Douglas Krantz - Technical Writer - Describing How It Works
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Does the N O Mean Normally Open or Normally Closed on a Tamper Switch?

When connecting up the fire alarm system to the sprinkler system, the labels on the swtiches can be confusing.
Which terminal does one use to connect the fire alarm panel to the switch? For the assembly, the one marked N C (Normally Closed) may be Normally Open or Normally Closed when used with the rest of the gatevalve tamper switch assembly or waterflow switch assembly.


By Douglas Krantz

To figure out the normally open and normally closed contacts for switches on fire suppression systems, use an ohmmeter.

Waterflow switches, gatevalve tamper switches, pressure switches, etc., are assemblies that interface between fire suppression systems (mechanical action) and fire alarm systems (electrical signals).

Basically, the assembly is a lever or pressure piston that pushes a button on a switch. The lever or piston part of the assembly is custom designed by a first party manufacturer. However, as part of the assembly, rather than redesigning the wheel, so to speak, the first party manufacturer uses a switch made by a third party manufacturer.

Markings by the Third Party Manufacturer

To indicate which contacts are which when the switch is by itself, the third party manufacturer puts labels on the switch. In other words, when you hold the third party's push-button switch in your hand, the contacts NC (Normally Closed) and NO (Normally Open) are the normal position of the contacts.

These contacts are often used differently in the assembly.

NO is Normally Closed or NO is Normally Open

Once the switch made by the third party is installed inside the waterflow, tamper, or pressure switch assembly, the lever or piston may normally push the button and quit pushing when there's an alarm or supervisory. In this case, the assembly's Normally Open and Normally Closed conditions will be opposite that of the third party's switch contact labels NO and NC.

Then again, the lever or piston may push the button on the switch only when there's an alarm or supervisory, so the label NO and NC are correct for the assembly.

Label Confusion

Confusing? Yes. These labels confuse me too.

A long time ago I started using an ohmmeter to determine which contacts to use. That's the only way I could get the wiring right the first time, every time.

Between the mechanical action of the suppression system and the electrical signals of the fire alarm system, the alarm or supervisory switch assembly is an interface. If the ohmmeter says that the NC and Comm contacts on the switch are shorted together when the fire suppression system is in alarm or supervisory, it's the NC and Comm contacts that I land the wires on.
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

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