THE RISK OF ARC FLASH IS REAL, BUT THERE ARE AFFORDABLE AND EASILY INSTALLED DEVICES THAT CAN PROVIDE THE NEEDED PROTECTION
By Jeff Glenney, P.Eng.
Electrical power is the lifeblood of coal mine operations. In fact, electrical infrastructure represents about 15% of capital expenditures for a typical mine. Little wonder, then, that the reliability and safety of that power is an ongoing concern.
The points at which electricity is distributed to loads throughout the mine are at risk for electrical arc flash, a dangerous electrical explosion. Coal mines are particularly susceptible to arc flash because coal dust is conductive and may allow electricity to arc from a conductor to ground or to another conductor.
The risk is real. According to the Centers for Disease Control and Prevention, in the U.S. mining industry non-contact electrical burns due to arc flash events are the largest single category of electrical injuries. In the period from 1990 to 1999 in U.S. mining there were 770 reported cases of heat radiation injury from electric arcs, three of them fatal — and this does not include injuries caused by blast effects.
While some industries have strict regulations concerning arc flash hazards, mining does not. In general industry, OSHA (Occupational Safety and Health Administration) recommends compliance with documented safety standards such as the National Fire Protection Association NFPA 70E, Standard for Electrical Safety in the Workplace.
These regulations are not mandatory in mining. Instead, MSHA requires compliance with 30 CFR (Code of Federal Regulations) Part 75 for underground coal and 30 CFR Part 77 for surface coal, which do not specifically cover arc flash. Yet, as awareness of arc flash has grown in recent years, MSHA has started to encourage mine managers to follow NFPA 70E, and many forward-thinking mine managers are starting to look for mitigation strategies.
|Figure 1: NPFA 70E requires this arc flash hazard information to be placed on a warning label affixed to the cabinet.|
Mines do use high-resistance grounding, which prevents phase-to-ground arc faults, and it works well. However, high resistance grounding does not protect against phase-to-phase faults. These are usually caused by worker error (such as dropping wrench on a conductor) or by an equipment problem. Mines are hard on electrical gear. Movement, vibration, shock or buildup of dust can cause equipment to fail in a way that causes a phase-to-phase fault.
Phase-to-phase faults are far less common, but it is arguably more dangerous if workers are present. In any case it poses an unacceptable risk in a coal mine. In addition to the immediate danger of injury and fire, there is the financial risk of interrupted operations, medical costs, equipment replacement costs, government fines and lawsuits.
Effects of Arc Flash
A phase-to-phase arc flash begins as an arc forms across an air gap. As it persists, the metal vaporizes and forms a region of highly conductive ionized gas. This allows the flow of available current to rapidly increase, producing extreme heat and the expansion of copper from a solid to a gas creates a shockwave or blast.
An arc flash can achieve temperatures of up to 20,000°C (35,000°F), hotter than the surface of the sun. The incident thermal energy (heat loading) on a worker standing inside the arc-flash boundary can exceed 120 cal/cm2, which can set clothing on fire and cause instantaneous third-degree burns to any exposed flesh. The visible and ultraviolet light can cause blindness.
The blast wave can reach 95 kPa (2,000 lb per square foot), which can rupture eardrums or crush a chest. It can throw molten metal and debris at speeds comparable to a bullet.
Fortunately, most arc flash incidents are not that extreme, but simple arithmetic shows that typical mine switchgear is at least theoretically capable of producing destructive arc flash events. The energy released by an arc flash can be calculated as voltage x current x duration. A phase-to-phase fault in a 480-volt system with 20,000 amperes of fault current provides 9.6 MW of power; if the fault lasts for 200 ms, then 1.92 MJ will be released, which is the energy equivalent of detonating almost half a kg (459 g) of TNT.
The mitigation of the dangers of an arc flash is accomplished with a variety of approaches. The minimum level is providing personal protective equipment (PPE) for workers to wear when approaching hazardous equipment. A more proactive approach is to lower the available incident energy by using circuit protection devices and high resistance grounding. Finally, there are arc flash protection relays that can be easily retrofitted into existing switchgear cabinets.
Start with an Arc Flash Assessment
The process begins with an arc flash assessment, a study of the entire facility’s electrical system, with a hazard level assigned to each electrical cabinet based on available fault currents and protection clearance times. Assessments are most commonly done using modern arc flash assessment software tools such as SKM, EasyPower and ETAP. For each cabinet, this analysis will determine the amount of incident energy generated during an arc event in calories/cm2 or Joules/cm2, determine the approach boundaries for qualified and nonqualified personal then determine the required level of PPE based on the incident energy.
Much of the information so derived must be placed on an arc flash warning label affixed to the cabinet. Figure 1 shows a representative label done in accordance with NFPA 70E standards.
|Figure 1: NPFA 70E requires this arc flash hazard information to be placed on a warning label affixed to the cabinet|
High Resistance Grounding
IEEE testing has shown that dangerous electrical arcs will not form at lower levels of current. That’s one reason why mines have adopted high resistance grounding, in which the neutral point is connected to ground through a resistor (see Figure 2) of value sufficient to limit ground fault current to a few amperes, generally 25 A or less, although a 5 A limit is typical. This approach prevents a phase-to-ground arc from forming. Unfortunately, it will do nothing to discourage the formation of phase-to-phase arcs.
That is not to say that nothing will happen, of course. For one thing, the voltage at the neutral point will immediately rise to the system’s phase-to-earth voltage; this can present a shock hazard and can cause voltage stress in some areas, for example to the filter capacitors of variable-frequency drives (VFDs) that may be on the system. Fortunately, the makers of most VFDs have switched to capacitors capable of withstanding this voltage, so any concern will be limited to older equipment.
The Canadian standard CSA M421-11, Use of Electricity in Mines, mandates ground conductor monitoring. When a grounding resistor circuit opens — by accident, by mistake, or even by someone stealing the copper ground wire — the system will become ungrounded. Without continuous monitoring it may remain so for some time, as there are no immediately noticeable symptoms.
A neutral grounding resistor monitor will detect if this happens and either trip a circuit breaker or provide an alarm. This alarm can notify mine maintenance to a condition that can be fixed in a timely fashion, and this may help those mines avoid an MSHA citation.
|Figure 3: An arc flash relay uses a series of light sensors mounted in areas to be protected.|
|Figure 4: Arc flash relays can shut off power before serious damage occurs.|
Current Limiting Devices
Current limiting fuses and circuit breakers are designed to open quickly when there is a high-value fault current. Because the total incident energy is a product of time, a fast-acting fuse or breaker will limit the total amount of current available to a fault.
While fuses and breakers can help reduce arc flash energy, they have a significant drawback; because the earliest moments of an arc flash may draw only a fraction of the current of a short circuit, overcurrent protective devices cannot distinguish them from a typical inrush current, and must wait until the current increases — during which time significant harm can be done to nearby personnel. If the current is low enough, an arc can develop and remain fairly stable for some time — seconds or longer — before it draws enough current to trip the overcurrent protective device.
In contrast, an arc-flash relay (see Figure 3) uses light sensors to detect light from an emerging arc flash and send a signal to the breaker. An arc flash relay can greatly reduce personnel hazard, equipment damage and downtime, and can also reduce the required level of PPE for personnel working on or near an open enclosure.
The relay and sensors can be used in transformer enclosures, substations, switchgear and motor control centers – any electrical gear, no matter the operating voltage. Arc flash relays are compact, economically priced, and can easily fit in retrofit projects and new switchgear with little or no re-configuration.
The arc flash relay is designed to operate extremely quickly. The light sensors will detect bright light almost immediately. The relay will then send a signal to the trip coil on the breaker feeding the panel. Arc flash relays are available that will send this signal in 1 ms or less, and most main breakers will open within about 50 ms, stopping further damage that would otherwise take place, as shown in Figure 4.
The light sensors can be either point type placed in each cabinet or a distributed fiber-optic type that can be threaded through areas to be protected. It is possible to use one arc flash relay for several cabinets, if one provides power to the other or both receive power from the same breaker, by putting sensors in both cabinets.
An arc flash relay may be fooled by light from other sources — such as welding arcs if the welder is not shielded and the cabinet is open. To prevent this, some arc-flash relays also sense current; if the sensors detect a flash of light but there is no corresponding overcurrent condition, then the relay will not trip.
It is worthwhile to point out that the main breaker must be equipped with a trip coil, or be retrofitted to add a shunt or undervoltage trip coil. The breaker should be cycled regularly and be maintained at a regular interval per the manufacturer’s recommendation.
|The arc flash photo sensor is visible at right.|
Arc Flash Relay Selection
Arc flash relays are available from a number of suppliers. There are several key factors to consider when making a selection. The reaction time varies between 1 and 9 ms, and a few have a backup trip circuit in case the main trip circuit fails to function (a situation that may occur at start-up, while the microprocessor is booting).
Installation requirements vary. Some arc flash relays require installation of PC interface software for their configuration; others may be easily installed by contract electricians and technicians simply by connecting the wired components and using default settings or simple on-board programming. Some relays will accept a mix of point and fiber-optic sensors; other require the relay to be ordered pre-configured for each type. This may not seem like a major concern, but it does limit flexibility and complicates installation.
Arc flash relays indicate the health status of connected sensors, but some also have local indication (blinking LEDs) on the sensors themselves, which are likely located in a different compartment than the relay. This feature gives confidence during installation that the system is set up correctly, and it warns workers during maintenance if protection may be defeated.
Some relays have the ability to trip a breaker further upstream if the main breaker does not open (a circuit breaker fail function), and the ability to be interconnected with other arc-flash relays. Some arc flash relays have multiple trip outputs, which simplifies zone protection.
Coal mine operations depend on the safe, reliable distribution of electric power. To protect against injury, equipment damage and economic loss, mine operators are increasingly protecting electrical distribution points with arc-flash relays. These easy-to-install and affordable devices provide protection that cannot be easily provided by other means.
Jeff Glenney, P.Eng. is a sales engineering manager for the protective relay products line at Littelfuse. He received his BSEE from the University of Saskatchewan, Saskatoon, Canada and is a registered professional engineer in Saskatchewan. In his capacities as sales engineer and sales engineering manager, he has worked with many system designers and end users to find solutions for protection relays. He now manages Littelfuse U.S. relay sales and spends the majority of his time in the western states. He can be reached at firstname.lastname@example.org.