ELECTRIC MOTOR SHORT CIRCUIT PROTECTION OVERVIEW
How are electric motors protcted from short circuits?
This can have a number of causes: damage to the varnish insulating the conductors, loose, broken or stripped wires or cables, metal foreign bodies, conducting deposits (dust, moisture, etc.), seepage of water or other conducting fluids, wrong wiring in assembly or maintenance.
A short circuit results in a sudden surge of current which can reach several hundred times the working current within milliseconds. A short circuit can have devastating effects and severely damage equipment. It is typified by two phenomena.
An electrodynamics phenomenon
An electrodynamics phenomenon between conductors producing intensive mechanical stress as the current crosses and causing:
1. Distortion of conductors forming the motor windings.
2. Breakage of the conductors’ insulating supports.
3. Repulsion of the contacts (inside the contactors) likely to melt and weld them.
These results are dangerous to property and people. It is therefore imperative to guard against short circuits with protection devices that can detect faults and interrupt the short circuit rapidly, before the current
reaches its maximum value.
Two protection devices are commonly used for this:
1. Fuses, which break the circuit by melting and must be replaced afterwards.
2. Magnetic circuit breakers which automatically break the circuit and only require to be reset.
Short-circuit protection can also be built into multifunction devices such as motor starter protection and contactor breakers.
How are electric motors protcted from short circuits?
A short circuit is a direct contact between two points of different electric potential:
1. Alternating current: phase-to-phase contact, phase-to-neutral contact, phase-to-ground contact or contact between windings in a phase.
2. Direct current: contact between two poles or between the ground and the pole insulated from it.
1. Alternating current: phase-to-phase contact, phase-to-neutral contact, phase-to-ground contact or contact between windings in a phase.
2. Direct current: contact between two poles or between the ground and the pole insulated from it.
This can have a number of causes: damage to the varnish insulating the conductors, loose, broken or stripped wires or cables, metal foreign bodies, conducting deposits (dust, moisture, etc.), seepage of water or other conducting fluids, wrong wiring in assembly or maintenance.
A short circuit results in a sudden surge of current which can reach several hundred times the working current within milliseconds. A short circuit can have devastating effects and severely damage equipment. It is typified by two phenomena.
A thermal phenomenon
A thermal phenomenon corresponding to the energy released into the electrical circuit crossed by the short circuit current I for at time t based on the formula I2t and expressed as A2s. This thermal effect can cause:
1. Melting of the conductor contacts.
2. Destruction of the thermal elements in a bimetal relay if coordination is type 1.
3. Generation of electrical arcs.
4. Calcinations of insulating material.
5. Fire in the equipment.
A thermal phenomenon corresponding to the energy released into the electrical circuit crossed by the short circuit current I for at time t based on the formula I2t and expressed as A2s. This thermal effect can cause:
1. Melting of the conductor contacts.
2. Destruction of the thermal elements in a bimetal relay if coordination is type 1.
3. Generation of electrical arcs.
4. Calcinations of insulating material.
5. Fire in the equipment.
An electrodynamics phenomenon
An electrodynamics phenomenon between conductors producing intensive mechanical stress as the current crosses and causing:
1. Distortion of conductors forming the motor windings.
2. Breakage of the conductors’ insulating supports.
3. Repulsion of the contacts (inside the contactors) likely to melt and weld them.
These results are dangerous to property and people. It is therefore imperative to guard against short circuits with protection devices that can detect faults and interrupt the short circuit rapidly, before the current
reaches its maximum value.
Two protection devices are commonly used for this:
1. Fuses, which break the circuit by melting and must be replaced afterwards.
2. Magnetic circuit breakers which automatically break the circuit and only require to be reset.
Short-circuit protection can also be built into multifunction devices such as motor starter protection and contactor breakers.
Definitions and characteristics
The main characteristics of short-circuit protection devices are:
1. Breaking capacity: the highest value in the estimated shortcircuit current that a protection device can break at a given voltage.
2. Closing capacity: the highest value a protection device can reach at its rated voltage in specified conditions. The closing value is k times the break capacity as shown in the table below.
The main characteristics of short-circuit protection devices are:
1. Breaking capacity: the highest value in the estimated shortcircuit current that a protection device can break at a given voltage.
2. Closing capacity: the highest value a protection device can reach at its rated voltage in specified conditions. The closing value is k times the break capacity as shown in the table below.
source: 2010 IIEE Technical Manual
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