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Ask Chris: What is the difference between contactors, motor starters, relays and circuit breakers?

A reader submitted a question through the Ask Chris form. I’ve expanded the original question from “what are the differences between relays, contactors and motor starters?” to “What is the difference between contactors, motor starters, relays and circuit breakers?”

We’ll look at the history of  electrical protection in this post to see how the difference between relays and circuit breakers evolved.


Switches allow the system operators to close electrical circuits so that electricity can flow into an electrical system.  They range from simple knife switches like you see in Frankenstein movies, to the light switches in your house, to complex mechanisms designed to open and close in high-voltage systems. Switches have the following limitations in an electrical system:

  • They are usually manually operated, which means someone must be present to energize or isolate a system.
  • They are typically very slow. This means that under high loads or voltages, a dangerous arc can form when the switch is opened or closed.
  • They are not typically designed to open a large amount of current.  This means that switches can explode if they are opened when the current flowing through the switch is greater than its rating.

Frankenstein SwitchMedium Voltage Switch


The early developers of electric systems quickly realized that you can get some spectacular explosions and damage when something goes wrong with electricity. The first protective device was probably unintentional as some part of the system simply melted away and isolated the rest of the system. This is the principle used in simple fuses.  A fuse has two conductive ends and some kind of material between them that is intentionally designed to melt when the current through the fuse exceeds its rating. Fuses can range from simple household fuses, to complex current-limiting fuses designed to limit the amount of fault current that can flow through it.

Low Voltage Fuses High Voltage Fuse

Circuit Breakers

Anyone who has replaced a fuse in their house or car can understand how inconvenient it is to:

  • Find out where the fuse is
  • See if you have the right fuse to replace it
  • Remove and replace it

Now imagine that you are a lineman who has to:

  • Find that fuse along power lines
  • Carry a large variety of fuses for all of the possible applications
  • Maneuver a stick long enough to reach the fuse on top of the power pole
  • Replace the fuse and re-energize the high-power circuit, which is guaranteed to make a nice spark and loud bang

Circuit breakers combine the benefits of switches and fuses.  A circuit breakers can energize or de-energize the system like a switch, but they have special mechanisms in place to open and close more quickly (<60-90ms).  They also have arc-quenching technology to prevent arcs from forming because arcs are the start of electrical explosions. Most circuit breakers have control relays that tell the breaker when to open and close.

Some low-voltage circuit breakers can also isolate parts of the system when something goes wrong using their own fault detectors. The circuit breakers in your house typically use heat and metallurgy to detect faults.  As current flows through the circuit breaker,  it flows through two different metals that warp at different rates. When the current is too high for too long, the different metals move apart and operate the circuit breaker’s trip mechanism, which isolates the problem from the power source to minimize damage.

Most medium- and high-voltage circuit breakers, however, have a protective relay to tell the circuit breaker to open when a problem occurs in the system.

Low voltage circuit breakerhigh voltage circuit breaker

Protective Relays

There are two different classes of relays; protection relays and control relays.  Protective relays monitor the electrical system and look for abnormalities. Some types of protective relays include:

  • Electro-mechanical relays that rely on magnetism and mechanisms to detect a fault and send a signal to open the circuit breaker. Electro-mechanical relays are typically single-function, single-phase devices, which means that you need a lot of relays to protect a single feeder or source.
  • Solid-state relays use electronic components to detect a fault, which then operate control relays to open the circuit breaker. Solid state relays are typically single-function, multi-phase relays which means you still need multiple relays to protect the system from different problems, but you can use fewer solid-state relays than electro-mechanical ones.
  • Hybrid relays combine different features from electro-mechanical and solid-state relays and have the advantages and disadvantages of both.
  • Digital relays use analog-digital converters, microprocessors, and algorithms to detect problems.  The microprocessor operates control relays that tell the circuit breaker to operate. Digital relays are multi-function and multi-phase, which means you can use a single relay to protect any feeder or source.

A protective relay that could monitor thousands of amps, and hundreds of thousands of volts, would be HUGE and expensive.  Most protective relays monitor the extremely high currents and voltages in a three-phase electric system through instrument transformers:

  • Current Transformers (CTs) transform the thousands of amps flowing through an electrical system into a maximum of five amps in North America under normal load conditions, and one amp for most of the rest of the world. However, power system faults can inject more than 100A into protective relays, so they need to be accurate over a very wide range of current. (<5A under normal conditions, and more than 100A in fault conditions.)
  • Voltage transformers (PTs or VTs) turn the extremely high voltages (480 to >500,000V) of an electrical system into a tolerable voltage (66 to 208V) that you could find in your house. Protective relays are able to fit into small panels because of voltage (potential) transformers.

Protective relays take the manageable voltages and current supplied by the instrument transformers and create a model of the current and voltage supplying a system. If the relay detects a problem,  it will send a trip signal to the circuit breaker. Determining whether the system is normal or abnormal is often the hardest part of protective relay testing and design. Protective relays and circuit breakers must work together to detect faults and isolate those faults from the rest of the system.

Basic Protective Relay Testing Course Relays

Control Relays

Control relays are used to relay messages (get it?) from one location to another. They can be used to turn one signal into many signals with multiple contacts, turn a low-voltage signal into a high-voltage signal, or visa-versa. Control relays come in many different flavors and can be found in nearly any electrical device more complicated than a toaster.

Control Relays


A contactor is installed to provide the same function as a switch or circuit breaker, but has a completely different operating principle. When you move a switch or a circuit breaker into the closed position and then stop, the switch or circuit breaker will stay in that position. If the person who moves the switch isn’t present, or the system that sends the open signal to the breaker stops working, the switch and circuit breaker will stay closed until they either melt down or are opened by some other means. A contactor is held closed by an energized coil.  If something happens to the power supply controlling a contactor, the contactor will immediately open.  This is called a fail-safe system because if something unexpected goes wrong with the control circuit, the contactor will open and the system will be “safe”.  If something happens to the control circuit of a circuit breaker, it will stay in whatever position it was before the problem, which may not be a “safe condition”.  Contactors are typically used in motor applications to protect the motor, or the process the motor is used to operate, when something happens to the control system.

High Voltage Contactor Low voltage contactor

Motor Starter

A motor starter is a system that energizes and de-energizes a motor.  A motor starter can have:

  • A single contactor or circuit breaker
  • A system of motor starters and auto-transformers for reduced-voltage motor starting
  • Some kind of solid-state device like a variable frequency drive (VFD) that will control the waveform sent to the motor to provide some kind of controlled motor starting

480V motor starter5kV Motor Starter

A summary of the difference between contactors, motor starters, relays and circuit breakers.

Switches, circuit breakers, and contactors are used to control what parts of an electrical system are energized or de-energized.

Fuses are used to protect the electrical system when something goes wrong, and can isolate the faulted parts from the normal parts.

Circuit breakers can control and protect the electrical system, but most need a protective relay to determine if a problem exists and tell the circuit breaker to open. Control relays tell the circuit breaker to open and close under normal conditions.

A protective relay protects the electrical system by trying to determine what is normal and abnormal and tells the circuit breaker to open when a problem is detected. Protective relays and circuit breakers must work together to isolate a fault.

A contactor energizes and de-energizes a motor based on information from the motor starter control system.

A motor starter energizes and de-energizes a motor using a contactor, system of contactors, or other means.


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About the Author Chris Werstiuk

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

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