Module 3 - Introduction to Circuit Protection, Control, and Measurement
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Module 3 −− Introduction to Circuit Protection, Control, and Measurement
Pages i, 1−1, 1−11, 1−21, 1−31, 1−41, 1−51, 1−61, 1−71, 2−1, 2−11, 1−21, 2−31, 2−41, 3−1, 3−11, 3−21, 3−31, AI−1, AII−1, AIII−1, IV−1, Index
Figure 2-16 - Circuit breaker components.
Figure 2-17 - Circuit breaker construction.
Figure 2-18 - Arc extinguisher action. The FRAME provides an insulated housing and is used to mount the circuit breaker components (fig. 2-17). The frame determines the physical size of the circuit breaker and the maximum allowable voltage and current. The OPERATING MECHANIsM provides a means of opening and closing the breaker contacts (turning, the circuit ON and ofF). The toggle mechanism shown in figure 2-17 is the quick-make, quick-break type, which means the contacts snap open or closed quickly, regardless of how fast the handle is moved. In addition to indicating whether the breaker is ON or ofF, the operating mechanism handle indicates when the breaker has opened automatically (tripped) by moving to a position between ON and ofF. To reset the circuit breaker, the handle must first be moved to the ofF position, and then to the ON position. 2-21 The ARC EXTINGUIsHER confines, divides, and extinguishes the arc drawn between contacts each time the circuit breaker interrupts current. The arc extinguisher is actually a series of contacts that open gradually, dividing the arc and making it easier to confine and extinguish. This is shown in figure 2-18. Arc extinguishers are generally used in circuit breakers that control a large amount of power, such as those found in power distribution panels. Small power circuit breakers (such as those found in lighting panels) may not have arc extinguishers. TERMINAL CONNECTORS are used to connect the circuit breaker to the power source and the load. They are electrically connected to the contacts of the circuit breaker and provide the means of connecting the circuit breaker to the circuit. The TRIP ELEMENT is the part of the circuit breaker that senses the overload condition and causes the circuit breaker to trip or break the circuit. This chapter will cover the thermal, magnetic, and thermal- 2-22 magnetic trip units used by most circuit breakers. (Some circuit breakers make use of solid-state trip units using current transformers and solid-state circuitry.) THERMAL TRIP ELEMENT A thermal trip element circuit breaker uses a bimetallic element that is heated by the load current. The bimetallic element is made from strips of two different metals bonded together. The metals expand at different rates as they are heated. This causes the bimetallic element to bend as it is heated by the current going to the load. Figure 2-19 shows how this can be used to trip the circuit breaker.
Figure 2-19 - Thermal trip element action: A. Trip element with normal current; B. Contacts open.
Figure 2-20 - Magnetic trip element action; Closed contacts
Figure 2-21 - Thermal-magnetic element action Figure 2-19, view A, shows the trip element with normal current. The bimetallic element is not heated excessively and does not bend. If the current increases (or the temperature around the circuit breaker increases), the bimetallic element bends, pushes against the trip bar, and releases the latch. Then, the contacts open, as shown in figure 2-19, view B. The amount of time it takes for the bimetallic element to bend and trip the circuit breaker depends on the amount the element is heated. a large overload will heat the element quickly. a small overload will require a longer time to trip the circuit breaker.
Magnetic TRIP ELEMENT A magnetic trip element circuit breaker uses an electromagnet in series with the circuit load as in figure 2-20. With normal current, the electromagnet will not have enough attraction to the trip bar to move it, and the contacts will remain closed as shown in figure 2-20, view A. The strength of the magnetic field of the electromagnet increases as current through the coil increases. As soon as the current in the circuit becomes large enough, the trip bar is pulled toward the magnetic element (electromagnet), the contacts are opened, and the current stops, as shown in figure 2-20, view B.
2-23
The amount of current needed to trip the circuit breaker depends on the size of the gap between the trip bar and the magnetic element. On some circuit breakers, this gap (and therefore the trip current) is adjustable. THERMAL-Magnetic TRIP ELEMENT The thermal trip element circuit breaker, like a delay fuse, will protect a circuit against a small overload that continues for a long time. The larger the overload, the faster the circuit breaker will trip. The thermal element will also protect the circuit against temperature increases. a magnetic circuit breaker will trip instantly when the preset current is present. In some applications, both types of protection are desired. Rather than use two separate circuit breakers, a single trip element combining thermal and magnetic trip elements is used. a thermal-magnetic trip element is shown in figure 2-21.
2-24 In the thermal-magnetic trip element circuit breaker, a magnetic element (electromagnet) is connected in series with the circuit load, and a bimetallic element is heated by the load current. With normal circuit current, the bimetallic element does not bend, and the magnetic element does not attract the trip bar, as shown in figure 2-21, view A. If the temperature or current increases over a sustained period of time, the bimetallic element will bend, push the trip bar and release the latch. The circuit breaker will trip as shown in figure 2-21, view B. If the current suddenly or rapidly increases enough, the magnetic element will attract the trip bar, release the latch, and the circuit breaker will trip, as shown in figure 2-21, view C. (This circuit breaker has tripped even though the thermal element has not had time to react to the increased current.) Q30. What are the five main components of a circuit breaker? Q31. What are the three types of circuit breaker trip elements? Q32. How does each type of trip element react to an overload? TRIP-FREE/NON-TRIP-FREE Circuit BREAKERS Circuit breakers are classified as being trip free or non-trip free. a trip-free circuit breaker is a circuit breaker that will trip (open) even if the operating mechanism (ON-ofF switch) is held in the ON position. a non-trip-free circuit breaker can be reset and/or held ON even if an overload or excessive heat condition is present. In other words, a non-trip-free circuit breaker can be bypassed by holding the operating mechanism ON. Trip-free circuit breakers are used on circuits that cannot tolerate overloads and on nonemergency circuits. Examples of these are precision or current sensitive circuits, nonemergency lighting circuits, and nonessential equipment circuits. Non-trip-free circuit breakers are used for circuits that are essential for operations. Examples of these circuits are emergency lighting, required control circuits, and essential equipment circuits.
2-25 TIME DELAY RATINGS
Figure 2-22. - use of circuit breakers in a power distribution system.
Figure 2-23. - Circuit breaker with an operating handle.
Figure 2-24. - Push-button circuit breaker. Circuit breakers, like fuses, are rated by the amount of time delay. In circuit breakers the ratings are instantaneous, short time delay, and longtime delay. The delay times of circuit breakers can be used to provide for SELECTIVE TRIPPING. Selective tripping is used to cause the circuit breaker closest to the faulty circuit to trip. This will remove power from the faulty circuit without affecting other, non-faulty circuits. Figure 2-22 should help you understand selective tripping. Figure 2-22 shows a power distribution system using circuit breakers for protection. Circuit breaker 1 (CB1) has the entire current for all seven loads through it. CB2 feeds loads 1, 2, 3, and 4 (through CB4, CB5, CB6, and CB7), and CB3 feeds loads 5, 6, and 7 (through CB8, CB9, and CB10). If all the circuit breakers were rated with the same time delay, an overload on load 5 could cause CB1, CB3, and CB8 to trip. This would remove power from all seven loads, even though load 5 was the only circuit with an overload. Selective tripping would have CB1 rated as long time delay, CB2 and CB3 rated as short time delay, and CB4 through CB10 rated as instantaneous. With this arrangement, if load 5 had an overload, only CB8 would trip. CB8 would remove the power from load 5 before CB1 or CB3 could react to the overload. In this way, only load 5 would be affected and the other circuits would continue to operate. PHYSICAL TYPES of Circuit BREAKERS All the circuit breakers presented so far in this chapter have been physically large, designed to control large amounts of power, and used a type of toggle operating mechanism. Not all circuit breakers are of this type. The circuit breaker in figure 2-23 is physically large and controls large amounts of power; but the operating mechanism is not a toggle. Except for the difference in the operating mechanism, this circuit breaker is identical to the circuit breakers already presented. 2-26 Circuit breakers used for low power protection, such as 28-volt dc, 30 amperes, can be physically small. With low power use, arc extinguishers are not required, and so are not used in the construction of these circuit breakers. Figure 2-24 shows a low power circuit breaker of the push-button or push-pull type. This circuit breaker has a thermal trip element (the bimetallic disk) and is non-trip-free. The push button is the operating mechanism of this circuit breaker.
2-27 You will find other physical types of circuit breakers as you work with electrical circuits. They are found in power distribution systems, lighting panels, and even on individual pieces of equipment. Regardless of the physical size and the amount of power through the circuit breaker, the basic operating principles of circuit breakers apply. Q33. What is a trip-free circuit breaker? Q34. What is a non-trip-free circuit breaker? Q35. Where should you use a trip-free circuit breaker? Q36. Where should you use a non-trip-free circuit breaker? The magnetic trip element makes use of a magnetic element (electromagnet). If current reaches a preset quantity, the magnetic element attracts the trip bar and releases the latch. The thermal-magnetic trip element combines the actions of the bimetallic and magnetic elements in a single trip element. If either the bimetal element or the magnetic element reacts, the circuit breaker will trip. Q37. What are the three time delay ratings for circuit breakers? Q38. What is selective tripping and why is it used? Q39. If the power distribution system shown in figure 2-22 uses selective tripping, what is the time delay rating for each of the circuit breakers shown? Q40. What factors are used to select a circuit breaker? Q41. What type of circuit breaker is used on a multimeter? Circuit BREAKER Maintenance Circuit breakers require careful inspection and periodic cleaning. Before you attempt to work on circuit breakers, check the applicable technical manual carefully. When you work on shipboard circuit breakers, the approval of the electrical or engineering officer must be obtained before starting work. Be certain to remove all power to the circuit breaker before you work on it. Tag the switch that removes the power to the circuit breaker to ensure that power is not applied while you are working. Once approval has been obtained, the incoming power has been removed, the switch tagged, and you have checked the technical manual, you may begin to check the circuit breaker. Manually operate the circuit breaker several times to be sure the operating mechanism works smoothly. Inspect the contacts for
2-28 pitting caused by arcing or corrosion. If pitting is present, smooth the contacts with a fine file or number 00 sandpaper. Be certain the contacts make proper contact when the operating mechanism is ON. Check the connections at the terminals to be certain the terminals and wiring are tight and free from corrosion. Check all mounting hardware for tightness and wear. Check all components for wear. Clean the circuit breaker completely.
When you have finished working on the circuit breaker, restore power and remove the tag from the switch that applies power to the circuit. Q42. What steps are to be taken before beginning work on a circuit breaker? Q43. What items are you to check when working on a circuit breaker? Summary This chapter has provided the information to enable you to have a basic understanding of circuit protection devices. The following is a summary of the main points in this chapter. Circuit PROTECTION DEVICES are needed to protect personnel and circuits from hazardous conditions. The hazardous conditions can be caused by a direct short, excessive current, or excessive heat. Circuit protection devices are always connected in series with the circuit being protected. A Direct Short is a condition in which some point in the circuit, where full system voltage is present, comes in direct contact with the ground or return side of the circuit. EXCESSIVE CURRENT describes a condition that is not a direct short but in which circuit current increases beyond the designed current carrying ability of the circuit. EXCESSIVE Heat describes a condition in which the heat in or around a circuit increases to a higher than normal level. FUses and Circuit BREAKERS are the two types of circuit protection devices discussed in this chapter. 2-29 PLUG-TYPE FUses are used in low-voltage, low-current circuits. This type fuse is rapidly being replaced by the circuit breaker. CARTRIDGE FUses are available in a wide range of physical sizes and voltage and current ratings. This type fuse is the most commonly used fuse.
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