Terms
What is the difference between a relay and a contactor?
A relay is generally used for low-current control, while a contactor is designed for higher-current
What distinguishes a control relay from a power relay?
Control relays handle low currents/signals, while power relays/contactors handle high currents.
When should a mechanically held contactor be selected instead of a magnetically held one?
Use a mechanically held contactor when the circuit must remain closed during a power failure.
What is the advantage of using low-voltage relays for lighting control?
Increased safety, smaller control wiring, easier switching.
A contactor fails to energize even though the control circuit has voltage. What are two possible causes?
Possible causes: coil burned out, open control circuit, loose wire, blown fuse.
Define "normally open" (NO) and "normally closed" (NC) contacts.
NO = contacts open when coil is de-energized, close when energized; NC = contacts closed when coil is de-energized, open when energized.
Identify one application where a control relay is preferred over a contactor.
In automation control circuits (e.g., controlling multiple signals).
How can you identify a solid-state relay compared to an electromechanical relay?
Solid-state relays have no moving parts and use semiconductors; electromechanical relays use coils and armatures.
What wiring consideration must be made when planning a low-voltage lighting system?
Ensure correct separation between low-voltage control and line-voltage lighting circuits
If a relay coil is rated for 24V but receives 120V, what will happen?
The coil will burn out or fail immediately.
What is meant by the “coil voltage rating” of a relay or contactor?
The rated voltage needed to energize the coil.
Describe the physical action inside a control relay when the coil is energized
The coil magnetizes an armature, pulling contacts open or closed.
What is the purpose of an overload relay in a motor control circuit?
To protect motors from overload by opening the circuit when excess current is detected.
When installing low-voltage relays for lighting, what device is typically used to control the relay coil?
Low-voltage switches or timers.
In a motor starter circuit, what would occur if the overload relay trips?
The motor circuit will open and the motor stops.
Define “mechanically held contactor.”
A contactor that stays latched mechanically without continuous coil power.
Explain why control relays are often used in interlocking circuits.
They allow one device to control or lock out another for safety.
Classify contactors according to the type of current they are used with (AC vs. DC).
AC contactors are designed for alternating current loads; DC contactors are designed for direct current loads.
Explain how a pilot device (such as a wall switch) interacts with a low-voltage relay.
The pilot device sends a small control signal to energize/de-energize the relay coil, which switches the line voltage.
Why might a contactor "chatter" when energized?
Coil receiving low or fluctuating voltage, poor connections.
What is an “auxiliary contact” and where is it commonly used?
An additional set of contacts used for signaling or interlocking.
Identify whether a relay is best suited for high current or low current control circuits.
Low current.
What type of contactor is typically used for lighting control applications?
Lighting contactors, often mechanically held.
What safety step should always be taken before wiring low-voltage lighting control systems?
Disconnect and lock out power.
In a low-voltage lighting system, why might lights stay on even after the control switch is turned off?
Relay contacts may be welded shut or coil wiring misconfigured.