Electricians are aware of the danger of arc flash incidents. What you may not know is that there’s a way to sense arc flash and shut down the equipment extremely quickly using fiber optics. I had never heard about it until an engineer contacted me asking some rather unusual technical questions about fiber optics that covered sensors, ruggedized components and some very different cable requirements.

When Glen Payne first contacted me, his questions told me his application for fiber optics was an unusual one, so I asked for some background information. I learned that an arc flash incident has plenty of light to sense. Light intensities can be thousands of times higher than the normal lighting.

ABB had developed an arc flash relay system using fiber optics as a sensor over 30 years ago. Since sensing arc flash and tripping relays needs to be done extremely quickly to mitigate the severity of the incident, fiber optics had the speed necessary to detect incidents. But fiber had another big advantage: it is immune to electromagnetic interference, so the electrical noise created by an incident does not affect the transmission of the information to the remote control unit.

Two types of sensors

There were two types of systems in use. One type used a “lens” sensor consisting of an optical fiber with a lens on the end that could be placed in an enclosure that could possibly have an arc flash incident. The lens would pick up the light from the arc flash and the fiber would transmit the light back to the control unit where it would be detected by a photoelectric sensor and trigger action to shut down the circuit.

A second type of sensor can be built using bare optical fiber that is mounted in the cabinets to be monitored. If an arc flash incident occurs, the intensity of the light is so high that sufficient light gets coupled into the bare fiber from the outside and is transmitted back to and then triggers the control unit. The advantage of the bare fiber sensor is that it can cover more areas at once using one sensor. A single fiber can be up to several hundred feet long, and that can cover a lot of equipment.

The control unit also senses overcurrent and requires that and an optical signal to initiate shutdown. These optical sensor control units can sense arc flash and overcurrent and initiate a shutdown in milliseconds, which is an important feature since reducing the arcing time is critical in limiting personal injury and equipment damage.

The solution

Payne was looking for solutions to several problems with the bare fiber sensor. The first was ruggedness. Bare fiber is stronger than steel, but can be crushed or broken if not handled carefully. Running it around inside electrical cabinets exposes it to some mechanical hazards. Regular fiber optic cable can’t be used because the jacket and strength members are not transparent to outside light.

The engineer’s solution was to develop a transparent cable that would protect the fiber like a regular fiber optic cable but still pick up the light from an arc flash incident. To increase sensitivity, he used a larger-core glass fiber surrounded by an optical gel and a transparent plastic tube for the cable jacket.

The next step was to make the connection to the equipment more rugged. A special type of all-metal fiber optic connector of the ST bayonet-lock type was used with a large strain-relief boot to protect the fiber at the connection point. The connector had another advantage: it was a no-adhesive connector that used a swaging process to terminate fiber in the metal connector ferrule. That meant the connector could be field installed if the user preferred, instead of buying prefabricated cables of fixed lengths. It also allowed using connections between cables, such as to enter and leave cabinets.

Payne’s work paid off. After several years of research and development, his sensor cable was recently granted a U.S. patent. Now, of course, the hard work begins: making people aware of this important safety system and the advantages of his special cable.

Header image by Shutterstock / Technicsorn Stocker / Nadinko.