Selection of MCCBs for Jet Fuel Hydrant Starters

"There are also intermittent trips that occur on the MCCBs whose cause we have not established."

Hellllooooo?

Start with the obvious my friend. MCCBs are not contactors, they are only designed to open under load in emergencies and be capable of resetting ONCE. It is assumed that the cause of the trip is going to be corrected. Multiple trips essentially exceeds the design duty of the device.

If you have eliminated all possible causes of short circuits and you are confident that the MCCBs are not tripping on thermal current, then the most likely remaining possibility is that you have the instantaneous magnetic trips set too low for the application. Some newer energy efficient motors can draw upwards of 20X FLC in initial energization when the windings are establishing the magnetic field. This only lasts a cycle or two, but it is sometimes high enough for long enough to cause the magnetic trips on MCCBs to activate. Were you in the US, our code now allows the instantaneous trips to be adjusted as high as 17X FLC if it is proven that an application will nuisance trip without that high of a setting. I cannot speak for Nigerian codes however,

You may need to replace those MCCBs now either way.

Hi hi,

Thanks for your suggestion.We have eliminated all possible causes of short circuit currents in our system.we have replaced the MCCBs with motor rated fuses of 350A and they are holding now for two consecutive days without fusing.We are also studying the settings of the MCCB s and we shall revert on our findings by next week.

It also worthwhile to note that we have a similar system at Moi international airport in Mombasa Kenya but runs on auto transformer starters unlike the star delta starting method employed at JKIA, Nairobi that is giving us problems.

Certainly VFDs/VSDs are the solution as opposed to cycling the pumps and we shall be budgeting for it in our next years financial budget. But in the meantime,we need to keep going.

I covet your comments and more suggestions,

Murwa

After checking out JRaef's suggestions first (they seem the most likely), you might want to look at how frequently the pumps are starting/stopping. I would think about installing some VFDs, modulating the speed to match the demand rather than cycling the pumps frequently.

In addition to the 400-440v fluctuation, can you tell if there are large voltage spikes and dips?

MCCBs? Eh?

The only installations seen here use motor-rated fuses. Now there's a hint!

LOL, over on this side of the pond and especially here in the wild wild west, the distance to the nearest supply house is sometimes greater than the distance across your entire country. We tend to use circuit breakers if we can and fuses only when a circuit breaker won't do the job...