Delaying a distribution panel load

[Rabalac]

Hi,

I need some advice. I am an electrical engineer in charge of a telecom facility electric distribution system. We have a generator that cannot manage the start up load of the facility.

It was suggested that a time delay be used to bring the load online 3 minutes after the generator has started to run. However, I have problem with mechanism the electrical contractor is proposing. The contractor is proposing to use a normally open 320Amp contactor and a time delay relay. My concern has to do with the use of a normally open contactor. I fear that if the normally open contact fails it will disconnect power from the distribution panel feeding the load with power. Quite frankly I would prefer a normally closed contactor. So that if the contactor failed it would fail close thus still keeping power on the panel.

Does anyone know of any good way that the distribution panel maybe delayed using a normally closed contactor or any other feasible solution.

 

[davidbeach]

Latching contactor? Bigger generator? Building control system that starts loads sequentially rather than simultaneously? All sorts of possibilities.

 

[waross]

So rather than lose part of your load if the contactor fails, you want to lose all your load when the device fails in the closed condition and the generator can't start? The MTB of normally open contactors is excellent, failure is rare. I don't know of any normally closed contactors for 320 amps. About the largest that I have seen is a 30 amp relay with normally closed contacts.
But all is not lost. Consider a breaker with a motor operator.Add a shunt trip to trip the breaker before the generator starts and then reclose the breaker with the motor operator.
I don't have numbers but I would think that the standard contactor may have a slight edge on the breaker for reliability if the breaker is being switched often.
My first solutio may be to take David's suggestion;

Building control system that starts loads sequentially rather than simultaneously?

This approach may be cheaper and more dependable.

Bill
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"Why not the best?"
Jimmy Carter

 

[Rabalac]

David,

Ideally a bigger generator would be the best option.However, that option is not available. The sequencing of the load is actually what we are doing.

What is ampacity of these latching contactors

 

[ScottyUK]

Have a look at ABB's R-Series contactors. They are the old fashioned shaft type with ratings up to about 2000A per pole, and are available NC. Personally I think using NC is a bad plan, but I have a sneaking suspicion that there is a magnetic latching variant of the R-Series which could be what you are looking for. Bill's idea of using the breaker - which you probably already have - with a retrofitted shunt trip or NVR is also a good idea.


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