Circuit Protection Definitions

Posted on 12/3/18 3:35 PM


Circuit protection is a major component to consider when designing electrical and electronic equipment. The main purpose is to provide protection to both the equipment and operator should a circuit overload occur.

Usually the protection occurs in the form of a circuit breaker or a fuse. A circuit breaker is an automatic switch that prohibits the flow of electric current when it is rapidly overloaded or unusually stressed. A fuse is a safety device that protects an electric current from excessive current. Both the circuit breaker and the fuse can open the circuit pathway in the event of an overload or short circuit, but the circuit breaker is reusable, while the fuse is not.

When the current is larger than the circuit breaker is designed to handle, the switch contacts open, breaking the current. This occurrence is often referred to as being “tripped.” When the issue is resolved (whatever “tripped” the circuit is repaired), the circuit breaker can be reset by a push of a button or a flip of a switch. The contacts will remain closed until another overcurrent fault condition occurs in the protected circuit, causing it to trip again. Circuit breakers can be designed to trip more rapidly than fuses, although they are typically larger and more costly.


Determining which form of circuit protection should be used in the product design is important. Following are a few of the circuit protection definitions.


It is the mechanism responsible for moving the internal components of a circuit breaker or a switch to manually turn it on and off. Additionally, in a circuit breaker, the internal mechanism responsible for tripping the breaker off in an overload state, can be referred to as an actuator.

Ambient Temperature

The temperature of the surrounding environment; technically, the temperature of the air surrounding a piece of equipment. The adjective “ambient” means “relating to the immediate surroundings.”

Breaking Capacity

The breaking capacity of a fuse is the maximum current that can safely be interrupted by the fuse, and generally, this should be higher than the prospective short circuit current.

Fault Protection

Protection against extreme high, instantaneous current spikes.

High Overcurrent Value

Overcurrent that is three times or more the value of the normal current.

In-Rush Current

Also referred to as input surge current or switch-on surge. The maximum instantaneous input current drawn by a piece of equipment when it is first switched on. This is the result of activating devices such as AC motors, transformers, power converters, and large capacitors or capacitor banks requiring charging of the capacitors. In‑rush currents can range anywhere from 1.5 times the steady‑state current to as much as 10 to 15 times the steady-state current for equipment utilizing large transformers or motors.

Internal Resistance

The resistance in ohms of the internal electrical pathway. 

Load Current

The typical current drawn by a device while in normal operation. 

Operating Temperature

The specified temperature range that a piece of equipment is limited to for proper, safe usage. Mathematically, this is the sum of the ambient temperature plus the typical heat rise generated by the equipment while in use. The lower limit of the operating temperature range is the absolute coldest that the equipment should be run in for proper operation and the upper limit is the absolute hottest temperature that the equipment should be run in for proper operation. It is generally controlled by safety standards. If the operating temperature range is not being met and the heat rise is within limits set by the applicable safety standards, then the ambient temperature must be changed accordingly to bring the system within proper operating guidelines.

Power Dissipation

A measure of the rate at which energy is lost from an electrical system, through heat transfer, into the circuits surroundings through conduction or radiation. Power dissipation is usually measured in watts.

Rated Power Acceptance

Also referred to as admissible power acceptance. The value of power acceptance of a fuse holder assigned by the manufacturer, which is the maximum power dissipation produced by an inserted dummy fuse link during testing, at the rated current tolerated by the fuse holder without exceeding the specified temperatures of the test. The rated power acceptance is referred to an ambient temperature of 23°C during testing.


Also referred to as overcurrent or excess current. In an electrical or electronic system, it is a situation where a larger than intended amount of current exists through the circuit, leading to excessive generation of heat and the possible damage of the equipment or components within the equipment, as well as the risk of fire. Common possible causes are short circuits, excessive load, and/or incorrect design.

Shock Resistance

The level of impact applied to a circuit breaker, measured in grams, under which the unit will still function correctly.

Short Circuit

A specific type of overcurrent event that creates excess current when the current travels along an unintended path where there is either no or very low electrical impedance.

Touchproof Fuse Holder

A touchproof fuse holder means more safety to the end user because a blown fuse can only be accessed from the outside of the equipment with the use of a tool and cannot touch any of the contact surfaces before the fuse carrier and fuse are removed.


This means the switch cannot be held in the closed position against overcurrent.

Typical Life Insurance

For a circuit breaker switch, this is the number of operations that the item is expected to last while connecting and disconnecting the nominally rated current of the device under a specified load condition, usually inductive.

Vibration Resistance

The level of continual vibration measured in grams under which the unit will still function correctly.

Topics: circuit protection, product design, terminology

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