Overload vs. Short Circuit

Overload or Short Circuit Protection? How to Protect your Design Against Both Dangers

Short circuits and overloads put different demands on circuit breakers.  It is imperative that engineers know how to protect their designs against both dangers.

Circuit breakers are used in a variety of ways.  They are mounted in panelboards to protect branch circuit wiring and they are built into equipment to protect it.  With this range of applications, it's not surprising that a circuit breaker must provide both short circuit and overload protection.

Interrupting a short circuit that is limited by the resistance of the wiring is a very severe test of a circuit breaker, and if the interrupting capacity of the breaker is not adequate, the device can literally explode.  Overload currents that reach 2 to 5 times the normal rating of the breaker are handled differently, and very often the circuit breaker must carry the current for an appreciable time without tripping.  This white paper will give pointers on how to determine the main job a breaker must do and how to make an appropriate selection.

Protection against shorts and overloads is the largest concern when choosing a circuit breaker.  Branch circuits fed from a 480V main need protection against short circuit currents measured in tens of thousands of amperes.  For that reason, panelboards are equipped with circuit breakers for branch circuit protection that are listed under UL 489 - "Standard for Molded-Case Circuit Breakers and Circuit Breaker Enclosures" - and are rated to interrupt fault currents from 5000 to 50,000 amperes or higher. 

A circuit breaker installed inside a piece of equipment is generally there to protect the equipment itself, and the applicable standard should be UL 1077 - "Standard for Supplementary Protectors for Use in Electrical Equipment".  In UL terms, UL 1077 compliant devices are called "supplementary protectors," and are labeled as "recognized components" (not "listed").  They are often called "circuit breakers for equipment" (CBEs).  While U 489 breaker and UL 1077 devices protect against both shorts and overloads, the UL 1077 devices tend to concentrate more on overloads - largely because they are always downstream of a UL 489 breaker.

Protection against Short Circuits

The other (generally much higher) interrupting rating specifies the maximum current that the breaker can interrupt safely (ie. without starting a fire) but may be rendered inoperable ("not fit for further use" or not "recalibrated after testing").  Under EN 60934 this is the PC1 rating, while under UL 1077 it is the SC 1 value.  Some manufacturers publish both ratings, but many do not.

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