Light-detection arc flash protection uses optical sensors inside switchgear and switchboards and, to avoid false trips, confirms the light with an overcurrent. When both conditions are present it trips the breaker feeding the arc within a few milliseconds. By shortening the arc duration it reduces the incident energy and the damage to the equipment. In IEC 61850 it is associated with the SARC logical node.
What arc flash protection protects
An internal arc in medium-voltage switchgear or in a low-voltage switchboard releases heat, light, pressure and molten metal within milliseconds. The energy received by a person standing in front of the equipment and the damage to the switchgear grow with the arc duration, and that duration is set by the time the protection takes to detect the fault and the breaker to clear it. It is the basis of the IEEE 1584-2018 incident energy calculation used in an arc flash study.
A time-overcurrent element coordinated with the feeders can take hundreds of milliseconds or more to clear a bus arc. Light-detection protection recognizes the arc by its light, confirms it with current and issues the trip within a few milliseconds. From then on, the breaker opening time governs.
It protects:
- People operating or working in front of the equipment, by reducing the incident energy.
- Equipment, by limiting the damage to one compartment instead of a whole switchgear section or switchboard.
- Service continuity, because repairs are shorter.
In relays that use ANSI designations the function is usually labeled AFD (arc flash detector).
Where it is applied
| Equipment | LV | MV | HV and EHV | Notes |
|---|---|---|---|---|
| Medium-voltage switchgear with circuit breakers | Not applicable | Yes | Not applicable | Main application, new or retrofit |
| Low-voltage main switchboards and MCCs | Yes | Not applicable | Not applicable | Internal arc testing per IEC TR 61641 |
| Generator and station service switchgear | Yes | Yes | Not applicable | Industrial and generating plants |
| Gas-insulated substations (GIS) | Not applicable | Depends on design | Not usual | Internal arcs are handled by design and other sensors |
| Switchgear without internal arc classification | Yes | Yes | Not applicable | A way to reduce risk without replacing the switchgear |
It is a protection for industry and distribution substations, where people operate switchgear and switchboards frequently.
How it measures
Light sensors. There are two types:
- Point sensors, one per compartment: bus, breaker and cables.
- Fiber sensors, a loop or run of bare optical fiber that detects light at any point along its route and covers several cubicles at once.
Current supervision. A fast overcurrent element, fed by the current transformers (CTs) of the main breaker or of the breaker that must open, confirms that fault current is present.
Logic. In the usual mode, the trip is issued when there is light AND overcurrent. A light-only mode exists for particular cases, with a higher risk of unwanted trips.
Outputs. The trip uses fast outputs to the breaker feeding the arc. Many systems also have a backup output to the next breaker upstream in case the first does not open.
The characteristic
There is no curve: the characteristic is a timeline in milliseconds.
- The arc appears and its light reaches the sensor.
- The current exceeds the threshold of the fast element.
- The logic confirms light and current and issues the trip.
- The breaker opens and the arc is extinguished.
The useful comparison is with time overcurrent: the area under the current curve during the arc is what accumulates as energy. Light detection shortens the first part; the second depends on the breaker and the relay does not change it.
- Arc. Light, pressure and fault current inside the compartment
- Detection. The compartment sensor sees the light
- Confirmation. Current exceeds the fast threshold
- Trip. Command to the breaker feeding the compartment
- Opening. The breaker opens and the arc is extinguished
Setting criteria
- Zones: each sensor is assigned to the breaker that feeds its compartment.
- Current threshold: below the minimum expected arcing current (which at low voltage can be well below the bolted fault current) and above load and inrush currents.
- Light sensitivity: according to the sensor type and the room environment.
- Backup: wait time before opening the upstream breaker if the first one fails.
- Study: the resulting arcing currents and times are confirmed in the arc flash and short-circuit study.
Interaction with other functions
- Overcurrent 50/51: the current element of the arc flash system is often the fast stage of the main breaker relay itself. Overcurrent 50/51 remains as backup.
- Breaker failure 50BF: if the breaker does not open, the arc continues. Breaker failure 50BF or the backup output of the arc flash system opens the next breaker upstream.
- Lockout 86: an arc flash trip does not reset by itself. The lockout relay 86 prevents closing again onto damaged switchgear.
- Automatic transfer and reclosing: they must be blocked on an arc flash trip so the faulted bus is not re-energized from the alternate source.
- Bus differential and zone selective interlocking: other ways to speed up bus fault clearing; the choice is made in the study.
- Relay-to-relay communication: in integrated systems, the light or overcurrent signal from one cubicle reaches the main breaker relay by hardwiring or by GOOSE.
How each manufacturer implements it
- Dedicated systems: ABB REA, Schneider Electric VAMP 121 and 221, and Siemens Reyrolle 7XG31, with central units and extension modules. We describe them in arc flash relays: ABB REA, VAMP and 7XG31.
- Built into the feeder or motor relay: the ABB Relion 615 series has arc flash protection with sensors connected to the relay and the ARCSARC logical node; the SEL-751 offers fiber-based arc flash detection as an option.
The practical differences are the sensor type, how many sensors each unit accepts, how the current condition is shared between units, and self-supervision of sensors and fibers.
IEC 61850 logical node
IEC 61850-7-4 defines the SARC node, for monitoring and diagnostics of arcs, with the fault arc detected data object (FADet) and arc counters. ABB models its arc flash protection with a node of that class (ARCSARC). Other manufacturers use their own nodes declared in their conformance documentation; SEL, for example, declares an extension node for arc flash detection.
In the trip chain, the operate signal goes through PTRC to the breaker (XCBR), and the light detection or the trip command can be published by GOOSE so that the main breaker acts on an arc detected in another cubicle.
How it is tested
In general the test verifies:
- Each sensor, with a test light source, confirming that it activates the correct zone.
- The current element threshold, with secondary injection.
- The light AND current logic: light alone must not trip in combined mode.
- The time from light to trip output.
- That each zone trips the correct breaker and that the backup output works.
- Self-supervision: a disconnected sensor or a broken fiber must raise an alarm.
During commissioning, the complete scheme is tested up to the breaker opening.
Typical mistakes
- Badly placed sensor that cannot see the compartment because of a barrier or a shutter.
- Zone assigned to the wrong breaker: a bus arc that trips a feeder instead of the main breaker.
- Current condition taken from a CT that does not see the arc, for example with on-site generation feeding the bus from another side.
- Light-only mode in a room with outside light or during maintenance with the doors open.
- Fibers bent below their minimum bending radius, or dirty sensors.
- Automatic transfer not blocked by the arc flash trip.
- Arc flash study not updated after installing the system.
We configure your arc flash protection
Are you installing or upgrading the arc flash protection of your switchgear, or did you have a trip that is not clear? We configure the relay remotely, worldwide: zones, thresholds, logic and GOOSE signals. Tell us the brand and model through remote relay configuration or contact us. If you also need the incident energy recalculated, we do it in engineering design and studies.
Related guides
References
- IEEE 1584-2018, Guide for Performing Arc-Flash Hazard Calculations: arc duration in the incident energy calculation.
- IEC TR 61641, Enclosed low-voltage switchgear and controlgear assemblies – Guide for testing under conditions of arcing due to internal fault.
- IEC 62271-200, AC metal-enclosed switchgear and controlgear: internal arc classification (IAC).
- IEEE C37.20.7, Guide for Testing Switchgear Rated Up to 52 kV for Internal Arcing Faults.
- IEC 61850-7-4, Compatible logical node classes and data object classes: SARC and PTRC nodes.
- ABB, REM615 Application Manual: ARCSARC arc flash protection.
Frequently asked questions
Why does an arc flash relay use both light and current?
Because light alone can come from another source: a flashlight, welding, or daylight with the compartment door open. Current alone cannot tell an internal arc from a fault elsewhere that another protection should clear. With both conditions at once the trip is fast and secure.
Does an arc flash relay remove the need for an arc flash study?
No. The study calculates the incident energy with the actual clearing time. Installing an arc flash relay changes that time, so the study must be updated to reflect it, together with its labels and PPE categories.
Which breaker should the arc flash relay trip?
The one feeding the compartment where the arc is. An arc in the cable compartment of a feeder can be cleared by the feeder's own breaker; an arc on the bus requires opening the main breaker; and an arc on the line side of the main breaker requires opening the upstream breaker, for example the one on the transformer primary.