E    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. This branch breaker is a combination thermal and magnetic or thermal-mag breaker. H    Privacy Policy For example, the time current curves for the protection devices in our laboratory example are shown superimposed in Figure 1.2. 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. We show a one-line diagram of the radial distribution of electrical energy, starting from the utility distribution substation, going through the industrial plant, and ending in a small laboratory personal computer. Proper grounding of service boxes and equipment can mitigate this risk. Circuit breaker manufacturers specifically address arc fault and ground fault in their custom designs. EDISON Multi-Purpose, dual element fuses, properly sized, provide excellent protection for similar applications. Industrial electrical equipment also uses overcurrent relays within equipment to directly protect against overcurrent damage. This level of current is 2000% (20 times) of the rated current of the plug strip thermal breaker, and is beyond the normal range of published trip time specifications for thermal breakers (100% to 1000% of rated current). 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. W    The Nominal Discharge Current Rating for Type 1 … As a simple example, consider the typical industrial laboratory electrical system shown in Figure 1.1. Protective devic… 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. Complex type: a variety of protection in line. The range of response for the thermal-magnetic breaker at 300 amps is 8 to 185 milliseconds. The basic types of overcurrent protection devices include fusible switches and circuit breakers. All other loads on the branch circuit, as well as the remaining loads within the laboratory, should continue to be served. Even with a finite width to the time-current curves, we can easily see the selectivity/coordination between the different protection devices. 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. Here's Why It Should Be, Hiring Contractors? Other devices like junction boxes, combiner boxes, pass-through boxes AC, and DC load centers also act as overcurrent protection devices among many other roles that they play in the solar power system. Loads on the other branch circuits within the laboratory still continue to be served. 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. If an OCPD is exposed to a level of voltage it is not designed to deal with, it may itself rupture or explode. Join thousands receiving the latest content and insights on health and safety industry. These conditions are typically caused by overloads, short circuits or ground fault events. How to Safely Rescue Someone from a Confined Space, 5 Myths About What Defines a Confined Space, What Your Confined Space Safety Program Needs to Cover, Mental Health and Suicide Prevention in the Construction Industry, How to Deal with Workplace Hazards that Have Latent Effects, 11 Workplace Wellness Efforts That Help Improve Workplace Safety, How Safety Professionals Can Overcome 'Old Dog' Attitudes, Safety View: The Myth of Crisis Leadership, Safety Talks #20: Rethinking PPE (or maybe it's PPP! M    The particular OCPDs necessary to protect against overcurrent vary depending on the hazards associated with a given electrical system. Unfortunately, overloads and short circuits do occur. Example 2:  Plug strip overload:  Assume that the computer operator has spilled a drink, and to dry up the mess plugs two 1500 watt hair dryers into the plug strip. Overcurrent devices shall not be connected in series with a grounded conductor. R    Consider the following three examples for the laboratory wiring, plug strip, and computer system. At these and higher currents, the magnetic detection mechanism within the thermal-magnetic unit is dominant. For example, in the laboratory system of Figure 1.1, a short within the computer power cord should be attended to only by the thermal breaker in the plug strip. Overcurrent The term “overcurrent” refers to abnormal current flow higher than the normal value of current flow in an … Example 3:  Short circuit within the computer power cord:  Assume a frayed line cord finally shorts during some mechanical movement. Protection Device : Types of Protection Devices Protection devices for electrical circuits accomplish two main functions namely consistency as well as protection. They are utilized in every conceivable electrical system where there is the possibility of overcurrent damage. Manufacturing tolerances and material property inconsistencies are responsible for these banded sets of responses. Very large fault currents are detected and cleared by the fast element within the fuse. 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 operator then flips them both on simultaneously, drawing a total plug strip load current of approximately 30 amps. Magnetic circuit breakers, fuses and overcurrent relays are commonly used to provide overcurrent protection. Most OCPDs (e.g., fuses) are found in the primary service panels (i.e., the “fuse box”), as well as associated electrical feeders and branch circuits, which are typically connected to their own breaker systems. The correct use of OCPDs (and their distribution in specific branch contexts) is necessary for safe OCPD use. Individuals working in proximity to devices at risk of overcurrent face dangers from electrical shock and fire, both of which can be caused by the damage to electrical equipment due to an overcurrent. OCPDs only work in relation to the amount of current flowing through them and will therefore not respond to an overcurrent isolated to another part of the circuit. Patents on mechanical circuit-breaking devices go back to the late 1800’s and the concept of a fuse goes all the way back to the first undersized wire that connected a generator to a load. Every electrical circuit in a home must be protected by its own OCPD that is properly rated for the circuit wiring. Thus the exact trip time range of the thermal unit is indeterminate. O    Overcurrent protection devices include circuit breakers and fuses. 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. 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). #    This main breaker is purely for back up protection. Instantaneous overcurrent requires that the current exceeds a predetermined level for the circuit breaker to operate. This is evidenced by the fact that the response curve for the thermal-magnetic branch circuit breaker knees downward sharply at current levels between 150 and 200 amps. The unit protection schemes is based on Kirchhoff’s Current Law – the sum of the currents entering an area of the system must be zero. The use of overcurrent protection devices is a standard part of electrical safety, and it is prescribed in the United States as part of the National Electrical Code (also known as NFPA 70). Playing type: over-current, the current voltage dropped to 0, and then began to … There are two types of overload protection: instantaneous overcurrent (IOC) and time overcurrent (TOC). K    Protection from overcurrent damage must be inherent to any new development in electrical apparatus. QUIZ: Are You Protecting Your Ears As Much As You Should Be? 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. It generally operates instantly. The thermal circuit breaker depends on differential expansion of dissimilar metals, which forces the mechanical opening of electrical contacts. Overcurrent Protection Devices The purpose of an overcurrent protective device is to provide protection to service entrance, feeder and branch circuit conductors and equipment. A power delivery system which is selectively protected will be far more reliable than one which is not. I    This information is contained in the “trip time vs. current curves,” commonly referred to as the trip curves. Fall Arrest Systems: Can You Tie Off at Your Feet? L    D    Receive updates on Mechanical Products and industry news and trends. The ability of a particular overcurrent protection device to interrupt a given level of overcurrent depends on the device sensitivity. 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. P    Types … Y    III Types of Overcurrent Protection. Should both the plug strip breaker and the branch circuit breaker fail to operate, the main laboratory breaker should clear the fault within 11 to 40 seconds. The steady-state trip current is referred to as the ultimate trip current. If the fuse fails to interrupt the current – or worse, if the fuse has been replaced with a permanent short circuit by a gambling repairperson – the thermal breaker in the plug strip should open the circuit within 0.6 to 3.5 seconds. One of the most important considerations in overcurrent and overvoltage protection is the speed of operation. Overcurrent Protection and Overcurrent Protection Devices. Here, the external accessory would be connected to the device input (bottom connector of system) and the device battery would be at the device output. The NEMA Fuse Section develops technical standards and serves as the industry voice for positively impacting product safety and performance requirements, and relevant … In a practical sense, we can say that no advance in electrical science can proceed without a corresponding advance in protection science. This research is used to decide the settings of the relay and circuit breaker, fuse sizes, or to choose new equipment that fits into the design. 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. Note that no back-up is provided for this particular fault after the branch circuit breaker. Without overcurrent protection, the entire system is out of service, and important life safety circuits may be lost because of a fault elsewhere as shown in Figure 1. Voltage relationships of the four supply lines from the secondary … The major function of the fuses and breakers is to protect the wires. 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 fuse will open the 120 volt circuit for any large fault within the computer. Protection against excess load at the plug strip, is provided by the thermal circuit breaker within the plug strip. In general, all overcurrent protection devices, no matter the type or principles of operation, respond faster when the levels of overcurrent are higher. The National Electrical Code 110.9 requires that equipment intended to break current at fault levels to have an interrupting rating sufficient for the current that must be interrupted. There is only a single feed line for each circuit. A trip time-current curve displays the range of, and the times of response for, the currents for which the device will interrupt current flow at a given level of circuit voltage. Standard fuses and circuit breakers are commonly used overcurrent protection devices (OCPD) to control overcurrent (overload and faults). Overcurrent protection is defined as any current above the generator rating. 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. N    With hinged armature relays, the time of operation of 0.01 second at three times the setting can be obtained. The breakers have many advantages … In cases where an overcurrent occurs, these devices will break the circuit through which the current is flowing, eliminating or re-routing the current flow. Z, Copyright © 2021 Safeopedia Inc. - Any deviation from this must indicate an abnormal current … More of your questions answered by our Experts. The general requirement for overcurrent protection of conductors is provided in Section 240.4, Protection of Conductors. The coordination delay is the principal means by which a back-up system is selective in its protection. Unit type schemes protect a specific area of the system, i.e., a transformer, transmission line, generator or bus bar. To protect a circuit against these currents, a protective device must determine when a fault condition … The correct use of OCPDs (and their distribution in specific branch contexts) is necessary for safe OCPD use. Soon after Volta constructed his first electrochemical cell, or Faraday spun his first disk generator, someone else graciously supplied these inventors with their first short circuit loads. 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. There are other network type distribution systems for utilities, where some feed lines are paralleled. 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