Fire Pump Power Supply Configuration

[guiyermo]

Hi everyone,

The reason I am consulting you is to ask if you know if there is any limitation or restriction given by NFPA 20 or NFPA 70 (NEC) for the power supply configuration of an electrical driven fire pump; I mean if it matters if it is Wye solidly grounded or Wye grounded with a NGR.

I have read these two standards and I have not found anything but I am not 100% sure.

In our design there is a 460 V 250 hp motor (fire pump) and a 460 V 5 hp (jockey pump) fed from a 1250 kVA 13.8/0.48 kV transformer Delta-Wye. The question will be if it matters if the Wye side is solidly gorunded or limited through a 5 A NGR - is there anything that rules this? In any case a ground fault protection will not be installed as per NFPA requirements.

Thanks and regards,

 

[7anoter4]

The core principle of Art. 695[NEC] is the fire pump motor must run no matter what — because it exists solely to protect the facility and people who work there. So keeping that fire pump running is mandatory, regardless of the consequences to the pump or its circuits. Saving the fire pump or its conductors is a hollow victory if you lose personnel and/or the facility.
As you said "in any case a ground fault protection will not be installed" .The both motors have to be delta connected and no grounding.
The wye side of the transformer may be grounded in order to limit the undamaged phase's voltage to ground but as no ground protection will be provided a suitable resistance has to be inserted between neutral point and ground in order to limit the fault current .The non live metallic parts of the motor will be grounded.

 

[rbulsara]

As far as I know, there is no restriction on whether the system is solidly grounded or resistance grounded. Ungrounded would be an issue, unless all requisite for an ungrounded system are met.

I have always seen and specified grounded sources.

 

[dpc]

I'm not aware of any NFPA fire code restrictions, but the NEC Article 250 puts restrictions on where high-impedance grounding can be used.

I would never implement HRG on any 480 V system in a commercial building. Only industrial facilities with an in-house electrical maintenance crew.

And never never ever use **low-resistance** grounding at 480 V.

"The more the universe seems comprehensible, the more it also seems pointless." -- Steven Weinberg

 

[ScottyUK]

7anoter4,

The both motors have to be delta connected and no grounding.

If there's no bond between the motor star point and the system neutral or earth, which would be the case in any normal star-connected motor, why would this matter? I'm curious because I see the occasional NFPA-compliant system destined for an overseas plant and wonder if I've been overlooking something during design review.


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If we learn from our mistakes I'm getting a great education!

 

[7anoter4]

I think the indicated starting method for a fire pump motor is DOL as -for the very low frequency of starting-the reliability of DOL is higher than VFD. In this case a star-delta controller may be inserted then the motor will be delta connected at the end. I think that is NFPA 20 requirement.

 

[ScottyUK]

Ok, I guess that makes some sense - I didn't realise that Y/D start was mandatory. Thanks.


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If we learn from our mistakes I'm getting a great education!