three methods of overcurrent protection
The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. A few of the symptoms of overcurrent in a motor are shorts, blown fuses and … 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). 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. 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. By paying attention to the details, OCP ratings can be selected to provide safe and reliable operations for the lifetime of the equipment. How to protect a circuit from dangerous overloads and short circuits. This rating falls between the standard ratings of 350 amps and 400 amps. Ascertain how to protect a circuit from dangerous overloads and short-circuits. 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. Consider the case of an office storeroom lighting circuit. 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. It protects the equipment of the power system from the fault current. If you'd like to know more about a specific topic relating to overcurrent, please share your thoughts in the comments section below. [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. 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. Phone apps are also available that provide the same function as the slide rule. 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. Overcurrent protection seems like a simple concept: Limit the current flow in a circuit to a safe value. 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. However, some equipment—motors, transformers, and conductors, for example—have overload-protection requirements set by the NEC. As in the case with the short circuit, the circuit breaker will allow the fault current to flow for only a short time. 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. The three major categories or types of overcurrent are overload, short-circuit, and ground-fault. Section 430.22 applies to this example in that it is a single motor circuit. NFPA 70: National Electric Code (NEC) gives little guidance regarding continuous and noncontinuous loads and why this is important. Which type is intended for all types of fault current conditions? Several of the NEC sections are modified for all the specific installations and equipment found in commercial buildings. 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. Ground-fault overcurrent can occur only if the electrical power distribution system of the building or structure is referenced to earth ground. Fig. An OCP rating of 300% higher than the full-load current is 3 x 124 amps = 372 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). Figure 4 shows the result of a short circuit condition. 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. This fuse will open the 120 volt circuit for any large fault within the computer. Some permit reviewers might require this to be considered a continuous load. 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. Figure 2 depicts a simple single-phase circuit operating in a normal configuration. 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. 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. Get an intro to overcurrent protection and OCPDs (overcurrent protection devices). Adjustments for the number of current-carrying conductors in a raceway are found in Table 310.15(B)(3)(a). This is the simplest of the ways to protect a line and therefore widely used. 240 III. 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. 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. Section 430.52(C)(1) has an exception that allows the next higher standard rating to be used. Short circuit: A short circuit is defined as flow of current outside the intended current path. Overcurrent protection (OCP) protects a circuit from damage due to an overcurrent condition. Distance protection. Secondary conductor overcurrent protection must comply with 240.21. In this example, the OCP rating cannot be increased above 350 amps. 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. NEC 450.3. Typical practice is to use an OCP rating less than the maximum calculated above. 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. 3: Trip characteristic of two-stage, non-directional, maximum-overcurrent time protection If several protective devices are connected in series across the network, this leads to a graded curve (Figure 4), the nearest protective relay being tripped in the event of a fault. 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. This protection scheme is further classified into two categories, the phase overcurrent protection and the ground overcurrent protection. Don't have an AAC account? For this reason, the manufacturer must provide data specific to the air conditioning and refrigerating equipment being used. Several articles applicable to commercial buildings modify the general NEC rule for overcurrent protection, as summarized below: In the following examples, the rating of the OCP trip value will be determined along with the ampacity of the conductors used in the circuit. This seems like an example of a noncontinuous circuit. The overcurrent relay is defined as the relay, which operates only when the value of the current is greater than the relay setting time. 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. Note that the maximum OCP rating is 350 amps, which is significantly higher than even the 90°C column ampacity of 195 amps. Once the circuit breaker opens or "trips" due to a short circuit or … This places a short-circuit across the supply-transformer winding. Differential protection. 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). 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). Symmetrical faults result in the same current flow in each phase during the fault condition. It has a limitation that it can be applied only to simple and non costly equipments. Branch circuits. 15.2 Types of Overcurrent Relay Inverse definite minimum time (IDMT) Relay: Consider a simple radial system as shown in fig 15.5. Overcurrent relays are the typical protection for conventional distribution systems. Thus overcurrent relays should be used over a limited length in the 3 feeder sections. 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. Feeder circuits. True. 2.1 Overcurrent protection. There are three principal components of an electrical circuit: a power source, a load, and a connection between the two. In cases where a load is certain to be noncontinuous, size the circuit to 100% of the load. 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. Temperature derated ampacity = 0.82 x 55 amps. 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. The main three methods of transmission line protection are – Time graded over current protection. Unsymmetrical faults are more common and usually result in less fault current than a symmetrical 3-phase fault. In this example, the circuit conductors (copper THHN) are routed through a boiler room where the temperature will not exceed 120°F. The operation of the consecutive time-inverse overcurrent relays are coordinated according to the discrepancy between the fault currents measured by these relays. 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. Overload conditions are usually not as time-critical as short circuits and ground faults. In this example, the air conditioning unit is fed using copper THHN wire in a conduit containing three current-carrying conductors. In this example, the wires are routed through a boiler room where the highest temperature is expected to be no higher than 120°F. By the same token, the rating or setting of the OCPD and the rated ampacity of the circuit conductors dictate the maximum full-load current that can be supplied from the service, feeder, or branch circuit. For motor circuits, overload protection is provided by the motor-overload protector (refer to Article 430 Part III). The load and cable ratings calculated above are minimum values. Figures 1 and 2 represent the more common transformer supplies to a structure. As with the previous example, the wire type selected is copper THHN, which is rated for 90°C. Though at first glance the NEC requirements might not seem straightforward, there is solid reasoning behind the overcurrent-protection code rules. for feeder, motor, and breaker failure protection. In this example, the ground-fault path adds approximately .012 ohms of resistance in parallel with the load resistance, resulting in a much lower circuit resistance. Information regarding equipment-overload capability often comes from the manufacturer. Unsymmetrical faults have different fault currents in each phase. The general rule for NEC ampacity selection is found in Section 310.15, which refers to the tables in Section 310.15(B). In the overloaded circuit, the load current is nearly 20 amps. [HEAD]. In this article, we will cover types of overcurrent, what overcurrent protection devices are, and their place in an electrical circuit. For this example, Table 430.52 allows a circuit breaker with a maximum rating of 2.5 times the full-load current of the motor. If circuit interruption will create a hazard (e.g., fire pump circu… Next, derating for the number of conductors in the conduit must be considered. Some examples of noncontinuous loads include food-waste disposers, sump/sewage ejector pumps, garage door operators, and electric pencil sharpeners. For four to six conductors in a raceway, the derating factor is 80%. The requirements for feeder-circuit overcurrent protection are found in Section 215.3 and are similar to the requirements for branch circuits. 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). 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. 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. Voltage relationships of the three supply lines from the secondary of a single-phase AC residential … This trip … Understand the three types of overcurrent conditions to consider in typical NFPA 70: National Electrical Code applications. 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. The circuit breaker will allow the overload condition to continue for approximately 2.5 minutes before opening the circuit. 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). 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. The overcurrent protection scheme is used to protect the distribution lines of electric grids integrated with DER. Again, if the fault current persists, the insulation will melt and the conductors will eventually be damaged. 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%. 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. 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. 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