Engineering Tutorials

We will discuss two types of products in this module. These are the Miniature Circuit Breaker and the Supplementary Protector.
We group these products together because they perform the same function. They both switch and protect the lowest common distribution voltage in an electrical system.
Some other similarities include:
• Both have molded case enclosures
• Both are used in low voltage (under 600 volts) systems
• Both devices are small: Typically 1" wide
The big difference between the two is that the supplementary protector is not UL 489 (Underwriters Laboratory) approved. For this reason, it cannot be used as a Branch Circuit Overcurrent Protective Device, or in the place of the branch circuit protector. A miniature circuit protector protects the whole branch circuit, but the protector is only used to protect a particular device.
Figure shows the difference between using just a miniature circuit breaker (shown on the left) and using a supplementary protector.
Figure Branch Circuit Overcurrent Protection (on left) Vs.
Supplementary Protector (on right)

Therefore, this module will concentrate on miniature circuit breakers. Supplementary protectors will be discussed only briefly