The ABA Engineering Academy starts its 2016 “Continuing Educational Series” with a basic review of proper grounding techniques. While this may seem elementary it never hurts to review the “why and how” of things we sometime take for granted.
When engineers talk about proper grounding and bonding most of it is centered around lightning protection and/or elimination of ground loops between equipment. However the first reason we should bond and ground equipment is Safety!
So lets look “under the hood” and understand what is grounding?
The purpose of grounding is to electrically interconnect conductive objects in order to minimize voltage differences between them. An excellent broad definition is that a ground is simply a return path for current. We must remember that current always returns to its source through either an intentional or accidental path – electrons don’t care and they don’t read schematics!
Bonding is the practice of intentionally electrically connecting metallic items which will bring all the bonded items to the same electrical potential as a protection from electrical shock. The bonded items can then be connected to ground to bring them to earth potential.
In a properly wired electrical circuit we have three wires. The neutral (white) and line (black) wires are part of the normal load current circuit shown by the arrows. Note that the neutral (white) and safety ground (green) wires
of each branch circuit are tied or “bonded” to each other and to an earth ground rod at the service entrance.
The green ground wire is actually call a “safety ground”. Any ac line powered device with exposed conductive parts (including signal connectors) can become a shock or electrocution hazard if it develops certain internal defects.
The outlet safety ground is routed, through the green safety ground wire to the neutral conductor at the main breaker panel. This low impedance connection to neutral allows high fault current to flow, quickly tripping the circuit breaker and removing power from the circuit.
As a result of this protection one should never use a 3 to 2 wire “ground lifter” to eliminate hum in the equipment.
As mentioned above the bonded safety grounds are normally connected to an “earth ground”, consisting of a copper ground rod of at least 10 feet driven in well compacted soil. We often think that if one rod does a good job then adding additional rods will be even better. Well…yes and no.
If multiple rods are used they must be bonded together. Since soil has resistance just like any other conductor, earth ground connections are not at zero volts, with respect to each other or any other mystical or “absolute” reference point. The soil resistance between separate ground rods can allow thousands of volts to develop between them if lightning strike current should actually flow in one of them. This is due to simple Ohms law…E = I x R.
Remember spacing of rods deals with a theory known as the Sphere of Influence. Basically you want to space your rods out 2-times (2X) the length of the rods. So, if you are installing 10-ft grounding rods, you would want them spaced at 20-ft intervals.
Notice in this drawing (courtesy of Nautel) how all the ground rods are bonded together and the various “safety grounds” are connect at a central point.
This single point (as it is called) ground system offers the best solution for routing stray currents to earth ground without going through the actual equipment.
In our next installment we will examine more in details stray currents caused by lightning and unbalanced loads.
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