Abstract: A voltage indicator is a low cost device that pre-verifies electrical isolation before maintenance personnel open a control panel and risk exposure to hazardous voltages. Typically mounted to the panel exterior and hardwired into the load side of the main disconnect, this device provides electrical maintenance immense safety value. Easy to apply, low cost, quick installation, and inherent reliability makes a voltage indicator a very effective electrical safety device.
Creating and insuring an electrically safe work condition is critical for electricians performing maintenance on de-energized systems. The presence of voltage is the only determining factor if an electrical accident or an arc flash can possibly occur --No voltage, No accident, No arc flash. "Voltage or no voltage" is also the basis of the NFPA 70E''s two primary themes: a) establishing an electrically safe work condition, and b) achieving safety while working on energized systems. A voltage warning indicator is like a hard-wired voltmeter with the advantage that it needs no power supply because it is powered from the same voltage that it indicates. This paper discusses its features, application criteria, critical reliability issues. A voltage indicator is an additional safety measure between maintenance personnel and hazardous voltage.
Electrical Safety is thrust into the crosshairs of every corporate safety committee after the publication of NFPA 70e Standard for Electrical Safety in the Workplace in 2000. Why the sudden interest in electrical safety? Electrical accidents have the highest fatality rate of all industrial accidents and the NFPA 70e was the tool OSHA needed to enforce electrical workplace safety. The NFPA 70e took electrical safety from black magic to a precise science. Furthermore, this document demanded new procedures, added personnel protection equipment (PPE); and required rigorous training standards, thereby making many practices obsolete.
Voltage Indicators: A Simple Concept with Striking Safety Value
Higher electrical safety standards inevitably create a need for new safety products. One of the best new products is an externally mounted voltage indicator that provides electricians with a flashing indication of voltages within a panel, before they open the panel. Elegantly simple, this product has immense safety value, because it addresses the core issue of electrical safety -- Is there voltage?. . . does the electrician know it?
Electricity: A Unique Danger
People and electricity are a lethal mixture. Electrical safety demands the correct answer to one question: Is voltage is present? Most electrical accidents happen when personnel inadvertently touch an energized conductor, which they think is de-energized. You can taste, smell, or see most other hazards (natural gas=rotten egg smell, fire=smoke, etc.), but electricity is invisible and gives no warning when it shows up for an accident.
"Electrical incidents, although a small portion of the total incidents, are disproportionately fatal when they occur."[1] Electrical accidents account for only 0.2% of all lost work days due to occupational accidents, but account for 1 in 20 (4.8%) occupational fatalities.[2] If a typical plant electrician has an electrical accident while working on 600 V or below, there is a 4.1% chance that it will be fatal.[3] This is 20 times the fatality rate when compared to the leading cause of occupational fatalities -- "Contact with Objects and Equipment" (fatality rate of 0.2% -- 873 deaths and 400,033 injuries). The National Safety Council (www.nsc.org/) estimates the direct cost for the employer of a fatality is $1,070,000[4].
Why is there such a high fatality rate for electrical accidents? Electricity can flow through people instantaneously -- in 1/60th of a second -- and it doesn''t take much current to kill a person. The risk of fatality increases with higher voltages because there is more force to move higher currents through the body. Consider these facts:
- 2-10 mA[5]: Minor shock, might result in a fall.
- 10-25 mA: Loss of muscle control, or not able to let go.
- 25-75 mA: Painful, may lead to collapse or death.
- 75-300 mA; For a ¼ second almost always results in death.
Current passing through the heart, chest, or nervous system is the most dangerous, and current passing through limbs results in severe burns. Current that hits the chest is almost always fatal, as is current passing between the hands and feet and involving the lungs and heart.
Essential Features
The electrical design of a voltage indicator demands accurate indication of Voltage or No Voltage. Every possible electrical scenario must be designed into the product. Lack of one critical feature may yield a Maybe voltage indication, thereby nullifying the voltage indicator''s benefit. Likewise, a voltage indicator, with the required functionality and reliability, provides a second independent verification of voltage in addition to a voltmeter. In other words, a second opinion (or voltmeter) is safer.
Redundant Circuitry, Design Reliability, and Procedures
Most safety alerts (i.e. smoke detectors) operate in a normally safe state (houses don''t burn everyday) and require external, yet fallible, power sources (i.e. batteries or 120 Vac), and the TEST button typically activates only part of the circuit (the horn). The only true test is creating enough smoke to activate the smoke alarm. Conversely, a voltage indicator that operates in a normally hazardous state (electrical systems are energized), is powered from the very hazard it indicates (no separate power supply to fail), and its TEST function is seeing it flash when powered. Lastly, external voltage indicators with redundant circuit designs and inherent reliability features, combined with a lockout tagout procedure that requires electricians to see it stop flashing after the isolator is opened, will increase safety tenfold.
Hard-wired voltage indicators check voltage all the time without using a voltmeter. They operate on either ac or dc over a wide range of voltage; warn personnel if one, two, or all three phases are live; and accommodate unique power sources.
Wide Voltage Range Operation
Electrical accidents and the NFPA 70E[6] consider voltages over 50 V to be hazardous. Therefore, a voltage indicator needs to indicate if there is any voltage on any phase above 50 V. Facilities with 600 V (or less) power systems could standardize on a voltage warning indicator with a rating of 40 Vac/30 Vdc to 750 Vac/1000 Vdc. This device could be installed in any panel in the facility.
Pilot lights and other devices operate at a narrow voltage range (i.e. 480 V+/- 10%). Outside this range, the device would be damaged and lethal voltage could still exist inside the enclosure.
AC/DC Operation and Stored Energy
A big concern with electrical safety is stored energy[7], which comes from several sources:
- power factor correction capacitors,
- machinery that "creates" electricity when slowing down, thereby feeding electrical power backwards into the electrical system.
- large power supplies or UPS systems,
- emergency back-up generators (stored energy is in the form of gasoline), and
- ac or dc drives.
In most cases, an electrical system is electrically safe when the isolator is open. However, stored energy can be a secondary lethal power source, present for a time, after the system has been electrically isolated. Stored energy, created under an ac power source, is dissipated by the flow dc currents to ground until the voltage level is zero. Because of this, the voltage-warning indicator needs to operate on dc and ac current and a wide range of voltage. This ensures that personnel have a visual warning until the stored energy has bled down to a safe level of 50 V or less.
Visual Indication of Multiple Power Sources
Unique safety risks occur when a control panel has multiple power sources. If an electrician is greeted with flashing voltage indicators showing the locations of all live power sources within the panel, he immediately knows how the panel is being fed. Even a de-energized voltage warning indicator provides the physical location of de-energized sources of power.
Systems that employ emergency backup generators inherently have multiple power sources and sometimes multiple isolators. A voltage indicator installed in these systems offers voltage indication to personnel in the event the generator is energized during maintenance. The safety issues are further exacerbated, because emergency generators are tested regularly. This means that the emergency power feed is energized intermittently, thereby providing an opportunity for an unsafe condition due to unexpected energization.
Identifies Even One Phase That''s Live
Occasionally isolators (disconnect switches and circuit breakers) fail and will weld the isolator blade(s) in the ON position, thereby leaving other phase(s) live. This is highly dangerous for an electrician, because the panel may look dead, but still has a live phase(s). In this case, a voltage warning indicator needs to warn electricians if there is power just on one phase. This hazardous situation affirms that the voltage indicator needs redundancy and exceptional reliability.
Arc Flash Risk
An arc flash explosion may occur whenever there is a phase-to-phase or phase-to-ground fault. The arc flash risk increases every time a voltmeter checks live voltage between phases and/or ground. An external voltage indicator shows an electrician that no voltage exists, while the panel is closed. The arc flash risk is reduced because the electrician uses his meter to check that the panel is dead, not live. Remember -- no voltage, no arc flash.
Once electricians open the disconnect and enter the panel, they must first verify isolation with their meter. This task is done as though the panel is live, and requires electricians to suit up in proper PPE. By employing a voltage indicator on the panel exterior, electricians can see it flashing, indicating Live Voltage. The indicator stops flashing once the disconnect is open, thereby pre-verifying that the panel is dead before it is opened by the electrician.
Conclusion
"Is there voltage?" and "How can we make sure that my electrician knows it?" Voltage is the single mitigating factor to electrical safety. Installing a permanent device that supplements the electrician''s voltmeter and provides 100% voltage warning all the time will result in more safety. One user said, "We haven''t had a single electrical accident since we started using [voltage warning indicators]." The presence of voltage determines if an electrical accident can occur -- no voltage, no accident. Therefore, the ability to see electricity all the time with a voltage warning indicator will provide constant awareness and keep personnel safer. Electrical fatalities will be reduced because a visual voltage warning device checks voltage 100% of the time.
Footnotes
[1] Page 244, Occupational electrical injuries in the United States, 1992-1998, and recommendations for safety research, Journal of Safety Research 34 (2003) 241-248, James C. Crawley, Gerald T. Homce
[2] Ibid. Section 2.2 page 244
[3] Bureau of Labor Statistics--Non Fatal and Fatal Injury Data for 2002
[4] secure.nsc.org/lrs/statinfo/estcost.htm#OTHER
[5] 1/1000 of an Ampere
[6] NFPA 70E 130.1
[7] NFPA 70e 120.2(F)(2)(b)