fire pump transfer switch basics

Power Transfer Swtiches

- for -

F.M. Approvals- by -

James S. Nasby

Columbia Engineering

C.E. F.M. Approvals 2

Topics to be Covered

Arrangement I (Combo. Unit) Arrangement II (Separately Listed Units) Power Supply Considerations Options


Note: “Alternate Source” may be a “Hot Emergency Bus” as often employed in hospitals.

In these systems, multiple MVA size gen-sets are used along w/ additional upstream Transfer Switches with a path to one or more Utility Connections.

This often results in very high fault currents on the Alternate Source Side.

Also note that two C/Bs will be required in all Arrangement I units per 2010 Edition of NFPA-20.

Two Arrangements


Power Supply Systems

Power Supply Weak = Voltage Drop Problems Stiff = Careful Coordination of WIC

Type of System Wye = Typical

Grounded Neutral = Typical Ungrounded = Care & Coordination

Delta = Great Care Needed. Ungrounded Corner Ground 4 Wire High Leg (240 / 120 / 208)


Options (Varations)

Added Ground Lugs (Some Jurisdictions)

Fourth Pole (Neutral) Usually “Overlapping” (Make Before Break) Supplied by Some E.F.P.C. Mfrs.

In Phase Transfer Delays Transfer Until Motor Back EMF Phase Angle

is Close to that of the Intended Source Completely Useless for Pump Motors due to Very

Low Inertia and Large Minimum Load on Motor


Arrangement I (Combo. Unit)

10.8.1 General10.8.2.1 Arrangement I

(Listed Combination Fire Pump Controller and Power Transfer Switch)

10.8.3 Power Transfer Switch Requirements


Arrangement I – Combo. Unit


Short Circuit (WIC)

Coordination

Note: “---” means no Upstream Protection is Required.

E.g.: Self Protecting (Blow Apart) Isolating Switch Provides Suitable Protection of the Internal Transfer Switch.


Arrangement II (Separate Units)

10.8.1 General 10.8.2.2 Arrangement II

(Individually Listed Fire Pump Controller and Power Transfer Switch)

10.8.3 Power Transfer Switch Requirements


Russelectric F.P. Transfer Switch


Arrangement II Requirements:Who Supplies What?

Section 10.8.2.2(2) "An isolating switch, or service disconnect where required, ahead of the normal input terminals of the transfer switch" 10.8.2.2(3) [and 9.2.3.4]: Xfer Switch OCP "shall be selected or set to carry indefinitely the sum of the locked rotor current of the fire pump motor" plus any additional connected loads. E.g.: 600% of motor FLA minimum10.8.3.4.2: transfer switch be sized at only 115% of the motor FLA current10.8.3.6.4: Phase Reversal Sensing


WIC Coordination ExampleTransfer Switch Sizing Example for 100 Hp at 460 Vac

X 115% Thermal X 600% Breaker WIC Coord.

Motor Hp Motor FLAThermal Sizing Upstream Size Switch Size

100 Hp 124 amps 143 amps 150 amp 744 amp 800 amp 400 amp

In this Example, a 150 Amp Xfer Switch meets the 115% criteria; but, it’s Short Circuit (WIC) Rating will be invalid if the upstream device is larger than 250 amps. However, a 250 Amp OCP doesn’t meet the 600% criteria. Hence, a larger Xfer switch is needed. Who does the coordination, and who supplies what?


Are All of These Supplied? 1) The two Position Indicator lights 2) The Engine Start delay timer and the Re-Transfer delay timer, 3) The Engine Start contacts, 4) The Test (Transfer) switch (pushbutton), maybe. 5) The Normal Side Service Disconnect and overcurrent protection fuses or breaker, 6) Otherwise, the required emergency side Isolating Switch which is: 7) Lockable "ON" 8) Having an external "Fire Pump Isolating Switch" placard, 9) A placard next to the controller giving the location of this Isolating Switch10) Supervision of the Isolating Switch by one of the four means given11) Audible and visible alarm to indicate Isolating Switch Open,12) Isolating Switch contacts for remote alarm,13) Means to prevent Higher-than-Normal motor in-rush currents,14) Phase reversal sensing and transfer circuitry.15) Although not required, the Bypass switch may or may not be standard equipment.


Arrangement II Requirements

See:

Application Guide - for -

Fire Pump Transfer Switches- for use with -

Electric Motor Driven Fire Pumps

Note: An updated copy is in the handouts.


Questions ?

fire pump transfer switch basics

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