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BASIC NEC 2023

Relays, Contactors & Basic Control Circuits Practice Test

Free online relays, contactors, and control circuits practice test. 25 basic electrical questions on NO/NC contacts, ladder diagrams, start/stop stations, overload relays, and motor starters. Good for electrician exam prep.

TOPICS COVERED

Relay Basics Contactor Basics Relay vs Contactor Normally Open Contact Normally Closed Contact Coil vs Contact Rating

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A relay is an electrically operated switch — a small control signal opens or closes contacts. A contactor is just a heavy-duty relay built for switching motors and other high-current loads. Contacts come in two flavors: normally open (NO), which is open until the coil energizes, and normally closed (NC), which is closed until the coil energizes. Control circuits are read on ladder diagrams, top to bottom and left to right. A classic start/stop station uses an NO start button with a "seal-in" contact to hold the circuit on, and an NC stop button that breaks the circuit no matter what.

25

Questions

38 min

Time

75%

Pass

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Relays, Contactors & Basic Control Circuits Practice Test — Question List

Question 1: A relay is best described as:

An electrically operated switch that uses a small control signal to open or close contacts
A device that only measures voltage
A type of fuse used in lighting circuits
A capacitor used for power factor correction

Question 2: A contactor is best described as:

A heavy-duty relay designed to switch high-current loads, such as motors
A small signaling device used only in electronics
A device that measures resistance only
A type of overcurrent protective fuse

Question 3: The main difference between a typical small relay and a contactor is that a contactor:

Is built to handle much higher current and includes features like arc suppression for heavier loads
Cannot be used in any AC circuit
Has no coil at all
Only functions on DC control voltage

Question 4: A "normally open" (NO) contact is a contact that:

Is open (not conducting) when the relay coil is de-energized
Is closed (conducting) when the relay coil is de-energized
Is always open, no matter what
Is always closed, no matter what

Question 5: A "normally closed" (NC) contact is a contact that:

Is closed (conducting) when the relay coil is de-energized
Is open (not conducting) when the relay coil is de-energized
Is always open, no matter what
Is always closed, no matter what

Question 6: A relay's coil voltage rating and its contact current rating are:

Separate ratings — the coil circuit and the switched (load) circuit are electrically isolated from each other
Always exactly the same value
Not real — relays have only one rating
Only relevant for DC relays, never AC

Question 7: In a motor control circuit, the "control circuit" (with pushbuttons and relay coils) is typically wired at:

A lower voltage and current than the power circuit that actually drives the motor
The exact same voltage and current as the power circuit, always
A higher voltage than the power circuit
DC only, and the power circuit is always AC only

Question 8: In a standard ladder diagram used for control circuits, each horizontal line is called a:

Rung
Rail
Loop
Branch box

Question 9: A ladder diagram is generally read in which order?

Top to bottom, and left to right across each rung
Bottom to top only
Right to left only
In no particular order

Question 10: A momentary pushbutton used to START a motor circuit is typically wired as a:

Normally open (NO) contact
Normally closed (NC) contact
Neither NO nor NC — it has no contacts
A contact that is always open, permanently

Question 11: In a standard start/stop control circuit, a "seal-in" (holding) contact is used to:

Keep the relay or contactor energized after the momentary start button is released
Disconnect the motor from power permanently
Prevent the start button from ever being pressed again
Reduce the control circuit voltage to zero

Question 12: In a classic start/stop station, the STOP pushbutton is wired as normally closed (NC) in series with the circuit, while the START pushbutton is wired as normally open (NO) in parallel with a seal-in contact. Pressing STOP:

Breaks the circuit, de-energizing the relay or contactor regardless of the seal-in contact
Has no effect at all on the running motor
Immediately reverses the motor's direction
Only works if the start button is also being pressed

Question 13: "Interlocking" two contactors, such as in a forward/reverse motor starter, is done to:

Prevent both contactors from being energized at the same time, which could cause a dangerous short circuit
Make both contactors close at exactly the same instant, every time
Disable both contactors permanently after first use
Increase the motor's running speed

Question 14: An overload relay, used alongside a motor contactor, is designed to:

Protect the motor from sustained overcurrent (overheating) conditions, separate from the contactor's job of simply switching power
Increase the motor's starting torque
Convert the motor from three-phase to single-phase
Replace the need for any circuit breaker

Question 15: A timer relay (TDR) with an "on-delay" function will:

Wait a set time after being energized before its contacts change state
Change its contacts instantly with no delay at all
Only function on DC control circuits
Permanently disable the circuit after one use

Question 16: A latching relay differs from a standard relay in that a latching relay:

Stays in its last position even after power is removed, until it receives another signal to change state
Requires continuous coil power at all times, just like every other relay
Cannot be used in any control circuit
Only works with three-phase power

Question 17: A solid-state relay (SSR) differs from a standard electromechanical relay in that an SSR:

Has no moving parts and switches using semiconductor components instead of a mechanical armature and contacts
Always requires a much larger coil than a standard relay
Can only switch DC loads, never AC
Has no coil or control input at all

Question 18: Auxiliary contacts on a contactor (small NO/NC contacts that move with the main contacts) are commonly used for:

Indication and interlocking with other parts of the control circuit
Carrying the full motor load current
Replacing the need for the main contacts entirely
Converting AC power to DC power

Question 19: A very common control circuit voltage found in US industrial control panels is:

120 V AC (or 24 V DC in many modern panels)
4160 V AC
13,800 V AC
480 kV AC

Question 20: A "motor starter" is generally defined as the combination of:

A contactor and an overload relay
A single fuse and nothing else
A capacitor and a resistor
A transformer and a rectifier

Question 21: Pushbuttons, selector switches, and limit switches used to send signals into a control circuit are commonly called:

Pilot devices
Power devices
Ground devices
Bonding devices

Question 22: A limit switch is a control device that is actuated by:

Physical/mechanical contact or movement, such as a door or a moving part reaching a certain position
A change in ambient light level only
A change in ambient sound level only
A change in the color of a nearby object

Question 23: A proximity switch differs from a limit switch in that a proximity switch:

Senses a nearby object without needing direct physical contact
Requires much harder physical contact than a limit switch
Can only be used in three-phase circuits
Cannot be wired into any control circuit

Question 24: Just like any inductive coil, a relay or contactor coil can produce a damaging voltage spike when de-energized. This is often controlled using a:

Flyback (freewheeling) diode, or a similar suppression device
Additional fuse rated for ten times the coil current
Extra length of uninsulated wire
Nothing — coils never produce a voltage spike

Question 25: On a control circuit ladder diagram, a rung contains a normally closed limit switch in series with a relay coil. If the limit switch is triggered (opens), the relay will:

De-energize, since the open limit switch breaks the current path to the coil
Energize even more strongly than before
Stay in whatever state it was already in, unaffected
Immediately weld its contacts shut

EXAM CARD BASIC

Questions25

Time limit38 min

Pass score75%

Code editionNEC 2023

DifficultyIntermediate

Last reviewedJun 2026

Timer starts immediately. Progress is auto-saved.

Source: NEC 2026 (NFPA 70)