Sunday, January 25, 2009

Need for Circuit Protection

Current and Temperature:
Current flow in a conductor always
generates heat. The greater the current flow, the hotter the conductor. Excess heat is damaging to electrical components and conductor insulation. For that reason, conductors have a rated continuous current carrying capacity or ampacity. Over current protection devices, such as circuit breakers, are used to protect conductors from excessive current flow. These protective devices are designed to keep the flow of current in a circuit at a safe level to prevent the circuit conductors from overheating.


An overload occurs when too many devices are operated on a single circuit, or a piece of electrical equipment is made to work harder than it is designed for. For example, a motor rated for 10 amps may draw 20, 30, or more amps in an overload condition. In the following illustration, a package has become jammed on a conveyor, causing the motor to work harder and draw more current. Because the motor is drawing more current, it heats up. Damage will occur to the motor in a short time if the problem is not corrected or the circuit is shut down by the over current protector.

Conductor Insulation:

Motors, of course, are not the only devices that require circuit protection for an overload condition. Every circuit requires some form of protection against over current. Heat is one of the major causes of insulation failure of any electrical component. High levels of heat can cause the insulation to breakdown and flake off, exposing conductors.

Short Circuits:

When two bare conductors touch, either phase to phase or phase to ground, a short circuit occurs. When a short circuit occurs, resistance drops to almost zero. Short circuit current can be thousands of times higher than normal operating current.

Ohm’s Law demonstrates the relationship of current, voltage, and resistance. For example, a 240 volt motor with 24 Ω of resistance would normally draw 10 amps of current.

I =E/R = 240/24=10 Amps

When a short circuit develops, resistance drops. If resistance drops to 24 milliohms, current will be 10,000 amps.

I = 240/0.024=10,000 Amps

The heat generated by this current will cause extensive damage to connected equipment and conductors. This dangerous current must be interrupted immediately when a short circuit occurs.

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