What is in this article?:
- The Basics of Circuit Breaker Maintenance
- Arc Extinguishers
Understanding circuit breaker maintenance and the basic components and operation of MCCBs, ICCBs, LVPCBs, MVACBs, & MVVCBs
Arc extinguishers (arc chutes) contain the arc, stretch it out, cool it, and deionize it. This occurs within one-tenth of a second or less and is critical to safe operation of the circuit breaker and power system. The time it takes to interrupt an arc is known as the “maximum total clearing time” — the time from the start of the arc until it is completely extinguished. This characteristic is used to properly coordinate power systems so they will trip in the right sequence (selective tripping). Selective tripping is also referred to as “power system coordination,” as the devices will operate in their proper sequence when this is performed correctly.
When the contacts begin to open, the hot arc will rise. Most air circuit breakers will have their arc chutes positioned above the contact assemblies, as the natural tendency of the arc is to rise and aid in extinguishing the arc. The arc is hurried along the process by various components in the arc chute, such as puffers, blowout coils, arc runners, and arcing horns. There are exceptions to this arrangement. One manufacturer’s breaker positions the arc chute to the rear of the breaker.
Photo 9 (right) shows an arc chute. Note that there are two sets of contacts per arc chute, due to the high load and arcing currents that must be handled. Photo 10 shows the vent and muffler assembly on the same arc extinguisher. Low-voltage arc extinguishers will not have blow-out coils or magnetic pole pieces, such as those found on MVACBs.
Photo 11 shows an arc chute arrangement on an air-magnetic circuit breaker. Photo 12 shows the detail of a blowout coil assembly of a circuit breaker. The blowout coil is energized when the contacts part, forcing the arc current through them. This creates a magnetic field that pulls the arc into the arc chute more quickly.
An alternative that has virtually replaced MVACBs is the MVVCB. Vacuum circuit breakers interrupt the arc by denying it air. In a pure vacuum, there can be no arc. Even though the vacuum in vacuum bottles is very good, it isn’t perfect — so some arcing does take place. The arc is interrupted very quickly, usually in two to three cycles, depending on the application. Photo 13 is a vacuum bottle cut-away to show its components.
Vacuum bottles require very little maintenance when compared to air-magnetic contact assemblies. The contact moves only about ½ in. in the vacuum bottle, and the opening springs are much lighter. This reduces wear on the assembly and also reduces the weight, because heavy metal supports and frame can be decreased. The primary components of a vacuum bottle include:
- Bottle —Made of extremely hardened ceramic or glass. The bottle must contain the explosive force of an arc.
- Flexible Metal Bellows — Soldered/welded to the moving contact stem, it maintains the seal between the moving contact and the bottle.
- Bellows Shield — Protects the metal bellows from the intense heat of the arc. Since the vacuum is not perfect, there will be some arcing inside the bottle.
- Contacts — No arcing and mains here, just one set of contacts. When an arc is interrupted, some of the metal is vaporized. Most recollects onto the contact surface, while some drifts toward the inside of the bottle.
- Metal Vapor Condensing (arc) Shield — Since some small quantity of the contact face does not recombine onto the contact face, it starts heading toward the bottle wall. The metal vapor condensing shield is designed so both ends are open and do not contact the bottle. Any metal vapor that drifts to it cannot make a short between the contacts.
Circuit breaker operation. Modern operating mechanisms are quick-make, quick-break. This means that the speed of contact operation is independent of the speed of the control handle. Operating mechanisms are also referred to as “stored energy” mechanisms, because there are both opening springs and closing springs. One set of springs usually has tension on it. For this reason, use extreme care when working on or near circuit breakers. They have heavy moving contact assemblies and powerful springs. If your hand was between the moving and stationary contacts when it closed, it could maim you.
Closing springs do not hold the contacts closed. Over a period of time, they would weaken, causing the contacts to bounce, vibrate, and burn. The contacts are held in the closed position by a prop and roller operating mechanism. The prop and roller puts the contact linkage into a mechanical bind, forcing the contacts to stay tightly closed. A typical prop and roller mechanism is illustrated in Figs. 2, 3, and 4.
Figure 2 shows an operating mechanism in the “closed” position. The insulated coupling (12) holds the contacts closed due to parts 2, 5, 6, 11, and 14 being placed in an interference fit. The trip latch (11) holds the secondary latch (14) from rotating clockwise. The secondary latch is positioned against the secondary latch roller (6 – yellow), which in turn extends the cam (main) roller (5 – red) against the prop (2). The opening spring (15) is not shown in this view, but is exerting pressure on the contacts to open.
Note that the secondary latch (14) is held against the secondary latch roller (6), which is pushing the main roller (5) and its linkage into a vertical position. The main roller is in turn held against the Prop (2), which prevents it from overextending. The centerline of the insulating coupling pin is in a straight line with the main roller through the camshaft. In this position, the contacts are unable to open until the linkage collapses, which cannot happen until the trip latch (11) releases the secondary latch (14).
Figure 3 shows the same mechanism in the “tripped” position. To open the breaker, the trip latch is rotated clockwise, allowing the secondary latch to rotate counter-clockwise. When it does, the main roller (5) and the secondary latch roller (6) collapse. This allows the opening springs to pull the contacts open. Part 7 is a bell crank lever, which is used to change motion in one direction into motion in another direction. As the linkage collapses, the bell crank rotates, allowing the contacts to open.
Figure 4 shows the mechanism in the “reset” position. This position is the condition of the mechanism just before closing. The trip latch (11) and the secondary latch (14) are reset to the same position as when the breaker is in the “closed” position. To put the mechanism in this position, the cam (3) has to be rotated slightly counter-clockwise until the prop (2) is lifted up, allowing the main roller (5) and linkage (green) to slip into the crook of the prop. By doing this, the linkage is slightly extended, and the secondary latch (14) engages the front of the frame, as shown (in blue), which allows a gap between the trip latch and the secondary latch. The prop and cam reset to their original position, and, if the breaker close button is pressed, the closing springs will accelerate the contacts closed. The cam and prop will rotate, extending the linkage and forcing the components into the same positions as shown in Fig. 2.
Circuit breakers are important for preserving the electrical system during abnormal conditions as well as protecting workers’ lives. Note the number of rollers and bearing surfaces in the operating mechanism. These require lubrication and, as the circuit breaker is in service, that lubricant dries out and becomes gummy. This slows the circuit breaker down, changing its operating speed. It’s important to note this change in operation speed will alter the results of a previously performed arc flash study.
White is director of training for Shermco Industries in Irving, Texas. He can be reached at firstname.lastname@example.org.