Fire Pumps

Seems an overkill to use ACB for protecting a fire pump motor. ACB are usually >630A with a high SC breaking capacity which may not be needed for such an application. All you need is probably a motor starter, rated for the motor kW or HP. In some parts of USA, i believe that fire pumps should not have overload protection, you need to check on the local regulations.

What is the motor rating ? Where are you located ?

The requirement is in UAE where NFPA codes are followed. I'm talking of a fire pump rated 100KW for which the locked rotor current requires a 1250A circuit breaker.

Many clients prefer diesel-drive fire pumps for the simple reason that the thing needs to run irrespective of whether there is electrical power or not during the fire. Diesel pumps run until either:

• The fuel tank runs empty, or
• An operative operates a manual shut-down device on the engine, or
• Self-destruction

whichever is the soonest.

Guest: Not sure if you are in Canada or the USA but section 32 of the electrical code for fire alarm systems and fire pumps will be a great help, rule 32-206 and 32-210 should help you out.

Jayman.

Located in Abu Dhabi. Tell me if there is any problem in using 1250A ACB (the local code demands for ACB it for the ratings higher than 800A) in place of 1250A MCCB for the circuit feeder protection for a fire pump with locked rotor around 1050A.

In my experience so far, the Contactor that is used to switch on a motor has an inherent ability to withstand the locked rotor current of that motor for a period longer than the starting time. A relay which is used to protect such a motor against overload, single phasing (and of course locked rotor) will have an appropriate trip curve to allow the LRA to flow until the motor reaches running speed.

Starting current (DOL) = Locked Rotor current, should not be more than 8 times the rated current, and should last, at best, about 6 secs. Why do you want to select a switching device based on this LRA? If you HAVE to use an ACB due to local rules, you can select one as per the FLA (Full load amps) and set the release as per the overload curve to allow starting without nuisance tripping.

You must be aware that there are motor starters available for much higher motor powers - even 900 kW.

An ACB is not usually used for motor switching. A contactor is best suited for that, with a life of millions of operations. An ACB has a life of 25000 operations at best.

Thanks for the explanation. The CB selection is based on the NFPA 20 - 9.2.3.4 'the overcurrent protection device shall be selected or set to carry indefinitely the sum of the locked rotor current of the fire pump motors...' . i'm aware of the fact that fire pump controllers have starting means and the transfer switches and cables are rated only for the full load amps.

My understanding is fire pump feeder protection breakers are not for over current , only for short circuit.

You seem to be getting confused a bit.

Not any manufacturer is allowed to manufacture fire motor pump controllers.

Only those manufacturers who submit a sample controller to say NFPA fulfilling all requirements of the code are certified to manufacture it and these manufacturers do have latest requirement and strictly comply to those as a minimum rather than the specifications being given by you .

The fire pump motor controllers (NFPA standard) I have used had standard functional control circuitory in addition a hand operated lever is there to bypass all interlocks and safeties to essentially run the motor in an emergency.

When I write the specifications I mention it that the controller must be from a certified manufacturer (NFPA) in strict accordance with the latest code requireqirements.

I never induldge in MCCB or ACB and never ever been disappointed.

This has bee covered before in here.

link

By the way, how do you get a motor with a LRC of 10X FLC? Are you confusing LRC from magnetizing inrush current? LRC is usually around 6X FLC. But to be honest even if it were 6X, I never interpreted that statement to mean that a 75HP motor will require a 600A circuit breaker!