Protect yourself from arc flash
Every year, more Canadian electrical workers are killed on the job than police officers or firefighters. In North America, an arc flash explosion in electric equipment sends victims to the burn unit at a rate of five to 10 times a day…an estimated one to two of those victims – per day -end up in the morgue.
How do you protect yourself from arc flash? The first step is to understand and appreciate the very real danger.
Arc flash temperatures can be as high as 2,800 to 19,000 degrees Celsius (5,000 to 35,000 degrees Fahrenheit). To give you a perspective, the temperature of the surface of the sun is an estimated 5,500 degrees C. An arc flash sends concentrated radiant energy, hot gases and melting metal outward to cause severe radiation burns or even death. Clothing melts and adheres to the skin. The accompanying pressure waves can be up to 2000 pounds per square foot, damaging hearing or brain function and the brilliance of the flash can damage eyesight. The pressure waves also hurl any loose equipment, tools, machinery or debris – causing injuries to anyone in the area.
Any work near live electrical circuits poses its share of risk, and electrical measurement jobs are no exception. In commercial and industrial settings in Canada, electricians commonly work with high-energy circuits up to 600 V. Though they are officially classed as "low voltage," these powerful circuits can deliver a deadly punch.
The danger of voltage transients
The presence of voltage transients is a characteristic of electrical supply systems that has important safety implications. When transients occur while a person is taking electrical measurements, they can lead to an arc blast.
Transients are present in almost every electrical supply system. In industrial settings they may be caused by the switching of reactive loads or by lightning strikes. Though such transients may last only microseconds, they may carry thousands of amps of energy.
For anyone taking measurements on electrical equipment, the consequences can be devastating. When such spikes occur while measurements are being made, they can cause a plasma arc to form — inside the measurement tool, or in the air outside. The high fault current available in 480 V and 600 V systems can generate an extremely hazardous arc flash.
Transients are not the only source of arc-flash hazard. A very common misuse of hand held multimeters can trigger a similar chain of events. If the multimeter user leaves the test leads in the amps input terminals and connects the meter leads across a voltage source, that user has just created a short through the meter.
Standards for protection
Arc flash is addressed legislatively at both the provincial and federal levels. Ontario's Construction and Industrial Regulations require workers to be protected against electrical shock and burns while working on or near the live, exposed parts of equipment or conductors. Alberta's Occupational Health and Safety Code requires workers exposed to electrical equipment flashover to wear flame-resistant outerwear and use other protective equipment appropriate to the hazard.
Federally, as of 31 March 2004, Bill C-45 established a duty under the Criminal Code of Canada for employers, managers and supervisors to ensure workplace health and safety. Under the code as amended by Bill C-45, there is no specific limit on fines against a corporation that's found guilty, and individual representatives of a corporation can receive a maximum sentence of life imprisonment if convicted of criminal negligence causing death.
The flagship safe work practices designed to prevent arc flash incidents are American in origin and contained in the National Fire Protection Association (NFPA) 70E, Standard for Electrical Safety in the Workplace (2004). The Canadian Standards Association (CSA) does not currently have a standard equivalent to NFPA 70E, although NFPA 70E is being considered for adoption in Canada. Many Canadian companies already adhere to the NFPA 70E standard.
NFPA 70E requires the calculation of a flash protection boundary inside which workers must be protected with personal protective equipment or PPE, such as eye and hearing protection, insulated hand tools, insulated gloves and fire resistant clothing. This boundary is different for various types of equipment and depends in part on the voltages involved. Incident energy during an arc flash is defined in NFPA 70E as the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. This incident energy is expressed in calories per cubic centimeter squared (cal/cm2). The flash protection boundary is the point at which the energy number is 1.2 cal/cm2 – or the level causing a second-degree burn.
As an integral component of PPE, test tools and equipment must also meet safety requirements. These standards are established by such organizations as ANSI, the Canadian Standards Association (CSA), and the International Electro-Technical Commission (IEC). Together they have created stringent standards for test equipment used in environments up to 1000 V (see sidebar for test tool safety information).
Predictive/preventative maintenance is the best protection
Knowing the health and history of the equipment in your facility is the first step to preventing serious incidents like arc flash. Poor connections, insulation failure, lightning, harmonics, overloading and wiring mistakes can all trigger electrical fires and other safety hazards – even arc flash incidents. The trick with detecting hazards is knowing what an abnormal reading looks like. The best solution is to gather baseline readings for especially important components and equipment:
Thermal imagers like the Fluke Ti30 can read the infrared energy emitted by an object and create a visible image of the object's surface temperature. Hot, loose connectors show up dramatically on these thermal pictures, especially in comparison to cooler, tight connections. This non-contact technique is perfect -- and safer -- for checking energized components and scanning operational equipment.
On a de-energized system, using a micro-ohmmeter will produce much more accurate results, ensuring that the connection will not dissipate excessive heat— or, identifying connectors that could be hazardous.
Insulation resistance is measured between phase conductors and between phase conductors and ground conductors. Good insulation should have very high resistance. An insulation tester applies a high dc voltage to de-energized, isolated components. The instrument then measures the resistance between the two points. This testing can be used to check large segments of insulation, including long lengths of cable, transformer windings, and motor windings. Low insulation resistance readings can indicate that somewhere on that length the cable is breaking down, potentially causing a short.
Circuit breakers are essential to electrical fire prevention. Proper testing of circuit breakers requires special equipment and specialized expertise. Testing is performed with the breaker removed from the circuit and the tests verify the trip current and delay.
Power quality studies can uncover symptoms that signal potential overheating. Periodically measuring harmonic distortion will alert you to potential heating problems due to excessive harmonic current. Voltage sags can be viewed as annoyance, but in systems service a consistent load they may be caused by deteriorating connections. Many wiring problems become apparent during a comprehensive power quality study.
Protecting yourself from arc flash comes down to respecting the danger, following safety procedures and knowing about the measures you can take to keep equipment working at safe and effective levels.