Passive vs. active protection

Virtual mains, zone-selective interlocking, and differential relaying are examples of protective device schemes that reduce arc duration, thereby reducing energy.

Methods that reduce clearing time via fixed devices or permanent settings are considered passive. Because they’re set (fixed), they don’t rely on having modified operation during an event.

Devices are considered active when they respond differently during an arc flash event than to other fault types and, for some schemes, operate to quench the arc. Active protection adds a degree of technical difficulty to the system because devices must communicate with each other to cause an action that either changes arc location or significantly reduces its duration. The increased complexity requires training employees to understand safety implications.

Another active protection solution employs a sensing relay with a current input and optical arc detection input to trip the line-side device directly. The trip signal is given only when the optical sensors detect a flash and the current relays simultaneously read a certain threshold. Depending on manufacturer, the trip signal is produced within milliseconds. Duration is reduced to the trip signal time plus the action/breaking time of the circuit breaker.

Relays can be retrofitted into equipment. Do not position optical sensors in ways that will yield a false flash signal.

Always perform a coordination study and arc flash hazard analysis to ensure the appropriate balance of system coordination and flash reduction. In some cases and for some equipment, a properly performed protective device coordination analysis may provide acceptable energy levels.

Operator involvement required

Interactive equipment requires employees to alter the normal coordination scheme to put the equipment in a secondary, or maintenance, operation mode. The maintenance mode generally sacrifices selective coordination for arc flash reduction.

To quantify the arc flash energy reduction, an arc flash analysis must first be performed. Values must be calculated for the possible maintenance setting to determine if any practical changes to maintenance procedures, such as different PPE, are possible. Because interactive systems rely on employees, thorough training, clear maintenance procedures and installation, and follow-up testing are paramount for success.

Where to begin

Hopefully, arc flash safety was considered when the facility’s electrical distribution system was designed. But if it wasn’t, many of these solutions are equally applicable as retrofits to existing systems.

An arc flash analysis is the first step in deciding how best to address the potential for and lessen the effects of electrical accidents, as is considering typical operating conditions and required maintenance tasks.

Products such as specialty relaying and remote operating mechanisms, intended to specifically address arc flash, are still relatively new. While they increase equipment costs, they may ultimately prove to be the wisest safety investment.

Terry Schiazza is business development manager for Low Voltage Equipment for Schneider Electric. E-mail terry.schiazza@schneider-electric.com. Mark Leinmiller is segment manager for Schneider Electric’s Water & Wastewater Competency Center. E-mail mark.leinmiller@schneider-electric.com. Visit www.schneider-electric.com.