MCB Tripping

[pchan12]

I have a rather unusual problem. I live in an Apartment which has a backup diesel generator (Third world - so erratic power supply). So we have 2 sets of circuit breakers one for the default power supply (25Amps) and one for the backup generator (6 Amps Type-C). The backup is to be used only for "light" appliances and so has a lower threshold.

When the main power is cut, the diesel generator takes about 30 seconds to come online during which there is no power after which we get power from the diesel generator. When this happens we sometime observe the backup MCB trip for certain apartments. This needs to be manually corrected. After the "flip" we don't observe further trips. The tripping happens only during a switchover and only intermittently so it is not deterministic.

One suspicion is "inrush" or "surge" current exceeding the Type C MCB trip curve. When the backup MCB is upgraded to 10A the trips are not observed. But we don't feel comfortable upgrading to 10A as the rating is only for 6A. Also it still doesn't explain how it doesn't trip as appliances should send surge current when the trip is corrected. Maybe some other factor causing it ?

How do we approach the problem ? Look at internal wiring in the individual apartment or faulty appliances, measure inrush current ? Does anyone have any pointers or insights ?

Please redirect if this is not the right forum, I chose this as it seemed to fit...

 

[BradtheRad]

backup MCB trip for certain apartments.

Did those apartments have air conditioners and refrigerators running at the moment of blackout? General advice says to wait a few minutes before turning them back on. The reason is because the compressors have liquid refrigerant in their cylinders during operation. If you try to re-start the compressor too soon again after shut-off, the liquid refrigerant resists being compressed. The motor has trouble overcoming this extreme load, thus it draws much greater startup surge current.

If you wait a few minutes after loos of power, the refrigerant turns into gas which is easily compressed, and causes less startup surge through the motor. Does it take a few minutes for you to reset the breakers manually? Then that could explain why you don't have further repeats of the problem.

 

[KlausST]

Hi

To detect the cause you need to measure and record the current.

A more perfect solution could be to bridge the 30s power loss with a battery powered UPS. 30s is not a big deal.

Klaus

 

[pchan12]

Did those apartments have air conditioners and refrigerators running at the moment of blackout? General advice says to wait a few minutes before turning them back on. The reason is because the compressors have liquid refrigerant in their cylinders during operation. If you try to re-start the compressor too soon again after shut-off, the liquid refrigerant resists being compressed. The motor has trouble overcoming this extreme load, thus it draws much greater startup surge current.

If you wait a few minutes after loos of power, the refrigerant turns into gas which is easily compressed, and causes less startup surge through the motor. Does it take a few minutes for you to reset the breakers manually? Then that could explain why you don't have further repeats of the problem.

Yes many of those apartments have refridgerators. Infact I am planning on moving the refridgerator out of the backup load. The power comes back in 30 seconds, the trip reset is done manually so that takes approximately 5-7 minutes. So this could be the reason. Some apartment don't have the refridgerator and they report trips too - the suspect equipment for them are chimney and led light setup.

 

[pchan12]

Yes many of those apartments have refridgerators. Infact I am planning on moving the refridgerator out of the backup load. The power comes back in 30 seconds, the trip reset is done manually so that takes approximately 5-7 minutes.

One option is using a voltage regulator as a power-on-delay for the refridgerator which buys me a few minutes. I will try this before removing the fridge from the DG load.

 

[KlausST]

Hi,

But I would need a much more powerful UPS for the entire setup to support fans/lights etc for hours.

As said: not for hours, just for the given 30 seconds until the diesel generator is up.

Klaus

 

[pchan12]

As said: not for hours, just for the given 30 seconds until the diesel generator is up.

Thanks for the insight. To support the whole apartment, I would need ~1000 watts (230 v). For a few minutes runtime, UPS options (Lithium ion / reputable brand) are expensive (> $600 + Taxes). I am not sure if I can buy a lower rated wattage UPS (say 600W/750VA) because my runtime requirement is lower. Besides, I have a mini-ups solution just for my TV/Router/NAS. I want to avoid the lead-battery inverter setup due to maintenance/noise.

So I will exercise this option as the last resort.