The most common cause of arc faults is insulation failure. These failures may be caused by defective or ageing insulation material, poor or incorrect maintenance, dust, moisture, vermin, and human error (touching a test probe to the wrong surface or a tool slipping and touching live conductors). Jakob Seedorff, head of R&D at Littlefuse Selco, explains.
Arc-flash events are dangerous, and potentially fatal, to personnel. Industrial arc-flash events cause most of electrically-related accidents and fatalities among qualified electrical workers. Even if personnel injuries are avoided, arc flash can destroy equipment, resulting in costly replacement and downtime.
Arc-flash safety standards
Standards for electrical safety in the workplace outline the practices and standards companies should follow, in order to protect workers and equipment from arc flash electrical hazards. They specify practices designed to make sure an electrically safe work condition exists.
The standards hold both employers and their employees responsible for creating a workplace for electrical workers that is not just safe but puts in place the best possible processes and procedures that are fully understood, practiced and enforced for optimal results. Using arc flash relays is one way to protect the functional reliability of the distribution board and, at the same time, comply with the requirements of, for instance, NFPA 70E and CSA Z462 in USA and Canada.
Current limiting fuses or current limiting circuit breakers help protect against arc flashes. They allow only a certain amount of energy to pass before they open a circuit. Because an arc flash can draw a fraction of bolted-fault current, circuit breakers cannot be relied upon to distinguish between the arcing current and a typical inrush current.
High-resistance grounding is another technique for protecting against arc flashes. If a phase faults to ground, then the resistance limits current to just a few amps; not enough to cause downtime by tripping the overcurrent protection device, and not enough to allow an arc flash. It is important to remember while resistance grounding prevents arc flash from phase-to-ground shorts, it has no effect on phase-to-phase shorts.
Another way to mitigate the dangers of arc flashing is by redesigning the switchgear. Switchgear cabinets can be designed to contain and channel energy away from personnel during an arc flash.
Arc-flash relays are microprocessor-based devices that use optical sensors to detect the onset of a flash. The sensors are strategically placed in various cubicles or drawers inside the switchboard.
Installing an arc-flash relay to rapidly detect developing arc flashes greatly reduces the total learing time and the amount of energy released through an arcing fault. In turn, there is less damage to equipment and fewer and less severe injuries to nearby personnel.
Arc-flash relay selection criteria
When selecting an arc-flash relay, there are six important criteria:
1. Reaction time
2. Trip reliability
3. Avoidance of nuisance tripping
4. Sensor design and installation
5. Ease of use
Since light is the earliest detectable indication an arc flash is occurring, arc-flash relays use optical light sensors to detect the arc that is forming. The output of the light sensor is hard-wired to the arc-flash relay, which trips a circuit that interrupts the energy supply in the arc.
The response time of an arc-flash relay is approximately 1-5 ms at light intensities of about 10000 lux or higher. Within that time frame, the optical sensor output can actuate a switch or circuit breaker to cut off current feeding the arc. The overall current clearing time depends on the protection strategy used and the performance of the external switch or circuit breaker used. The breaker will typically take an additional 35-50 ms to open, depending on the type of breaker and how well it is maintained.
Reliable tripping is the most important characteristic of an arc flash relay, because this ensures mitigation of an arcing fault. Two aspects of reliability should be considered: trip redundancy and system-health monitoring.
Arc flash relays should offer a redundant tripping feature, which means it has both primary and secondary trip path logic. The primary path is controlled by the internal microprocessor and its embedded software, and works by activating the coil of the primary trip relay. A microprocessor can require 200 ms or more before it is able to start scanning the optical sensors. However, a solid-state trip path can detect an arc and send a trip signal in as little as 2 ms. In addition, there are fail- safe features that alert operators when, for example, the microprocessor fails.
Health monitoring makes sure the system is in good operating condition and should extend from the light sensors to the output of the arc-flash relay trip circuitry. Health monitoring starts on the sensors. A signal is sent from the relay to the light sensors, where a test light is detected by the sensor and sent back to the relay. Following the path of a trip signal from the sensor, internal monitoring must also include the primary and redundant trip circuit.
Avoidance of nuisance tripping
A typical arc-flash relay system has an integrated three-phase current measurement function that detects and reacts to short circuit and overcurrent conditions. Although this is not a requirement for the system to operate, this option will increase the reliability of the system (minimise unwanted tripping). If the microprocessor logic receives an input from a light sensor, it checks for a rapidly rising input from the current transformers. Two conditions need to be fulfilled before the trip is sent to the circuit breaker: a certain current flow that exceeds the normal operating current of the system (the threshold level is adjustable from 10-1000% of the full load current) and a signal from the arc-flash sensor, implying that the sensor has reacted to a high intensity light source.
Sensor design and installation
Arc flash relay installations utilise multiple fixed-point light sensors near vertical and horizontal bus bars where arcing faults are apt to occur in feeder switchgear cabinets. Sufficient numbers of sensors should be installed to cover all accessible areas, even if policy is to only work on de- energised systems. At least one sensor should have visibility to an arc fault if a person blocks another sensor’s field of view. Light sensors may also be installed in other electrical cabinets and on panels that are subject to routine maintenance and repairs. A fiber-optic sensor, which has a 360° field of view for detecting light, allows more flexible positioning of the light sensing locations, as the fiber-optic strands can be looped throughout an enclosure or panel to cover challenging component layouts.
Easy to use hardware and software
Another important factor to consider is ease of use. Some relays may require field assembly, calibration, or advanced configuration before installing. It is critical to consider those extra steps and the capabilities of the operators who will be using the devices. Often, very complicated devices can be misused because of incorrect set-up or configuration, which can defeat the purpose of the device altogether. A few arc-flash relays have software that provides event logging. To make troubleshooting easier, this software should record the specific sensor that initiated the fault in the data records.