Published · Vili Power Solutions
Stray voltage is a small voltage between two things a cow can touch at the same time, such as a metal waterer and the concrete under her feet, or a stall divider and the floor. When she touches both, a small current flows through her body. People usually cannot feel it. Cows are far more sensitive to it, and they learn quickly which places to avoid.
It is a real and well-documented problem, and it is also blamed for a lot of things it did not cause. The only way to know which situation you are in is to measure it correctly.
Signs in the herd and the parlor
The behavior that makes people suspect stray voltage includes:
- Cows reluctant to enter the parlor or certain stalls, or crowding back at the entrance
- Kicking at milking units, stepping, or general nervousness during milking
- Uneven or slow milk-out, and more units kicked off or reattached
- Reduced water intake, or cows lapping at the water instead of drinking normally
- More manure and urine in the parlor than usual
- Avoidance of one feeder, waterer, or area of the barn while others are used normally
None of these prove stray voltage. Liner condition, pulsation, vacuum level, handling, footing, and herd health problems cause the same symptoms. Your milking equipment dealer and veterinarian should be part of the conversation. If the problem lines up with a specific location, a specific piece of equipment, or a specific time of day when big loads cycle on, that points more toward an electrical cause.
Where stray voltage comes from
Every grounded electrical system carries current on its neutral conductor, and current through any conductor with resistance produces some voltage. Because the neutral is bonded to the grounding system, and the grounding system is bonded to metal equipment and water lines, part of that voltage can show up on the metal a cow touches. This is called neutral-to-earth voltage. A small amount is normal on any farm. Problems start when it gets high enough at cow-contact points.
The common causes we look for:
- Undersized, corroded, or damaged neutrals. A neutral with a bad splice or a corroded connection has more resistance, so the same current produces more voltage. Barns are hard on connections because of moisture, ammonia, and manure.
- Load imbalance. On single-phase services, large loads that are not balanced across the two legs push more current onto the neutral.
- Poor grounding and bonding. Missing bonds between metal equipment, water lines, and the grounding system leave pieces of equipment at different voltages.
- Wiring faults on the farm. Neutrals bonded to ground in the wrong place, such as at a subpanel or outbuilding, let neutral current flow on grounding conductors and metal piping. Equipment with damaged insulation can leak current to its frame.
- Utility-side contribution. The utility distribution system also uses a grounded neutral, and current on it can raise the voltage at your service. Some of what shows up in the barn can come from off the farm, which is why the utility often needs to be involved.
On many farms the answer is a combination: some contribution from the utility side and some from on-farm wiring. Good testing separates the two.
How it is measured
A voltmeter by itself does not tell you much, because a high-impedance meter will read voltage on points that cannot push meaningful current through a cow. The standard approach is to measure cow-contact voltage with a shunt resistor across the meter leads, typically 500 ohms, to represent the resistance of the animal. The reading then reflects what a cow would actually experience at that spot.
Measurements are taken at the points where cows make contact: waterer to floor, stall hardware to floor, the parlor deck, feed rails, and entrance and exit lanes. Readings are compared against the neutral-to-earth voltage at the service and at the transformer.
Stray voltage moves around with load, so a single reading at one moment can miss it. Thorough testing usually includes:
- Readings taken while the vacuum pump, feed equipment, fans, and water heaters cycle on and off
- A recording meter left in place for a day or more to capture milking times and peak loads
- A load test, where a known load is switched on and off, to help separate on-farm sources from utility-side contribution
As for how much is too much: the level of concern commonly cited from USDA and agricultural engineering guidance is about 1 milliamp of current through the cow, which works out to roughly 0.5 volt measured across a 500-ohm shunt at a cow-contact point. Some states and utilities use their own testing protocols and action levels. Treat the commonly cited figure as a reason to investigate, and rely on actual testing at your site before making decisions.
What fixes look like
The right fix depends on what testing shows. In general it falls into a few categories:
- Correcting on-farm wiring. Repairing or replacing bad neutrals and connections, removing neutral-to-ground bonds where they do not belong, running proper equipment grounding conductors to outbuildings, and fixing equipment that leaks current to its frame.
- Balancing loads. Moving large single-phase loads so the two legs of the service carry similar current, which reduces neutral current.
- Bonding. Tying metal stalls, waterers, pipelines, and equipment together and to the grounding system so they sit at the same voltage.
- Equipotential planes. A grid of reinforcing steel or wire mesh in the concrete, bonded to the equipment the cows touch, keeps the floor and the equipment at the same voltage. Article 547 of the National Electrical Code covers agricultural buildings and includes equipotential plane requirements for livestock confinement areas. Entrances and exits get a gradual transition so cows do not step across a sharp voltage difference. Adding a plane to an existing barn means cutting or replacing concrete, so it is usually done during remodels or when a parlor is rebuilt.
- Coordinating with the utility. If testing shows significant off-farm contribution, the utility has to evaluate its side. Their options are their decision, and having good test data in hand makes that conversation much more productive.
Be wary of anyone selling a single device that promises to eliminate stray voltage without testing first. Disconnecting grounds or bonds to make a reading go away can create a shock hazard for people and animals and violates code.
Prevention on new and remodeled facilities
Stray voltage is much cheaper to prevent than to chase. On a new parlor, barn, or remodel, plan for equipotential planes in the concrete, proper feeders and grounding conductors to every building, balanced loads, and connections and enclosures that hold up in a wet, corrosive environment. See our dairy and agricultural electrical page for the rest of what goes into wiring a dairy.
When to call
If cow behavior points to a specific location or piece of equipment, or if you have had a shock complaint from anyone working in the parlor, have the electrical system looked at. We work on dairies across Northern Colorado, including around Greeley and Windsor, and can test, trace the source, and correct on-farm wiring and bonding. Contact us or call (970) 685-2068.
