Douglas Krantz - Technical Writer - Describing How It Works

Can I Leave the Automatic Transfer Switch in MANUAL?

By Douglas Krantz | Suppression

Can I Leave the Automatic Transfer Switch in MANUAL?


Can I Leave the Automatic Transfer Switch in MANUAL?


Greetings Douglas,

In our condo community with 5 buildings, we have an AFSS [Automatic Fire Sprinkler System] & a FDAS [Fire Detection and Alarm System].

The fire pump is provided with a generator set equipped with ATS [Automatic Transfer Switch]. In one occasion the generator set suddenly turned "ON". However, at that time there was no brown out.

My question is, what caused the generator set to operate? From that time, our maintenance person changed the ATS from Automatic to Manual.

Thank You, TO

Here are some definitions of power sources for both the Fire Detection and Alarm System and for the Fire Pump.

FDAS Power Sources

The Fire Detection and Alarm System (FDAS) is a stand-alone system. It requires two sources of power, the primary power source and the alternate power source. The power supplies in the FDAS are for the FDAS only, and will not power any other system.
  • The primary power source for the FDAS is the utility power into the building

  • The alternate power source for the FDAS is the backup batteries for the panels. Commonly the alternate backup power will last at least 24 hours

AFSS Fire Pump Power Sources

To increase water pressure in the sprinkler system, the Automatic Fire Sprinkler System (FDAS) has a fire pump. This increased pressure is especially needed for the higher floors of many buildings.
  • The primary power source for the fire pump in the AFSS is the utility power into the building

  • The alternate power source for the fire pump in the FASS is the generator set - In case there's a power blackout or power brownout to the building, the generator set, through the Automatic Transfer Switch (ATS), provides power for the fire pump - As long as there's fuel for the generator set, the alternate power will continue


The fire pump has an emergency water-bypass. In case the fire pump is not working, it's a pipe that allows water to go around the fire pump. If the fire pump isn't working, the city water pressure is all that is available in the Automatic Fire Sprinkler System.

ATS - Automatic Transfer Switch

The Automatic Transfer Switch (ATS) has three positions - MANUAL-AUTO-ON.
  • MANUAL Position - The ATS allows only utility power to get to the fire pump - Even when there's a power blackout or power brownout, the ATS will never automatically switch from primary power to alternate power

  • AUTO Position - The ATS will automatically switch from primary power to alternate power any time there's a power brownout or power blackout

  • ON - Also Known as HAND (the system is turned on by Hand) - The ATS is switched to alternate power - It will stay in alternate power until switched back to Auto or Manual

Conditions to Turn-On the Generator

The Automatic Transfer Switch (ATS) controller, or else the Fire Pump Controller may be what detects the trigger to turn the generator on and switch to alternate power, but I'm not sure how they are actually wired together.
  • Power Blackout - The primary power for the building is totally blacked out - The alternate power from the generator set is needed to replace the primary power

  • Power Brownout - Power Phase Loss - One of the phases for the power for the primary power is lost - the other two phases are still operating - The pump won't operate efficiently, so the alternate power from the generator set is needed to replace the primary power

  • Power Brownout - Power Phase Reversal - The phasing for two of the primary power phases is reversed - the fire pump will operate in reverse, becoming very inefficient - the alternate power from the generator set is needed to replace the primary power

  • Power Brownout - Low Voltage - One or more of the three phases into the building is below the required minimum to keep the fire pump totally efficient - The building's lights can still be just about as bright as normal, but the generator turns on because the utility power voltages for the fire pump, if there's a fire, isn't "good enough". The alternate power from the generator set is needed to replace the primary power

  • The ATS switch is Turned to "Manual", "ON", or "Hand" - This may also be what turns on the generator set

  • Trouble with the ATS or Generator Set Controllers - Trouble almost never happens, but something going wrong with either controller could possibly cause the generator set to turn on

Results of Leaving the ATS Switch on Manual

The original cause of the generator set turning on may be low voltage on one or more phases of the utility power into the building.

I don't know what happened in your case. However, I've seen a fire alarm system that went into a brownout condition whenever its utility voltage dropped more than 10%. That time it was very intermittent, so that took a lot of investigation to even find; basically, the brownouts never happened during the day. Eventually, though, the cause of the intermittent brownouts was found, and fixed.

Leaving the Automatic Transfer Switch in the Manual position means that the alternate power for the fire pump is never turned on, even if the utility power to the building blacks-out or browns-out during a fire.

When the fire pump's ATS switch is in the MANUAL position, if there's a fire, and the primary power is impaired, the only way to use the alternate power for the fire pump is for someone to enter the burning building, in the dark, and switch the ATS switch to either AUTO or ON.

Find the Cause

I suggest that an electrician is involved to troubleshoot both the generator set and the ATS to find out what happened.



Douglas Krantz

facpdoug@gmail.com
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