3 Phase power keeps on tripping

[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