From the days of Westinghouse and Edison, when “power distribution” was by a single, naked copper wire strung through trees and around glass knobs, to today’s civilization, which would not be possible without computers and tablets, people have struggled and died to provide adequate grounding systems. “The electrical intersection of man and machine has resulted in injuries and deaths,” said Terry Colleran, international expert, DCS, instrumentation and process control, Air Liquide. Improper grounding also leads to early demise of many control system components.
Together with Jesse Godwin, service specialist, Control Southern, Colleran presented the session, “Grounding vs Grounding: What’s the Difference?” at the Emerson Global Users Exchange 2016, this week in Austin, Texas.
Today’s automation systems use transistors and integrated circuits, components whose lives can be reduced or ended by capacitive discharge, overvoltage and power supply AC ripple. A capacitive or static electricity discharge that jumps a ¼-in gap at less than 60% relative humidity is 10,000 volts. “Wires store electrons. The amount depends mainly on the length of the wire, just like a capacitor,” Colleran said. When the capacitance is exceeded, the voltage discharges. Surges and arcs are caused by motor starts and stops, RF transmitters, proximity to wires carrying more than 50 VAC, and of course, lightning. “Unless constantly drained away to ground, electrons gather in wires until the capacitance is exceeded,” he said.
Beware AC ripple on DC power—“sour power, we call it,” Colleran said. You’ll have more or less, depending on the rectification and power quality. “Capacitor filters prevent problems, but they deteriorate. Replace them every five to seven years.” Three volts of ripple means the 24 VDC electronics will see only 21 VDC, and will draw higher current. “Higher current means more heat, more resistance, and more current draw—a vicious cycle,” he said.
Protective vs Instrument Grounds
A proper plan requires two separate grounds, a protective earth (PE) ground for power, and an isolated ground for instrumentation. “Both must be at the same potential to protect man—a technician might touch both ground bars in a cabinet—so the National Electrical Code requires ‘at least one connection,’ but they are not the same thing,” Colleran said.
Provide PE grounds in the form of a triad—a set of three rods spaced so the Hall-effect areas intersect to maximize current capacity. Use a separate, star point ground, and connect all the instrument grounds to it. “Then provide only one connection between the star point and the triad. Do not connect the power and instrument grounds in the cabinets. If you do, you don’t have a separate instrument ground.”
“You think electricity takes the path of least resistance, but it doesn’t. It takes all the paths. So in a lighting strike, it burns through everything. Wires, steel structure, the voltage in everything goes high, then over time, it dissipates,” Colleran said. “You want the lighting to go to ground first and mainly through the triad, so keep instruments isolated.”
The effectiveness of ground depends on the quality of the substrate—the soil beneath and around the facility in contact with ground rods. The conductivity of many substrates is bad and varies with time, seasons and weather. “When the plant is built, they take one sample at one time, 30 inches down,” Colleran said. “Sandy soils resist taking a charge.” Plants built on sandy soil, chert (flint) and loam, like many in the southeast and southern U.S., experience increased instrument failures during dry seasons and droughts when soil water content is low.
Electrical events can cause sealed controls—contacts don’t open, safety systems actuate, equipment operates “in an uncontrolled manner, causing destruction of machinery and property, and sometimes loss of life,” Colleran said. “What’s the cost of a tripped process? The estimated cost of a life is more than $10 million, a good triad and star point cost about $3,000.”