Proper safety procedures are critical when testing high-voltage systems.
As the following examples prove, mistakes happen. However, when the safety of you and your coworkers is at stake, neglecting strict safety protocol can result in serious injury and even death.
An electrician troubleshooting a live 480V motor control center receives serious second- and third-degree burns from an electric arc. The cause? He had incorrectly placed the function switch on his multimeter to read resistance instead of voltage.
Two electricians are severely burned — one fatally — while testing for voltage in a motor starter. One held a multimeter while the other applied the meter's probes to energized terminals. One electrician's unexpected movement caused a test lead banana plug (energized from the circuit under test) to separate from the multimeter jack. The plug made contact with the starter's grounded metal enclosure and initiated a high-energy arc.
An electrician is sent spiraling from a steel tower while checking a circuit with a multimeter. He was using a meter rated for 1000V to verify a 15kV circuit was dead. He didn't know the circuit was hot. Therefore, when he applied the multimeter, a phase-to-ground fault occurred. Fortunately, his safety harness stopped his fall and coworkers on the ground rescued him.
An electrician receives second- and third-degree burns from a high-energy arc while measuring the voltage to ground on each phase of an ungrounded, 2.3kV, 3-phase circuit. Using a medium-voltage voltmeter, he and his crew had measured zero voltage to ground and assumed the circuit was dead. Then, they began attaching temporary safety grounds to each phase. After connecting one phase to ground, the crew attached another safety ground, resulting in a high-energy arc.
These case histories come from an IEEE paper entitled “Personnel Safety and Plant Reliability Considerations in the Selection and Use of Voltage Test Instruments,” (IEEE Transactions on Industry Applications, Vol. 33, No. 2, March/April 1997), written by H. Landis Floyd II and Brian J. Nenninger.
Guidelines for increased user safety. These four horror stories are not isolated incidents. According to Floyd and Nenninger, the underlying cause of such situations is usually a combination of one or more hardware deficiencies, human errors, and the shortcomings of systems that manage the selection and use of test instruments.
Following the guidelines outlined below can keep you and your staff from becoming electrical accident statistics:
Know your instrument's limitations. Thinking you know everything about test instruments can be dangerous. As the IEEE paper points out, most solenoid-plunger type testers have thermal duty cycle limitations (not rated for continuous service). If you exceed these limits, you could see turn-to-turn failures in the solenoid coil. These instruments also have a voltage-indicating threshold of approximately 60V and a relatively low-impedance inductive load that can cause undesirable transients or distort the waveform of the circuit under test.
The paper also explains that limitations on noncontact-type voltage-sensing instruments (operation is based on capacitive coupling) include the inability to detect DC voltage or accurately indicate voltage magnitude. Variations in the test circuit's capacitive coupling may affect the performance of certain makes of these instruments. In fact, one such instrument would not indicate voltage on the secondary of an ungrounded 2.3kV transformer when it was isolated from the bus and cables. This suggests the capacitive coupling of the secondary cable system was a variable in the instrument's performance.
To err is human, so be extra careful. Floyd and Nenninger note that analog and digital multimeters require manual setting of the function selector switch and manual test lead plug connections, but these tasks increase the opportunity for operator error. As we've seen from our case histories, setting your multimeter to work as an ohmmeter or ammeter and then connecting it to a voltage source can damage the instrument and cause injury.
Almost all multimeters have thermistors, varistors, and fuses that are supposed to limit or prevent instrument damage from internal short circuits. But these devices may not protect you from exposure to electric shock or a high-energy electric arc flash. Why? Because the arc flash can occur before the protective devices can clear the circuit.
Have a management system in place. Because of hardware deficiencies and human error, you or your company must have a system in place to manage the selection, purchase, maintenance, and overall control of test instruments used in the workplace. Without such a system, you may easily identify equipment failures and human errors, but miss the underlying cause of electrical accidents involving test instruments.
An effective management approach. Floyd and Nenninger list the following points for managing personnel safety as it relates to voltage test instrument use.
Consider voltage test instruments to be personal safety equipment. This implies a higher level of attention and control.
Converge suppliers and set minimum specifications. This simplifies training and helps keep undesirable instruments off jobsites.
Set training requirements. Each make of instrument has unique features that even experienced personnel may be unfamiliar with. Training should cover the hazards and limitations of each instrument you plan to use.
Retire obsolete instruments. Review your inventory to identify these instruments. Many instruments currently available don't meet the rigid standards needed to prevent accidents. In fact, older models of currently acceptable instruments may predate recognized safety standards.
Restrict multimeter use when voltage testing is the only need. For circuits where a multimeter configuration error could cause a high-energy arc flash, restrict the use to single-function voltmeters or other instruments that don't depend on manual settings.
Audit, test, and inspect. Make sure you regularly review the hardware, personnel, and management system elements noted here for compliance and improvement opportunities. Also, ensure you audit the qualifications of your personnel and provide training when needed. What's the bottom line? Inspect your test equipment and keep it in top condition.
Users provide snapshot of current practice. In developing their paper, Floyd and Nenninger conducted a survey of common practices and experiences within the petrochemical industry. Table 1 on page 33 (not available on Web version) summarizes the survey results for a cross section of companies.
Questions 1, 2, and 4 deal with company policies and practices regarding voltage test instruments. Question 3 deals with unwanted events attributed to the malfunction or improper use of these instruments. More than 18% of facilities had a personal injury claim in the past five years. Also, more than 36% reported near misses, while almost 12% experienced a plant upset.
Of the companies surveyed, almost 72% offer training on the use and hazards of voltage test instruments, while 65% have documented safety procedures to clear electrical equipment. However, only 35% have a safety policy specifying the types of voltage test instruments allowed for clearing or troubleshooting electrical equipment. Of those companies, 71.5% limit the types allowed to five or less. Note: More than 76% of those same companies apply the policy to contractors.
Table 2 on page 33 (not available) summarizes the correlation of survey results between Policy Questions 1, 2, and 4 and Unwanted Events Question 3. The results show that almost 91% of all reported personal injuries occurred at facilities that did not have a policy specifying the types of voltage test instruments to be used. In addition, more than 68% of all near misses and more than 85% of all plant disruptions occurred within these same facilities.
The development of safety standards and specifications is not a task to take lightly. As illustrated by Floyd and Nenninger's study, significant opportunities for adopting a policy of test instrument specification exist at today's facilities. Developing company training programs and clearing procedures that address the proper use of voltage test instruments is just as important. With safety guidelines in place and employees trained on proper test procedures, you'll be less likely to become a statistic.