In most cases and early when the plant is built, a ground stake is driven into the earth a certain depth and into earth whose chemical content meets certain conditions of electrical conductivity. When equipment is first added to the plant, this ground does a pretty good job of acting as a zero volt reference.
Over time, and as additional current hungry equipment is added, this original ground is less able to represent a confident zero volt reference. Also a factor is the manner in which this ground is distributed around the plant floor. For example, take the case where a single ground wire is run from the stake and all equipment needing grounding is connected to this wire which snakes around the plant floor. The equipment nearest the stake in a large plant may see a reasonable and steady zero volt reference but the equipment at the far end certainly will not due to all the current hungry heavy equipment connected between it and the stake.
All too often, the Lear equipment is at the far end, or at least, far from the stake or located right next to heavy high horsepower motors. The first indication of a problem to the Lear equipment is usually the HVC function where a string of insulation faults is indicated even though the wire insulation is OK. A quick test of this is to lift the wire out of the detector head and observe whether the faults continue. On a round wire detector, simply rest the wire on top of the detector using a felt pad to support the wire. If the faults continue as before, poor grounding is probably the cause of the HVC indications. However, the enamel coating has often turned out to be the problem and the faults are legitimate.
On a flat wire detector, use the spreaders (which were supplied) to hold the reeds away from the surface of the wire. First, the two restraining posts which limit the reed arm travel must be covered with an insulating sleeve or a small square of fiberglass or plastic to prevent the reeds from arcing to the restraining post.
Going back to the case where the Lear equipment is located at the far end of the ground wire and suffering from the heavy current equipment connected between it and the stake, an obvious improvement is where an additional wire is run back to the stake and heavy equipment is not allowed to be connected to this wire. Even better, is a new ground stake located closer and used only by the sensitive equipment such as in the QC lab and computer room--and the Lear equipment.
Digressing a bit: During the past several decades, these power distribution problems have become steadily worse due to the increasing use of variable speed DC motors running off of PWM (pulse width modulated) current waveforms which have become the common form of speed control. Additionally, Silicon Controlled Rectifiers (SCR's), as used to control oven heat, really do a job on the plant voltage waveforms since they turn on and off a heavy current 120 times per second and with very fast rise and fall times. Wicked transients and harmonics result.
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