Criteria for selecting arc flash protection techniques, Part 3
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.