Preventing arc flash danger in mining applications
04 April 2013
According to the Centers for Disease Control and Prevention, in the US mining industry non-contact electrical burns due to arc flash events are the largest single category of electrical injuries. This paper examines the phenomenon of arc flash, looks at its causes, and explores measures that can minimise the likelihood of an arc flash occurring and greatly reduce the danger and damage if one does occur.
Mines are heavily dependent on electric power, so the distribution of electricity is a big part of any mining operation, and electrical safety is an on-going concern. Moisture and dust are ever-present, threatening to coat insulators with conductive deposits. Vibration, movement of equipment and even shock waves from blasting can damage or degrade equipment. As with any facility that uses high-power electrical equipment, a mine can hold the danger of arc flash, which can injure or kill personnel, damage or destroy equipment, and shut down production.
What is an arc flash?
An arc flash is the result of a short circuit through an air gap between one energised conductor and another, or (more commonly on a solidly earthed system) between an energized conductor and earth. It can occur on any system operating at 240 V and above with 125 kVA or more available.
An arc flash can be caused by a person simply dropping a tool or touching a test probe to the wrong place. But it can also occur with no people present — from a loose connection, wildlife getting inside an outdoor cabinet, failure of a short circuit protective device, or even by an accumulation of dust on insulators. There was a case some years ago in which dust accumulated on phase conductors on old wiring that ran through a pit beneath an electrical cabinet. An arc developed between two of these phase conductors and tracked up the cables into the cabinet, where it initiated an arc flash.
Effects of arc flash
As an arc flash begins it creates a rapidly-expanding ball of high-temperature conductive plasma consisting of ionised air and vaporised metal. As it expands, it produces a combination of light, radiated heat, blast effects (overpressure) and shrapnel.
The energy released by an arc flash can be calculated as Energy = (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 1.92 MJ will be released, which is the energy equivalent of detonating almost half a kg (459 g) of TNT.
Figure 1 - For each cabinet an arc flash 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 personnel, and much else
The blast wave can reach 95 kPa (2,000 lb per square foot), which can rupture eardrums, crush a chest or throw a person across a room — not to mention damaging other equipment in the vicinity. It can throw blobs of molten metal and bits of other debris at speeds comparable to a pistol bullet, causing direct injury to nearby personnel and setting things on fire.
The arc flash reaches temperatures of up to 20,000 ºC (35,000 °F), more than three times the absolute temperature of the surface of the sun. It produces an intense burst of broad-spectrum radiation ranging from radio wavelengths to ultraviolet that can cause damage to anything it reaches. The incident thermal energy (heat loading) on a person 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 eye damage and even permanent blindness. And the vaporised metal in the plasma cloud, along with the smoke from any fire caused by the incident, can cause a breathing hazard.
Countermeasures to arc flash
Mitigation of the dangers of arc flash can be done by changing the electrical system to make arc flash less likely to occur, replacing switchgear with arc-resistant equipment, by adding protective equipment to minimise or stop any arc flash that does occur, and by the proper use of personal protective equipment (PPE).
Conduct an arc flash study
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 non-qualified personal, determine the hazard/risk category and determine the required level of PPE. 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 US practice.
Change to a high impedance earthed system
Figure 2 - In a high-impedance earthed (IT) system the neutral point is connected to earth through a resistor of value sufficient to limit earth fault current to a few amperes
Many mines have adopted high impedance earthing (IT earthing, per the IEC), in which the neutral point is connected to earth through a resistor (Figure 2) of value sufficient to limit earth fault current to a few amperes, generally 25 A or less, although a 5 A limit is typical. As with an unearthed system, an earth fault to an energised phase will not operate an overcurrent protective device, requiring supplemental ground-fault detection . In addition, this type of system is not subject to the transient overvoltage problems of an unearthed system.
Of course a high impedance earthed system is not totally immune to all arc flash danger. As with an unearthed system, a phase-to-phase fault can still cause an arc flash, but statistically very few first fault failures are phase-to-phase, and even fewer involve all three phases.
That is not to say that nothing will happen, of course. For one thing, an earth fault will cause the voltage at the neutral point to rise to the system’s phase-to-neutral voltage; this can present a shock hazard and causes 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.
A high impedance earthed system shares one potential drawback with an unearthed one: since some systems are configured to continue to operate if one phase faults to earth, how can one know that such a fault has occurred? Arguably the best way to sense ground faults in a high impedance grounded system is to use a current-sensing earth fault relay (EFR, or ground fault relay — GFR — in American parlance), which uses one or more core-balanced zero-sequence current transformers (CTs or ZSCTs) to detect currents flowing where they should not.
This permits selective coordination on a tripping system and makes it easy to find a fault on an alarming system. It can be implemented using a core-balance zero-sequence current transformer, which involves passing all current-carrying conductors through the window of a window-type CT; if there is an earth fault the currents will be unequal, and the difference can be detected and used to set an alarm. The Canadian standard CSAStd. M421-11 Use of Electricity in Mines, requires ground fault protection. M421-00 also mandates ground conductor monitoring and earthing-resistor monitoring. If the earthing resistor opens — by accident, by mistake, or even by someone stealing the earthing wire — the system will become unearthed, and may remain so for some time, with the potential for equipment damage caused by transient overvoltages. A neutral earthing resistor monitor will detect if this happens.
Consider an arc flash relay
Even a high-impedance earthing system can experience a phase-to-phase arc flash; for this reason there is a need for a method to directly detect an arc flash and instantly shut down power. This device, an arc flash relay, is arguably the most advanced arc flash protective measure. It is useful on any system with sufficient voltage/current to produce an arc flash. In addition, it can be used with zone-selective interlocking protection systems.
Figure 3 - Arc flash relays can shut off power before serious damage occurs
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.
What an arc flash relay does
An arc flash relay uses photo sensors mounted inside cabinets or enclosures to detect the onset of an arc flash and sends a signal to an upstream breaker to shut off the power. 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 3.
The photo sensors can be either point type (Figure 4) placed in each cabinet or a distributed fibre-optic type that can be threaded through areas to be protected. Arc flash relays should be installed to protect every cabinet or enclosure that has sufficient power present to cause an arc flash. Note that 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.
Under some conditions an arc flash relay may be fooled by light from other sources —flash photography, nearby welding, or even direct sunlight. To prevent this, an arc flash relay can use current transformers to provide a trip-permissive signal; if the sensors detect a flash of light but there is no corresponding increase in current, the relay will not trip.
It is worthwhile to point out that the main breaker must be equipped with a relay trip coil, or be replaced by one that does, and further that the main breaker should be cycled on and off every few months to make sure that it will open when the arc flash relay signals it.
Arc flash relays are available from a number of sources; make sure that the one selected has such features as very fast response, internal health monitoring, 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.
Figure 4 - An arc flash relay uses a series of photo sensors mounted in areas to be protected. Photo credit: Salisbury by Honeywell
In the mining industry arc flash is the most common source of electrical injury. In the period from 1990 to 1999 in the US mining industry 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, shrapnel, etc., nor does it include economic losses.
Modifications to electrical systems, such as converting to a high impedance earthed system, can help to reduce the rate of accidents, and a very effective improvement is to install arc flash relays, which can greatly reduce personnel injuries, equipment damage and economic losses.
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