Overload is a controlled overcurrent situation, normally of low magnitude. Even though this example is using copper THHN wire, rated for 90°C, the 60°C column must be used due to the requirement of Section 110.14(C)(1)(a). As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. Next, select a conductor in accordance with sections 215.2 and 310.15. Consider a 208 V, 3-phase feeder supplying a panelboard with a noncontinuous load of 10 kVA and a continuous load of 30 kVA. For this example, the wires are routed through an office environment where the highest temperature is expected to be 85°F during times when the cooling systems are turned off. In general, the conductors of all of these circuits must be provided with a means of overcurrent protection at the point where they receive their electrical supply. [nextpage title=”Over Current Protection (OCP)”] There are a lot of misconceptions about the over current protection (OCP) and an explanation of why this protection exists is in order. Either a circuit breaker or fuse could be used since the manufacturer listed the equipment with both types of OCP device. Section 215.2 requires the conductor to be sized in the same manner as the OCP-no less than 125% of the continuous load and 100% of the noncontinuous load. The requirements for determining the maximum rating or setting for motor-branch-circuit short circuit and ground-fault protection can be found in Table 430.52. There are several applicable sections of the NEC that set the requirements for the selection of OCPs and conductors for commercial buildings. Overcurrent Protection in AC Power Systems, Historical Engineers: George Westinghouse and the AC Power Grid, Improving Home Appliance Safety with IEC Standards, Common Analog, Digital, and Mixed-Signal Integrated Circuits (ICs). The conduit will contain three current-carrying conductors. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. As with branch circuits, the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the noncontinuous load. A fault, such as a short circuit or ground fault, is not an overload. There are three principal components of an electrical circuit: a power source, a load, and a connection between the two. Using Table 310.15(B)(16), the 75°C column, the minimum wire size is #8. This waveform is divided into six periods based on the state of operation of the MOSFET, the drain current relative to the current limit threshold and the output state of the protection circuit. But there is much more to it. 517. Transformer primary protection— Overcurrent protective devices (OCPDs) are required as primary protection for transformers when the primary voltage is greater than 1,000V [Tables 450.3(A)] and for transformers with primary voltage of 1,000V or less [Table 450.3(B)], with no Exceptions. However, many authorities having jurisdiction have made voltage drop a code requirement. This may be caused by excess friction within its internal bearing surfaces, excess heat (due to high ambient temperature or another failure), or by the binding or some other mechanical overload in the utilization equipment it drives. With coordinated overcurrent protection, the faulted or overloaded circuit is isolated by the selective operation of only the overcurrent protective device closest to the overcurrent condition. Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). Protecting the circuit conductors against overheating by limiting the amount of current the conductors are required to convey inherently protects the supplied electrical distribution and utilization equipment (the connected load) from the effects of overcurrent. Consideration should be given to load growth. To protect a circuit against these currents, a protective device must determine when a fault condition develops and automatically disconnect the electrical equipment from the voltage source. Once the ampacity is determined, the voltage drop should also be considered. The overcurrent protection required for transformers is consider for Protection of Transformer only. Thus overcurrent relays should be used over a limited length in the 3 feeder sections. Some examples of continuous loads include office lighting, exterior lighting, data center equipment, fixed storage-type water heaters with capacities of less than 120 gal (450 l; as per NEC 422.13), and chilled/hot-water circulating pumps. Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. Unfortunately, overloads and short circuits do occur. They also protect circuits downstream from the transformer. Overcurrent relays are the typical protection for conventional distribution systems. In this article, we will cover types of overcurrent, what overcurrent protection devices are, and their place in an electrical circuit. Search Products And Discover New Innovations In Your Industry. Also, minimum circuit amperes (MCA) must be used to determine the minimum conductor rating. It protects the equipment of the power system from the fault current. In this example, the wires are routed through a boiler room where the highest temperature is expected to be no higher than 120°F. The motor-overload protector is usually a device, located in the motor starter, that responds to motor current and is set to trip open the motor controller when the motor current exceeds 125% of the nameplate current for 1.15 service factor motors or 115% of nameplate current for motors without a service factor. However, the next higher rating allowance does not apply to Exception (c). Further, Section 240.4(D), Small Conductors, requires the OCP for #14 wire to be rated at 15 amps. Where the primary overcurrent protective device does not exceed 125 percent of the primary full load current, secondary overcurrent protection for the transformer is not required. Instantaneous clearing of 450. His expertise includes 29 years of designing electrical systems for industrial and commercial projects including high-rise buildings, hospitals, schools, theaters, museums, hotels, convention centers, manufacturing facilities, water treatment plants, and nuclear power facilities. Determining whether a load is continuous or noncontinuous is not always clear. The circuit is protected by a 15-amp circuit breaker. A three-phase AC electrical power distribution system, as shown in Figure 2, will normally have a higher value of short-circuit overcurrent because the short will normally involve more than one single-phase AC transformer winding. Both the conductors and the connected load they supply must be protected at the correct amperage. This section requires the use of the 75°C column in Table 310.15(B)(16), because the terminations for equipment rated 100 amps or higher are required to be rated for 75°C unless listed and labeled otherwise. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. The circuit breaker will allow the short circuit current to flow for only a short time. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. The fuses or circuit breakers located downstream provide full overcurrent protection for any circuits or equipment located on their load side while providing only overload protection for their line- or supply-side circuit. When determining the overcurrent protection requirements for transformers in a circuit, other requirements from Article(s) _____ may also need to be investigated. 230 II. Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. Both types of control restrict the amount of current that can flow in the circuit. For an ambient temperature of 85°F, the correction factor for copper THHN 90°C wire is 1.0, so no ampacity adjustment is required. Figure 2 depicts a simple single-phase circuit operating in a normal configuration. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment. This is the simplest of the ways to protect a line and therefore widely used. If the motor starting torque and time to reach operating speed is such that the motor will not start, Section 430.52(C)(1), Exception 2, allows the OCP rating to be raised even higher. Section 210.19(A) requires the conductor to be sized in the same manner as the OCP-no less than 125% of the continuous load and 100% of the noncontinuous load. The motor-circuit OCP is providing only short-circuit and ground-fault protection. If circuit interruption will create a hazard (e.g., fire pump circu… The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. In this example, the OCP is simply equal to the manufacturer-supplied MOP value of 50 amps, since 50 amps is a standard OCP rating per Table 240.6(A). Since there are only three current-carrying conductors in the conduit for this example, derating for the number of conductors is not required. Depending on the time of operation the overcurrent relay is categorized into following types. This places a short-circuit across the supply-transformer winding. Article 310 provides the general requirements for conductors, insulation, markings, mechanical strength, and ampacity rating. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. This data will be found on the equipment nameplate and can also be obtained from the manufacturer in the form of a data sheet. A few of the symptoms of overcurrent in a motor are shorts, blown fuses and … 1. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Unsymmetrical faults have different fault currents in each phase. A utilization-equipment load will operate at 120 volts when connected between either end of the two series-connected 120-volt windings and the third wire shared by the two windings (see Figure 1). Then . The requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. Protection 2 •Zones of Protection •Ground Fault Protection •Phase Fault Protection ... 51V Voltage Controlled/Restrained Overcurrent 24 Volts per Hertz 32 Reverse Power 40 Loss of Field ... this method converts three unbalanced phases into three independent sources, which makes asymmetric fault analysis more tractable. Once the circuit breaker opens or "trips" due to a short circuit or … Search Products And Discover New Innovations In Your Industry!! Don't have an AAC account? Calculation of the available short-circuit current is outside the scope of this discussion. Some electrical distribution manufacturers provide slide-rule-type guides to help select motor-circuit ratings. Though at first glance the NEC requirements might not seem straightforward, there is solid reasoning behind the overcurrent-protection code rules. Time Graded Over Current Protection The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time. The circuit diagram for the same is simple and can be seen in the below image. The calculated ampacity for the #2/0 copper THHN wire used in this example is: The conductor ampacity derated for ambient temperature is higher than the 75°C ampacity column, so it is acceptable for use in this example. In this case, the 75°C column must be used due to the requirement of Section 110.14(C)(1)(a). 427 Fixed electric heating for pipelines and vessels, 430 Motors, motor circuits, and controllers, 440 Air conditioning and refrigerating equipment. How do you limit the current flow? 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. Ascertain how to protect a circuit from dangerous overloads and short-circuits. These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. The NEC has informational notes concerning voltage drop for branch circuits and feeders, but it is not a code rule. Ground faults include a single line-to-ground fault and multiple-line-to-ground faults (Figure 1). Therefore, the minimum conductor size is 10 AWG, and the overcurrent protection or … It owes its application from the fact that in the event of fault the current would increase to a value several times greater than maximum load current. If a transformer serves a fire pump (Art. Circuit breakers protect transformers from overcurrent situations and short circuits. The NEC requires a more conservative selection (125%) in the case of continuous loads due to increased heat dissipated by the circuit conductors as compared with noncontinuous loads. A: Overcurrent protection for the transformer is covered by 450.3(B) and Table 450-3(B). In the overloaded circuit, the load current is nearly 20 amps. In this example, the air conditioning unit is fed using copper THHN wire in a conduit containing three current-carrying conductors. Instant, definite time, direction, and non-directional, IDMT (Inverse Definite Minimum Time Relay), are some of the overcurrent characteristics that are often used in complex distribution systems. Number of conductors. Fig. The conductors will begin to heat up, but will not be damaged. This should not be mixed with ‘overload’ relay protection, which typically utilizes relays that function in a time-related in some degree to the thermal capacity of the equipment to be protected. In an over current relay or o/c relay the actuating quantity is only current.There is only one current operated element in the relay, no voltage coil etc. Table 310.15(B)(2)(a) provides the ambient-temperature correction factors, which must be applied to the ampacities given in Table 310.15(B)(16). Branch circuits. If designed to current energy code requirements, it should have a vacancy sensor to automatically shut off the lights when there are no occupants detected. As with branch circuits, the general requirement is to size the OCP no less than 125% of the continuous load and 100% of the noncontinuous load. How to protect a circuit from dangerous overloads and short circuits. The basic rule for overcurrent protection of conductors—other than using flexible cords, flexible cables, and fixture wires—is to protect the conductor in accordance with the ampacities specified in Section 310.15. Typical practice is to use an OCP rating less than the maximum calculated above. Any current magnitude that is greater than the rated ampacity of the conveying wires or the rated-load current of the electrical utilization equipment—such as light fixtures, motors, or transformers—is described as an overcurrent. As with motors, the requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. [HEAD]. These simplifying assumptions make it straightforward to write equations for voltage, current and power, and integrate the power equation to determine the energy dissipated in the … Action current of low voltage circuit is 1.2 times of starting current of electric motor. As shown in Figure 3, full overcurrent protection for the conductors and the connected load can only be provided by a fuse or circuit breaker installed at the point where the circuit originates (or where it receives its supply). And 400 amps protection seems like a simple concept: limit the flow. 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