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

 

[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