Circuit Protection: The Difference Between Circuit Breakers and Supplementary Protectors
To ensure safety in manufacturing plants, it’s important to understand the difference between a circuit breaker and a supplementary protector—and the specific applications for each device.
Electrical power and control systems require circuit protection to prevent fires, short circuits, and equipment damage. As part of their system design process, the electrical engineering team specifies the required circuit breakers and supplemental protectors.
Your Home Has Both Circuit Breakers and Supplementary Protectors
Most homes have a circuit breaker box, typically with separate 15-20 Amp breakers to protect the wiring for each home circuit (kitchen, living room, etc.) from overheating and damage. (Older homes may still use fuses.) The breaker opens the circuit when the amperage exceeds its rated capacity. They have a reset mechanism and can be manually turned on and off.
Residential supplementary protectors stop electricity from reaching an appliance before it can cause damage. They are found, for example, on kitchen garbage disposals, hair dryers, and surge protectors that shield the delicate and expensive electronics of your computer and entertainment systems. They have a reset button, and provide additional protection beyond the circuit breakers.
UL (Underwriters Laboratories) provides standards to protect homes as well as promote safety in manufacturing plants. UL 489 applies to circuit breakers, UL 1077 covers supplemental protectors, and UL 508A applies to industrial control panels.
Circuit Breakers and UL 489
UL 439 defines a circuit breaker as a device designed to open and close a circuit by non-automatic (i.e., manual) means, and to open the circuit automatically on a pre-determined overcurrent, without damage to itself when properly applied within its rating. They provide service feeder and branch circuit protection for wiring, and are designed for short circuit, overcurrent, and overvoltage protection.
UL tests these devices for proper operation, and certifies compliance with the standard by assigning a Listing Mark that identifies the device as a “Listed Circuit Breaker.”
Supplementary Protectors and UL 1077
This standard defines a supplementary protector as a manually resettable device designed to open the circuit automatically on a pre-determined value of time versus current or voltage. Sometimes located within an appliance or other electrical equipment, they may have a device to manually open and close the circuit. They provide overcurrent and overvoltage equipment protection but limited short-circuit protection, and are always located after the branch circuit protection.
These devices are tested by UL in combination with the equipment being protected. Approved devices receive a Component Recognition Mark, which is the mirror-image reverse of UR. It may also include the words “Supplementary Protector,” which is very different from a Listing Mark.
UL 508A: Circuit Protection in Industrial Control Panels
The key to selecting the type of protection is the type of circuit being protected.
Circuit breakers are required for the following circuits:
- Feeder Circuit: All circuit conductors between the service equipment, the source of a separately derived system or another power supply source, and the final branch-circuit overcurrent device.
- Motor Branch Circuit: The portion of the electrical distribution system that extends beyond the final branch circuit overcurrent protective device. They serve lighting, appliance, motors, and/or other individual loads.
The following control circuits may require a combination of circuit protection.
- Class 1 Control Circuit: A control circuit on the load side of an overcurrent protective device where the voltage does not exceed 600V and the power available is not limited. Also, it can be a control circuit on the load side of power limiting supply, such as a transformer. The circuit primary (line side) must have branch circuit protection (circuit breaker or fuse). It may also have supplemental protection on the line side. Secondary (load side) protection can be provided by a supplementary protector.
- Class 2 Control Circuit: A control circuit supplied from a source having limited voltage (30V rms or less) and current capacity, such as from the secondary of a Class 2 transformer, and rated for use with Class 2 remote-control or signaling circuits (DC power supply). Supplementary protectors can be used for both primary (line side) and secondary (load side) protection as this type of circuit is downstream of a branch circuit device.
Safe system design for electrical power and control systems is complex and often requires obtaining engineering consulting services to provide the needed expertise. Just remember the following guidelines.
Use circuit breakers for:
- Branch Circuit Protection
- Power Transformer Primary and Secondary Protection
- Load Protection for Motors, Heaters, Lamps, etc.
Use supplementary protectors for:
- Control Transformer Primary Protection
- Control Transformer Secondary Protection
- Protection of control devices, such as relays, starter coils, and solenoids
Matrix Technologies is one of the largest independent process design, industrial automation engineering, and manufacturing operations management companies in North America. To learn more about our electrical engineering capabilities and manufacturing process control solutions, contact Brian Haury, Department Manager, Power, Instrumentation, Control Department.
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