Skip the NI Direct Bar
Skip navigation

Preventing problems from stray electricity in your milking parlour

Michael Garvey, Dairying Development Adviser Armagh, CAFRE, Greenmount Campus

Stray voltage in excess of 1.5 volts AC has been associated with increases in the level of clinical mastitis in dairy cows.  The clinical infections usually occur in cows that already have sub-clinical infections.  Finding the cause of stray voltage is generally not simple, because the on-farm and off-farm sources act together and can vary with time.  In order to minimise stray voltage a high standard of electrical installation is required.  All electrical installation in milking parlours should comply with the National Electrical Safety Standard – BS 7671 - Requirements for Electrical Installation. This standard relates to wiring, socket outlets and lighting fittings.
The following rules should be adhered to:-
  1. All metallic cow and milker contact objects should be bonded to form an equipotential cage.  Independent bonding of each major object to an earth bar is advisable.  At least 4 mm2 covered copper wire with non-corroding attachment screws is required.  
  2. An earth rod should be provided outside the parlour or dairy and joined to the earth bar by labelled plastic coated 16 mm2 copper wire with non-corroding secure attachment.  It is essential that this is well constructed and protected from disconnection.
  3. An RCD (ELCB) Earth Leakage Circuit Breaker should always be fitted to the main fuse board in the parlour.  A 30 mA RCD will give very good protection.  All sockets must have a 30 mA RCD.
  4. As an insurance against stray current, a secondary earth may be advisable in the milkers’ pit.  As this area is normally damp or wet during milking, a single 10 mm galvanised bar driven 1.5 m into the floor of the pit with moisture access around it and a permanent connection onto the main frame of the parlour will provide a very effective earth.
  5. Never install an electric fence controller in or near a dairy or milking parlour.  Also, electric fences should always have a separate earth at least 20 m away from the mains earth.
  6. No socket or live switches should be located in the milking parlour itself.  A separate power room or blind wall in the dairy is suggested.
  7. In new parlours an earthed 100 mm grid of reinforcing mesh less than 50 mm deep in the concrete cow standings should be used as precaution against stray voltage.
Procedure for the detection of stray voltage in milking parlours:-
(a) Wet floor.
(b) Switch on all electrical equipment.
(c) Measure voltage AC between wet floor and rump rail, pipeline, feeders and other conductible metal objects.
(d) Voltage greater than 0.75 V AC indicates a possible problem.
(e) Switch off equipment by removing fuses until voltage is reduced.
(f) If voltage remains, the cause is probably not in the parlour or dairy. Have a more comprehensive installation inspection carried out by a National Inspection Council for Electrical Installation Contracting approved electrical contractor.