Timing is everything

Engineers must consider arc flash prevention in the electrical systems. How important is timing?


Because an arc flash relay depends on an existing circuit breaker to interrupt the current, does it make a difference in overall reaction time? And if the relay must wait until an arc forms, will the relay take longer to trip than the circuit breaker would trip on its own? Finally, how does the reaction time of an arc flash relay compare to an overcurrent protection relay? 

Circuit breakers must react somewhat slowly to low-level faults; otherwise, they would nuisance trip. Arc flash relays avoid nuisance tripping by sensing light. Light is present even at the early stages of an arc flash. 

The current drawn by an arc flash event will vary depending on circumstances; a tool across two live phases will have very low resistance and may draw as much current as a bolted short, while a momentary ground fault can have much higher resistance and may draw only 1/3 the current of a bolted short. If the arcing fault current is less than the instantaneous trip level of a circuit breaker or overcurrent protection relay, then it falls into the short-time trip on the overload curve. In contrast, an arc flash relay detects the light from an emerging arc flash and sends a trip signal to the existing circuit breaker in approximately 1 ms. 

After receiving a trip signal from an arc flash relay, a typical circuit breaker used in switchgear or a motor control center may take about 50 ms to open, depending on make and model. This is the breaker’s mechanical clearing time and is the same or slightly less than the time it takes for it to react to a high-value fault. But many arcs begin as fairly low value faults that draw less than the breaker’s instantaneous trip current, and only reach that level after several milliseconds. An arc flash relay will detect an arc as soon as it begins and trigger the breaker directly, reducing the time to clear the fault to the arc flash relay’s 1-ms response time plus the breaker’s mechanical clearing time. Because arc flash incident energy grows exponentially with time, those saved milliseconds can make the difference between damage that is hardly noticeable and a catastrophe.

A similar comparison can be made with an overcurrent protection relay. Compared to the 1 ms reaction time of an arc flash relay, a typical overcurrent protection relay can take between 25 and 45 ms to detect a fault and send a signal to the circuit breaker. A digital or numeric overcurrent protection relay requires time for sampling the signal, conversion to digital, calculation and operation of an output, while an analog unit (still found in many older installations) may take many ms, depending on make, model, and the type of fault. And either type, even when set to its shortest possible time, will still take several milliseconds to respond.

Do arc flash relays really work? What’s your opinion? Join the conversation below. 

Justin Mahaffey is sales engineer for Littelfuse, where he helps customers improve uptime and worker electrical safety. His experience includes heavy industrial applications such as oil and gas drilling and electric power utilities. Early in his career, Mahaffey worked as both a test and product engineer.

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