 bedrooms, two kitchens, and three bathrooms, but it has just not happened.

There are devices to switch off items when the load gets too heavy, this is common with EV charging, my own house will only heat domestic hot water either off-peak or with excess solar power. But 3 kW water heater and a 4 kW battery charger can mean overnight a 7 kW demand, add the dishwasher, washing machine and tumble drier, (3+3+0.8) = 13.8 kW or 60 amp. So it would only take one of use to forget and go for a shower, and the fuse could pop.

In the main, the dishwasher and washing machine only use 3 kW for a short time, to heat water, and the start timer is ½ or 1 hour increments to the time it was being set, so unlikely, they will all start together, and the water heater uses approx 14 kWh per week, both solar and off-peak, so each night it will not run very long.

However, with storage heaters, a common way to heat a flat, one could see a huge demand 11:30 pm until fully hot, and it would be easy to over step the mark.

We had the reverse where I worked, the factory used a lot of power 7 am to 6 pm but hardly anything at night, so storage heaters worked well, until someone switched one off, and then was cold in the office during the day and turned on a fan heater during the day.

 

[securespark]

I have just skimmed this, but could phase imbalance come into it at all?

 

[ebee]

I have just skimmed this, but could phase imbalance come into it at all?

That is quite a reasonable question

 

[martygturner]

if you look at the earthing arrangement there are two different connections, one from the sheath and one from a block... are you seeing a high resistance / lower resistance earth path as you state that sparks appear from the sheath connection. If you are getting a voltage big enough to spark the earth then has a TNCS been connected to an TNS for example when a fault appears eg load is applied.

 

[securespark]

Could there a PD between the two?

 

[ebee]

Another valid point.
A bloke once said to me "An Earth is an Earth is an Earth!".
Of course I contradicted him.
An Earth right here might not be the same protentional as an Earth Derived a few metres or so away. Therin can lie the problem.
By the time we increase our distances a bit that potential difference begins to stabilise and we soon get to a position where the additional difference is almost zero whether to the other side of the Earth or the other side of the Universe.

 

[martygturner]

Could there a PD between the two?

exactly... why do we bond external pipe work or in this case are there two main earthing arraignments, one with a spark inducing voltage and one without.

 

[SUNRAY]

I have just skimmed this, but could phase imbalance come into it at all?

The only way that would be an issue is if the neutral were lost, in which case both supplies would effectively be in series across 2 phases. The chances of a flat and a shop being balanced is extremely low, in which case one of them would be receiving substantially more than 230V.

An example would be the shop closed so low powered devices such as router, intruder alarm, fridge etc. let's say 500W worth and the flat washing machine and cooking dinner plus the indedentals; say a peak of 7KW worth would result in a current of 3.53A and 370V across the shop equipment and 208V between E and N star point.

However 3½A will not be blowing any fuses.

 

[bernardgreen]

Who is replacing the fuses when they have blown ?