Common static grounding mistakes
(Contents of this article first appears in Electrical Business Magazine in Oct 2013, and are used with permission of the author, Brian Astl of Lind Equipment)
The most common mistake when designing a static grounding system is to use clamps that are not designed for static discharge. For example, battery clips are used extensively by people who have not had proper static bonding and grounding experience. The problem with battery clips is they have sharp points and weak springs that will not withstand regular use. Quite quickly, the spring will cease to provide a strong grip, which means there is poor metal-to-metal contact, and the clip is prone to falling off during operation.
Another incorrect choice is to use welding clamps. While welding clamps have stronger springs than battery clips, they are built with flat ends to the clamp that cannot assure good metal-to-metal contact. Without strong sharp points on the end, you cannot be sure the clamp will be able to penetrate paint, rust or dirt.
A common mistake is the lack of verification for the static grounding system. Most operators do not know whether their static grounding clamps and wires will actually drain away static electricity when in use. Typically, a static grounding clamp is attached to a plated steel or copper cable that terminates at a known ground point (e.g. ground rod, ground bus). The concept is the static electricity will flow through the clamp and cable to ground. However, wear and tear, weather, corrosion and other factors can quickly degrade that connection between the clamp and the known ground point. Many facilities do not have proper maintenance procedures in effect to verify this connection, which is typically done with an ohmmeter measuring the resistance from the known ground point to the tip of the clamp.
Even where a maintenance program is in place for the static grounding systems, the verifications are often only done intermittently, which means a problem with the static grounding system can go undetected for months at a time. Recently, a facility verified an outdoor static grounding assembly that was connected to a known ground point using very thick, insulated copper cable. It was always assumed the cable could not fail
as it was mechanically very robust. Visual inspections confirmed the system was in place and should operate. However, the ohmmeter showed there was no electrical continuity between the clamp and the known ground point; the static grounding system was completely ineffective. Upon removing the insulated jacket, it was discovered the cable had been severed in an area that could not normally be seen. Were the assembly monitored by a static monitoring system, workers would have been aware of the danger immediately.
