Criteria for selecting arc flash protection techniques, Part 3
A variety of arc flash mitigation systems and active arc flash resistant switchgear are available to boost protection against arc flash incident energy.
- Understand the causes and consequences of arc flash incidents.
- Properly categorize different types of arc flash mitigation techniques.
- Learn the pros and cons of different types of arc flash mitigation techniques.
The second installment of this three-part series discussed using passive arc resistant switchgear to mitigate arc flash. This final part in the series outlines the advantages and disadvantages of other mitigation methods: arc flash mitigation systems and active arc flash resistant switchgear
Arc flash mitigation involves advanced detection techniques for arc flash. Arc flash mitigation methods attempt to detect arc flash events and signal breaker tripping faster than normal circuit protection methods. There are a variety of arc flash mitigation systems, including zone selective interlocking, bus differential relay protection, arc flash reduction maintenance switch, and arc flash mitigating relay.
Zone selective interlocking
The objective of zone selective interlocking (ZSI) is to trip the breaker closest to the fault without time constraints and still maintain system coordination. This is accomplished by communication between the feeder breakers and the main breaker or relay. If the feeder breakers detect an overcurrent condition, they send a restraint signal to the upstream breakers. The main then follows its normal time-current characteristics and serves as a backup. However, if the main breaker detects an overcurrent condition above its short time (or ground fault) pickup setting, but the feeders do not (e.g., main bus fault), the main breaker will trip with no intentional delay.
Benefits of ZSI include:
- Standard protective relays
- Applicable to both low- and medium-voltage systems
- Allows for selective coordination while potentially lowering arc flash incident energy
- Cost of this method is small if you utilize standard relays with a ZSI feature.
One of the disadvantages is that ZSI may not let you obtain your desired arc flash energy level because of the need to coordinate settings by allowing time for the downstream feeder restraint signal to reach the upstream main.
Additionally, it can be application intensive for some breaker arrangements. The speed of operation is dependent on the relay plus the breaker speed. While ZSI can reduce the arc flash incident energy level, an arc flash hazard study must be conducted to determine the level obtained.
Bus differential relay protection (87B)
Bus differential protection (ANSI Device Number 87B) is a common protection scheme constructed with protection relays monitoring current transformers at every incoming and outgoing connection to a switchgear lineup, which is shown in Figure 2. The currents from all outgoing connections are summed and subtracted from the summed currents of all incoming connections. If the relay sees a difference that exceeds the discrepancy threshold, a trip signal is sent.
Because of the unique sensing conditions, little, if any, intentional time delay is required, and the fault is cleared. Arc flash energy is linearly dependent on time; therefore, this reduction in clearing time can reduce arc flash incident energy. This type of protection typically does not affect system coordination and may allow for additional protection.
Differential protection is common in medium-voltage configurations, but less common in low-voltage applications due to the size of relay class current transformers, protection relay requirements, and wiring complexity. The costs associated with low-voltage differential protection are substantial when compared to the cost of the base equipment. Equipment damage is still an issue, and recovery time will be dependent on the magnitude of fault. The placement of the CTs and breakers defines the protective zone of the 87B protection, and there may be gaps (e.g., the line side of a low-voltage main breaker) where good protection is not provided. Generally, bus differential schemes are more expensive than ZSI, but less expensive than passive arc resistance.
Arc flash reduction maintenance switch
Switchgear and relay manufacturers have developed a protection function activated by a switch or button that lowers the trip delays on protection relays to be used during times of maintenance. As Figure 3 shows, the short time delay is reduced to its lowest level so that any overcurrent situation will trip the breaker more quickly.
An arc flash study should be performed to determine the new available arc flash incident energy under the new trip settings. This method has the potential to reduce arc flash incident energy during maintenance of this equipment. However, there is higher potential for nuisance tripping and loss of loads so critical they must remain energized while being maintained. As with previously discussed methods, relay and breaker response speed are important in determining the level of arc flash incident energy that would be experienced, and the recovery time for equipment is dependent on the level of fault magnitude. The cost of this method is small relative to the base cost of the equipment and other arc flash protection methods.
Arc flash mitigating relay
There are now relays and relay protection functions designed specifically for the detection of arc flash events. They use new detection techniques and algorithms. The most common detection technique is a combination of overcurrent detection and light intensity detection inside of a switchgear compartment. The light detection is done with either point light sensors mounted in different switchgear compartments or with a fiber optic loop running through several compartments or along the bus. When the relay detects both an overcurrent situation and a fast, substantial increase in light intensity, it concludes that an arc flash event is occurring. The relay will then signal for an upstream breaker to trip. This process greatly reduces the time it takes to both detect an arc flash event and signal a trip to upstream breakers when compared with traditional circuit protection methods. Traditional systems can require 100 to 400 msec to detect the arc flash, signal the breaker to trip, and open the breaker. Arc mitigating relays can reduce this time to 52 to 57 msec. This can greatly reduce the amount of damaging energy because arc flash incident energy is linearly proportional to the duration of the arc flash.
These relays can be used in both medium- and low-voltage applications, but like bus differential protection, they can be more costly in low-voltage applications compared to the base cost. In medium-voltage applications the cost increase is small compared to other arc flash protection methods.
Active high-speed switch
An active high-speed switch solution takes the arc flash mitigating relay a step further. It relies on the arc flash detection from relays as mentioned before (using the overcurrent plus light detection technique), but also incorporates a fast-acting switch that redirects the energy from the wayward path through ionized air to a lower resistance metal conductive path. This effectively extinguishes the arc. The system simultaneously signals for an upstream breaker to open to interrupt the fault. This method has often been referred to as a “high-speed shorting switch.”
The biggest advantage of this method is that it does not rely on an upstream circuit breaker to extinguish the arc. Fast-acting switch systems have been developed that can detect and extinguish the arc in less than 4 msec. High-speed switch solutions are designed to protect both personnel and equipment. Of all the protection methods available, active high-speed switches offer the highest reduction in arc flash incident energy and the shortest recovery time.
A variety of methods are available for implementation into switchgear for increasing protection against arc flash incident energy. Many criteria must be considered in choosing which methods to use, including application, cost, targets of protection (personnel and equipment), standards, and recovery time. In some cases multiple methods can be the best choice for reducing incident energy to protect equipment.
Ken Joye is staff marketing specialist at Schneider Electric. He has worked for Schneider Electric for 39 years, specializing in medium-voltage equipment applications for more than 20 years. Joe Richard is senior marketing specialist at Schneider Electric. He graduated with a BSEE from the Georgia Institute of Technology in 2007. He has worked for Schneider Electric for 6 years, specializing in medium-voltage switchgear and applications.