Arc-flash protection (ANSI 50AF / AFD) — optical arc detection in switchgear

The arc-flash guide covers how incident energy is calculated per IEEE 1584 and which PPE follows from that calculation. That is a passive assessment: given that an arcing fault can occur, how much energy is released, and what PPE is needed to survive it? This article covers an active protection function that doesn't determine PPE, but instead drastically shortens the actual duration — and therefore the energy — of an arcing fault: the arc-flash relay, commonly referred to as ANSI 50AF or, in more recent documentation, AFD (Arc Flash Detector).

Why a conventional overcurrent relay is too slow

A conventional overcurrent protection function (ANSI 50/51) detects an arcing fault purely through current, and often must carry a time delay to remain selective with downstream protection. For an arcing fault, however, it is not just the current but above all the time that determines the released energy: the IEEE 1584 incident-energy formula is directly proportional to arc duration. Every extra tens of milliseconds of clearing time translate directly into a proportionally higher energy at the fault location — and therefore a heavier PPE requirement or, beyond a certain point, an unworkable risk.

Operating principle: light plus current, not light alone

An arc-flash relay detects an internal arcing fault via optical sensors that pick up the sudden, extremely intense light flash of an arc (in the visible and near-infrared spectrum), combined with current supervision: only when both a light flash and a simultaneous current rise above a set threshold are measured does the relay conclude that an arcing fault is actually in progress, and issue a trip command to the upstream circuit breaker.

Point sensors: individual light sensors per switchgear compartment (busbar compartment, cable compartment, breaker compartment). These give the best zone selectivity: the relay can pinpoint exactly which compartment the arc originated in, a goal comparable to the zone-discriminating ability of bus differential protection (87B), even though it rests on a completely different principle (current comparison instead of light detection).
Fiber-optic loop: a continuous fiber loop running along the busbar or through several compartments, detecting light anywhere along its length. Simpler to install than multiple point sensors, but with less precise zone assignment — the loop reports that light was detected somewhere along its path, not necessarily in which exact compartment.

The reason current supervision is indispensable: purely optical detection would also react to welding flashes, camera flashes, or direct sunlight falling on a sensor, causing nuisance trips. The combination of light AND current prevents those false trips while the actual clearing time for a genuine arcing fault remains extremely short.

Clearing time: a few milliseconds instead of cycles

Where a conventional overcurrent protection often needs tens to hundreds of milliseconds (including any time delay for selectivity), the detection decision of an arc-flash relay typically takes no more than about 2 to 2.5 ms, measured from the start of the light flash to the trip command. The total clearing time of the installation still depends on the breaker's own opening time — but the relay portion of the chain, which is often the longest step in a conventional overcurrent scheme, is essentially eliminated.

Note: "50AF" is a widely used but originally informal designation — a combination of ANSI number 50 (instantaneous overcurrent) with the suffix "AF" (arc flash), not originally an officially registered ANSI/IEEE C37.2 function number. More recent editions of IEEE C37.2 have since introduced a formal AFD (Arc Flash Detector) function designation; in practice, both terms circulate side by side.

Zone selectivity versus a single, blanket trip

As with differential protection, it is desirable for an arc-flash relay to trip only the compartment or zone where the fault actually occurred, rather than the entire installation. With point sensors per compartment, combined with a logical assignment of each sensor to the nearest breaker, that selectivity can be achieved. A fiber-optic loop without further subdivision does not deliver that precision by itself: when the exact fault location is uncertain, designers often opt for a broader, guaranteed-safe trip of multiple breakers at once.

Practical relevance

For new-build or retrofit medium-voltage switchgear with a high available fault level, an arc-flash relay is one of the most cost-effective measures to reduce incident energy — and therefore the PPE category and residual risk to personnel — often with a bigger impact than shortening the time delay of the underlying overcurrent protection alone. The optical sensors must, like any other protection component, be functionally tested periodically, typically with a calibrated test light source that simulates a known light flash.

Common mistakes

Thinking the arc-flash relay is the same as the IEEE 1584 calculation or the PPE label — the calculation determines the protection needed given a fault; the relay shortens the actual duration (and therefore energy) of that fault.
Applying optical detection without current supervision, risking nuisance trips from welding flashes, camera flashes, or direct sunlight on a sensor.
Placing point sensors so that coverage gaps exist between the detection cones of neighbouring sensors, leaving an arcing fault at an intermediate location undetected — or detected only via the slow overcurrent backup.
Not periodically function-testing the optical sensors, leaving a defective sensor undiscovered until an actual arcing fault occurs — exactly when fast detection is needed most.