Stray electricity is known by a many different names: stray voltage, stray current, neutral-to-earth voltage, transient voltage, and metal structures-to-earth voltage. In Ireland it is mainly referred to as stray voltage. It occurs as small voltage differences, generally less than 10 volts, between various parts of buildings. It can exist between the structure and the floor, the structure and installed equipment or between the floor and the equipment. It can also be transmitted through liquids. When cows come in contact with two surfaces that are at a different potential (voltage) a current will flow through the animal.
A variety of cow responses to stray voltage have been reported from farm surveys in other countries. Commonly cited cattle responses include, unexplained production drops, increased evidence of mastitis, elevated somatic cell counts, longer milking times, incomplete milk let down, nervousness while in the parlour, rapid exit from the parlour, reluctance to use water bowls or metallic feeders and altered drinking habits, e.g. lapping of water from bowls.
However, other factors such as mistreatment, uncomfortable stalls, milking machine problems, slippery floors, disease, sanitation and nutritional disorders can create problems similar to the responses outlined above.
Cows are much more susceptible to stray voltages than humans because their bodies are better conductors of electricity. The ease at which current can flow through a cow's body depends on which path it takes. Also, different individuals, either cows or humans, react differently to voltage differences due to their physiological makeup.
Humans have a much higher body resistance and so are not as affected by small voltage differences in the same way as cows. Human body resistance can be more than 30 times that of cows.
The sources of these relatively small electrical currents that pass through animal’s bodies are often difficult to locate. Stray electricity can arise because of poor electrical connections, corrosion of switches, defective wiring, frayed insulation, faulty equipment or heavily loaded power lines.
Stray electricity may also be "imported" from other buildings connected to the same electrical installation, or from other users connected to the same ESB transformer
Sources of stray voltage can be varied and complex. Finding the cause is generally not simple because different sources may act together and at different times. It is even possible that on-farm and off-farm sources may act together and at different times.
The main earth wire and the neutral wire are connected together near the ESB's main fuse beside the meter. Therefore, the neutral is connected to the main earth and any bonding earth beside the parlour. The neutral is also connected to earth at the transformer and at certain ESB poles. A voltage difference can occur between the ends and various parts of the neutral along its path between the milking shed and the transformer. Through the earth connections this voltage difference becomes distributed over the ground surfaces around the transformer and around the milking shed. It can result in voltage differences between different parts of the shed.
The voltage drops when the electrical load is heavy. It drops usually by about 5%, which is fine and it shouldn't be above 10%. I came across a few parlours, where with everything switched on around milking time the voltage dropped by about 20%, to the point where motors were starting to trip out. This is an indication that the load may be too great on the transformer, the cables too small or the transformer too far away from the parlour. Whatever the cause, a voltage drop of this magnitude must be rectified. Installing bigger cables, a more powerful transformer, or bringing the transformer closer to the parlour or combinations of these solutions will rectify the problem. Voltage drop shortens the life of electric motors and can also lead to stray electricity.
In and around the milking parlour various items of electrical equipment with different loads give rise to different voltage drops in the cables supplying them. This causes neutral to earth voltages leading to stray electricity.
In situations without equipotential bonding, only the milking machine and stalls are connected back to the earth wire in the electric motor, which is in turn connected to the main earth. Therefore any voltages that have to be distributed over the ground surfaces at the earth can affect the area in and around the parlour. By connecting the floor and metal parts of the parlour together the parlour and its immediate surrounds are all kept at the same potential.
Isolation of the milking machine and any of the metalwork around the parlour from the earth system is unsafe for humans and is not practical because of the array of electrically operated equipment in modern parlours. As well as that, isolation is not a fully effective method of controlling stray voltage.
Preventing or minimizing Stray Electricity
In order to minimize stray voltage a high standard of electrical installation is required. All electrical installations in milking parlours should comply with the ETCI (Electro-Technical Council of Ireland) rules for agricultural and horticultural premises. Engage an electrical contractor to carry out the work. The ESB booklet “Farm Well…Farm Safely” outlines the types of fittings and the standard of installation required. Little or no maintenance will be required if things are done properly, although regular checking is recommended. On many farms electrical installations are poorly maintained. Temporary makeshift repairs are common, often using electrical fittings unsuited to outdoor or damp conditions
A good standard of electrical installation is the first step in preventing a stray voltage problem. If the electrical installation is not up to standard, problems such as insulation breakdown on wiring and fittings can cause fault currents to earth causing stray electricity. This poses a threat to human safety as well. Get this potential source of stray voltage out of the way before any attempts are made to locate the source of a suspected problem.
The most permanent solution to stray electricity is to connect (bond) all metallic cow and milker contact objects in the parlour and the dairy together to form an equipotential cage. Independent bonding of each major object to a bonding busbar is advisable. This is normally done using “4 square” (4mm2 cross-sectional area) earth wire (green/yellow PVC coated) and special non-corroding bonding straps. Bonding wires should be looped from item to item and then connected to the bonding busbar. The busbar is connected back to the earth in the distribution board.
The bonding is only as good as the soundness of the electrical connections between all the items that are joined together. If the metal is dirty or corrosion takes place the resistance of the connection will not allow these small currents to flow and bring everything to the same potential. There is definitely a need for a very high standard of installation and maintenance where equipotential bonding is concerned.
An earth rod should be provided outside the parlour or dairy and joined to the bonding busbar by a labeled 10mm2 earth wire. Provide the earth rod with a non-corroding secure attachment. It is essential that any connection to an earth rod is well constructed and protected from disconnection.
In the interests of reducing stray voltage to a minimum it is recommended that a metal equipotential grid is located in the floors of milking parlours. The floor is a conducting surface and the metal grid allows it to be bonded to the rest of the equipment in the parlour.
In new parlours A142 steel reinforcing mesh about 200mm x 200mm should be laid in the floor within 40-50mm of the surface. The mesh should be laid in the cow standings, in front of and behind the pit and on the floor of the pit. Extend the mesh as far as possible into the yard to eliminate possible shocks as the cows move into the parlour. The mesh should be turned down into the ground at 450 in the form of a ramp for about 1.8m at its extremities near the entrance and exit of the parlour. In some countries 100mm x 100mm mesh is recommended. All sheets of mesh must be welded together at several points and ideally welded to all uprights and pipes fixed into the floor. In this way, all metal-work in the parlour and dairy, including the milking machine, is connected electrically via the upright pipes to the mesh in the floor. Thus, all surfaces in and around the parlour and dairy can be maintained at or very near the same potential.
In an existing parlour the mesh may be located in either a new floor or, where suitable, a screed on the existing floor. Another alternative is to lay copper bonding conductors in slots cut in the floor and grouted.
Detailed information and specifications are contained in Section 705 (and supplements to it) of the ETCI Wiring Rules
Electric fence Controllers
Never install an electric fence controller in or near a dairy, milking parlour or any livestock buildings. Also, electric fences should always have a separate earth at least 10m (some fencer suppliers recommend 100m) away from ESB poles, the parlour earth and the main earth. Never locate the fencer earth between protective earths. Increase the efficiency of the fencer earth by using more earth rods and locating them in damp ground. Fix up any shorts around the farm. When a fence wire shorts out on the farm, or a neighboring farm, a high voltage may appear on the fencer earth. If the fencer earth is inadequate or is too near other earths, this voltage may end up in the parlour. The recommendations outlined above and the use of equipotential bonding will provide effective protection. A short-term solution is to turn off the fencer at milking time.
If you suspect a problem with stray voltage ask your electrician or other competent person to check your installation and take measurements.
A suggested procedure for the detection of stray voltage in milking parlours is as follows:
- Wet floor
- Switch on all electrical equipment
- Measure AC voltage between wet floor and rump rail, pipeline, feeders and other conductible metal objects
- Measure AC voltage between a neutral wire and an earth wire
- Voltage greater than 0.75 V AC indicates a possible problem
- Switch off each item of equipment to see if the voltage is reduced
- If voltage still remains disconnect the live by removing each fuse or switching off each MCB (miniature circuit breaker) in turn to see if the voltage is then reduced. If a voltage reduction can be attributed to any particular electrical item it suggests that faulty wiring could be the problem.
- If voltage remains, the cause is probably not in the parlour or dairy. In that case, a more comprehensive detection needs to be carried out. Ask the E.S.B. or your electrical contractor to investigate the problem further possibly by fitting a voltage recorder over a longer period.
Tom Ryan, Teagasc, Kildalton