What's new in low-voltage power circuit breakers
The trend toward higher reliability requirements in plant electric power systems is raising demand for economical and effective protection for those systems.
The trend toward higher reliability requirements in plant electric power systems is raising demand for economical and effective protection for those systems. Specifiers are seeking products that meet ANSI standards, simplify maintenance, and reduce the overall number of system components.
Much of the electric power in a typical plant is distributed at low voltage (up to 1000 V). The circuit breaker, a familiar protective device, plays a pivotal role in the movement and use of electric power. This article discusses the basics of a particular type of breaker -- the low-voltage power circuit breaker (LVPCB), explains advances in technology, and presents an example application for which newer breakers are well suited.
Circuit breaker basics
A circuit breaker has two functions: It serves as a switch to turn electric power on and off, and provides protection against various overcurrent conditions. Sounds simple enough. But defining the switching and overcurrent conditions often turns out to be a complex undertaking.
The common 120-V breaker found in household and office panel boards provides a good example. A 20-A breaker doesn't simply trip whenever more than 20 A flows through it. It must be able to pass, without tripping, brief (but sizable) current surges such as occur during motor starting.
On the other hand, if a short circuit occurs, the breaker must open the circuit quickly. Even so, before the breaker can trip (open), the short-circuit current can build to a very high level. The breaker must be capable of interrupting this current. Breakers used in 120/240-V panelboards are commonly rated to interrupt currents up to 10,000 A.
Industrial power systems are considerably more complicated. A 2500-A breaker may feed various 800-A breakers, which feed 200-A breakers, which feed 50-A breakers. Through the process of selective coordination, the time-vs-tripping current characteristics of each breaker must be examined to be sure that there is no overlap.
If there is overlap, some faults can trip not only the nearest breaker, but one or more upstream breakers as well. That condition is undesirable because it causes blackout of areas that shouldn't be involved with the fault.
Low-voltage circuit breakers are available in three types:
- Molded case circuit breakers (MCCBs)
- Low-voltage power circuit breakers (LVPCBs)
- Insulated case circuit breakers (ICCBs).
ICCBs contain features of both MCCBs and LVPCBs, but are less often used than either. In the preceding paragraph, the 50-A and 200-A breakers would likely be MCCBs and the 800-A and 2500-A breakers would likely be LVPCBs.
Although molded case breakers offer the fastest trip times, they are UL tested for a specific number of operating cycles per unit and are not field maintainable. LVPCBs are the most rugged and in several regards the most versatile devices.
Several characteristics distinguish power breakers from other types.
- Continuity of service. LVPCBs are able to withstand the stresses of faults for up to 30 cycles (1/2 sec) instead of opening immediately. This ability to delay opening allows for a breaker nearer the fault to clear the fault, preventing unnecessary outages.
- Maintainability. LVPCBs are field maintainable for long service life. The ability to inspect and replace parts onsite is especially attractive for heavy, repetitive-duty applications.
- Safety. LVPCBs are typically "drawout" devices mounted in an enclosure. As a power breaker is withdrawn (by cranking a racking mechanism) from its connected position, the power connections are broken. The racking out cannot be done unless the breaker is open. An intermediate position with power only to the breaker's control circuits allows for testing, and the fully withdrawn position permits inspection and maintenance.
- Reliability. LVPCBs must meet high electrical and mechanical endurance ratings required by ANSI and UL.
- Remote operation. LVPCBs are designed for both opening and closing by remote control. This feature allows rapid reclosing after a fault.
The characteristic above labeled "continuity of service" is the primary differentiating feature between a power breaker and molded case breaker. Two parameters are required to characterize this feature: short delay current and short delay time.
Short delay current is the maximum current the breaker can handle safely for a period of time up to the short delay time.
Short delay time is a period of time up to 30 cycles (1/2 sec) during which the breaker can handle current up to the short delay current value.
Short delay current and time ratings are the maximum values for a circuit breaker. The breaker's trip unit can be set for lesser values as needed.
Another parameter of concern is the interrupting rating. This rating is the highest current that a breaker is intended to interrupt, with or without delay. If the amount of fault current available to flow through a breaker exceeds its interrupting rating, current-limiting fuses must be installed ahead of the breaker.
Advances in technology
Several trends are influencing LVPCB technology. Distribution transformer size is increasing, resulting in demand for higher interrupting and short time ratings. Communication with breakers is sought for both control and information gathering. Reduction of maintenance requirements (staff or outsourced) is a goal. Space is often at a premium, so smaller footprint equipment is desirable.
New circuit breaker technology has increased short time and interrupting ratings to levels that previously required the use of fuses. Nonmetallic case materials are partly responsible for the higher electrical ratings. These materials also allow a product that is smaller, lighter, and easier to install and maintain than traditional LVPCBs.
Traditional power breakers are built by assembling components to a welded steel frame; therefore, they are often referred to as metal-frame breakers. Newer multimaterial units blur this distinction, but it should be noted that LVPCBs are breakers that meet ANSI Standard C37.50, regardless of the materials of construction.
Interchangeable, microprocessor-controlled, plug-in trip units make power breakers into "smart" devices. These trip units greatly facilitate coordination with downstream breakers and offer monitoring, control, and diagnostic capabilities. The monitoring functions can include power quality parameters as sophisticated as total harmonic distortion and waveform capture. Other modern features include the ability to make all accessory changes in the field, and simple, visual, contact-wear indicators.
A typical industrial application requiring a higher-than-normal short-delay current rating is double-ended switchgear with two main service lines and tie, with tie breaker normally open (closed transition retransfer) or tie breaker normally closed. (See diagram.)
Tie breaker normally open
This system is normally operated with tie breaker T open, and a closed transition retransfer is used. Both 480-V sources must be suitable for parallel operation, and properly protected. The available fault current on the load side of each 2000-kVA transformer to each main breaker, M1 and M2, is 39,100-A symmetrical rms.
Under fault conditions, motors, driven by their loads, can briefly act as generators that contribute to the fault current. The worst-case motor contribution to fault current can be estimated from ANSI standards at four times the transformer secondary rated full-load current. Based on kVA and voltage ratings, the transformer secondary rated full-load current is 2406 A. Thus, 4 X 2406 A, or approximately 9600 A, would be added at Bus 1 and Bus 2 for a total available fault current at each bus of 48,700 A.
If voltage to Line 1 is lost, M1 automatically opens. Since the tie breaker T is open, the loads on Bus 1 experience a complete voltage loss. When the tie breaker closes, voltage is returned to Bus 1, and motor circuits on the bus typically have to be restarted in sequential order.
Many industrial users do not want to lose the loads to Bus 1 a second time by making an open transition retransfer (opening the tie breaker and then closing the main breaker M1) when voltage is restored to Line 1. A closed transition retransfer is achieved by first closing the main breaker M1 and then, a few cycles to a few seconds later, opening the tie breaker. During the time of retransfer when both mains and the tie breaker are closed, if a fault occurs on the load side of one of the feeder breakers -- such as location A near the load side terminals -- then feeder breaker F2 is required to interrupt approximately (2 X 39,100) + (2 X 9600) A, or 97,400 A.
NEC 110-9 requires, "Equipment intended to break current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment." In order to avoid an NEC violation, the feeder breakers need an interrupting capacity of 97,400 A, or nominally 100,000 A. This amount is beyond the ratings of traditional LVPCBs. (If a full short-circuit study were performed, the motor contribution at point A would likely be somewhat less than the 2 X 9600 A utilized in the above calculation. This reduction is due to the cable impedance between the motors and point A, and the fact that the motor contribution from MCC-2 does not pass through breaker F2.)
To ensure a coordinated system, if a fault occurs at point B on the load side of a combination starter in MCC-2 during the closed transition retransfer, breaker F2 must have a short time rating equal to the available fault current at point B. The fault current flowing through breaker F2 during a fault at point B is the available fault current at point A reduced by the cable and combination starter impedance from A to B.
For increased coordination, a high short-delay current rating of 85,000-A rms symmetrical is desired for the feeder breakers. At a minimum, all feeder breakers and equipment downstream from the main switchgear should have interrupting capacity ratings for the higher fault current available when the sources are paralleled with the tie closed. Since the mains do not experience fault currents higher than the current provided by one source, they can have interrupting capacity and short-time ratings the same as for a single-feed source.
The tie breaker experiences only the fault current from one source and motor contribution from one side, and can have a rating equal to the highest combination of one source, plus motor contribution on that bus. It is also recommended that reverse current protection be provided at the main breakers.
Tie breaker normally closed
This system could be operated with the tie breaker normally closed -- a configuration that creates paralleling sources. Many industrial systems are operated with both sources paralleled due to the critical natures of processes, where even a few cycles of lost power to the load could have expensive consequences. Systems supplying welding equipment are also often operated with two or more sources in parallel in order to generate adequate voltage and current during the welding process.
Reverse current and reverse power relaying, as well as a synchronism check, must be included with the main LVPCBs M1 and M2. It is recommended under this continuous parallel operation to consider replacing M1 and M2 with network protectors equipped with forward-phase and ground-fault time overcurrent protection, in addition to network relaying.
Recent advances in performance and physical size of low-voltage power circuit breakers are giving plant engineers more protection and coordination capabilities, with a resulting decrease in electrical distribution problems and plant downtime. -- Edited by Gary Weidner, Senior Editor, 847-390-2689, email@example.com
Plant electrical distribution system protection is imperative for safety, and can be a crucial factor in operating costs.
New-generation power circuit breakers pack more performance into less space.
Breakers can be programmed for system coordination.
Technical questions concerning this article can be directed to the author at 630-789-4999; e-mail nochucj@ ch.etn.com. The company web site is www.cutlerhammer.eaton.com.