What Causes Stray Voltage ?
Stray Voltage is an extraneous voltage that appears on grounded surfaces in buildings, barns and other structures. Stray Voltage is classified as a low frequency form of conductive Electro-Magnetic Interference (EMI). In most buildings stray voltage is not a problem because the levels are generally below the perception level of humans and usually there is no sensitive electronic equipment which can be affected by it. However, Stray Voltage or EMI is a major problem for Hospitals, Manufacturing Plants and Farms. Hospitals are concerned with the Microshock Electrocution hazard in addition to the problem that EMI can cause with sensitive electronics. Farms are a special case because the stray voltage level can become so high that people and animals can actually feel the electrical shock..
In the 1960's EMI was recognized as a serious problem for Hospitals and other facilities that used sensitive electronics. EMI became and continues to be a specialty field within electrical engineering. In the early 1970's Bass Associates Inc. received an award from the Consulting Engineers Council for their pioneering work developing methods for controlling EMI in general and specifically Microshock Electrocution hazards. John Bass personally holds the patent for a tester that he developed to measure the very low voltages and currents associated with Microshock. We continue to be active in the field today.
Ohms Law, the most basic law in electrical engineering, predicts that a voltage will occur whenever current flows through a resistor. All physical materials including wires, pipes and soil have some resistance. Stray Voltage is the extraneous voltage that is generated on grounded surfaces when current flows through the resistance of a ground path.
The most fundamental requirement in controlling stray voltage is to rigorously maintain the distinction between Ground and Neutral (the grounded conductor). The Neutral wire is a conductor that provides the return path from the load back to the power source. The neutral has White insulation and is grounded only once at the service entrance. The neutral wire is a conductor and must never be grounded at a second place in the system. If it is grounded a second time, part of the neutral current will flow through the building structure and will cause stray voltage.
The Ground wire is either bare or has Green insulation. The Ground is connected to the service entrance ground but it also connects to and grounds all of the metal components of the electrical system and the building structure. The Ground does not normally carry current. The only time the ground should carry current is to trip out the circuit breaker in the event of a ground fault. There are many other factors that come to play in the control of EMI, however maintaining the distinction between neutral and ground will eliminate 90% of the stray voltage problems.
The transformer has inherent isolation that can be used to control the magnitude and path of leakage currents or ground faults that originate from devices that are powered from the transformer. It is important that you never connect the primary and secondary windings of a transformer together. The secondary neutral of a transformer must be established by grounding it to the building structure or some other point that has electrical continuity with the building service entrance ground. When in doubt, run a separate ground wire to the service entrance. Grounding the case of a transformer is a little tricky because if done incorrectly it can compromise the integrity of the ground system or prevent primary ground faults from tripping the primary circuit breaker.
One of Kirchhoff's Laws states that the current that flows out of a node must exactly equal to the current that flows into the same node. A transformer winding is a piece of wire that can be considered to be a node. Therefore the total current flowing out of a transformer winding must exactly equal the current flowing into the winding. In practical terms this means that all ground currents must return to the specific transformer coil from which they originated. This is a big help because it tells us where the ground currents are going even though we don't know the path they take to get there. Ground currents always generate a measurable voltage gradient because of the ground path resistance. Plotting the voltage gradient provides a vector (pointer) that can be used to track down the source of the ground current.
Farmers need to select electrical equipment, including lights, that will not generate excessive ground currents. Lighting ballast, electrical motors and auto-transformers are primary sources of leakage current even when they are UL approved and working correctly. All electrical equipment needs to be tested for leakage before it is installed. If the leakage currents is in excess of 1 milliamps (0.001 Amps), then the equipment should not be used. If you must use it, then special precautions must be taken in the electrical system design.
The electrical utilities do all of the wrong things relative to stray voltage. They consider ground to be the same as neutral and they connect the primary neutral and the secondary neutral together at their distribution transformers. The neutrals are then connected to water pipes and building structure of the building. All this results in the potential of the building structure and piping being elevated to the voltage level of the utilities neutral conductor. This is the primary cause of stray voltage on a farm. The good news is that this kind of stray voltage can be eliminated quickly and cheaply by insisting that the electrical utility separate the primary and secondary neutrals on your transformer.
The practice of installing expensive equal-potential grounding grids in the building floor is generally not necessary and in some cases can even make the problem worst. There are products on the market that attempt to dynamically generate an equal and opposite voltage to cancel the stray voltage. These systems are expensive and usually are not effective because they are unable to keep the whole system in balance. The simplest and most cost effective solution is to track down and eliminate ground currents in your facility.
Stray Voltage can be caused by either the electrical utility or the on farm wiring or both. Farmers must protect themselves by requiring the utility to isolate their neutrals and by the design of a robust electrical system for their farm.
Visit the following pages for a more detailed study of stray voltage.
How Can Stray Voltage be Caused by Farm Wiring?
How Can Stray Voltage be Caused by The Utility?
How Do I Test for Stray Voltage?
How Do I Protect Myself from Bogus Stray Voltage Testing?
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Copyright 1998 Bass Associates Inc. Last modified: March 31, 2010