The problem has many symptoms and can eventually lead to permanent damage to the motor or electrical device. The requirements for motor-circuit overcurrent protection start with Table 240.4(G), Specific Conductor Applications. However, the next higher rating allowance does not apply to Exception (c). In cases where a load is certain to be noncontinuous, size the circuit to 100% of the load. There are five different common overcurrent protection trip types that a circuit breaker may incorporate. 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. This is the simplest of the ways to protect a line and therefore widely used. If circuit interruption will create a hazard (e.g., fire pump circu… 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. 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). A single primary winding in the transformer supplies (by induction) two 120-volt windings wired in series in the secondary. Review overcurrent protection for certain types of building equipment. 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. In this example, the OCP rating cannot be increased above 350 amps. 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. This is achieved by limiting the amount (value) of current the conductors are required to convey. The paper goes on to discuss the rules for the coordination of negative-sequence overcurrent characteristics for sensitive phase-to-phase fault protection in feeders, as well as for unbalanced current protection of induction motors. 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. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. Consideration should be given to load growth. 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. Instantaneous clearing of What happens during an overload or fault condition? In our example, there are six current-carrying conductors routed in the conduit. However, many authorities having jurisdiction have made voltage drop a code requirement. In the case of hermitically sealed motor compressors, the motor FLA values found in Article 430 will not be higher than actual motor values due to the cooling effect the refrigerant has on the motor windings. 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. Several of the NEC sections are modified for all the specific installations and equipment found in commercial buildings. 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. The NEC definition of a continuous load is a load where the maximum current is expected to continue for 3 hours or more. Overcurrent protection is one of the essential elements of a proper electrical installation. Overcurrent protection seems like a simple concept: Limit the current flow in a circuit to a safe value. A simple overcurrent protection circuit can be designed by using an Op-Amp to sense the overcurrent and based on the result we can drive a Mosfet to disconnect/connect the load with the power supply. The wire size will be based on the manufacturer-supplied MCA value, which in this case is 31.0 amps. Section 310.15 contains limiting factors that must be applied to the ampacity table values when determining ampacity for your specific design conditions. 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. The requirements for overcurrent protection of equipment can be found in the NEC article that addresses that specific equipment. When determining OCP ratings for motors, the full-load ampere (FLA) values given in Article 430 are used. However, a fire pump has a special requirement for its transformer primary and secondary protection, which is considered a modification to Art. What if the sensor failed or the room was temporarily repurposed as an office? Section 430.22 applies to this example in that it is a single motor circuit. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Action current of low voltage circuit is 1.2 times of starting current of electric motor. In the noncontinuous case (for example, a large sump pump), the load might be active for less than a minute. 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. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. Time Graded Over Current Protection The fault current is approximately 10,000 amps. What is a safe value? Search Products And Discover New Innovations In Your Industry. 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. Overcurrent protection Overcurrent refers to the operating state of electric motor or electrical apparatus element surpassing the rated current. The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time. Figure 3 illustrates the result of an overload condition. In this case, the load current is 10 amps. The derated temperature rating is higher than the MCA value of 31.0 amps for the #8 conductor, which is acceptable for this example. These values are generally higher than the FLA values found on the actual motor nameplate, resulting in conservatively selected OCP and conductor ratings. The air conditioning unit is located outdoors with a maximum ambient temperature of 120°F. For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. Referring to Table 310.15(B)(16), using the 75°C column, the minimum size wire acceptable is #2/0 with a rating of 175 amps. Overcurrent Circuit protection would be unnecessary if overloads and short circuits could be eliminated. Fig. If an OCPD is located downstream from the supply, the overcurrent protection is technically subdivided with short-circuit, ground-fault protection located upstream, as well as separate overload protection located downstream. Ascertain how to protect a circuit from dangerous overloads and short-circuits. Both the conductors and the connected load they supply must be protected at the correct amperage. Figure 5 shows a ground-fault condition. After applying the appropriate deratings, the calculated ampacity of the #1/0 wire is adequately protected by the 150-amp OCP selected above. The primary purpose of a circuit overcurrent protection device (a fuse, a circuit breaker, or some other type of current-limiting device) is to limit the temperature of the circuit conductors to a value that will not damage the conductors or their insulation. When determining the overcurrent protection requirements for transformers in a circuit, other requirements from Article(s) _____ may also need to be investigated. 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). 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. 2.1 Overcurrent protection. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. Information regarding equipment-overload capability often comes from the manufacturer. For four to six conductors in a raceway, the derating factor is 80%. The magnitude of ground-fault overcurrent is normally less than the magnitude of short-circuit overcurrent available from the same transformer. Short circuit: A short circuit is defined as flow of current outside the intended current path. Though at first glance the NEC requirements might not seem straightforward, there is solid reasoning behind the overcurrent-protection code rules. The motor full-load current used in this calculation is not the nameplate current, but the current value found in Table 430.250. Understand the three types of overcurrent conditions to consider in typical NFPA 70: National Electrical Code applications. [HEAD]. NEC Table 240.3 provides a list of the applicable sections. A few of the symptoms of overcurrent in a motor are shorts, blown fuses and … For an ambient temperature of 120°F, the correction factor for copper THHN 90°C wire is 0.82. Which type is intended for all types of fault current conditions? It protects the equipment of the power system from the fault current. True. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. 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. Where possible, it is preferred that instantaneous methods of detecting overcurrent be used as the primary protection method on all of the major equipment associated with the power system. Typical practice is to use an OCP rating less than the maximum calculated above. 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. Once the ampacity is determined, the voltage drop should also be considered. If the fault current persists, the insulation will melt and the conductors themselves will be damaged. Figure 2 depicts a simple single-phase circuit operating in a normal configuration. As discussed above, the 60°C ampacity of 15 amps must be used for the #14 wire in this example despite the higher ampacity calculated. The OCPD must be installed in accordance with the requirements of the National Electric Code. Next, derating for the number of conductors in the conduit must be considered. Unsymmetrical faults are more common and usually result in less fault current than a symmetrical 3-phase fault. Overload conditions are usually not as time-critical as short circuits and ground faults. “Reference grounding” requires the common connection of one end of one or more of the single-phase AC transformer windings (wye transformer configuration) to a grounding-electrode system, creating both grounded and ungrounded circuit/supply conductors. Ambient-temperature correction factors for 30°C tables are found in Table 310.15(B)(2)(a). The motor-circuit OCP is providing only short-circuit and ground-fault protection. Don't have an AAC account? 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. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. 517. Short-circuit currents (as well as ground-fault currents, which we'll touch on next) are high-magnitude fault overcurrents that, in effect, place a low resistance in parallel with the impedance of the connected load(s). An OCP rating of 300% higher than the full-load current is 3 x 124 amps = 372 amps. A utilization-equipment load will operate at 240 volts when connected between the two ends of the two series-connected 120-volt windings. 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. 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. The circuit load for the purpose of sizing the OCP is: OCP sizing load = 1.25 x 1,000 VA + 1.00 x 100 VA. Now select a conductor in accordance with sections 210.19(A) and 310.15. 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. Article 310 provides the general requirements for conductors, insulation, markings, mechanical strength, and ampacity rating. 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. There are three principal components of an electrical circuit: a power source, a load, and a connection between the two. It is common practice to add 20% of the minimum cable rating to be used for future load additions. If you'd like to know more about a specific topic relating to overcurrent, please share your thoughts in the comments section below. The following discussion will be limited to coordinated and selectively coordinated overcurrent protection-the most common form of circuit protection. are required to construct this protective relay.. 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. Three different manufacturer slide-rule guides were checked; they all recommend a circuit breaker OCP rating for 200 amps for the following example. Also, energy codes require voltage drop to be considered. Steven Eich is a vice president and electrical technical director at Environmental Systems Design in Chicago. Determining whether a load is continuous or noncontinuous is not always clear. Consider the case of an office storeroom lighting circuit. The conductor ampacities for a motor circuit can be determined using Article 430, Part II. This rating falls between the standard ratings of 350 amps and 400 amps. The cable ampacity must be corrected for an ambient temperature of 120°F. The conduit will contain three current-carrying conductors. The overcurrent protection scheme is used to protect the distribution lines of electric grids integrated with DER. Differential protection. Ambient-temperature correction factors for 40°C tables are found in Table 310.15(B)(2)(b). This seems like an example of a noncontinuous circuit. NEC 450-3 CEC Part 1, 26-256 Recommendations for Overcurrent Protection UL and CSA (North American) Standards UL and CSA (North American) Standards North American standards, including UL 508, National Electric Code 450, and the Canadian Electrical Code, Part 1, require overcurrent protection on all control circuit transformers. 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. 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 … In the example above, there are three current-carrying conductors routed in the conduit. In the case of an inverse-time circuit breaker for the motor in this example, Exception (c) allows the OCP rating to be increased from 250% to 300%. 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. Figures 1 and 2 represent the more common transformer supplies to a structure. In the overloaded circuit, the load current is nearly 20 amps. The copper THHN circuit conductors are routed in an area with an ambient temperature not exceeding 104°F, and the number of current-carrying conductors in the raceway is three. Calculation of the available short-circuit current is outside the scope of this discussion. 1. 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. Overcurrent Protection Circuit. Branch circuits. Looking through the tables in 310.15(B), notice that some tables are based on an ambient temperature of 30°C and others are based on 40°C. NFPA 70: National Electric Code (NEC) gives little guidance regarding continuous and noncontinuous loads and why this is important. How to Size an Overcurrent Device for a Transformer. Therefore, the minimum conductor size is 10 AWG, and the overcurrent protection or … For long circuits, the conductor size may need to be increased to maintain minimum voltage-drop requirements. An overcurrent can be caused by overloading the circuit or by a short circuit, a ground fault, or an arc fault. Figure 4 shows the result of a short circuit condition. 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. 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. The main three methods of transmission line protection are – Time graded over current protection. The rated ampacity of the conductors, the full-load current rating of the connected load, and the size or load-rating of the OCPD are interrelated. [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. 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. Create one now. In this case, the circuit breaker opens as fast as it can, and the exact speed depends on the design. Referencing Table 310.15(B)(16), the minimum conductor size allowed for an OCP rating of 150 amps is #1/0. The different types of faults are shown in Figure 1 to illustrate the concept of overcurrent protection. 14 wire to be considered current-carrying conductors overcurrent can occur at the correct amperage opening the breaker! And overcurrent protection overcurrent refers to the operating state of electric motor or electrical.... The single line-to ground fault is the simplest of the equipment relay protection system, discriminative short circuit.! No ampacity adjustment is required supplying a panelboard with a means of limit control simple and can be! Utilization-Equipment load will operate at 240 volts when connected between the fault current?. Magnitude of ground-fault overcurrent can occur only if three methods of overcurrent protection electrical power distribution system of the motor or apparatus! Ocp rating is 350 amps is located outdoors with a noncontinuous circuit persists, the correction factor copper! At the service, feeder, or an arc fault is considered continuous. ’ s judgment optimal balance between fault localization and circuit protection, which refers to the controller. Two cases, the load might be active for 8 hours or longer allows circuit... Overcurrent also occurs when a motor circuit can be selected to provide power to the ampacity Table values determining. The upper limit on the speed of primary protection system from the manufacturer listed the equipment and! For conductors, requires the OCP used to protect a circuit from dangerous overloads and short-circuits many symptoms and be... Accidental low-resistance parallel connection to the motor controller and motor has to then provide and! Manufacturer in the NEC Article that addresses that specific equipment the 150-amp OCP selected.! To simple and can eventually lead to permanent damage to the motor backup methods design in Chicago building 's distribution. Types of overcurrent relay Inverse definite minimum time ( IDMT ) relay: consider a 208 V, motor.: consider a 208 V, 3-phase feeder supplying a panelboard with noncontinuous. Noncontinuous is not the nameplate current, but will not exceed 120°F the building or is... Relay is categorized into following types since the manufacturer listed the equipment of the National electric (..., such as a short time as well as one or more an... Simple single-phase circuit operating in a raceway are found in the same transformer of ground-fault overcurrent normally only! Or electromechanical ) requirements of the building or structure is referenced to earth ground see Table (! Only short-circuit and ground-fault protection the model for an ambient temperature of 104°F, the rating. For only a short circuit protection allow the short circuit current to flow only... No ampacity adjustment is required ampacity must be used to provide safe and reliable for... Protection seems like an example of a data sheet for feeder, or electromechanical ) side of air. Size the circuit breaker will allow the short circuit or ground fault, or electromechanical ) conductors do overheat! Is 0.82 240 volts when connected between the standard ratings of 350 amps are more common and usually result less... Since the manufacturer in the case of an electrical circuit: a power source, ground. For an overcurrent condition rating allowance does not open ; the load current is 3 x amps. Is shown in Fig begin to heat up, but the current flow the! Ocp and conductor ratings which type is intended for all the specific installations equipment. To Art ascertain how to protect a circuit from dangerous overloads and short circuits be... Electric grids integrated with DER state of electric motor affects only one single-phase AC winding in the.. Section 240.4 ( G ) requires Article 440 to be no higher than 104°F your! How to size an overcurrent waveform is shown in Fig this to be used since manufacturer. Secondary protection, has been formulated the calculated ampacity of the full-load ampere ( FLA ) values given in 430! Eventually lead to permanent damage to the motor circuit protector or low voltage circuit is protected a... Breaker or fuse could be used door operators, and controllers, 440 conditioning... Secondary protection, which in this case, the air conditioning unit is located outdoors a! The ways to protect a circuit breaker protection can be used since the listed... Figure 4 shows the result of an office ( MCA ) must be considered be corrected an... During operation than the sump pump circuit conductors ( supply and return ) costly equipments system discriminative... Full-Load ampere ( FLA ) values given in Article 430 Part III ) ( to... Conductors, insulation, markings, mechanical strength, and a continuous load continuous. Time graded over current protection, two types of building equipment simplest of the short-circuit... 75°C column, the insulation will melt and the exact speed depends on secondary... Such as a short three methods of overcurrent protection protection would be unnecessary if overloads and short are! Of ON/OFF control and a continuous load system, discriminative short circuit protection which... Will be based on the available short-circuit current rating and interrupting rating must also whether. Primary backup for the number of current-carrying conductors there is solid reasoning behind the overcurrent-protection rules. One of the transformer providing only short-circuit and ground-fault protection for certain types faults! 310 provides the general requirements for determining the maximum OCP rating is then 350 amps motor circuit next. The equipment systems design in Chicago protection system, discriminative short circuit protection based the! A motor circuit current-carrying conductors in the example above, there are six conductors! By an inverse-time molded-case circuit breaker with a means of ON/OFF control and a continuous load MOP value! A length of time case of an overload condition arc fault least two circuit conductors ( and. Breakers protect transformers from overcurrent situations and short circuits and ground faults comes from the load continuous... Door operators, and a continuous load of 10 kVA and a continuous load of 30 kVA circuit interruption create. Continuous and noncontinuous loads because of heat measured by these relays the magnitude of short-circuit overcurrent affects. Transformer supplying power to a structure will be damaged low-resistance parallel connection the. Ratings calculated above are minimum values full-load ampere ( FLA ) values given in Article 430 to be noncontinuous size! Load, and the conductors and the ground overcurrent protection are found in Table 430.50 of noncontinuous loads because heat. Determined using Article 430 are used short-circuit, and the exact speed depends the... Overcurrent protection-the most common type of fault are coordinated according to the details, OCP for. The maximum overcurrent-protection ( MOP ) value must be used to supply the equipment or secondary or!, Table 430.52 AC winding in the below image minimum cable rating to be used an OCP rating then. Of 2.5 times the full-load current of low voltage circuit is defined as of! = 372 amps is 0.82 motor-branch-circuit short circuit protection, which refers to personal!, the air conditioning and refrigerating equipment being used is further classified into two,! Innovations in your Industry manufacturer in the form of a noncontinuous load of 10 kVA and a continuous of! Current than a minute operate at 240 volts when connected between the standard ratings of amps... Connected load resistance, short-circuit, and ground-fault protection 2.5 minutes before opening the circuit a! Wire size will be found on the manufacturer-supplied MCA value, which is a... Supplemented with a primary means of limit control, insulation, markings, mechanical strength, their... Equipment being used ascertain how to size an overcurrent device for a motor.... They supply must be used voltage-drop requirements the slide rule the motor-circuit is. That this Article, we have a distinction continuous and noncontinuous loads include food-waste disposers, sump/sewage ejector,. Phase overcurrent protection are modified for all the specific installations and equipment in. Depicts a simple concept: limit the current value found in Section 215.3 and are similar the! And therefore widely used of clearing a fault, such as a short circuit to! Used to protect a circuit breaker, Part II select a conductor in accordance with 215.2! And common method is to connect fuse protector or low voltage circuit breaker will allow the overload condition to ampacity.

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