Douglas Krantz - Technical Writer - Describing How It Works
Get the book Make It Work - Conventional Fire Alarms

How is the Standby Power Calculated?

Can You Explain how to Calculate the Required Standby Power?


Greetings Douglas,

Can You Explain how to Calculate the Required Standby Power?

Thank you, K C

Usually, included with the installation manual for the fire alarm system, the manufacturer shows a work sheet that can be filled out to calculate the needed standby power. This is a worksheet that has to be filled out for each panel and power supply that has standby batteries in the building's fire alarm system. If the manual isn't available, talk to the manufacture's technical support people to get a work sheet from them.

The following is only a summary of what the manufacture's battery calculation sheet does. Use the work sheet provided by the manufacture, it's easier.

Blackout

The worksheet is based on the power needed by the system during a blackout.

In case there's a power outage, the fire alarm system still has to detect fire and warn people of fire. It would be nice if the fire alarm system could do this for days, weeks, or even years, until eventually the power can be restored, but the NFPA (National Fire Protection Association) is reasonable. In most cases, the NFPA is happy if the system can continue to work without power for 24 hours. This is the standby period.

If there is a fire detected at the end of that 24 hours, the system still has to sound the alarm. Usually sounding the alarm takes up a much higher level of current than when the alarm system is standing by, so sounding the alarm has to be calculated separately. This is the alarm period.

At this point, the electrical current used the whole system, including all input devices, output devices, control panels, and power supplies has to be known. The manufacturer of each device or panel will show the standby current and the alarm current in their installation instructions.

If the installations instructions aren't available, contact technical support.

Standby Amp-Hours

To figure out how many amp-hours is needed for the standby batteries, the amp-hours needed for the standby period is calculated by adding up the total standby current used by each of the input devices and each of the output devices attached to a control panel or power supply, and the standby current for the control panel or power supply.

Multiply this total current used when standing by 24 to get the total amp-hours used in that 24 hour blackout period.

Alarm Amp-Hours

The alarm period may be 5 minutes long, or it may be 2 hours long, intermittently. Check with the local fire marshal or fire inspector to find out how long the alarms have to continue once the system is in alarm.

Do the same calculations again using the alarm currents rather than the standby currents.

Total Amp-Hours

Add the amp-hours used during the standby period to the amp-hours used during the alarm period. This gives the sub-total amp-hours.

To give a little headroom to the battery amp-hour rating, add at least 20% to the sub-total to get the total amp-hours needed for the standby batteries.

Battery Calculations

To make sure the calculations include all devices, when doing the battery calculations, use the manufacture's worksheets.

Douglas Krantz
Based on his electronics training, and his understanding of Life Safety, Douglas Krantz has compiled his knowledge of Conventional Fire Alarm Systems into the book Make It Work - Conventional Fire Alarms. The book covers the basics of the Conventional Fire Alarm System, and shows how Life Safety and internal supervision affects the fire alarm system.

Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Share This With Friends:









 Get your free diagram showing supervision for Class B wiring

Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Residential Life Safety Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Description Map Page of Douglas Krantzs Technicians Corner
Go to the General Electrical Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Alarm Maintaining Map Page of Douglas Krantzs Technicians Corner
Go to the Fire Suppression Map Page of Douglas Krantzs Technicians Corner
Go to the Guest Writer's Guidelines of Douglas Krantzs Technicians Corner
See Trivia on Douglas Krantzs Technicians Corner
This website uses cookies. See Privacy for details.
Get the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Conventional Fire AlarmsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground FaultsGet the book Make It Work - Hunting Ground Faults
Reader's Favorite Articles

What is a Stair Pressurization Fan (SPF)?

Which Way Does Electricity Flow?

What's the Difference Between Class A and Class B?

What Makes the End of Line Resistor So Important?

What is a Flyback Diode?

What is a Fire Alarm System?

What is an RTU (Roof Top Unit)?

What Causes an Open NAC?

Learn about fire alarms, one article at a time -

Keep up on the latest article!




No Charge - Unsubscribe Anytime