Overcurrent Protection Circuit. What is a safe value? Specifically, the maximum overcurrent-protection (MOP) value must be used to determine the rating of the air conditioning or refrigerant circuit. An overload occurs when an electrical circuit, whether by the original design of a new circuit or by modification of an existing circuit, is required to convey load current in excess of the rated-load ampacity of the circuit conductors. Note that there is an exception to Table 310.15(A)(2) that allows the higher ampacity to be used for cables having different ampacities where the lower ampacity does not exceed 10 ft or 10% of the total circuit length. To use this table, you must know the type of motor used in the circuit and the type of OCP used to protect the circuit. Overcurrent Protection Circuit Working The derated temperature rating is higher than the MCA value of 31.0 amps for the #8 conductor, which is acceptable for this example. From this basic method, the graded overcurrent relay protection system, discriminative short circuit protection, has been formulated. 2.1 Overcurrent protection. 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. A fault, such as a short circuit or ground fault, is not an overload. In this example, the OCP rating cannot be increased above 350 amps. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. For example, a 20-amp branch circuit is modified with an additional lamp, which increases the load current to 22 amps: this would be a circuit overload. The short circuit can be across two or more transformer single-phase AC windings. This fuse will open the 120 volt circuit for any large fault within the computer. 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). True. Temperature derated ampacity = 0.82 x 55 amps. Both types of control restrict the amount of current that can flow in the circuit. The following discussion will be limited to coordinated and selectively coordinated overcurrent protection-the most common form of circuit protection. Fortunately, the NFPA 70: National Electric Code (NEC) gives requirements for most of the applications that electrical engineers and designers encounter in their work. The overcurrent protection scheme is used to protect the distribution lines of electric grids integrated with DER. There are some conditions under which the derating factors do not apply, as seen in 310.15(B)(3)(a)(2) through (4). 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. A utilization-equipment load will operate at 240 volts when connected between the two ends of the two series-connected 120-volt windings. Step#1: Size the overcurrent protection device in accordance with Sections 215-3 for “feeders Overcurrent Protection” . The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. Secondary conductor overcurrent protection must comply with 240.21. This rating falls between the standard ratings of 350 amps and 400 amps. The means of ON/OFF control is normally in the form of a switch (either manual, automatic, electronic, or electromechanical). Table 1describes current, voltage and protection circuit assumptions for each period. By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. A single primary winding in the transformer supplies (by induction) two 120-volt windings wired in series in the secondary. The general rule for overcurrent protection is to select an overcurrent protection device (OCPD) that protects conductors based on their ampacities (after ampacity correction and adjustment per 310.15). Figure 3 illustrates the result of an overload condition. Consider an example of an air conditioning unit with a MOP (circuit breaker or fuse) of 50 amps and an MCA of 31.0 amps. The requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. 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. Distance protection. Ascertain how to protect a circuit from dangerous overloads and short-circuits. 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. Table 310.15(B)(3)(a) is used to determine the appropriate derating factor. The circuit breaker will allow the short circuit current to flow for only a short time. Article 310 provides the general requirements for conductors, insulation, markings, mechanical strength, and ampacity rating. In this case, the circuit breaker opens as fast as it can, and the exact speed depends on the design. Thus, the calculated ampacity for the #1/0 copper THHN wire used in this example is: #1/0 copper THHN ampacity = 170 amps x 0.82 = 139.4 amps. Consider a 25-amp load flowing in a circuit. Either a circuit breaker or fuse could be used since the manufacturer listed the equipment with both types of OCP device. An overcurrent occurs when the current exceeds the rated amperage capacity of that circuit or of the connected equipment (such as an appliance) on that circuit. For example, consider a single-phase 120 V circuit feeding an open-office lighting load (continuous) of 1,000 VA and a small cooling unit’s condensate pump load (non-continuous) of 100 VA. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). Starting from the load end, we have a dual-element or slow-blow fuse at the input of the power supply to the personal computer. The fault current is approximately 5,000 amps. Adjustments for the number of current-carrying conductors in a raceway are found in Table 310.15(B)(3)(a). Figure 5 shows a ground-fault condition. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. When determining OCP ratings for motors, the full-load ampere (FLA) values given in Article 430 are used. Search Products And Discover New Innovations In Your Industry!! In this example, the wires are routed through an environment where the highest temperature is expected to be no higher than 104°F. Although Table 310.16 shows a 12 AWG conductor as having an ampacity of 25A, the asterisk directs us to Section 240.3(D) where we find that a 12 AWG conductor is limited to 20A for our purposes. I. The cable ampacity must be corrected for an ambient temperature of 120°F. Ground-fault overcurrent is also a short-circuit condition that normally affects only one of the circuit conductors and the grounded metal raceway or electrical distribution or utilization equipment enclosure. How to Size an Overcurrent Device for a Transformer. As with the previous example, the wire type selected is copper THHN, which is rated for 90°C. 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. You must provide the protection where the conductors receive their supply [240.4 and 240.21], but several exceptions to this rule exist [240.4(A) through (G)]: • Power loss hazard. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. The overcurrent protection required for transformers is consider for Protection of Transformer only. Instantaneous clearing of 11.4.3.1 Fault Detection Methods. Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. Several of the NEC sections are modified for all the specific installations and equipment found in commercial buildings. For example, a 1.5-hp motor used in a hermetically sealed compressor might be able to do 2 hp of work because heat is being removed from the motor windings, allowing higher currents to flow without exceeding the winding-conductor temperature rating. The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time. For an ambient temperature of 120°F, the correction factor for copper THHN 90°C wire is 0.82. And common method is to connect fuse protector or low voltage circuit breaker. The three major categories or types of overcurrent are overload, short-circuit, and ground-fault. The conduit will contain three current-carrying conductors. Coordinated protective devices provide an optimal balance between fault localization and circuit protection based on the responsible engineer’s judgment. Ground fault: A ground fault is a specific type of short circuit involving at least one of the phase conductors encountering a grounded conductor or surface. Again, if the fault current persists, the insulation will melt and the conductors will eventually be damaged. The air conditioning and refrigeration equipment manufacturer must also indicate whether a fuse or circuit breaker can be used to supply the equipment. For four to six conductors in a raceway, the derating factor is 80%. If the fault current persists, the insulation will melt and the conductors themselves will be damaged. 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. It is common practice to add 20% of the minimum cable rating to be used for future load additions. In the overloaded circuit, the load current is nearly 20 amps. 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). In a 3-phase circuit, two types of short circuits are possible: symmetrical 3-phase faults and unsymmetrical single-phase faults (Figure 1). This condition is allowed by the NEC because overload protection is provided by the overload protector in the motor starter, which is set at 125% of full-load nameplate current for a motor service factor of 1.15. 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. Requirements for motor-circuit protections. By Steven Eich, PE, CDT, REP, LEED AP; Environmental Systems Design, Chicago, Specifying electrical distribution systems, Risk management: Commissioning electrical systems, Understanding the changes to NFPA 70-2020, Designing flexible, safe labs: Electrical, power and lighting, Designing flexible, safe labs: Fire and life safety, Circular economy ideas are leading to better real estate investments, Designing flexible, safe labs: HVAC and plumbing, Top 5 Consulting-Specifying Engineer articles: December 11-17, 2020. This trip … This seems like an example of a noncontinuous circuit. This places a short-circuit across the supply-transformer winding. 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. The next highest standard OCP (see table 240.6(A)) is 15 amps. Ambient-temperature correction factors for 40°C tables are found in Table 310.15(B)(2)(b). Without some form of overcurrent protection device installed in series with the circuit conductors, the only limit of the fault overcurrent is the conductor resistance and the amount of power available from the transformer. The means of limit control is normally an overcurrent protective device, which at the electrical-power distribution level is a fuse or circuit breaker (as seen in Figure 4). It is important to have a distinction continuous and noncontinuous loads because of heat. Overload conditions are usually not as time-critical as short circuits and ground faults. Overload overcurrent is self-defining: Any current in excess of rated-load current is, in effect, an overload. It has a limitation that it can be applied only to simple and non costly equipments. 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). An overcurrent can be caused by overloading the circuit or by a short circuit, a ground fault, or an arc fault. Table 240.4(G) requires Article 430 to be used for selection of motor-circuit overcurrent protection. 240 III. There are three principal components of an electrical circuit: a power source, a load, and a connection between the two. Figure 1 is the drawing of a single-phase AC, 3-wire, 120/240-volt supply to a building such as a home or small industrial facility). This section requires the use of the 60°C column in Table 310.15(B)(16), because the terminations for equipment rated 100 amps or less is assumed to be rated for 60°C unless listed and labeled otherwise. 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. max be the upper limit on the speed of primary protection. An electrical overload overcurrent also occurs when a motor is mechanically overloaded. The operation of the consecutive time-inverse overcurrent relays are coordinated according to the discrepancy between the fault currents measured by these relays. Action current of low voltage circuit is 1.2 times of starting current of electric motor. After applying the appropriate deratings, the calculated ampacity of the #1/0 wire is adequately protected by the 150-amp OCP selected above. Determining whether a load is continuous or noncontinuous is not always clear. 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. As shown in Figure 5, the electrical power distribution system within a building or other structure has three major classifications: the service, the feeder circuits, and the branch circuits. 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). 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. Such overcurrent protection will not necessarily protect the primary or secondary conductors or equipment connected on the secondary side of the transformer. The model for an overcurrent waveform is shown in Fig. Feeder circuits. Overcurrent Circuit protection would be unnecessary if overloads and short circuits could be eliminated. How to protect a circuit from dangerous overloads and short circuits. Consider the case of an office storeroom lighting circuit. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. 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. Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). Figure 2 depicts a simple single-phase circuit operating in a normal configuration. But there is much more to it. Information regarding equipment-overload capability often comes from the manufacturer. For an ambient temperature of 104°F, the correction factor for copper THHN 90°C wire is 0.91. In the example above, the circuit conductors (copper heat-resistant thermoplastic (THHN[A1] [A2] )) are routed through the office environment in a conduit containing six current-carrying conductors. If circuit interruption will create a hazard (e.g., fire pump circu… 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. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. The full-load current rating of the connected load dictates the size (by rated ampacity) of the supply conductors and the rating or setting of the OCPD. Also, minimum circuit amperes (MCA) must be used to determine the minimum conductor rating. Of the factors to be considered, two commonly encountered factors, or deratings, are the ambient temperature and the number of conductors in a raceway. In this case the relay R 1 would have to backup the fuse. Number of conductors. For motor circuits, overload protection is provided by the motor-overload protector (refer to Article 430 Part III). The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. In the noncontinuous case (for example, a large sump pump), the load might be active for less than a minute. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. Which type is intended for all types of fault current conditions? The OCP used to provide power to the motor controller and motor has to then provide short-circuit and ground-fault protection for the motor circuit. Referring to Table 310.15(B)(2)(a), the correction factor for 90°C-rated copper THHN wire with a maximum ambient temperature of 120°F is 0.82. Overcurrent protection schemes are generally designed with a primary means of clearing a fault, as well as one or more backup methods. Phone apps are also available that provide the same function as the slide rule. Differential protection. However, many authorities having jurisdiction have made voltage drop a code requirement. Depending on the time of operation the overcurrent relay is categorized into following types. The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). Based on the relay operating characteristics, overcurrent relays can be classified into three groups: definite current or instantaneous, definite time, and inverse time. 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. Note that the maximum OCP rating is 350 amps, which is significantly higher than even the 90°C column ampacity of 195 amps. For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. These above mentioned requirements cause protection of transmission line much different from protection of transformer and other equipment of power systems. If you'd like to know more about a specific topic relating to overcurrent, please share your thoughts in the comments section below. When determining the overcurrent protection requirements for transformers in a circuit, other requirements from Article(s) _____ may also need to be investigated. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Overcurrent protection Overcurrent refers to the operating state of electric motor or electrical apparatus element surpassing the rated current. This protection scheme is further classified into two categories, the phase overcurrent protection and the ground overcurrent protection. The main three methods of transmission line protection are – Time graded over current protection. NEC 450.3. Air conditioning and refrigerating equipment circuits. Typical practice is to use an OCP rating less than the maximum calculated above. Comparing the two cases, the chiller circuit conductors will be at a higher temperature during operation than the sump pump circuit conductors. The motor-circuit OCP is providing only short-circuit and ground-fault protection. In this case, the maximum OCP rating is then 350 amps. Next, derating for the number of conductors in the conduit must be considered. Table 240.4(G) requires Article 440 to be used for selection of motor-circuit overcurrent protection for air conditioning and refrigeration equipment. Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. Symmetrical 3-phase faults rarely occur, but their analysis is useful in understanding a system’s response to a fault and usually results in the worst-case fault levels. In the example above, there are three current-carrying conductors routed in the conduit. The feeder overcurrent protection device must be sized not less than 125% of 184 amperes, So, overcurrent protection device size = 184 amperes x 125% = 230 amperes. Thus overcurrent relays should be used over a limited length in the 3 feeder sections. In this example, the wires are routed through a boiler room where the highest temperature is expected to be no higher than 120°F. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. 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. The circuit is protected by a 15-amp circuit breaker. For long circuits, the conductor size may need to be increased to maintain minimum voltage-drop requirements. The different types of faults are shown in Figure 1 to illustrate the concept of overcurrent protection. Symmetrical faults result in the same current flow in each phase during the fault condition. Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. This is the simplest of the ways to protect a line and therefore widely used. The #10 wire has sufficient ampacity, but per Section 240.4(D), it must be protected with an OCP with a rating of 30 amps or less. Sections for articles pertaining to equipment typically found in commercial buildings include: The general requirement for overcurrent protection of conductors is provided in Section 240.4, Protection of Conductors. In the continuous case (for example, a chiller), the load might be active for 8 hours or longer. I hope that this article has helped you achieve a better understanding of overcurrent and overcurrent protection devices. Fig. Search Products And Discover New Innovations In Your Industry. Next, derating for the number of conductors in the conduit must be considered. 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. The ground-fault overcurrent normally affects only one single-phase AC winding in the transformer supplying power to the faulted condition. Would be unnecessary if overloads and short-circuits any current in the conduit 85°F the... Repurposed as an office storeroom lighting circuit less fault current than a minute president and technical... Limiting the amount ( value ) of current outside the intended current path ) requires 430! Overcurrent protection scheme is further classified into two categories, the requirements for branch circuits simple... ( for example, Table 430.52 allows a circuit breaker or fuse could be eliminated appropriate deratings the. 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