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Short Circuit Protection For Motors

Motors are employed in providing the bulk of the power needed to convert raw materials into the finished goods that we all use. It is estimated that over 50% of all work done in industry, is done by electric motors, and it is thought that this percentage will continue to rise. With so much of our industry relying on electric motors, it is imperative that motor circuit protectors be used so the motors have maximum up-time and minimal breakdowns.

Improper protection of motors can cost many hundreds of thousands of dollars in lost productivity and output. A tremendous amount of effort has been employed by manufacturers of motor circuit protectors to improve their already excellent products, to provide even better circuit-protection and short-circuit protection.

Requirements:

Every industrialized country, be it in North America, Europe or Asia has an electrical code that specifies the protection that is to be provided to electric motors. In the case of the US, the National Electrical Code is the regulator, and it specifies that a typical circuit should include a means of disconnection, a device to prevent short circuit overload (circuit breaker or fuses), a motor controller and motor overload protection.

Short circuit protection:

There are two systems that can be used to protect motors against short-circuits; fuses and circuit breakers. There are four basic types that are recognized, and their use is based on the type of motor to be protected.

 

  • Non time delay fuse protection
  • Time delay, dual element fuse
  • Magnetic instant trip circuit breaker
  • Inverse time breaker

Each of these devices has certain strengths and weaknesses, let’s look at each device.

Fuses

A fuse is a device which is located in the circuit; it provides protection from overload of wiring, insulation, controls and the motor. Over-current is considered any continuous increase above the nameplate level. Over-currents in motor circuits are classified into two distinct categories. Locked-rotor current is 6 to eight times full load current; this is protected by relay devices, short circuits can be in excess of eight times full load current and if they are not interrupted in nano-seconds, severe damage to the motor and the circuit can be expected. All fuses used as motor circuit protectors are nothing more than a wire embedded in an insulated sleeve, when the current reaches a dangerous level, the wire burns through and the current is interrupted.

Circuit breakers:

Circuit breakers do the same job as fuses, the primary difference is; they can be re-set and used again whereas a fuse is a one-time protective device. The circuit breaker has the ability to be reset allowing the process facility to commence production again with minimal downtime.

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