From: Gary Jones(firstname.lastname@example.org )
Date: Wed, 3 Mar 2004 11:57:42
Big Batteries? Big Electric Shock Potential!
Those who install, service, or operate these stored power systems sometimes overlook or underestimate the hazard they pose. by Bill Ramsey
Large industrial and stationary back-up batteries are used to "store" electrical power. The sudden and unplanned release of their direct current (DC) electricity can cause problems ranging from electric shock to battery fires or explosions.
The voltage required to power motive "forklift type" equipment begins at 36 volts and ranges upward. These high-voltage batteries are used in applications at distribution centers, for airport operations, in underground mining, and in all-electric passenger vehicles. Of course, with every battery there is the need for a high-voltage charger and power source connections for charger operation. The charger provides yet another source of a potential electrical injury or lethal shock.
With an increase in voltage, the accompanying amperage also increases into a potentially lethal range. With stationary "back-up" battery systems, the voltages range in the hundreds of volts. These big "back-up" batteries are used to provide emergency power when normal sources of electricity are interrupted without warning.
Those who install, service, or operate these stored power systems sometime overlook or underestimate the electrical hazard they pose. The hazard of direct current shock is thought by some to be below the injury or electrocution threshold level. Some also think DC shocks are rare or that the human body can somehow tolerate a DC shock better than it could an AC (alternating current) shock. In truth, DC and AC both present significant hazards to those who are not trained and equipped to deal with them.
Electrocutions rank fourth in workplace fatalities. Half of those occur when the victims are working at 600 volts or less. Additionally, there are 8,000 electrical injuries annually, and nearly half of those are disabling. The difference between a mild shock and respiratory and heart paralysis is actually quite small. At only 30 milliamps, an electric shock can be life-threatening. This hazard must be kept in mind when working with "live" electricity, including that found in stored power sources such as large batteries and chargers.
Minimizing the Risk
What can be done to minimize the risk of electrical shock when servicing large batteries? The first line of defense begins with the selection and use of insulated service tools.
Insulated tools prevent short-circuiting the battery or charger by arcing across two conductive parts of the system. Insulated service tools also prevent electrical current from entering the body at the point where the tool contacts a conductive portion of the circuit. Insulated tools are certified for use up to 1,000 volts. OSHA's 29 CFR 1910.331-335 requires the use of certified insulated tools "where 50 volts or more may be present."
Next, the selection and use of electrical safety wear is important when working on or around batteries. Protection of the eyes, face, and hands top the list. Personal protective wear should match the type and the level of electrical hazard that may be present. When certain battery-related operations occur, consideration should also be given to the use of additional equipment such as insulated rubber safety shoes, floor mats, and draping blankets. An example of an elevated risk would be in the installation and powering up of a high-voltage battery system.
It should also be mentioned that batteries contain corrosive electrolyte that requires the selection and use of personal protective equipment appropriate to chemical hazards. Exposure to corrosive electrolyte is most likely when operations such as the moving or watering of batteries occur. Battery electrolyte is either dilute sulfuric acid in lead acid batteries or potassium hydroxide in alkaline batteries. Both of these corrosives deserve considerable respect for the exposure hazard they represent.
Chemically rated protective equipment may not be designed or certified for electrical shock safety. For example, gloves rated for corrosive chemical protection usually are not the same as those rated for high-voltage electrical work. One should always assess which hazard is most likely to be encountered before selecting and wearing personal protective equipment. The hazard would be based upon the operation being undertaken.
Explosion and Fire Hazards
With large batteries, there is an additional hazard that is so closely related to the electrical hazard that it must not be excluded here. That is the hazard of battery fire or explosion. The recharging of a battery causes hydrogen to be produced. In the final stages of recharging, this is known as "gassing." Hydrogen is an explosive gas when found in the air in a concentration above 4 percent. That is known as the LCL, or lower combustion limit. Fortunately, hydrogen is also a light gas that is easily dispersed by air movement and exchange. A hydrogen gas monitor could sound the alarm indicating the increase of hydrogen concentration. However, air movement and exchange is the first line of defense.
The hydrogen level inside the battery cells themselves will always be greater than the level found in normal outside air. This elevated hydrogen level is created by the internal chemistry of the battery itself. That is why any area where batteries are recharged or stored in racks always should be posted with cautionary signs warning that no smoking, sparks, or open flames should be present in the area. Measures also should be taken to safeguard against static electric build-up and potential spark discharge. Additional signs warning of electrical shock hazard also should be present.
The fire and explosion hazards are real. While battery fires and explosions are not common, neither are they rare. Most often, post-incident investigation indicates the cause to have been an operational error or safety short cut.
There is one additional and often overlooked electric shock and battery fire hazard: The failure to maintain cables, cable connectors, and plastic insulators can create an electric shock or spark. The hydrogen-enriched area just inside the battery cells or just above the battery could be ignited by a single spark. Immediate replacement of damaged or missing exterior battery parts is imperative for safe operation. There are no known or accepted methods available to make safe temporary repairs to damaged cables. Wrapping damaged battery cables with "electrician's tape" will not afford protection.
Finally, it is important to point out that large batteries are a safe source of power when they are installed and operated properly by people who are trained and equipped to do so. Any stored power source has hazards that must be respected. Batteries are no exception to that rule.