Selectivity is the property of a protection system by which only the minimum amount of system functions are disconnected in order to alleviate an overcurrent situation. This is because, for these levels of currents, the thermal portion of the detection mechanism within the thermal-magnetic branch breaker is dominant. The main laboratory 100 amp thermal-magnetic unit would respond only if the other loads within the entire laboratory totaled greater than 30 amps at the time of the 70 amp power supply fault. An example would be a circuit that is improperly routed through an extension cord with a lower voltage tolerance than the rest of the circuit. Anything less leaves the apparatus or circuit susceptible to damage or total destruction within a relatively short time. There is only a single feed line for each circuit. Even with a finite width to the time-current curves, we can easily see the selectivity/coordination between the different protection devices. Limited power type: limited output of the total power. Industrial electrical equipment also uses overcurrent relays within equipment to directly protect against overcurrent damage. The ratings for the dual-element fuse in the computer power supply, the plug strip thermal breaker, the branch circuit thermal-magnetic breaker and the main circuit thermal-magnetic breaker are 2, 15, 20, and 100 amps, respectively. The particular OCPDs necessary to protect against overcurrent vary depending on the hazards associated with a given electrical system. Protection from overcurrent damage must be inherent to any new development in electrical apparatus. COm safe anD reliaBle ProteCtion in two DistinCt ProDuCt CategorY offerings Mersen offers a broad and comprehensive line of protective devices for the protection of power electronics and DC applications. In general, all overcurrent protection devices, no matter the type or principles of operation, respond faster when the levels of overcurrent are higher. This fuse will open the 120 volt circuit for any large fault within the computer. A fuse is an overcurrent protective device containing a calibrated current-carrying member that melts and opens a circuit under specified overcurrent conditions. Type 1 SPDs include watt-hour meter socket enclosure type SPDs. With hinged armature relays, the time of operation of 0.01 second at three times the setting can be obtained. A device’s interrupting rating is the maximum amount of short-circuit current at rated voltage that an overcurrent protective device, such as fuses or circuit breakers, can safely interrupt under specific test conditions. The ability of a particular overcurrent protection device to interrupt a given level of overcurrent depends on the device sensitivity. The overcurrent fault protection relays on the primary circuit of the transformer will provide some protection to the secondary circuit, but earth fault protection devices on the upstream side will not provide any secondary circuit protection because there is normally no zero-sequence path through the transformer. G    Overcurrent protection is protection against excessive currents or current beyond the acceptable current rating of equipment. The most common of these protection devices are fuses, circuit breakers, and overcurrent relays. Overcurrent protection and allowable loads for branch circuits and feeders that do not serve as the main power feeder to the dwelling unit load shall be in accordance with this chapter. K    An electric utility company would never connect a new generator, a new transformer, or a new electrical load to a circuit that cannot automatically open by means of a protective device. This main breaker is purely for back up protection. There are other network type distribution systems for utilities, where some feed lines are paralleled. Fire Extinguisher Classifications: What They Mean and Why They Matter, ISO 45001: Key Points Every Safety Professional Should Know, 12 Things to Do During an OSHA Investigation, The Top 7 Electrical Hazards and How to Prevent Them, Why Stress Should Be Treated as a Serious Workplace Hazard. 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). Types of Fault Current Protective Devices & Characteristic Curves Coordination Time Intervals (CTIs) Effect of Fault Current Variations Multiple Source Buses Partial Differential Relaying Directional Overcurrent Coordination Transformer Overcurrent Protection Motor Overcurrent Protection 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 … More of your questions answered by our Experts. Note that, except for the fuse curve, each time-current curve is shown as a shaded area, representing the range of response for each device. EDISON Multi-Purpose, dual element fuses, properly sized, provide excellent protection for similar applications. The rated current for a device is the highest steady-state current level at which the device will not trip for a given ambient temperature. Overcurrent Circuit protection would be unnecessary if overloads and short circuits could be eliminated. These conditions are typically caused by overloads, short circuits or ground fault events. Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Rewound type: the initial current is constant, the voltage drops to a certain value current began to decrease. If it should also fail, then the branch breaker should back them both up, again after a certain coordination delay. In order for a fault within the computer power cord to cause a total blackout within the laboratory, two series-connected breakers would have to fail simultaneously – the probability of which is extremely small. The electrical system reliability, component and circuit protection, and overall safety are directly related to the reliability and performance of the overcurrent protective device and can depend upon whether the required testing and maintenance are performed as prescribed for the overcurrent protective device … Note that no back-up is provided for this particular fault after the branch circuit breaker. But the radial system is the most common and the simplest to protect. Safety Talks #16 - Mask Safety: Vetting Your Distributors and Suppliers! Unfortunately, overloads and short circuits do occur. Example 1: Component failure within the computer power supply:  Assume that a power component within the computer power supply has failed – say two legs of the bridge power rectifier – and that the resulting fault current within the supply, limited by a surge resister, is 70 amps. From the thermal breaker trip curve, we see that the plug strip unit should clear this overload within 5 to 30 seconds. Covering ratings of 0.5 – 300 amps, the selection guide is a summary overview of the Series of circuit breakers made by MP. QUIZ: Are You Protecting Your Ears As Much As You Should Be? Download the Circuit Breaker Selection Guide. There are two types of overload protection: instantaneous overcurrent (IOC) and time overcurrent (TOC). The thermal circuit breaker depends on differential expansion of dissimilar metals, which forces the mechanical opening of electrical contacts. Protective devic… In a purely radial system, such as the laboratory system in Figure 1.1, we can easily see the cascade action in which each overcurrent protection device backs up the devices downstream from it. The system is said to be radial since all branch circuits, including the utility branch circuits, radiate from central tie points. Loads on the other branch circuits within the laboratory still continue to be served. Thus the exact trip time range of the thermal unit is indeterminate. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection. The large inrush current that occurs for a very short time when the computer is first turned on is masked by the slow element within the fuse. Protection is assured through detaching power supply in a circuit through overcurrent protection , which removes fire hazards and electrocution. ), A Sweaty Situation: PPE, Hydration, and How to Manage Both. An overcurrent protection device protects the circuit by opening the device when the current reaches a value that will cause an excessive or dangerous temperature rise in conductors. The National Electrical Code (NEC) has established basic power system overcurrent protection requirements and recognizes fuses and circuit breakers as the two basic types of OCPDs. 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