Circuit breaker technology reduces arc flash risk
Circuit breakers have protected electrical circuits since their introduction in 1927. While their function hasn’t changed, today’s circuit breakers are equipped with new technologies to meet safety and reliability goals. Arc flash can be mitigated, ground fault protection is possible, interrupting and current limiting capabilities have increased and communication options are now ava...
Circuit breakers have protected electrical circuits since their introduction in 1927. While their function hasn’t changed, today’s circuit breakers are equipped with new technologies to meet safety and reliability goals. Arc flash can be mitigated, ground fault protection is possible, interrupting and current limiting capabilities have increased and communication options are now available.
Lowering arc flash risk
Arc flash is a dangerous condition caused by the energy released during a fault, and typically occurs in systems with voltages above 240 V. The tremendous amount of heat energy released during an arc flash incident can cause severe burns, as well as loss of hearing or eyesight from the pressure waves and light emitted during the flash. IEEE 1584 is an accepted guide for calculating arc flash incident energy. Arc flash testing for power circuit breakers and molded case circuit breakers is conducted based on the same criteria used to establish IEEE 1584.
The 2005 National Electric Code Article 110.16 requires arc flash warning labels on panelboards, switchboards and switchgear that may require service or maintenance while energized. Changes to the NEC-2008 code are expected to expand the requirement to include other electrical equipment such as enclosed breakers and transformers. The label is meant as a warning against working on energized equipment and a reminder to wear appropriate personnel protective equipment.
Using circuit breaker electronics, arc flash incident energy can be reduced up to 30% with an arc flash reduction maintenance switch in the trip unit. In one such unit, an analog circuit is used to trip the breaker when in arc flash mode and provides the fastest tripping possible by eliminating the 50-millisecond delay typically required by the microprocessor to calculate the circuit’s conditions.
A maintenance switch typically has multiple discrete settings for levels of protection between 2.5 and 10 times the continuous current of the breaker. The lowest setting corresponds to the maximum arc flash protection setting. A range of settings is provided for flexibility and allows the plant engineer to select the setting that maximizes worker safety. The adjustments are provided to take into account other events such as motor-starting inrush current that could otherwise trip the breaker if the lowest arc flash setting is used.
With the arc flash maintenance mode set at the maximum protection, the incident energy is reduced below the requirements for Hazard Category 1. This means that workers can be protected without requiring a full multi-layer flash suit. For example, a 1600-A power breaker on the secondary of a substation feeds a downstream MCC and a worker needs to replace a bucket in that MCC. The worker would set the power breaker to arc flash maintenance mode. If a fault occurred downstream of the power breaker, the power breaker would trip without any intentional delay, based on its maintenance mode setting and regardless of the trip unit’s other settings. This high speed clearing limits the available arc flash energy to levels below NFPA 70E Hazard Category 1, providing increased worker safety. During normal operation, the maintenance switch is off and coordination settings are retained.
Arc flash reduction maintenance switches can be activated locally using a selector switch, armed via a remote switch or via communications. This technology can be retrofitted into existing power breakers using an upgraded trip unit. This technology is also available on medium voltage circuit breakers up to 15 kV.
Advancements in breaker electronics
Ground fault detection is also important in enhancing reliability and safety. A ground fault is a short circuit between one phase and ground. Ground faults comprise the majority of faults that occur in industrial and commercial power systems, according to the IEEE Buff Book, ANSI/IEEE Std. 242-2001 %%MDASSML%% “IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.” However, because they are of low magnitude, they are not detected by the standard circuit breaker or fuse. If left unnoticed, a ground fault can escalate, but by the time the fault level reaches the threshold needed to trip the standard breaker, damage to downstream equipment may have already occurred.
The miniaturization of electronics and incorporation microprocessors has enabled even small-frame industrial circuit breakers to be equipped with adjustable electronic long, short, instantaneous and ground-fault protection. Ground-fault protection in electronic trip units is accomplished through residual sensing using individual current transformers, and can detect currents as low as 20% of the breaker frame rating. Earth-leakage breakers provide higher sensitivity by using a zero-sequence current transformer to sense line currents and detect ground faults down to 30 mA. These devices have adjustments for both the ground fault pick-up level and time delay to coordinate with other devices in the system and account for transient conditions.
Earth leakage protection is now readily available in industrial breakers as an add-on module. Critical applications should incorporate ground fault protection to quickly isolate faults, thereby preventing extensive equipment damage %%MDASSML%% and the resultant inadvertent downtime. The NEC requires ground fault protection for service entrance equipment, heat tracing and snow melting applications and feeder circuits in healthcare facilities. In addition, installing ground fault protection beyond those circuits can prevent damage to transformers and motor insulation in any circuit. Besides protecting equipment, isolating ground faults quickly before they intensify also reduces hazardous arc flash energy.
Communications capabilities have existed in circuit breakers for more than 20 years. Recently emerging is the ability to monitor the power system and its breakers from anywhere in the world through a standard Internet connection. Real-time breaker statistics include:
Trip unit maximum temperature
Number and type of faults detected
Number of operations
Date and time the breaker was last operated.
The trip unit can be remotely programmed, adjusted, controlled and the breaker operation indicators can be viewed remotely without requiring personnel to enter the switchgear room. Furthermore, architecture is now available that enables all the electrical equipment in a facility %%MDASSML%% or across multiple locations %%MDASSML%% to be managed over an Ethernet network. Access to system and device information arms the plant engineer with the ability to respond to issues before they result in extensive downtime. In the future, breakers within a switchboard will use wireless communications and wireless sensors to detect changes in temperature, humidity and vibration within the equipment.
Fault interrupting, current-limiting capabilities
Breakers are reliable alternative to fuses in any facility. High performance molded case and power breakers are now available with up to 200 kA interrupting capacity at 480 V, and with current limitation. The lower I2t let-through energy results in a corresponding decrease of hazardous arc flash energy. These breakers match the high interrupting levels of fuses, but offer the safety and enhancement benefits of a circuit breaker.
Breakers can be reset after a fault without replacing parts; are encased in a molded housing that prevents access to live parts; and can easily incorporate ground fault protection, status indication, accessories and remote tripping. MCCBs are available with higher interrupting capacities than they were previously. Now you can use a molded case breaker in a small footprint for these applications without fuses or limiters.
Circuit breaker manufacturers use various contact and arc chamber designs to achieve high interrupt and current limitation. The reverse-loop stationary contact arrangement directs current flow in a way that generates a strong magnetic repulsion force on the movable contact arm as well as strong magnetic force to push the arc into the arc chutes under a high fault condition. This is known as 'blow apart’ contact action.
Some circuit breakers employ a double-break design. The double-break design uses a moving contact arm with two contact points that rotate around an axis. The two arcs generated across the two sets of contacts can provide high arc voltage quickly. To achieve the higher interrupt ratings, the reverse-loop stationary contact design sometimes uses an extended movable contact arm to increase the opening speed and contact gap. In the single-break design, slot motors are added around the stationary conductor and movable contact arm assembly. The slot motors are steel laminations that increase the magnetic force acting on the movable contact arm and the arc for fast contact opening, and push the arc into the arc chutes.
Breakers designed to interrupt high fault currents must also be able to handle high internal pressures in order to interrupt without damaging the molded housing. The breaker housing or arc chamber housing molding is typically made of glass fiber-filled thermoset resin because of its strength and resistance to cracking under high pressures. Every MVVB that is UL-489-Listed must pass up to seven short-circuit tests (including two high interrupt tests), and still be able to trip in calibration.
<table ID = 'id4698466-0-table' CELLSPACING = '0' CELLPADDING = '2' WIDTH = '100%' BORDER = '0'><tbody ID = 'id4700296-0-tbody'><tr ID = 'id4700298-0-tr'><td ID = 'id4700300-0-td' CLASS = 'table' STYLE = 'background-color: #EEEEEE'> Author Information </td></tr><tr ID = 'id4700310-3-tr'><td ID = 'id4700312-3-td' CLASS = 'table'> Patricia Chandler is a product manager for MCCBs at Eaton Corp., Pittsburgh, where she has worked for 10 years. Chandler holds a Bachelor of Science in Commerce and Engineering Sciences from Drexel University in Philadelphia. She can be reached at firstname.lastname@example.org . </td></tr></tbody></table>
The Bottom Line...
Plant engineers can design and operate a safer and more reliable electrical system by taking advantage of the latest circuit breaker technology.
Arc flash energy can be significantly reduced with an arc flash reduction maintenance switch.
Ground fault or earth leakage protection can be used to isolate ground faults and prevent equipment damage.
Breaker diagnostics, along with information on the entire power chain, can be accessed from virtually anywhere.