Stray current

Stray current refers to unintentional electricity flowing through the aquarium water. Malfunctioning and deteriorating equipment are the usual sources and can represent a danger to the aquarist and livestock. Stray current is closely related to stray voltage - a voltage potential between the aquarium water and ground.

Stray current vs stray voltage
In typical household circuits, current is far more dangerous than voltage.

Stray voltage is an electrical potential difference between the water and earth ground. A voltage difference does not indicate any electricity is actually flowing through the water. It indicates the possibility of current to flow if a suitably low resistance path between the water and ground is introduced. So stray voltage on its own is not directly harmful. It is common to have some stray voltage as pumps can induce a voltage potential with their rotating magnets.

Stray current refers to flowing electricity. It should be considered dangerous at even the milivolt range. When wet, the body's electrical resistance is dramatically reduced allowing more current to flow. At the 100ma range, current can cause ventricular fibrillation (uneven, uncoordinated pumping of the heart), muscular contraction, nerve damage, death is likely. A properly functioning tank should not exhibit any measurable stray current (< 1mA). Stray current may or may not trip a GFCI receptacle depending on the path it takes.

Symptoms

 * shock and/or tingling when touching the water, in particular on cuts
 * shock and/or tingling when touching the water and a metal lighting fixture
 * GFCI outlet tripping
 * odd behaviour from fish or coral
 * possible cause of Head and Lateral Line Erosion (HLLE) disease
 * erroneous readings from pH probes

Leaks
An energized conductor is exposed to the aquarium water. This usually occurs when equipment is worn or damaged. For example, an electrical cord where the insulation is cracked or worn away. Other examples includes cracked housings on pumps and heaters. This is dangerous as the full current of the outlet may flow if the aquarist touches the water and a ground (like a grounded metal fixture). In rare cases, under the right circumstances (Murphy's Law), when there are multiple electrical cords/components in the water (which is the case with most of our systems), there is the possibility of current flowing from Line to Neutral or Ground from one cord to another. This current will flow from one component to the other, through the water & may not necessarily be measurable with a meter to ground. It will most probably affect your livestock though.

Magnetic Induction
Moving magnetic fields can induce a voltage and current in the aquarium water (or any conductor). Pumps and powerheads often have spinning magnets and may generate up to 50v of voltage but only very small amount of current. Flourescent bulbs (PC, VHO, T5) will also induce voltage and current. The current induced by these sources is small, in the nanovolt range.

Static electricity
This may possibly be a small source of stray voltage. The large conductive surface film will likely dissipate any charge.

Identifying
''' WARNING - This procedure deals with mains power and is potentially fatal. If in doubt, contact an electrician. Consider performing a visual inspection as decribed above instead. '''
 * Visual Inspection
 * Remove all equipment (perhaps as part of regular cleaning and maintenance)
 * Inspect cords for worn, chewed or damaged sections
 * Inspect where the cord enters the equipment and connects to the electrical plug. Is it secure and undamaged
 * Look for cracked housings on pumps
 * Look for water inside glass heaters
 * Measuring with multimeter
 * Ensure all equipment is connected to a GFCI outlet. While this is not required for to measure current it will significantly reduce the risk of harm and/or death if a mistake is made.
 * Ensure you have a clear, dry area to work. Reduce the risk of accidentially touching anything in particular the water or ground.
 * Turn on all electrical equipment in contact with the water
 * Configure the multimeter to measure current (mA)
 * Some multimeters have multiple probe sockets, ensure your probes are in the correct sockets for measuring current
 * For older multimeters without auto-ranging: select the highest range
 * Place black probe in the third (ground) prong of the GFCI outlet (or an attached three prong extension cord)
 * Place red probe in the water
 * For older multimeters, if the meter reads 0 select the next lowest range and repeat until the meter reads non-zero or you select the lowest range.
 * If the multimeter reads 0ma, you are done, there is no stray current
 * Disconnect equipment one by one (don't forget float switches)
 * When the current reading drops, the last piece of equipment you disconnected is a source of stray current
 * Immediately replace all equipment contributing to stray current

Recommendations

 * Connect all equipment to one or more GFCI protected outlets
 * Periodically inspect equipment using the Visual Inspection procedure above
 * An aquarium should have no detectable stray current (detectable by a common multimeter). Replace any equipment contributing stray current.
 * Minimize contact between equipment and the water
 * Keep the control dial and cord on heaters above the water line
 * Keep electrical cords out of the water
 * Use external (non-submerged) pumps
 * Use drip loops
 * Consider low voltage DC pumps (EcoTech, Tunze, Koralia?) and LED lighting to reduce risk
 * Keep high voltage AC equipment as far away from the water as practical
 * Consider using moisture resistant electrical receptacles such as those used outdoors
 * Consider installing a ground probe (only if GFCI is present). There is debate as to the benefits of a ground probe.
 * At current levels above the GFCI trip current, the ground probe will trip preventing the current from flowing through someone contacting the water later.
 * At current levels below the GFCI trip current, the ground probe will allow current to flow where it would not have before potentially affecting livestock.
 * Do not use a ground probe to solve a problem with low levels of stray current, instead identify and replace the source.

Links

 * Technical Considerations Regarding Static Potentials
 * Ground Probes