Overcurrent Introduction

An Introduction to Overcurrent Protection

Overview - Whitepaper Articles 1.1 – 1.4

Potentially damaging overcurrents occur in electrical circuits due to either sustained overloads or inadvertent transient (fault) conditions. Natural, non-damaging transient overcurrents occur during start-up conditions in many circuits.

This series of four whitepaper articles discusses:

1.1 Overcurrent Protection and Overcurrent Circuit Protectors,
1.2 The Physics of Circuit Interruption,
1.3 Overcurrent Clearing Times, and
1.4 Device Physical Characteristics.

Article Contents

1.1 Overcurrent Protection and Overcurrent Circuit Protectors


Low voltage (300 Volts and below) network overcurrent protection consists of a series connection of circuit protectors that both detect the presence and interrupt the flow of circuit overcurrents. Overcurrent circuit protectors operate in a cascading manner, with each designed to function at a set level and duration of overcurrent flow. Standard low voltage protection devices are: fuses, thermal circuit breakers, magnetic circuit breakers, thermal-magnetic circuit breakers, and solid-state switches.

1.2 The Physics of Current Interruption


All electro-mechanical overcurrent circuit protectors require physical separation of current carrying contacts; and utilize the rapid collapse of the conduction mechanism within an electric arc at a zero current condition to interrupt the flow of circuit current.

1.3 Overcurrent Clearing Times


The total clearing time or operating time of an overcurrent circuit protector is defined as the time duration from overcurrent initiation to complete cessation of current flow. This total operating time is divided into two principal sub-periods: the detection time period and the interruption time period. The detection time period is engineered to be commensurately shorter as levels of overcurrent are higher. It is this period that determines the ability of circuit protectors to coordinate with, or back up, one another. The interruption time period is the "action" period in which the device forcibly opens the path of overcurrent flow.

1.4 Physical Characteristics


Overcurrent circuit protectors are available in a variety of different housings and functional arrangements. Whether or not a particular circuit protector is suitable for a particular application is dependent both on the device performance capability and the available physical characteristics for the device.