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.