20 www.megger.com2. Proper grounding of metallic enclosures and support structures that are part of the electrical system and may be contacted by personnel. Also to be included are portable electrically operated devices. Consider that only a small amount of electric current — as little as 0.1 A for one second — can be fatal! An even smaller amount can cause you to lose muscular control. These low currents can occur in your body at voltages as low as 100 V, if your skin is moist.3. Protection against static electricity from friction. Along with this are the attendant hazards of shock, fire and explosion. Moving objects that may be inherent insulators - such as paper, textiles, conveyor belts or power belts and rubberized fabrics - can develop surprisingly high charges unless properly grounded.4. Protection against direct lightning strokes. Elevated structures, such as stacks, the building proper, and water tanks may require lightning rods connected into the grounding system.5. Protection against induced lightning voltages. This is particularly a factor if aerial power distribution and communications circuits are involved. Lightning arresters may be required in strategic locations throughout the plant.6. Providing good grounds for electric process control and communication circuits. With the increased use of industrial control instruments, computers, and communications equipment, accessibility of low-resistance ground connections in many plant locations — in office and production areas — must be considered.National Electrical Code Maximum ValuesThe National Electrical Code, Section 250-56 states that a single electrode with a resistance to ground greater than 25 Ω shall be augmented by one additional electrode. (Other standards may prevail elsewhere in the world.)We recommend that single-electrode grounds be tested when installed and periodically afterward.
