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.