Don't Be Shocked
Protect your equipment from static damage--and your workers from the risk of electric shock--with matting designed specifically for the job.
- By Erin L. Wade
- Jan 01, 2003
ALL of us have experienced it--that tiny shock we receive when touching a doorknob after walking across a carpeted floor. The shock is the transfer of static charge, or a balancing of our charge to that of the object. This transfer is called Electrostatic Discharge, or ESD. To understand electrostatic discharges, you must first understand the source.
Ernie Harper, a safety and forensics engineer at Hewlett-Packard Co. in Idaho who has worked with ESD for many years, said there are three basic types of ESD. ESD can come from a source (our clothing, for example) directly to the device needing protection, the device generating a charge within its own automated part movements, and field coupled or induced electric transfer. "Whenever an object is electrostatically charged, there is a field produced around that object," says Harper. "If a device can be placed in that field, a charge is created, and if that device then becomes grounded, the original charge can be transferred over."
Any ESD device, such as matting, has the same purpose: (A) to dissipate or prevent electric charges from being built up on the surface of a targeted device or surface, and (B) to do it without creating a safety hazard or damaging sensitive electronic devices because of a too-quick discharge, said Harper.
ESD can cause any degree of damage to your sensitive equipment, ranging from total or a catastrophic failure to a degrading or partial failure. In some cases it may be referred to as a latent failure initially undetected, but one that will shorten the lifespan of the device or make it perform improperly. Recent research indicates ESD may be responsible for more than 25 percent of all integrated-circuit failures over the complete life cycle of parts in manufacturing and testing, as well as in the field.
Protecting Your Equipment
Conductive mats are used to protect sensitive equipment. They come in two categories based on their level of resistance to electric current. According to Lisa O'Dell, vice president of marketing for Tennessee Mat Co., conductive mats have a very low electrical resistance of 1x10(3)-1x10(6) ohms that allows ESD to flow across its surface. (An ohm is a measure of resistance the mat will have to the static charge. Therefore, the lower the ohms, the less it resists the static charge, and the faster the charge is taken off the worker and through the mat.) However, "this mat may not provide protection from a serious shock hazard if you encounter a live electrical circuit," said Harper.
Static-dissipative or anti-static mats, with an ohm reading of 1x10(6)-1x10(10), have a higher resistance, which means they take the charge off the worker a bit slower. Harper said these mats provide enough resistance to slowly dissipate the static charge without risk to more sensitive microcircuit devices. "Anti-static mats and similar ESD-rated devices are capable of providing reasonable protection for life-threatening encounters with voltages up to 480 volts by limiting current flow from the body through the mat to survivable levels," he said. "Conductive mats may cost you more, but as you can see, the higher cost does not equate to the best mat. The anti-static mat, with its 'slower rate of neutralization,' will protect the more sensitive devices while improving safety to the employee."
According to O'Dell, there are a few things to consider when choosing a mat. First, determine how sensitive the equipment is. Leaders in the mat industry say conductive mats would be best for very sensitive equipment because the charge would move quicker and there would be a lesser threat to the equipment. However, end users of the mats say very sensitive equipment cannot handle the fast-moving static charges; they prefer to use anti-static mats, which move the charge a bit slower. In deciding which mat to use, be sure to shop around and get several opinions.
If one of your workers must stand on the mat for long periods of time, you also need to consider the comfort provided by the mat. Some ESD mats have a sponge backing and a separate layer that aids in comfort. Molded rubber mats are second in comfort, but if comfort is not an option, runner mats would work fine, O'Dell said.
"The problem with sponge backing is that the sponge is not conductive," she said. "It is expensive to make a conductive sponge and it usually increases the cost of the mat."
There are many applications where ESD matting could be useful, but mostly in industrial facilities. "Some workers may be packaging materials in shrink wrap, which is loaded with static charge, then they walk over and key information into a computer," O'Dell said. "They will need a mat in front of that computer to help conduct the charge."
Many manufacturing plants also use assembly lines that are run by computers. "Static also creates a spark, so you don't want static around anything flammable," O'Dell said. Because of this, pharmaceutical companies and people who work with paints might want to place ESD mats in work areas.
All matting must be grounded to be effective, said O'Dell. If it is not grounded, there is nowhere for the charge to go. Grounding accessories such as a grounding cord, which will connect the mat to a building ground, should be purchased with the mat to ensure it will be effective. Crown Mats and Matting also recommends using a grounding cord for every 10 feet of matting.
Despite all efforts to ground the mat, many people make the mistake of wearing tennis shoes or other comfortable shoes with insulative soles, which make it impossible for static to be drawn into the mat, O'Dell said. "One option is to buy conductive shoes, which are expensive, or buy a heel grounder," she said. A heel grounder is a V-shaped piece of conductive rubber that fits over the heel of the shoe. A piece of conductive fabric is attached to the grounder and then tucked into the shoe, providing a vessel for the charge to travel through the worker to the mat. "Skin also produces moisture, especially in the shoe, which helps in the conductivity," she said.
Computer workers might also need a wrist strap that attaches to the mat. Computers generate lots of static, and this will draw the static directly to an ESD tabletop mat.
According to Harper, many other ESD devices are in use, including ESD floors, ESD chairs, ESD work surfaces, and more. "All these devices share some common features in that they have a minimum electrical resistance of 1 meg-ohm or more and should connect to a common grounding point," he said.
Protecting Your Workers
According to OSHA, someone is electrocuted in the workplace every 36 hours and more than 700 people lose their lives every year because of accidents associated with electricity and electrical products.
Mats used to protect workers are in a completely different category called non-conductive, switchboard, or dieletric matting. This type of matting prevents ESD from flowing across its surface and insulates the worker against high-voltage electric shock. In these high-voltage environments, any movement or attraction of electricity could be life-threatening.
Non-conductive mats usually start with 10,000 meg-ohms or higher resistance, said Harper. "These mats will not allow static charges to dissipate," he said. "Non-conductive mats are usually used around very high-voltage environments where electrical current flow can threaten life."
According to O'Dell, the two types of non-conductive matting are based on test specifications that the materials meet. One spec is by ASTM and another is a military specification; end users buy mats based on one or the other.
Non-conductive matting is insulative and tends to be more expensive because it is made of denser materials and has to be completely pure in form. Any defects or small fibers in the material could conduct electrical charge and pose a life-threatening risk. This type of matting is usually made of dense rubber or a dense vinyl compound. Once the material is made, every square foot is tested with a high dielectric amperage to make sure there are no defects.
"The dielectric strength of these mats is usually 30,000-40,000 volts," said O'Dell, "but the recommended usage is much lower to ensure a gigantic margin of safety. That way there is absolutely no chance of a charge ever being conducted."
There are several applications for this type of matting, according to the Stanford Linear Accelerator Center's Environment, Safety, and Health Division. First, it must be placed around all test benches that are used for testing equipment with exposed, energized parts. It must be placed so that personnel are standing only on the matting and are never in direct contact with the floor or any other grounded metal parts while working on or near exposed, energized parts. It must be used in addition to all other personal protective equipment that is required by OSHA when working with exposed, energized parts, and it must be inspected regularly to ensure it is not damaged.
Other mat issues are relevant to safety beyond their electrical properties, Harper said. First, many mats may not work well with chair or cart traffic. "They may roll or bunch up with wheel movement and the backing material may not hold up over time, especially if it's rubber or vinyl," he said.
If mats are placed over carpet, they often cause tripping hazards if not selected specifically for that type of surface. It must be remembered that mats are only a part of an ESD system. An ungrounded anti-static mat can actually hold a static charge, making the problem even worse.
Cleaning also is an important part of maintaining the mat's effectiveness. "It is always correct to take the manufacturer's recommendations," said Harper, "but be sure to control this aspect because carbon or other conductive components can be degraded with harsh cleaning methods.
"On the other hand, not cleaning mats may increase or decrease the effectiveness of the mat over time, depending upon whether the contamination is conductive or resistive in nature," he continued.
A key point to remember is that complete elimination of static in the workplace is not possible. That's why all companies subject to unfavorable static discharge should apply a coordinated method for controlling the problem.
This article originally appeared in the January 2003 issue of Occupational Health & Safety.