Relays vs. Circuit Breakers For Circuit Protection
Relays, diodes, circuit breakers, MOSFETs, and fuses are all mechanisms used to break a circuit whenever certain overvoltage or overcurrent events arise. These components are not universally usable, they have to be selected to target specific ranges of required protection as well as types of transient voltage or overcurrent events. Some industry standards require these components be included in a PCBA to provide a minimum level of circuit protection.
In many pieces of industrial equipment and power systems, the common tools used to protect important devices are circuit breakers and relays. These devices are typically much larger than any of the other components that will appear in your devices or PCB assembly as these components need to be very rugged. They provide some of the highest levels of circuit protection available and they will be the first line of defense against large power surges, transients, and current surges.
Two Switchable Circuit Protection Devices
Relays and circuit breakers are both used for circuit protection, but they are not interchangeable. There are some important differences between these components, which are highlighted in the sections below. Sometimes, the differences between relays and circuit breakers can be unclear due to the following:
- Circuit breakers and relays are both switches, but they don’t open in the same ways
- Relays and circuit breakers can be reset by hand, but only relays can be modulated electrically
- A circuit breaker could contain a relay that controls when the circuit breaker opens/closes
- Circuit breakers are typically intended for overcurrent protection, while relays can provide both overcurrent and overvoltage protection
Relays
A relay is a switchable device that can be toggled electrically, so they are often used in switching and control applications. The central idea behind a relay when used for circuit protection is to toggle a switch in the event of an overvoltage or overcurrent condition, thereby opening the circuit and halting current flow to a load. This is done through magnetic toggling, where a current is used to generate a magnetic field that toggles a mechanical switch. This is shown in the image below.
Relays generally come in two varieties: normally open (NO) and normally closed (NC). An NC relay is typical and it will provide a closed circuit as long as there is no fault in the system. Some relays, such as that shown above, will have an NO contact (A) and an NC contact (B); the NO contact will only become part of the circuit in the event that a fault trips the relay.
|
Relay Type |
Description |
|
Electromagnetic Relay |
Use an electromagnetic coil to control switching, with contacts opened or closed based on coil current. |
|
Reed Relay |
Consists of magnetically actuated reed switches within a hermetically sealed glass tube. |
|
Solid-State Relay (SSR) |
Uses a transistor to modulate current through the magnetic coil and induce switching. |
|
Thermal Relay |
Opens/closes contacts with a bimetal bending mechanism, which is heated by the applied current. |
|
Time Delay Relay |
Provides a delay between coil activation and contact switching, which is useful in timed control operations. |
|
Latching Relay |
This relay has two stable states that maintain a contact position until a switching pulse is applied. |
|
Mercury Wetted Relay |
Uses a pool of mercury to make and break contacts. |
|
Voltage Monitoring Relay |
Monitors voltage and detects undervoltage or overvoltage conditions. |
|
Current Monitoring Relay |
Senses current and detects undercurrent or overcurrent conditions. |
Circuit Breakers
Circuit breakers are very large switchable devices that react to faults leading to overcurrent conditions. When the overcurrent condition occurs, it forces the circuit breaker to open, which ceases current flow to the protected device.
Circuit breakers are typically panel-mounted, meaning they are affixed to a panel or enclosure so they are not accessed inside the product or on the PCB. This is certainly the case in residential settings, where a home’s main panel contains a circuit breaker to sense and prevent excessive current draw by devices in a home. However, some circuit breakers can be mounted directly onto a PCB. An example from E-T-A (PN: 1410-L210-L2F1-S02-5A) is shown below.
These devices are mounted in series with the protected device in your system, and they contain a mechanical contact that opens automatically in the event of overcurrent. These components are then reset manually by the user, which closes the contact and allows current to begin flowing again. An example schematic showing the symbol and typical placement for circuit breakers is shown below.
Typical wiring for a circuit breaker. [Source]
Although circuit breakers primarily provide overcurrent protection, some circuit breakers can provide limited overvoltage protection. These circuit breakers include additional features such as surge protection modules or built-in surge arresters. These features help suppress transient voltages or power surges originating from a variety of causes, such as lightning strikes or power grid fluctuations.
A complete list of the types of circuit breakers is shown below.
|
Circuit Breaker Type |
Description/Function |
|
Miniature Circuit Breakers (MCB) |
Provide protection against overcurrent and short circuit conditions in low-voltage circuits. |
|
Molded Case Circuit Breaker (MCCB) |
Larger circuit breakers for higher current ratings, commonly used in industrial and commercial applications. |
|
Residual Current Circuit Breaker (RCCB) |
Protect against ground faults or leakage currents, preventing electrical shock. |
|
Arc Fault Circuit Interrupter (AFCI) |
Detects dangerous arc faults to prevent fires, commonly used in residential applications. |
|
Ground Fault Circuit Interrupter (GFCI) |
Provides protection against electric shock from ground faults, commonly used in areas with water or moisture. |
|
Thermal-Magnetic Circuit Breaker |
Provides thermal protection against overcurrent and magnetic protection against short circuits. |
|
High-Voltage Circuit Breaker |
Protects against overcurrent and short circuit conditions in high-voltage transmission and distribution systems. |
|
Oil-filled Circuit Breaker |
Uses oil as an arc suppression medium, commonly used in high-voltage applications. |
|
Vacuum Circuit Breaker |
Uses a vacuum as an arc suppression medium, compact and reliable for medium-voltage applications. |
|
Sulphur Hexafluoride Circuit Breaker |
Uses SF6 gas as an arc suppression medium. |
Electronic Circuit Breaker With Overvoltage/Overcurrent Protection
The circuit breakers listed above are primarily electromechanical, meaning a mechanical switch is being modulated. In some cases, this modulation is applied through a transformer and a relay; when the current output across the internal transformer is too large, it can open a relay and thus open the protected circuit. This is done with hand-reset relays, which are normally associated with transformers.
It’s also possible to build an electronic circuit breaker that uses a relay, where the relay is tripped by a set of transistors offering two possible thresholds. A popular example involving a transformer and a resettable relay is shown below.
In this example, power is being drawn from AC mains into a light bulb that acts as our AC load. There is a relay in series with the load that is capable of breaking the circuit if it is tripped. In parallel, the AC input is rectified to DC and regulated at a standard voltage (5V). The output on the secondary side of the transformer is then stepped down across a voltage divider and used as the input in a pair of op-amp circuits.
By comparing the stepped down and divided AC line voltage with the +5V stable reference voltage, the op-amp will switch between high or low voltage values. The outputs from each op-amp will then trigger a transistor, which forces switching action in the relay and causes it to open. The circuit currently triggers based on overvoltage, but this could be modified to sense overcurrent by using a current-sense resistor to determine the secondary current instead of using a voltage divider.
More Than Relays and Circuit Breakers
If your system requires higher levels of transient voltage suppression or ESD protection, then relays and circuit breakers aren’t the only option. Additional pieces of equipment that can be used for circuit protection can target some other type of transient voltage, inrush current, or power surge that might damage or disable a system. Some of these components/circuits include:
- Diodes (TVS and Zener diodes)
- Crowbar circuits
- MOSFETs
- Metal-oxide varistors (MOVs)
- Resettable fuses
Circuit breakers are often found away from the actual PCB as they are typically panel mount devices. The items in the above list can be placed directly on the PCB in order to handle any transients or faults that might not cause a circuit breaker or relay to open and protect a circuit.
Whenever you need to implement circuit protection from transient voltages in your PCB design, make sure to design your system with the complete set of simulation tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.
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