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?