What is an overcurrent protection device?Understand the basics of overcurrent
For a circuit to work properly, the current should be limited to a safe level. This safe current level is determined by the current handling capabilities of the load, conductors, switches, and other system components. Under normal operating conditions, the current in the circuit should be equal to or less than normal current levels. However, sometimes a circuit may have a higher than normal current flow (overcurrent).
What is overcurrent?
Overcurrent is a condition that exists in a circuit when the normal load current is exceeded. Overcurrent conditions can be caused by short circuit or overload conditions.
In a short circuit condition, the current bypasses the normal current path.
Although a partial short circuit will increase the current level, it may or may not cause damage, depending on the ratings of the circuit components. However, if there is a complete short circuit, the load resistance will be completely eliminated from the normal current path. Figures 1a and 1b illustrate this.
Figure 1a. Local short circuit.
Figure 1b. Complete short circuit.
If the power supply has enough stored energy when a complete short circuit occurs, circuit components may be damaged or explode. Switches can melt or evaporate, conductors can overheat, and insulation can burn. It may also damage the power supply.
Temperatures generated due to partial or complete short circuits may cause fires, resulting in loss of property and life. Because the risk is so great, all circuits must be protected from short circuit conditions.
Overload conditions may also cause overcurrent conditions. For example, consider a situation where too many loads are connected to a given power source. Even if each of these individual loads draws its normal current, the total current may exceed the power supply's rating.
If the overload lasts only a short time, the temperature rise is small and has little or no effect on the equipment or conductors. However, sustained overloading is destructive and must be prevented.
Unlike a short circuit, an overload does not cause a sudden arc, and the system may survive the overload situation even if we do not remove it from the system immediately. Over long periods of time, however, overloading can cause fires due to overheating of equipment and conductors.
Figure 2 depicts an overload circuit. At this time, the rated current capacity of the branch is 15A; however, the total current consumed by the parallel loads is 17A. The circuit was overloaded by 2 A, resulting in the circuit breaker tripping.
Overcurrent protection circuit
The resistance of a fuse or circuit breaker is very low and usually accounts for only a small portion of the total circuit resistance. Under normal circuit operation, it functions only as a conductor.
Both fuses and circuit breakers are in series with the circuit they protect. Generally, these overcurrent devices must be installed where the protected conductor is energized; for example, at the beginning of a branch circuit, as shown in Figure 3.
Figure 3. Connection of overcurrent protection device.
If an overcurrent condition occurs, the fuse will blow or the circuit breaker will trip. Although these devices protect the circuit from overcurrent conditions, they only open the circuit and remove power. They often fail to solve the problem. Therefore, we need to find and correct the problem before replacing the fuse or resetting the circuit breaker.
Common overcurrent protection devices (OCPD)
An overcurrent protective device (OCPD) is an electrical device used to protect services, feeders, branch circuits and equipment from overcurrent by interrupting the flow of electricity.
Overcurrent protection simply means using a fuse, circuit breaker, or fusible link to protect equipment, the circuits in the equipment, or the wiring of the equipment. These terms are often used interchangeably because they share some similarities. Circuit breakers or fuses are typically used to protect an entire device from excessive current, but they can be sized to protect one component within the device. This provides overcurrent protection for equipment and optional protection for components such as transformers or circuit boards.
Figure 4 shows two common types of fuses used in control circuit boards: plug-in fuses and glass (Buss) fuses. These types of fuses can also be found on the secondary side of the transformer.
Figure 4. Plug-in fuses are used to protect circuit boards from overcurrent conditions. Glass fuses are available as plug-in fuses or in fuse holders. (Pennies included for size reference.) Figure 5 shows the circuit board with the plug-in U-shaped fuse.
Figure 5. This is the circuit board of the air handler, with optional heating tape. Note the 3 A plug-in fuse located on the upper left side of the circuit board.
A circuit breaker or fuse with the correct amperage and voltage rating should be within easy reach of the heating system. Typically, the circuit breaker is rated the same as the amperage listed on the nameplate of the electric heating unit.
The installation contractor may need to analyze the ampere rating of the installation to apply the correct size circuit breaker. In some cases, a circuit breaker can be specified at 115% of the device's amperage.
Oversized circuit breakers should not be used. Circuit breakers are designed to protect equipment and electrical wiring. A circuit breaker with too much amperage will not shut off the power if the current is too high. A circuit breaker that is too small will shut off the power before the device can draw current.
A fusible link (see Figure 6) is usually connected in series with the electric heating element. The purpose of this link is to open when high current or heat is encountered.
Figure 6. This common fuse is in series with the heating circuit.
The fuse cannot be reset and must be replaced if open. The cylinder is silver and has the manufacturer's information printed on it. This information can include temperature and current rating. The cylindrical device has a square end and a tapered end. The cones can be black or red, depending on the color of the material used in the manufacturing process. The link resistance can be checked to determine if it is an open circuit (its resistance should be zero ohms).
Both fuses and circuit breakers have current and voltage ratings.
Continuous rated current
The continuous ampere rating marked on a fuse or circuit breaker represents the amount of current the device can carry without blowing or tripping. The rated current must match the full load current of the circuit as closely as possible. For example, an undersized fuse may blow easily, while an oversized fuse may not provide adequate protection.
The voltage rating of a fuse or circuit breaker is the voltage it is designed to safely interrupt the flow of electricity. Specifically, the voltage rating determines the device's ability to suppress internal arcing that occurs when current is turned on during overcurrent or short-circuit conditions. The rated voltage must be at least equal to or greater than the circuit voltage. It can be higher, but never lower. Low voltage circuit breakers use power below 1000 V to protect circuits.
Interruption current rating
The interrupting current rating (also called the short-circuit rating) of a fuse or circuit breaker is the current that it can safely interrupt. If the fault current exceeds the interrupting capability of the protective device, the device may actually rupture, causing additional damage.
The interrupt current rating is many times greater than the continuous current rating and should far exceed the current the power supply can deliver. Typical interrupting ratings are 10,000 A, 50,000 A and 100,000 A.
Current limiting capability
Current limiting capability is a measure of the amount of current an overcurrent protection device can allow through the system. The operating time of the current limiting protection device is less than half a cycle. For example, a current-limiting fuse that supplies a short-circuit current will begin to melt within one-quarter of a cycle of an AC wave and blow the circuit within one-half of a cycle.
The time-current characteristic or response time of a protective device is the length of time required for the device to operate under a fault current or overload condition.
Fast-acting rated protective devices may respond to an overload in less than a second, while standard types may take from 1 to 30 seconds, depending on the magnitude of the overload current. Fast-blow fuses are very sensitive to increased current and are used to protect extremely delicate electronic circuits through which current flows steadily.
The important role of circuit overcurrent protection
Circuit overcurrent protection is an important part of every circuit. If a circuit's voltage and current levels exceed the safe levels for which it is designed, it can damage or even destroy the circuit. Generally speaking, fuses and circuit breakers are designed to protect people, conductors, and equipment. Both work on the same principle: interrupt or disconnect a circuit as quickly as possible before damage occurs.
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