What are Stray Currents and Stray Voltage?

Answer 1

Stray voltage and stray current refer to a variety of different phenomenon which cause undesirable voltage or current to exist on normally non-energized or non-conducting parts. Some causes:

1. An electrical fault in a wire's insulation can allow direct contact or create a conductive path between supply conductors and normally non-energized parts. If the protective devices (ie fuses) do not clear the fault, this condition will persist and present a potential shock hazard. The fault current will flow along all available paths, including water, and voltages may be measurable on surfaces far from the fault. The cause may be insulation degradation, installation damage, or miswiring (an accidental swap of the "hot" and neutral wires is relatively common and referred to as reversed polarity.) This type of stray voltage, caused by electrical fault, is also called contact voltage. It is almost exclusively a low voltage (<600V) problem, usually affecting 120V circuits. The solution to contact voltage is to repair the fault. Better grounding may increase the likelihood of protection devices isolating the fault, but is no guarantee since fault impedances are high, limiting fault current. Utilities and private electrical system owners concerned with the problem sometimes opt to use a proactive detection program to find hidden electrical faults.

2. Another type of electric fault occurs due to corrosion or mechanical failure of a wire or connection, creating an open circuit. If the device is grounded, the return current flows on ground conductors and along any other parallel paths back to the source. This can cause shock hazards and also causes current to flow on water pipes. It can cause interference with audio/video equipment due to the magnetic fields induced by the unwanted current. This type of stray voltage is called contact voltage when it is a shock hazard, but may also be referred to as stray current when referring to the unwanted currents on parallel paths.

3. A third condition not due to faults is a difference in potential, or voltage, between the earth and the grounded conductor in an electrical circuit. The current flowing in the circuit encounters a small resistance between the load and the source, which creates a small voltage drop between the load and the source. This small voltage can be measured between the grounded neutral wire and earth at any point along the circuit. If the circuit is long or there is some artificially high resistance in the return path due to corrosion or damaged conductors, this voltage could be high enough to affect livestock, or cause a tingle to people when exposed to both the earth and grounded electrical parts, as in a swimming pool or outdoor shower. The same thing could occur if an older circuit's neutral simply is undersized for the loads added over the years. This type of voltage is called neutral to earth voltage or NEV. Solutions to NEV are either electrical isolation from the supply system neutral or the establishment of an "equipotential plane" by bonding metal parts with the earth and other metal parts in the problem area (barn, pool deck, outdoor shower area, etc.)

4. A final condition worth mentioning is voltage or current present due to the influences of electric or magnetic fields. Electric fields of sufficient magnitude will give rise to significant voltages on ungrounded metal parts. This is called capacitive coupling and is not dangerous because, though the voltage is measurable, no current can flow from this source. The impedance of the impromptu capacitor formed by the electric field and ungrounded metal object is too great to drive any current. Voltage from capacitive coupling is commonly encountered by electricians when a spare set of wires is run adjacent to a live circuit. The spare wires, disconnected from any source or load, still have a voltage on them, measurable with a voltmeter. A shunt resistor in parallel with the test leads will bleed off this voltage and give an accurate measurement, 0V. Magnetic fields can also induce current on a grounded loop. One example of this is a metal fence placed underneath and parallel to a high voltage transmission line. If the fence is continuous over a long length, grounded at both ends, and built in particularly conductive soil, a current will flow in the loop created by the fence and the ground. In rare cases, people have been shocked by this kind of condition. The solution is to break up the loop into smaller loops or add insulating couplings so that no loop exists at all. Another example of this can occur on buried metal infrastructure running parallel with overhead distribution lines. These kinds of scenarios can be frustrating to fix and often non-insulating sections of pipe are inserted to stop current from flowing.

This author is an experienced distribution engineer familiar with problems faced by electric utilities. I am not familiar enough with the stray voltage reported by aquarium enthusiasts to know which phenomenon is most responsible. Numerous forums discuss the problem in detail.