Sizing MCCB & Generator for Fire Pump with a Soft Starter

[sw88]

Hi guys, basically my team had designed a system where the pump and control panel would be served by a 150A isolator. However, now the electrical contractor is suggesting that a 250A breaker is required instead.

Basically it is a 45kW (60HP) fire transfer pump, 3-phase, 415V, with a soft starter control panel.

The guy's input is that the FLC will be at 100A and the maximum starting current will be 350A. The isolator suggested is 250A. So is this guy's suggestion valid?

Regarding the problem about the generator, the generator is currently sized at 300 kVA and the fire pump proposed apparently requires a generator capacity of at least 300kVA.

45kW, with a power factor of 0.85, would mean that the pump has a FLC of 74A. How does one size properly a breaker with a soft starter?

So the issue now is whether my breaker and generator are sufficient?

 

[davidbeach]

Where, what codes? Typically a fire pump has to be allowed to run to destruction, no tripping at LRA. Things can vary when on a generator, but I'm not sure about the use of a soft starter unless it can carry LRA forever.

 

[sw88]

It's Singapore fire code.

 

[jraef]

Here in North America, the use of a soft starter to control a fire pump requires that the soft starter be capable of being bypassed and the motor started across the line (DOL) if the soft starter should fail. What that means is, having the soft starter cannot be used to change the basic circuit requirements for the feeder and generator ahead of it. I can't imagine Singapore having a less stringent approach, but even if then do that does not make it right from the standpoint of engineering prudence.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[KuanYau]

sw88 (Mechanical)

What is the Singapore fire code SSxx you are referring to?
Do you have a copy of the SSxx Code and read it?

Info: You can buy the Singapore standard SSxx on-the-spot in Singapore or "read" it, if you are a member of the library.

Some Singaporeans may be be able to chip in with some opinions on this matter.

 

[waross]

Caterpillar offers free generator sizing software. You may want to resize the generator. It looks a little large for a 45 kW motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sw88]

I do have access to SS 575 regarding the fire pump but my problem is electrical. I just want to know if the my current 150A breaker is sufficient for the fire pump. Currently, I've requested the starting current graph of the pump and the breaker's characteristic curve to see if it will trip. I guess only with detailed specs then I can make a better analysis.

 

[johannes60]

Hi all,

Singapore being a member of the IEC an IEC standard describing CB sizing may be used for answering the question. This would be IEC 60947-2 'Low-voltage switchgear and controlgear - Part 2: Circuit-breakers'.
Assuming that overload conditions during pump starting are limited to ~60 sec at typically 3 * I flc (common time limit and current setting on a soft starter) a 100 Amp breaker would be sufficient.
Typically the soft starter will monitor the load current and such provide over-current protection; the CB only needs to provide short-circuit protection.
If a contactor is used to bypass the soft starter after ramp-up over-current protection of the load has also to be provided by the CB's protection unit.
Usually a set of inverse time-delay over-current curves comes with the CB's protection unit so the one matching the pump's thermal damage curve best may be activated.

Johannes

 

[Parchie]

@Johannes,
The OP is talking about two things here: a fire pump and a soft-starter. Do you have any link where IEC specifies how fire pumps should be powered? AFAIK, fire pumps are to be designed to run to destruction because tripping these motors could mean success or failure of a fire fighting activity-->meaning, lives/properties are at stake!

 

[krisys]

As a thumb rule, the breaker size will be 250% of the motor full load current (FLC).

Thus, for the motor having FLC = 74 Amps (45 kW, @ 0.85 pf),
the expected breaker size would be 2.5 x 74 = 185 A.
The offered 250 A rated breaker should be adequate. This is based on the Direct On Line (DOL) starter.

Since the soft-stater would reduce the current, the suggested breaker rating should be adequate.

Prima facie, the 300 kVA generator appears to be good to handle the 40 kW motor starting. However, for the enhanced performance, you shall ask the contractor the following:

a) Explore the possibility of supplying the higher (say 400 kVA) alternator, without increasing the engine rating.
b) Selecting the alternator impedance parameters (as low as possible)
c) Proper tuning of AVR and governor responses to improve the voltage and frequency recovery during the motor starting.

In view of the small rating of the motor, all the above measures may not be necessary. These are the possible ares to attack.