It is necessary to study the static buildup potential of any workplace and institute protective measures to control such buildup.
The following are the dangers posed by static electricity:
- Ignition causing fire or explosion
- Damage to sensitive electronic components
- Electric shock to humans followed by accidents such as a fall
- Damage to mechanical components such as bearings due to sparking through the oil films on bearing surfaces.
Charge buildup takes place when two surfaces, which are in contact and across which electrons migrate, get suddenly separated.
For moving objects, a ground brush of metal, brass or carbon can be used to provide the required leakage path. This method is commonly used for shafts of rotating machines to prevent bearing surface damages (refer Figure 1 above).
For objects, which are in contact with ground already, no separate grounding or bonding is necessary.
Grounding cannot, however, provide a solution in all cases, especially where a bulky non-conducting material is involved. In this case, the part of the substance, which is a distance away from the grounded portion, can retain sufficient charge, since movement of charge will not be fast enough in an insulating material. This charge can result in a spark.
Many insulating materials such as fabric, paper, etc. can absorb small quantities of water when the atmospheric humidity is sufficiently high. Even in the case of materials that do not absorb water, a thin layer of moisture gets deposited on the surface due to humidity (e.g., plate glass).
If the environment has a humidity of over 50% moist insulating, materials can leak charges as fast as they are produced. This prevents high charge buildup thus avoiding sparks. Conversely, most of the materials become dry when the humidity becomes lower than 30% since they tend to lose moisture to the atmosphere. This results in increased charge accumulation, which can cause sparking.
In some cases, localized humidification using steam ejectors can be useful particularly where the large space involved makes increase of humidity in the entire space a difficult proposition.
This method is however unsuitable where:
- The processed material can be adversely affected due to high humidity.
- If the area involved is air conditioned or humidity controlled for process reasons or human comfort.
- In cases where humidity increase does not cause appreciable drop in resistivity.
In all such cases, other methods of static control may have to be resorted to.
Ionization consists of forced separation of electrons from air molecules by application of electric stress or other forms of energy. The air thus ionized becomes conductive and can drain charges from charged bodies with which it is in contact.
The positive ions and electrons are also attracted by the negative and positive charges respectively thus resulting in charge neutralization.
Ionization can be produced by high-voltage electricity, by ultraviolet light or by open flames. Various devices using a step up transformer operating on mains supply and producing high electric fields are commercially available. Due care is needed however to address safety issues arising out of the use of high voltage.
A simpler device is the static comb, which does not use electricity at all. It consists of a metallic bar with a row of sharp points projecting from it and bonded to ground. When this device is placed near the charged surface, the electric stress due to accumulation of charge near the sharp points causes ionization and helps to drain the charge from the surface.
This method is commonly used in belt-driven equipment near the point of separation of the belt and pulley (refer Figure 2 above).
Another method of ionization is by using a row of small open flames. This method, however, requires caution where combustible materials are handled.
Since the buildup of charge requires at least one of the surfaces to be non-conductive, it follows that by making the non-conductive surfaces conductive even slightly would reduce charge buildup.
For example: coating a belt using conductive film on the side where it is in contact with the metallic pulley can prevent charge buildup.
Similarly, conductive footwear and suits will prevent static buildup in the workplace. Also, materials with lower static producing properties should be used wherever necessary.
Static accumulation and discharge can destroy integrated circuit (IC) devices. Facilities, which handle these devices, or components that are made using them, should be designed with adequate precautions. Conducting cuffs connected with ground using metallic bonding conductors is a common device used in assembly shops to avoid transfer of charges from the operators’ body to the circuit components.
Reference: Practical Grounding, Bonding, Shielding and Surge Protection G. Vijayaraghavan, Mark Brown and Malcolm Barnes (Buy hardcopy from Amazon)