3 Phase power keeps on tripping

[Daveh36]

Hi Guys any advice on this one will be a major help.
Was called out to a loss of power turned up expecting a breaker down as usual but not the case.
so here goes the problem- an apartment block with 315 amp incoming fuses feeding 400 amp mccb set to trip at 220. This in turn feeds a rising busbar with several tap off units and 250amp mccb feeding the apartment fuses and meters. No faults show when tested out but every night it trips taking out the 400amp mccb and all three main fuses.
Anyone any ideas on why this is happening. Please help as at my wits end with this one

 

[westward10]

Taking out all of the three main fuses suggests overcurrent as opposed to a fault. Sounds like you need to put some monitoring in place as well as asking if someone has recently installed an item of three phase equipment which is tipping the balance. EV chargers?

 

[Daveh36]

Taking out all of the three main fuses suggests overcurrent as opposed to a fault. Sounds like you need to put some monitoring in place as well as asking if someone has recently installed an item of three phase equipment which is tipping the balance. EV chargers?

Sorry meant to to add that I was originally thinking overload. Fitted some monitoring and max load was around 200amps. But it did register a huge spike of 1900amps but only for a duration of 2 cycles

 

[pc1966]

A crude check using Schneider's on-line tool confirms that a 220A MCCB has limited selectivity with a 315A fuse but it ought to be able to protect against typical overload scenarios depending on model/settings. Below shows that model and guessed settings could be selective to the 2kA surge you report

However a 2kA surge should take at least 1s to blow those fuse, i.e. about 25 times the period your monitoring shows!

 

[UNG]

Hi Guys any advice on this one will be a major help.
Was called out to a loss of power turned up expecting a breaker down as usual but not the case.
so here goes the problem- an apartment block with 315 amp incoming fuses feeding 400 amp mccb set to trip at 220. This in turn feeds a rising busbar with several tap off units and 250amp mccb feeding the apartment fuses and meters. No faults show when tested out but every night it trips taking out the 400amp mccb and all three main fuses.
Anyone any ideas on why this is happening. Please help as at my wits end with this one

Is the supply to each apartment single or three phase and how many apartments are there

Sorry meant to to add that I was originally thinking overload. Fitted some monitoring and max load was around 200amps. But it did register a huge spike of 1900amps but only for a duration of 2 cycles

So does this happen at roughly the same time everyday and what voltage do you have when it occurs

 

[pc1966]

Just to add to @UNG questions:
Have you actually seen it go when the monitoring was on it?

Did that 1900A / 2 cycle event cause anything to trip?

 

[Daveh36]

Hi Guys thanks for the responses
To update and answer the other questions.
Yes all apartments are single phase total of 66 split over the phases.
The three main fuses blew at the same time as the monitored spike. It has been tripping and blowing the fuses at a similar time ever day this week but strangely didnt trip last night Saturday

 

[marconi]

New apartment block or a few years old? Just started happening? What space and water heating - off peak time switched electric? EV chargers of 7kW or upwards and are they time controlled to use off peak leccy? Fresh water pumps topping up roof top tanks overnight? Phase currents and voltages balanced? Are phase to N PSSC similar? Are time switched loads all turned on at the same time perhaps and all drawing current until each load starts to randomly cycle on/off ie no staggering or sequencing to promote diversity?

 

[Daveh36]

New apartment block or a few years old? Just started happening? What space and water heating - off peak time switched electric? EV chargers of 7kW or upwards and are they time controlled to use off peak leccy? Fresh water pumps topping up roof top tanks overnight? Phase currents and voltages balanced? Are phase to N PSSC similar? Are time switched loads all turned on at the same time perhaps?

Apartment block about 8 or 9 years old. Had a couple of issues in the past. The feed in question only serves the apartments water pumps lifts etc on a different incomer

 

[pc1966]

Hi Guys thanks for the responses
To update and answer the other questions.
Yes all apartments are single phase total of 66 split over the phases.
The three main fuses blew at the same time as the monitored spike. It has been tripping and blowing the fuses at a similar time ever day this week but strangely didnt trip last night Saturday

Any voltage surge/outage then?

I'm wondering if something upstream is causing voltage to cycle off/on and surge killing it?

Still, it suggests the MCCB is not set to be selective with the fuses, and it still is not enough of a surge for 2 cycles to take out those fuses. What is the max current the monitor's CT can actually record?

 

[pc1966]

Yes all apartments are single phase total of 66 split over the phases.

Hmm, that is 22 per 315A fuse / 220A MCCB feed which is around 14.3A/10A per property which is quite small unless they are all small flats.

If all had something like a fridge motor on and you tried to switch simultaneously I can see it taking that out!

To me it looks like supply surge but no idea why it is doing it, or something is really arcing but after a couple of occurrences that would be vaporised. Still, it ought not to be taking out the supply fuses so info on the incomer MCCB and its settings would be of interest.

 

[pc1966]

Actually if the surge was really big, so CT saturated, you would see fuse and MCCB go no matter what. But also looked at some other curves for 315A BS88 fuses and pre-arcing is around the 2kA point so they could well have been melted in ~2 cycles by something in the 3kA region.

 

[marconi]

For pc1966

If it is a similar time each day - your post #7 - my money would be on radio/internet or time controlled overnight electric space and water heating all coming on at the same instant - (and perhaps some series resonance effect along supply, busbars and loads causing the circa 2kA (or more?) couple of cycle surge current).

An estimate

66 flats but say 50 of them turning on a 3kW storage heater heater element (say) = 50 x 3 = 150kW. 50kW per phase.

Inrush current is about 1.5 times normal current for a second as cold elements warm up.

50 x 1.5 = 75kW.

75000/230 = 326A

So a surge of 330A per phase for every 3kW of off peak heater load in 50 flats.

So if a storage heater and immersion in each flat this would be a surge of 660A.

And on top of prevailing load current of all 66 flats - assume 50 x 0.5kW per flat = 25kW or 8kW per phase = 8000/230 = 36A - call it 40A per phase.

Communal lighting 66 flats x 25W = 1.65kW - ignore.

Peak surge current 660 + 40 = 700A ish (or more) for one second.

pc1966 - How would 315A fuses and 400A mcb set to 220A trip cope with 700A for one second?

 

[pc1966]

For pc1966

If it is a similar time each day - your post #7 - my money would be on radio/internet or time controlled overnight electric space and water heating all coming on at the same instant - (and perhaps some series resonance effect along supply, busbars and loads causing the circa 2kA (or more?) couple of cycle surge current).

It certainly is suspicious that all go at once and regularly. Unless they have suicidal rats in the busbar chamber that like to go out at the same time of day!

pc1966 - How would 315A fuses and 400A mcb set to 220A trip cope with 700A for one second?

The MCCB "depends" as they can be set to all sorts of curves, so more information is needed.

But what bothers me is the 315A fuses ought to have blowing times of 5s at 2kA (Fig 3A3(c) in the regs, page 369 of BBB) and to go in 0.1s needs something like 5.4kA

So the recorded 2-cycle surge of 1.9kA ought not to bother the fuses at all, though it could trip an electronic-curve MCCB depending on settings.

So with 22 flats on each fuse, either they are running a lot of current for long time (so the 220A set MCCB would have gone) or they are managing to all draw ~150A surge at once, which seems unlikely. I would be more inclined to suspect some big 3-phase load comes on but is faulted, though it seems nothing like that is in the flats!

TL;DR: The observed figures don't add up to me.

 

[snowhead]

Right no-ones mentioned it yet so I'll just have to bring up the subject of,,,, "Smart Meters".

Are any / all on "Smart Meters"?
Have there been any recent upgrades downgrades to "Smart Meters" in any apartments? (thinking about Off Peak times now being set to identical switch times)

Is the heating Central boiler or individual Electrical heating?
And is the hot water central source or individual?

 

[pc1966]

Certainly it is possible that a "smart" time-switch is doing it by switching many locations simultaneously but it still looks rather odd.

A check on valid instrument range and more logging would be useful, as well as more details of the MCCB and its settings. At the very least the MCCB ought to be able to protect the fuses against overload to 2-3 times nominal current if correctly set.

 

[R-fur]

I fitted quite a few radio teleswitches when they first came out in the early 80s. They had a random time delay built in so that once they recived an on signal there was a delay before the contacts closed, this was to soften the bump to the network. Dont know for certain but I would think smart meters have something similar.

 

[Lucien Nunes]

What can we infer about the fact that the replacement fuses don't blow at the moment the system is re-energised? Whatever is causing them to blow doesn't immediately kick back in after the outage.

 

[pc1966]

What can we infer about the fact that the replacement fuses don't blow at the moment the system is re-energised? Whatever is causing them to blow doesn't immediately kick back in after the outage.

That is a good point, it seems unlikely then it is simply switch on to normal loads. Also the time-switch idea seems to be pushing it for lots of per-flat loads.

Some photos of the incoming arrangement and how it is then distributed would be interesting, along with a bit more info of how it is split to flats (e.g. Ryefield board(s) or various MCCB sub-boards, etc) and any other locations.

 

[marconi]

Carrying on from my #13 and thinking about LN's post. I return to the effect caused by block wide bulk time-switched overnight space and water heating.

From what PC1966 said in response to my query fuse rupture does not then seem likely to be due to the simultaneous switch on surge of 50 flats worth of 3kW load each.

Noting that all three fuses rupture and the flats are all single phase it may well still be something all flats have in common like their space and water heating. For the first period after switch on (first hour at least) it may be a storage heater (3kW) and immersion heater (3kW) are all energised and thermostats closed; after a few hours thermostat starts cycling which will/would reduce average load per phase of incomer. Until this happens current per phase could be......

Worst case: 66 x 6kW = 396kW or 132kW per phase. 132000/230 = 575 A per phase.

With background load of 40A per phase as estimated before, current per phase could be 575 +40 = 615A

315A BS88 type gG at 615A ruptures after about 700-800 seconds or 13 minutes. see page 5:

https://espm.co.uk/BS88 fuse info.pdf

We do not know trip time delay for 400A mccb set to trip at 220A. It may not be set up correctly either which seems likely if 3 fuses rupture before mccb trips.

So apartment block has a problem with overcurrent in the guise of overload.

Replacement of the fuses shortly afterwards do not then suffer rupture until following night because new fuses are cold and thermostats are now cycling so mean phase current is less than there rated current. The once nightly epoch of overload has passed.

 

[R-fur]

Despite what I said above about teleswitches having delays, it is possable that as this is a block of flats that there is only one switch controling an off peak contactor in each house. That would switch on all the off peak supplies at the same time but does not explain Lucians point about why dont the fuses blow when replaced.

 

[marconi]

Pc1966 - I am pondering what the OP said about the trips and fuse ruptures ‘at the same time as 2kA spikes’ - see OP’s #7. You mention these spikes ought not to trouble the fuses unless perhaps the spikes occur as the fuses begin to rupture, arc and open circuit - all short term events at and after rupture not before them. BS88 gG fuses are current limiting. The inductance of each line’s circuit carrying high phase current generating a forward induced emf of high voltage attempting to maintain the phase currents at these amperages. These line currents increase briefly markedly - a spike - during the rupture phase but are limited by the fuses limiting properties as they open circuit and extinguish the arc? Could this explain the 2kA spiking?

 

[pc1966]

It is all rather odd. The first issue is the fact both MCCB and fuses go, typically if well designed that means a very big fault, i.e. so the MCCB let-through is above the fuses pre-arcing I2t and so even though it trips, the fuse is already a goner.

Now we don't know the MCCB type or settings in any detail here, so that current where selectivity is lost might be as low as 0.8kA or so if bandly designed...but the time duration of the 1.9kA as 2 cycles or 0.04s is not enough to explain the fuse going unless it was already really hot.

However, the currents prior to that were only around 200A so fuses would be effectively cold then?

So did the monitoring get something wrong? Has it some upper limit where the CTs saturate and really the surge was really closer to 5-10kA for 2 cycles?

I guess there might be somewhere that has something causing a major fault current such as water getting on to terminals or similar and it going all arc-flash as typically a L-anywhere arc are migrates so you end up with bolted 3-phase style fault. But surely someone would notice the sound and/or damage? Doing that a couple of times I would expect to burn off metal and make it permanently shorted or open enough to stop.

The OP has not said what time it is tripping at, maybe that would give us more ideas?

 

[marconi]

My think-piece does not explain 3 fuses rupturing at the same time. Even if passing an overload current for minutes it would be surprising if they ruptured at the very same time. The OP may not have been perfectly accurate in this regard - the fuses ruptured about the same time maybe.

I am veering more toward a cabling or busbar fault which is producing a short-circuit or earth fault or both. What about treeing in feeder and sub main cables? I am no expert nor have any experience of it but seem to remember it can be made worse or a cable fault event triggered when the cable warms up and cools down regularly which is a particular problem for well loaded underground cables in winter. Even newish cables can be prone to treeing if they have manufacturing defects.

Electrical treeing - Wikipedia - https://en.wikipedia.org/wiki/Electrical_treeing

 

[marconi]

Might the 2kA spike be caused by supply transformer tap-changing in response to overnight mains voltage sag when electric heating is switched on in the neighbourhood? Perhaps tap change is not swift and clean. At night few flat dwellers would notice if it was not. Tap change causes a significant voltage and current step upwards.

 

[UNG]

Apartment block about 8 or 9 years old. Had a couple of issues in the past. The feed in question only serves the apartments water pumps lifts etc on a different incomer

This makes me a bit curious if the past history has a bearing on this fault
So what were the past issues you mention and for how long has the current fault been occurring

 

[Daveh36]

Thought I'd post an update on my head ache. For some reason the power stayed on from Friday until 6.00 pm tonight that is and as usual mccb tripped as well as taking out all three incoming fuses. Still cant find any reason for this so was just wondering. Is there any way that this could be a supply fault from the substance by any chance ? Or am I just clutching at straws now ?

 

[marconi]

What was the previous issue you mentioned? How far away is the sub-station? What business or industry is nearby? Is it electric water and space heating - I assume it is since you have not said otherwise. Do any tenants report their lighting sagging/swelling or flickering because of voltage disturbances? Has it only just started to happen or been a problem that has become worse over time? Any major water leaks, vermin infestation, flooding, ingress of rain, fires, building damage near switchgear, busbars and cabling since which this problem has occured? Might you rig up power analysis monitor again at incomer? Solar pv on the roof? Electric vehicle chargers? Are nearby premises suffering similarly? What are phase-neutral voltages during day and at night?

It sounds like the time of outage is now earlier than before but is it always in the evening and overnight?

Suggest you contact the DNO to find out if they know of any network/supply faults in the vicinity and to inform them of this problem.

 

[marconi]

Rats can gnaw cables:

There’s a Rat in the Switchgear, what are we going to do? | BASEC - https://www.basec.org.uk/news/2018/5/8/theres-a-rat-in-the-switchgear-what-are-we-going-to-do/#:~:text=Regular%20cable%20such%20as%20PVC,sheathing%20itself%20may%20be%20attacked.&text=Choosing%20the%20right%20cable%20can,help%20the%20rat%2Dproof%20outcome.

Rats are nocturnal and most active at dusk and dawn as they set out to forage and return. In the UK rats prefer to nest in places that are warm, secure and with easy access to food and water. Rats need regular access to a water supply, unlike mice who can, if necessary survive on just the moisture in their food. See:

https://www.rentokil.co.uk/blog/where-do-rats-nest/#:~:text=Rats are nocturnal and most,buildings in search of food.

They also need material to make nests; the sheath of swa cables for example.

 

[UNG]

It is difficult to see from the OP's profile what level of experience they have, and this is possibly one of those faults that requires a bit more on the ground in depth analysis of the issues surrounding it
I'm beginning to wonder why the OP isn't answering some of the questions being asked when they may be pertinent to identifying the problem
If a PQA has been monitoring the supply is this long term monitoring and are there any trends that can seen that may give any indications as to the timing of the fuses blowing / MCCB tripping
With all the bad weather recently especially the heavy rainfall has that had any effect on the frequency of fault occurring

 

[R-fur]

Is this a private network or is the DNO coming out to change the fuses? If the fuses belong to the DNO Iw ould expect them to start charging or refuse to replace them until the customer side issue is resolved.
As per UNG why wont the OP answer direct questions?

 

[marconi]

I have just read some posts in the iet forum about mice finding their way into trunking, busbars and switchgear and ‘setting up home’ with baby mice appearing in due course because the TV reception was lousy.

It might be an idea to just open up the switchgear and trunking to see what might be going on Inside.

 

[Daveh36]

I have just read some posts in the iet forum about mice finding their way into trunking, busbars and switchgear and ‘setting up home’ with baby mice appearing in due course because the TV reception was lousy.

It might be an idea to just open up the switchgear and trunking to see what might be going on Inside.

Hi Guys sorry been a tad busy trying to sort this out to reply often enough but I'll try answering some of the questions .
The 400amp MCCB which incidentally has fried today is a Siemens see photos.The supply is from it's own substation and the dno inform me it's a clean supply with 400 amp fuses. I questioned any spikes on their side and they ruled it out although they fitted monitoring on it tonight. The previous issues I've been told about are that a tap of unit had been replaced on the 11th floor a few weeks ago not sure why at this stage and am making enquiries as to why. Going to strip that out tomorrow just to be on the safe side as dont know who fitted it. Really am just clutching at straws now.

 

[marconi]

Daveh36. Thank you for the update on this interesting to me yet fraught for you problem. Looking at the voltage and current plots it seems to me that there is a spike of voltage upwards to 272V immediately followed by a spike in current in all three phases. I wonder if the problem is on the incoming supply side ie causing the spike upwards of 22V but it is troublesome to the block's electricity installation because this very short spike when presented across a very low surge impedance inevitably results in a momentarily very high surge current in all three lines. I mentioned before it may be some quirky trick of fate that the supply impedance connected in series with the building wiring surge impedance are resonating at one of the frequencies excited by the spike. You know that at series resonance the current is only limited by the copper Ohmic resistance which may be very very small because the block is close to the supply transformer and only fed by a short cable. With such large currents flowing there will be significant short term overvoltages which may lead to flashover between exposed copper conductors and effectively a three lines short circuit until the fuses rupture.

Why the voltage disturbance happens may be because of an upstream transformer tap change to regulate network voltage.

I think you could sensibly press the DNO to investigate this observation. The coincidence of the voltage spike, on all three phases and generally overnight at the same time. pc1966 mentioned you should check the peak current specification of the current clamps and analyser because the recorded spike may well be much higher than 2kA.

 

[pc1966]

Looking at those plots, yes there are odd voltage spikes, but in the grand scheme of things they are negligible. I don't see this is a DNO problem.

Looking at the current waveform then clearly it is not overload or too little diversity, this is a major 3-phase short taking place somewhere, with a hint the Y phase kicked off and then its friend joined it. I'm still not convinced the logger has the true peak as ~1.7kA or so won't take out MCCB trip and 3 * 315A fuses, but if it was really 5-10kA then it makes sense.

You need to inspect the outgoing circuit's cable and various DBs as a couple of faults like that is going to paint the inside with copper vapour!

What & why is a mystery, but I can't see anything other than a major short/arc event explaining it.

 

[pc1966]

The 400amp MCCB which incidentally has fried today is a Siemens see photos.

A few high-current faults and that happens, contact life can be as low as a few dozen and max rated breaking. It points to the faults being way over 1.9kA though.

 

[Daveh36]

Daveh36. Thank you for the update on this interesting to me yet fraught for you problem. Looking at the voltage and current plots it seems to me that there is a spike of voltage upwards to 272V immediately followed by a spike in current in all three phases. I wonder if the problem is on the incoming supply side ie causing the spike upwards of 22V but it is troublesome to the block's electricity installation because this very short spike when presented across a very low surge impedance inevitably results in a momentarily very high surge current in all three lines. I mentioned before it may be some quirky trick of fate that the supply impedance connected in series with the building wiring surge impedance are resonating at one of the frequencies excited by the spike. You know that at series resonance the current is only limited by the copper Ohmic resistance which may be very very small because the block is close to the supply transformer and only fed by a short cable. With such large currents flowing there will be significant short term overvoltages which may lead to flashover between exposed copper conductors and effectively a three lines short circuit until the fuses rupture.

Why the voltage disturbance happens may be because of an upstream transformer tap change to regulate network voltage.

I think you could sensibly press the DNO to investigate this observation. The coincidence of the voltage spike, on all three phases and generally overnight at the same time. pc1966 mentioned you should check the peak current specification of the current clamps and analyser because the recorded spike may well be much higher than 2kA.

Thanks for that info. I've replaced the MCCB again this morning and re- energised and now that the DNO have monitoring I'm hopeful of a resolution although I'm expecting my phone to ring at about 2.00ish in the morning. I'll keep you updated when I get some results back from my monitoring or the DNO

 

[plugsandsparks]

This is indeed very odd. It looks like a typical direct short but suspect very high current (higher than recorded).
What is odd and has been mentioned you would expect the fault to still be there.
Not sure how many apartments there are and how doable this would be but.
If you could isolate all the apartments and switch off the main incoming isolator and IR test to see if there is residual evidence of L-L, L-N and L-E resistance such that it is low for normal expected result but high enough for you to replace the fuses until the next event.... e.g. you may get one reading that is significantly different to the rest
Keep us posted, its a good one

 

[pc1966]

Thanks for that info. I've replaced the MCCB again this morning and re- energised and now that the DNO have monitoring I'm hopeful of a resolution although I'm expecting my phone to ring at about 2.00ish in the morning. I'll keep you updated when I get some results back from my monitoring or the DNO

Really this looks like a bigger fault inside the installation!

What does the MCCB feed?
How is it cabled?
How are the flats fed from the main supply?

However, if you are going to inspect stuff please make sure the supply MCCB is off! You really do not want to disturb something with a history of high kA faults as it can lead to life-changing or life-ending injuries!

 

[Daveh36]

Really this looks like a bigger fault inside the installation!

What does the MCCB feed?
How is it cabled?
How are the flats fed from the main supply?

However, if you are going to inspect stuff please make sure the supply MCCB is off! You really do not want to disturb something with a history of high kA faults as it can lead to life-changing or life-ending injuries!

Think I'm a little wiser than to work with out turning the MCCB off but hey.
There are 66 apartments spread over the 12 floors. Rising main with tap off box ever other floor feeding fuse cabinets for the apartments. Which are in turn fed via split concentric to individual boards. Bear in mind these were fitted around 9 years ago so no surge protection. Interestingly though I was informed today that the apartment block next door it one had power outage yesterday as well might be coincidental but who knows

 

[pc1966]

Think I'm a little wiser than to work with out turning the MCCB off but hey.

Its a public forum, we can't assume anything!

I would rather risk hurting someone's pride but underestimating them than have to live with overestimating somone by not saying something that ultimately led to an injury.

There are 66 apartments spread over the 12 floors. Rising main with tap off box ever other floor feeding fuse cabinets for the apartments. Which are in turn fed via split concentric to individual boards. Bear in mind these were fitted around 9 years ago so no surge protection. Interestingly though I was informed today that the apartment block next door it one had power outage yesterday as well might be coincidental but who knows

It would take a very high level of lightning hit to trigger an arc, and this is not a country that sees that every day!

I'm guessing the flats are all on 60A fuses and if so they have total selectivity with 100A or more, so anything downstream of those fuses cannot be responsible for blowing the incoming 315A fuses.

So from my armchair view it comes down to a fault the rising main or in one of those boxes on the supply side of the flat's fuses.

Or there is some secret unknown big 3-phase load somewhere!

 

[pc1966]

Incidentally have you been able to speak to folks in various apartments? Has anyone heard a loud noise at the time things tripped out?

Might just help get to which floor(s) are close to a fault.

 

[Spaldingsparks]

someone hasnt got a funny kind of farm in the flats have they?

 

[marconi]

pc1966: I don't feel so ready to dismiss something on the DNO side of the fuses because the timing and regularity of the outages hints at something happening during the quieter hours or lower load periods - so something being done for technical reasons and necessary during these periods and regularly. My money now is on a DNO overvoltage mechanism triggering a busbar arc fault perhaps at an insulator defect. (I discovered by the way high current arcs between busbars tend to move away from their point of initiation due to the usual electrodynamic forces at play.) As ever I could be wrong...

I got myself immersed in this problem to avoid the miserable news about what is happening in Ukraine. This is what I read this afternoon:

https://www.studiecd.dk/cahiers_techniques/Fault_arcs_on_busbar_sets_and_switchboards.pdf

 

[pc1966]

pc1966: I don't feel so ready to dismiss something on the DNO side of the fuses because the timing and regularity of the outages hints at something happening during the quieter hours or lower load periods - so something being done for technical reasons and necessary during these periods and regularly. My money now is on a DNO overvoltage mechanism triggering a busbar arc fault perhaps at an insulator defect. (I discovered by the way high current arcs between busbars tend to move away from their point of initiation due to the usual electrodynamic forces at play.) As ever I could be wrong...

Very true the regular timing of the fault suggests something is a trigger for it.

I can't see high over-voltage (i.e. 8-10kV as mentioned in the article) being the cause as otherwise we would have lots of flat residents complaining of damaged electronics. Still, it would be a good idea to consider having some decent-sized SPD fitted to the incoming supply (e.g. a "type 1+2" and some 100A fuses/MCCB or similar for fault protection).

But the idea that DNO changes are causing electrodynamic forces in a busbar somewhere is very intriguing to me. I was pondering on how you could have a system working perfeclty for a period and then BANG! go the incomer fuses. I was wondering if the rises or one of the fuse cabinets was adjacent to a lift or something that was occasionally hitting it? But maybe there is a loose bar and DNO tap-changing is making it flap?

I got myself immersed in this problem to avoid the miserable news about what is happening in Ukraine. This is what I read this afternoon:

https://www.studiecd.dk/cahiers_techniques/Fault_arcs_on_busbar_sets_and_switchboards.pdf

Yes, a very depressing event all round

 

[Lucien Nunes]

At this point I would be camping in the riser listening carefully around fault time.

 

[pc1966]

At this point I would be camping in the riser listening carefully around fault time.

I wonder if you could get a few long-lasting audio recorders set up in strategic locations?

Easier than camping out for a day or so!

 

[marconi]

At this point I would be camping in the riser listening carefully around fault time.

I would arm myself with a spanner or screwdriver, as required, and some electrician's gloves and goggles and then open up the riser and switchgear and simply have a look with my eyes during daytime!

 

[Daveh36]

someone hasnt got a funny kind of farm in the flats have they?

My first thought lol as come across lots of farming but wouldn't account for it

 

[marconi]

Very true the regular timing of the fault suggests something is a trigger for it.

I can't see high over-voltage (i.e. 8-10kV as mentioned in the article) being the cause as otherwise we would have lots of flat residents complaining of damaged electronics. Still, it would be a good idea to consider having some decent-sized SPD fitted to the incoming supply (e.g. a "type 1+2" and some 100A fuses/MCCB or similar for fault protection).

But the idea that DNO changes are causing electrodynamic forces in a busbar somewhere is very intriguing to me. I was pondering on how you could have a system working perfeclty for a period and then BANG! go the incomer fuses. I was wondering if the rises or one of the fuse cabinets was adjacent to a lift or something that was occasionally hitting it? But maybe there is a loose bar and DNO tap-changing is making it flap?


Yes, a very depressing event all round

I was not clear enough. I meant the electrodynamic forces act on the arc - Fleming's left hand rule. This is illustrated by some time-lapse photos in the reference in one of my earlier posts.