electric supply to fire water pump 11

[jacphi]

i have a controlgear rated for 1200A,480v. one of the feeder is a fire water pump.having a FLA of 240A(locked rotor current 1500A Approx. i am extracting the local std requirement for fire pump unit package.
"QUOTE"
The protection schemes for Fire Pump Unit (FPU) packages shall comply with NFPA 20 and NFPA 70. In general, these standards require that the motor be allowed to operate under locked rotor condition continuously and is only automatically disconnected under short-circuit conditions.
Exception :
The exception to these requirements is that where a FPU package is fed from a bus that has ground overcurrent protection on the incomers or supply transformers, the feeder to the FPU package shall have ground overcurrent protection, and all other feeders connected to the same buses or transformers shall have ground overcurrent protection.
"UNQUOTE"

my question is will this controlgear be sufficient to supply the locked rotor condition mentioned? does the exception above mean that by providing groung fault protection i do not have to comply with require that the motor be allowed to operate under locked rotor condition continuously. please comment.

 

[buzzp]

My feeling is no it will not be sufficient. Rated 1200A and motor Locked rotor is 1500A. The idea is if you have a fire you do not want the motor to be shut-off in the event of an overcurrent. The fire is more important than the motor. I do not see how installing ground current protection will eliminate the potential need for the motor to run under locked rotor currents. I am certain others will have additional info to add or corrections to make to my assumptions.

 

[buzzp]

My feeling is no it will not be sufficient. Rated 1200A and motor Locked rotor is 1500A. The idea is if you have a fire you do not want the motor to be shut-off in the event of an overcurrent. The fire is more important than the motor. I do not see how installing ground current protection will eliminate the potential need for the motor to run under locked rotor currents. I am certain others will have additional info to add or corrections to make to my assumptions.

 

[peebee]

I agree with buzzp.

And if you have other loads on the switchgear (which it sounds like you do), then your situation is even worse.

And to compound the problem even further, there's a voltage drop limit in NEC 695.7 of 15% during starting conditions. There's a really good chance your gear and feeders and upstream transformer would impose a more severe undervoltage than 15%.

 

[uno]

Hold on guys. Do not forget that we are talking about an abnormal condition when the motor may be required to run at locked rotor condition. Refer to NEC Section 695. Even the feeders which provide power to the fire pump are sized per the full load current and not per the locked rotor current.Only the overcurrent protective device will be selected & set to carry indefinitely the locked rotor current.

The exception may be related to providing a ground fault protection which is also not required as per NEC.

It may be too early to write off the 1200A Controlgear.

 

[jbartos]

Suggestion to the previous posting:
The feeders to motors are sized higher than the motor FLA, e.g. 125% of FLA by NFPA 70 NEC.

 

[jbartos]

Suggestion: Adhere to current NFPA 70-2002 National Electrical Code Article 695 Fire Pumps

 

[buzzp]

Hmm, if the OCPD is sized for LR then should not the rest of the control gear be sized for the same (ignoring what the NEC says)? Seems like a potential safety hazard to me. If the OCPD does not shut down at LR then the equipment could potentially run indefinately with LR or near that. Seems like sparks and arcs (fire) may be the result. Maybe I am missing something here. What explanations are there for this reasoning?

 

[jbartos]

Suggestion to the previous posting: The intent of such design is to have the pump functioning as much as possible regarding additional fire hazard posed by the fire pump electrical installation run. Apparently, the fire pump is assumed to do more "good" than its feeder being on fire doing more "bad."

 

[buzzp]

Yes I understand the need to keep the pump running but why create another fire with undersized equipment?

 

[Sparkyonline]

Reposted from response at alternate listing for this subject:

No. The exception deal with the disconnecting portion of overcurrent protection. In other words, if you have the ground fault protection, you are 1) required to have it present at all the locations listed, and 2) now allowed to have the ground fault protection automatically disconnect in the ground fault instance and not only in the short-circuit instance.

With this requirement, the locked rotor "operation" could easily exceed the capabilities of the gear in question, even without considering the other loads involved.

Consideration should also be given to emergency requirements for this service.

Also, some locations will allow you to have separate service for the fire pump and associated equipment.

Hope this helps.

 

[Difference]

According to my knowledge LRC is a static condition before the rotor of the motor starts moving. The LRC will last on until the motor has reached full speed and go down to rated current.
Full load current FLC is 240A Means motor will start with about 6 x rated currrent.
Why should a 1200 A rated switchgear not being able to start a 240 A rated current. Also the starting current is a little higher. This is quite normal. All breakers, bus bars and components are designed for continous current. Starting a fire pump will last < 10s.

 

[jbartos]

Suggestion to buzzp (Electrical) Oct 17, 2003 marked ///\\Yes I understand the need to keep the pump running but why create another fire with undersized equipment?
///Yes, this is an excellent question with no easy answer. Apparently, the smaller size of fire pump motor feeder and inadequately protected with respect to the immediately upstream located protector is viewed as significant cost and labor savings.\\\

 

[jacphi]

with all the sugestions posted i am convinced that the feeder breaker to the fire pump should be rated for 1500A with short circuit and groung fault protection. the only doubt still remaining is the rating of the controlgear,1200 or 1500A!!!!!!!!!!

 

[jbartos]

Suggestion to the previous posting: Would you clarify what you mean by &quot;controlgear&quot;? The feeder to the fire pump shall be rate 125% of the pump motor full load amps (FLA), i.e. 1.25 x 240A = 300A. Obviously, the higher ampacity fire pump motor feeder, the better. The 125% of FLA is a statistically determined number. The actual fire pump installation should have evaluated the motor feeder voltage drop, if the feeder is long, e.g. over 100 feet or so since the motor Locked-Rotor Amps (LRA) do also count.

 

[jacphi]

controlgear is a term used for motor control centres(MCC).

 

[uno]

jacphi

What are the other loads on the MCC Bus. Are these continuous or intermittant loads?

 

[jacphi]

some continuous some intermittant..there are 5 different feeders on this MCC.

 

[buzzp]

Difference,
I am certainly no expert on fire pump controls but I know these controls should allow the pump to run no matter what, hence the locked-rotor current is the maximum it can draw (not just on start-up) under virtually any condition. So under whatever circumstances (single phase, undervoltage, etc..) the pump has to run. Hence, the discussion on locked-rotor current is not just for start-up conditions.

 

[barac]

The motor will only last around 30 secs running at locked rotor current before it burns out and is useless anyway!