A coil moves a contact mechanism.
In a typical electromagnetic relay, energising the coil produces a magnetic force that moves an armature. That movement changes the state of one or more contacts. A spring returns many non-latching relays when the coil is de-energised.
The coil circuit and switched circuit are distinct. Electrical separation depends on the device’s construction and ratings; a relay is not automatically suitable for every isolation duty.
Try normally open and normally closed contacts.
For this non-latching example, “normal” means the coil is de-energised. Normally open (NO) contacts are open in that state; normally closed (NC) contacts are closed.
- CoilDe-energised
- Normally open contactOpen
- Normally closed contactClosed
Coil off: NO contact open; NC contact closed.
What makes a contactor different?
Contactors are switching devices commonly designed for power loads and frequent operation, with contact and arc-management arrangements suited to their rated duties. Relays cover a broad range of signalling and switching applications.
A contactor may have main contacts for a load and auxiliary contacts used by the control system. The coil voltage says nothing by itself about the load the main contacts can switch.
Read ratings in context.
A device with a 24 V DC coil can switch a separate circuit, but only within the contact ratings and specified load duty. The coil voltage is not the switched voltage rating.
AC and DC switching, resistive and inductive loads, and starting currents can impose different demands. Two devices that physically fit need not be interchangeable.
Commands and feedback are different.
Lift controls may use auxiliary contacts to monitor a switching device. Hearing a click does not prove that every contact has changed correctly or that the connected equipment has moved.
Contacts can wear or weld. Ordinary relays should not be treated as equivalent to devices with particular monitored or safety-related characteristics.
A closer look
Latching relays can retain a state after the operating signal is removed. Solid-state relays switch electronically rather than with the moving-contact mechanism illustrated here. Neither should be assumed to behave exactly like this basic model.
Check your understanding
In the demonstration, what happens to the NC contact when the coil is energised?
Reveal the explanation
It opens. “Normally closed” describes the de-energised state of this non-latching relay, not a contact that is always closed.