Is there some standard way to distinguish between the normal state of a particular component and the environment in which it is used?

For example, a reed switch would be considered normally open if it results in an open circuit when a magnet is not nearby. If this switch is used in an alarm system to monitor the state of a door, however, it is likely expected that the door is usually closed which would put the switch in its abnormal state. This means that the monitoring system is expecting that the circuit is closed under normal circumstances and alarms when the door is open.

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    \$\begingroup\$ Other than switches being labelled NO for Normally Open and NC for normally closed, not really, you select the switch which does what you want in a particular application \$\endgroup\$
    – Sam
    Commented May 2, 2016 at 3:44

1 Answer 1


Yes. The so called 'environment' and its conditions are accounted for by the design engineer or technician. This person dictates what the default state is for all switch's and relays based on the devices they are driving, especially in a room where mistakes could be fatal.

Generally this means what is 'normal' is the safest state possible until humans or computers or firmware takes control. A contactor to discharge a capacitor bank would be in an 'off' state until needed, but may be in an 'on' state until software takes control after booting up.

Part of a POST sequence for ATE would be to check the status of switch's, relay's, power supplies and heaters/coolers before allowing main power to come on in a room that simulate lightning, as an example.

Terms like 'Normally open' (NO) and 'Normally closed' (NC) become too abstract in a room filled with them. If you want the details you look at the schematics posted on a wall in the room.

If the door to the room was opened, the power to the room would be cut off and any stored high-voltage would be dumped into a bleed resistor.

I was in charge of such a room and developed the software (LabView) and did much of the wiring, then testing, usually at 20,000 amps and about 30,000 volts.

All things considered the 'Standard Way' is always about safety, starting with the default settings of the hardware before it was under software control.


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