HEALTH & SAFETY
This common phenomenon is not to be taken lightly—under the right conditions, it can produce dangerous results.
STATIC ELECTRICITY has been recognized for many years as a possible fire or explosion hazard in environments in which flammable gases, vapors, and dusts are present. It can be generated easily, and poses a serious hazard in hospital operating rooms, industrial areas, and most rescue and extrication operations.
When two like or unlike materials are in contact, electrons flow across the interface in both directions. The flow is usually not equal in both directions, and one surface acquires an excess of electrons while the other shows an electron deficiency. As long as the surfaces are in contact, no charges are generated. When the two conductive materials are separated there will be no static charge; their conductivity produces a back-flow of electrons at the last point of contact that restores the surfaces to electroneutrality. However, if one or both of the materials are poorly conductive, the surfaces will retain static charges, one surface positive and the other negative.
The static charges generated can be quite large. A person walking across nonconductive carpet in low humidity can accumulate a charge of 10,000 volts. A static charge as low as 3,000 volts can ignite gasoline-air mixtures. Therefore, under the proper conditions, dangerous static charges can be produced easily.
Low humidity is one of the conditions conducive to the generation of static charge. Many materials are hygroscopic (will absorb water). If the relative humidity is high, these materials will absorb water from the air. The absorbed water will cause the surfaces of the materials to become somewhat conductive. This will minimize or eliminate the buildup of static charge. Examples of hygroscopic materials include cotton, wool, silk, and rayon.
Many synthetic fibers and fabrics are not hygroscopic or conductive. Among these are materials such as polyethylene, polystyrene, polyesters, polyamides, and polyacrylonitriles. Since these materials can generate static electricity easily, antistatic treatments have been devised. In most cases, the materials are treated with compounds that cause their surfaces to become hygroscopic. The materials are then manufactured into various items such as clothing, boot covers, and mats. The final products are then advertised as being “antistatic” or “static-free.” This advertising is both misleading and dangerous
Ihe terms “antistatic” and “staticfree” are accurate in some respects. The materials themselves have been made hygroscopic and, therefore, conductive. However, it must be emphasized that the antistatic treatment is most effective only when the relative humidity is fairly high (greater than 35 percent). At lower humidities, the effectiveness of the antistatic agent may be negligible.
More importantly, “antistatic” means that the material itself is conductive, but does not guarantee that a static charge will not build in a nonconductive material in contact with the antistatic product. Static charges are created in both materials, but the charge is dissipated in the conductive, antistatic material. However, if the other material is nonconductive, the static charge will remain and will possibly lead to a static discharge.
In an incident recently investigated in our college laboratory, operators were cleaning sludge from an underground gasoline tank. All personnel were wearing antistatic coveralls. Shortly after cleaning began, the gasoline-air mixture ignited at the sleeve of one of the operators wearing antistatic apparel. The person was removed from the tank, and the fire was extinguished. However, the victim received serious burns. Static electricity was obviously the cause of the ignition, since there was no other ignition source. All clothing and equipment were brought to the laboratory in order to determine the origin of the static charge.
Various combinations of materials were rubbed together, and their surfaces were tested with an electrometer to detect the presence of static charge. Static charges were produced in several cases. Charges resulted in a few instances when two nonconductive materials were rubbed together. This is to be expected. These cases were eliminated as causes of ignition because the materials tested had not been in contact at the time of the fire. Tests with other materials more closely resembled the situation prior to the ignition.
It was found that the treated, antistatic surface of the coveralls generated a large static charge on the inside of a rubber glove and the outside of a rubber boot worn at the time of the fire. The antistatic material itself did not show a charge because the charge was dissipated by its conductive surface. The point to be emphasized is that the opposite charge generated on the nonconductive material was not dissipated. This charge was capable of giving a static discharge that could ignite the gasoline-air mixture. It was concluded that the static charge on the glove was, in all probability, the source of ignition. In further tests it was found that the inside of the antistatic material produced a static charge with a variety of nonconductive surfaces. In these cases, the inside of the antistatic material held a static charge also. This is not surprising, since the inside of these garments had not been treated with antistatic agents.
In another case recently investigated, an operator was adding acrylonitrile/ butadiene/styrene (ABS) resin to a stainless steel tank containing methyl ethyl ketone (MEK). The MEK vapors ignited above the surface of the MEK tank, and the operator was severely burned. The resin had been introduced to the tank by pouring it from a paper bag through an opening in the tank. The operator had been wearing a polyester shirt at the time of the fire. The dissolving of the resin in the MEK was aided by stirring with a wooden paddle. The source of ignition was apparently static electricity (since there was no other source of ignition). The static electricity in this situation could have been generated by contact of the following: the MEK with the sides of the tank; the MEK with the paddle; the ABS with the surface of the MEK; the ABS with the sides of the tank; or the ABS with the bag in which it was contained. The static charge could also have been generated on the operator’s body or clothing. The clothing, the ABS contained in a heavy paper bag, the wooden paddle, a portion of stainless steel, and the MEK being used were all brought to this laboratory.
Small static charges were produced through contact and rubbing of various combinations of the materials mentioned above. The largest charge and probably the only one sufficient to ignite the MEK was generated by rubbing of the polyester shirt with the paper bag containing the MEK. It was therefore concluded that this was the most likely source of ignition, since sufficiently large charges were generated on both the shirt and the bag.
A question then arose: Could the fire have been prevented if the operator had been wearing antistatic clothing? Tests were run with the paper bags and garments treated with antistatic agents. As before, no charges were generated on the antistatic material, but charges sufficient for ignition were produced on the paper bag. It was concluded that materials treated with antistatic agents would probably not have prevented this accident.
Members of the fire service and paramedic units should be especially aware of the possibility of generating static electricity. Firefighters and paramedics may be called to assist in cases where a small fire has occurred but has been extinguished by nonfirefighting personnel. Although the original fire has been extinguished, conditions may still be such that a second ignition could occur from a static charge. This would undoubtedly have been the case in the first incident cited in this article. If the victim had not already been removed from the gasoline tank, rescuers would certainly have been at risk.
Firefighters should also be aware that ignition of any flammable or combustible substance or mixture from static charge is a possibility at any emergency scene. For example, a nonfire-related injury at a grain elevator may require the presence of paramedics. Dust at the injury site may be ignited by static charge.
Fire personnel should be acutely aware of the hazards of static electricity, since many of their outer garments are poorly conductive and therefore good sources for static charge. These include rubber boots and gloves and suits made of Kevlar, PVC, and other synthetic fibers. This is not to say that these are not good materials under normal firefighting conditions, but they should be worn with caution when static electricity is a possible source of ignition. Caution should also be exercised in wearing antistatic materials in contact with this type of clothing.
Antistatic materials should not be worn in contact with nonconductive materials if static electricity is to be avoided. The antistatic material will dissipate the static charge, but the nonconductive material cannot. This can lead to an extremely dangerous situation when flammable gases, vapors, and dusts are present. Therefore, antistatic materials should be used with considerable caution and never as a substitute to grounding or bonding.