What is a nc contact?

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.