Stray Voltage
What causes it, and what to look for
Electricity in the Milking Parlour
The effect of small voltages on milking cows (called stray voltage
by some people) is a significant and widespread problem. It is
electrical current caused by voltage differentials that upsets the
cows.
Resistance and Sensitivity to Electricity
Cows are 5-10 times more sensitive than humans, and some are more
affected by the current than others due to different levels of
resistance. The electrical resistance of a cow determines the
current flow resulting from a given voltage differential (explained
by the formula V= I x R). Resistance of cows (measured in ohms) to
stray voltage can vary from 250-400 ohm to 1000 ohm maximum, ie a
low resistance cow would be exposed to twice the current of a
high-resistance cow at the same voltage, or half the resistance to
double the current.
Causes
The cause of the voltage can be from several factors:
- Unbalanced three phase loads. An imbalance on the three-phase system can result in higher resistance on the neutral line causing a voltage drop
- High single-phase loads, e.g. water heaters. Having the single-phase water heater on during milking is often a source of imbalance
- Poor earth spikes including one at the transformer
- Improper wiring
- Faulty equipment, e.g. water heater elements
- Corroded connections e.g. aluminium neutral wires
- Rotary platforms can also generate static electricity. Bonding can dissipate the charge.
- Teat sprayers and drenching guns can also give cows an electric shock.
Electric fence systems with a return earth through the farm
dairy are also known to cause problems. Electric fences on backing
gates can be regarded as one way to get cows into the shed quickly
and milked; but at what cost to milk in the vat and
profitability?
What to look for
Electrical shocks are associated with adverse animal behaviour -
difficult to get into the bail, dunging and erratic milk let down
responses. The symptoms of small voltage and electric shocks
include:
- Nervousness and surplus cow dung in the yard and bails
- Reluctance to use metal feeders and touch the pipe work, e.g. front gate in herringbone
- Incomplete milk let down
- 2-3 minutes delay until letdown
- Increased mastitis as a result of incomplete milking
- Elevated cell counts in infected cows caused by the stress
- Faulty liners and pulsators or when teat sprayers and drench guns are used or touch the cows can cause some of the symptoms.
Cows will show a response (flinching and behavioural changes) at a
current of 3mA or a voltage of 0.7 volts. However some cows flinch
at 0.2 volts. A kicking response is likely at voltages greater than
1.4 volts and currents of 4mA. Transient voltage pulses of 10-volt
amplitude for 1 millisecond are sufficient to cause a reaction, 40
volts for 100 milliseconds cause violent reactions.
The most important voltages to look for in the milking area are
those between the bail pipe work and the floor, and those due to
voltage gradients over a wet floor, or moving on to a rotary
platform. Two volts over 1.5 meters are sufficient to cause a
problem.
A stray voltage problem is often only detected during milking time
because of power loadings. Testing needs to be done at the end of
milking when the shed is still wet with urine, dung and leftover
chemicals from milking. The resistance of the floor that will
control the current will affect the readings.
Conclusion
Identifying and rectifying a stray voltage problem will result in
more relaxed, at ease cows in the parlour, less mess and increased
milk yields.
