In my career in the power utility business I have had many different jobs. In the winter of 1981-82, in the wake of the Three Mile Island accident, I was a field engineer on a ladder against a utility pole. I was doing field checks on the installation of emergency sirens for a nuclear plant, and as part of those checks, I did a “bump check” for direction of rotation on the siren motor. The rotation was incorrect and so I carefully “swapped” two leads at the motor contactor to obtain the correct rotation, WITH THE LEADS ENERGIZED.
When I got back to the plant, I told the guys that I worked with what I had done, and they looked at me in disbelief. I had never been told about the consequence of an arc flash accident. I was given a quick education on the subject and thanked my lucky stars that I had not killed myself.
This story illustrates the need for general electrical safety training, which I had never had, in spite of over 10 years of work in power plants at that point in my career. Note that at that time, there was no such thing as arc flash training as we know it today. If anyone needs an object lesson on the need for arc flash safety, just enter “arc flash accidents on You Tube” into your web browser to see gruesome videos of the consequences of arc flash.
Much has changed in safety regulations and training since my brush with arc flash danger. One of the landmarks of arc flash safety is a white paper by IEEE member Ralph H. Lee (PE) in 1981, “The Other Electrical Hazard: Electric Arc Blast Burns.” This was followed by the first version of NFPA 70E, Standard for Electrical Safety in the Workplace, which was released in 1995, and IEEE 1584—IEEE Guide for Performing Arc Flash Calculations which was published in 2002. Also in 2002 there is the first mention of arc flash labels in the National Electrical Code (NEC). OSHA covers requirements for arc flash safety in standard 1910.269.
An arc flash is essentially an explosion caused by very rapid heating of air and ionized gases to temperatures as high as 35,000°F; the explosion occurs from the rapid expansion of air and vaporized components as they are heated by the arc flash; for reference, copper vapor produced from an arc flash expands to 67,000 times the volume of solid copper. There is also considerable radiant heat transfer. The net result is an explosion of hot gas with a supersonic shockwave and extreme radiant energy, with potential for severe burning.
The amount of energy released in an arc flash depends, principally, on two parameters:
Engineers have developed a parameter called incident energy for arc flash incidents. There are detailed guidelines for calculation of incident energy through a process called an arc flash study. The principal factors that determine incident energy are:
The incident energy is expressed as energy per unit of area, using metric units, which is calories per square centimeter, (cal/cm2). For reference, the threshold value of incident energy for a 2nd degree burn of human skin is about 1.2 cal/cm2. PPE requirements depend on the incident energy calculated. NFPA specifies four arc flash categories as follows:
At Category 1, a person must wear AR (Arc Rated) shirt and plants (or overall), and AR face shield along with more “standard” PPE, including hard hat and so on only. At Category 4 in addition to the Category 1 PPE, it is necessary to wear AR flash suit hood, AR gloves and AR jacket (often collectively called a moon suit) as well. Anyone who has worn Category 4 PPE can attest to the fact that the PPE makes performing many tasks more difficult then Category 1 PPE. This means that, in addition to safety, reducing incident energy aids personnel by making work easier to perform.
There are many ways to keep personnel safe when working on or around energized electrical equipment. Many of the measures used to make people safe are procedural in nature, such as implementing effective lock-out-tag-out procedures and wearing appropriate PPE. Another approach involves the designing electrical equipment to make it safer. To understand this some more background is necessary
An arc flash is generally caused by a short circuit (fault), which may be phase-to-phase or phase-to-ground. Protective devices, including protective relays, were developed over 100 years ago to trip circuit breakers to de-energize (clear) faults. Perhaps the earliest protective device is the fuse, which interrupts current in a circuit when current exceeds a setpoint. In most power utility applications fuses have been replaced with overcurrent relay.
Over time, protective relaying has become an increasingly sophisticated art with the objectives of providing protection for personnel and equipment while at the same time minimizing disruption to electrical distribution system operation. One aspect of protective relaying in electrical distribution systems is the use of time delays in protective relays.
One example of such a time delay is seen in what is commonly known as an inverse time overcurrent relay. In the inverse time overcurrent relay, there is a time delay for the breaker trip that is inverse to the magnitude of the current; simply stated, the greater the current is, the shorter the trip time delay is.
One reason for the use of time delays like that seen in the inverse time overcurrent relay is to prevent “false trips” that result for normal electrical phenomena such as motor starting current. The starting current for a motor may be as much as ten times normal running current for a few seconds, with no harm to the motor or the electrical equipment (conductors and so on) that feed the motor. Time delays are also applied to other types of protective relays, such as differential relays as well.
Another reason for the use of time delays is to coordinate the activities of multiple circuit breakers in an electrical system. The objective of this coordination is to allow the circuit breaker that is electrically closest to the fault to trip and clear the fault, while other circuit breakers remain closed to avoid loss of power to the entire electrical distribution system.
In the last several years, there have been two innovations in protective relaying for switchgear that is designed to reduce incident energy in order to reduce hazards to personnel. These innovations are in response to requirements of the 2014 update of NEC Section 240.87. This update requires some means of reducing the clearing time of any potential arc fault in “installations” (e.g. switchgear) involving overcurrent devices that are rated, or can be adjusted to, 1200 amps or higher. This blog describes two common means of complying with this requirement, energy-reducing maintenance switching with a local status indicator, and energy-reducing active arc flash mitigation systems.
Energy-Reducing Maintenance Switching (ERMS) is a system that sets circuit breaker electronic trip unit(s) and/or protective relay(s) to operate faster when there is arc fault than with normal setting. There is a local switch that personnel can operate to place the system in service (ERMS is ON); there must be a “local status indicator” to allow personnel to verify that the system is in service. ERMS systems have different names depending on the manufacturer; two common names are RELT (Reduced Energy Let Through) and Quick Trip Switch. In one example, the use of an ERMS reduces the fault current from 12,000 amps to 8,000 amps and the time delay is reduced from 0.20 seconds to 0.05 seconds; the incident energy fell from 25.8 to 2.3, and thus the PPE category fell from 4 to 1.
The NEC requires that the ERMS switch be on anytime a worker is working within the arc-flash boundary as defined in NFPA 70E. A typical task that would call for the ERMS switch to be turned on is racking a circuit breaker in or out a breaker in switchgear that is energized. After personnel are done with their work and are outside of the arc-flash boundary, the ERMS switch can be turned off.
A potential disadvantage of the ERMS system is that it must be placed in service manually. This means that there is potential for human error, failing to turn the ERMS system on.
One type of energy-reducing active arc flash mitigation system is often referred to as a crowbar system. The crowbar system creates a low impedance current path for an arc fault while an upstream circuit breaker clears the fault. The disadvantage of this system is that like the ERMS it must be placed in service manually.
A common alternative to the crowbar-type energy-reducing active arc flash mitigation system is Arc Flash Detection (AFD) devices, also called arc flash relays. The advantage of these devices is that they are in service continuously and operate automatically, thus reducing the possibility of human error compromising arc flash protection as compared to the ERMS.
While there are many variations of energy-reducing active arc flash mitigation system, depending on the manufacturer, most AFDs use optical sensors to detect the bright light that accompanies and arc flash. When the AFD senses the light from an arc flash, it generates a circuit breaker trip signal that generally has no time delay, which opens circuit breaker that feeds the fault more quickly than without the AFD, thus reducing the incident energy. Typically the electronics in the AFD use special fast-acting transistors to minimize the time required for the generation of the tripping signal. Using AFDs can reduce incident energy by as much as 80%.
Many AFDs have an additional feature and overcurrent relay. For these AFDs, there must be excessive current AND optical indication of an arc flash to generate a trip signal. This feature is provided to prevent “false trips” for sources of light like a camera flash. AFDs that have this overcurrent function typically have a switch that allows the overcurrent relay function to be disabled so that the light from the arc flash alone generates the trip signal.
Protecting the people in your plant from the hazards of arc flash required training so that they understand both how arc flash can cause severe injury and death, and the means to stay safe. Arc flash training requires understanding “generic” safety requirements such as the PPE required based on the NFPA arc flash categories. Truly effective arc flash training requires covering the specifics of your plant, including features like as ERMS systems and AFDs that are installed on your equipment.
FCS has a long and successful experience in providing effective, plant-specific arc flash training, including the relatively new equipment used for arc flash protection. For more information, contact us at fcs@fossilconsulting.com.
