Electrical Engineering
Electrical Wiring

What is a nc contact?

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Wiki User
2008-09-09 03:11:21

An N.C. contact is 'Normally closed'. This applies to relays

(electronic switches). You can use either a normally closed

contact or a normally open contact to do what you need,

provided the relay has both and depending on your situation. Here's

a recent example of mine: A building's fire alarm is wired to the

HVAC system to shut down ventilation in case of a fire. In this

case we want the HVAC to have power normally but remove power (with

the relay) during a fire alarm. We would connect the HVAC power to

the relay's N.C. (normally closed) contacts, because they are

exactly that: normally closed. This would allow the HVAC to

function. When power is applied to the relay, it would switch

contacts, disconnecting power to the HVAC. An important note is

that contacts and other electrical connections that can change

condition (such as a relay) are described in a de-energized

state. Thus, the N.C. contacts are normally closed, while the power

to the relay is off. Likewise, a N.O. contact is open with relay

de-energized but will close upon supplying power to the relay.

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