Electrical components, such as switches, circuits and relays, are equipped with contacts and come in various configurations – depending on the functions of the electrical device. Common options for contact configurations in circuits include: normally open and normally closed.
What’s the Difference?
Comparing the two options, a normally open contact (NO) refers to a configuration without current flow in its ‘normal’ state. During use, closing the contact will complete the connection and facilitate current flow, resulting in an energized contact.
A normally closed contact (NC) is simply the opposite of a normally open contact. In the contact’s ‘normal’ (closed) state, its components are connected and facilitate current flow. Engaging the contact will open the components, resulting in a disruption of current flow.
Below is a table that compares the two types of contacts:
|Normally Open (NO)
|Normally Closed (NC)
|No Current Flow
|No Connection (Diagram)
A doorbell is a great example of a normally open contact being put to use. When the doorbell is not activated, the unit’s contacts are open. As a result, one does not hear the buzz. When the doorbell is activated (by pressing the button), the normally open contacts are closed or connected, which energizes the contacts, creating a loud chime.
If the doorbell was equipped with a normally closed contact, individuals would continuously hear the chime until the button is pressed or the connection is broken. Engaging a doorbell (pressing the button) with a normally closed contact would stop the device from generating a buzz, as such action would cause the contacts to open, interrupting the current flow.