Terms and Definitions
3 Overload and Overcurrent Protection
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Inrush Current
When a motor is first started, before the shaft has a chance to pick up speed and begin to rotate, the characteristics of the stator coil are that of a short circuit. As such the motor starts to draw very high values of current. This current creates a magnetic field that causes the motor shaft to spin, and that spinning action creates a counter-EMF (CEMF), which limits the current to its normal running value.
The initial high value of current is called inrush and can cause severe line disturbances and nuisance tripping if fuses and circuit breakers are not sized accordingly.
Overload
The term “overload” describes a moderate and gradual rise in the value of current over a relatively long period of time. It is caused by excessive amounts of current drawn by a motor, which may be as high as six times the rated current. This is caused by too much load on a motor. Systems are protected by overload protection relays. While overloads are allowed for a short time (usually minutes), prolonged overloads will use thermal action to cause a protective device to trip.
Overcurrent
The term “overcurrent” (sometimes called a short circuit or a ground fault) describes a sharp and fast rise in current over a short period of time (fractions of a second). Circuits and equipment are protected from overcurrent situations by fuses or circuit breakers.
In these cases, the value of current is far greater than the nominal line current and can indeed be anywhere from six times to many hundreds of times greater the normal rated current value of current.
There are several causes of overcurrent situations. For example, when a bolted fault occurs—either a line to ground or a line to line fault. This causes a very large value of current to be drawn because of the inversely proportional relationship between the resistance of a circuit and the current drawn.
Another less intuitive cause of short circuits is when an induction motor starts. When a three-phase induction motor is first energized, the stator windings consist of a very low resistance path. This draws a very large inrush current which is indistinguishable from a standard short circuit, except that it quickly drops down to the rated value of current drawn by the motor. This is due to the CEMF (counter-electromotive force) developed by the rotating shaft of the motor. When the motor is spinning, a CEMF limits the current to safe values. When the motor is not spinning, a very large value of current is drawn from the source. This current is sometimes called locked-rotor current, and motor starters and overcurrent devices must be rated to safely handle this value of current.
Effects of short circuits
Two of the main negative outputs of overcurrents are:
- Thermal energy: High values of current will create lots of heat, which can damage equipment and wires. Thermal energy can be expressed by I2t (current squared times time)—the longer the fault persists, the greater the potential thermal damage.
- Mechanical forces: High-fault currents can create powerful magnetic fields and exert huge magnetic stress on busbars and equipment, sometimes warping them out of shape and creating other problems.
Large values of fault current can cause damage very quickly, so overcurrent protective devices must act very quickly to clear the fault. There are two main categories of overcurrent protective devices: fuses and circuit breakers.
Fuses
A fuse is a simple device that protects the conductors and equipment of a circuit from damage due to higher than normal fault values. It is designed to be the weakest link in a circuit.
A fuse is an insulated tube containing a strip of conductive metal (fuse-link) that has a lower melting point than either copper or aluminum. The fuse link has narrow, resistive segments that concentrate the current and cause the temperature at those points to rise.
In a short circuit, the fuse elements burn open in just a fraction of a second. The higher the values of fault current, the faster the fuse will react.
In an overload situation, the fuse elements can take many seconds or even minutes before thermal actions cause the fuse link to melt open.
Fuses come in two categories: Fast-acting fuse (Type P) and time-delay fuse (Type D).
Fuses used in motor circuits have to withstand the intense inrush current when the motor is started, and so we use time-delay fuses, also known as “dual-element fuses.”
Common ratings
All overcurrent devices must be operated within their rated values. Three of the most important ratings are voltage, current, and interrupting capacity.
Voltage rating
Fuses and circuit breakers must be rated for at least the value of the voltage of the circuit they are designed to protect.
When a fuse or circuit breaker interrupts a fault current, it must safely extinguish the arc and prevent it from reestablishing itself. Therefore, the voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.
For example, a fuse rated at 240V RMS will be acceptable for use in a 120V circuit. However, it would exceed the fuse’s voltage rating to use it in a 600V circuit.
Continuous-duty rating
Continuous-duty rating describes the maximum rated value of RMS current that the overcurrent device is designed to handle on a continuous basis without tripping. Generally speaking, the ampere rating of the fuse or breaker should not exceed the current carrying capacity of the circuit, but there are exceptions, such as certain motor circuits.
Interrupting capacity
When a short circuit or ground fault occurs, the circuit resistance drops to effectively zero ohms, causing very large values of current to flow. This extremely fast rise in fault current can cause damage to wires and equipment through overheating and must be extinguished as quickly as possible.
The interrupting-capacity (IC) rating of an overcurrent device is the maximum fault current that the device can interrupt without damage to itself. Most circuit breakers and fuses have an IC rating of 10,000 amps.
For systems capable of larger fault currents, high-rupture capacity (HRC) fuses can interrupt currents up to 200,000 amps by using an arc-quenching filler such as silica sand to help interrupt the fault.
The rate of flow of an electric charge, measured in amperes (or amps). When one coulomb of charge moves past one point in once second, current is said to flow at a rate of one ampere. Current flows from negative potential to a positive potential through a load.
The initial high value of current produced when an inductive load is first energized.
An insulated tube containing a strip of conductive metal that has a lower melting point than either copper or aluminum. It protects a circuit from damage because it will melt in overload or overcurrent situations and break the connection with the rest of the circuit.
An automatic device that is designed to safely disconnect circuits under fault conditions. Most circuit breakers provide Overload and Overcurrent protection, and are rated in Volts, Amps and Horsepower.
A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor.
A heater element paired with normally-closed contacts that open once the heater gets too hot. Two types of relays are the bimetallic strip and the melting solder pot.
A sharp and fast rise in current over a short period of time (fractions of a second) where the value of current is far greater than the nominal line current.
The opposition to the flow of current in an electric circuit, measured in ohms (Ω).
The current drawn by a motor when the motor is not spinning.
A device that controls the flow of electrical power to a motor. It is designed to safely start and stop a motor, and provide overload protection.
The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points. It is measured in volts (V).
The maximum amount of voltage that a fuse, circuit breaker, switch-gear or motor starter can handle. The voltage rating of a fuse or circuit breaker must be equal to or greater than the system voltage.
The maximum rated value of RMS current that the overcurrent device is designed to handle on a continuous basis without tripping.
The unit used to measure electrical current. It is equal to a flow of one coulomb per second. It may also be called "amp."
The unit used to measure electrical resistance (Ω). It takes one volt to push one amp through one ohm of resistance.
The maximum fault current that an overcurrent device can interrupt without damage to itself. Most circuit breakers and fuses have an IC rating of 10,000 amps.