The leg-shackle-earth interface

Shackle maintenance

The contact points between a bird’s legs and the shackle is likely to contribute the most resistance within each electrical circuit. Therefore birds must not be shackled with any items around their legs (eg straw) that might further increase resistance.

If the total current passing through a constant voltage waterbath begins to drop as stunning progresses throughout the day, it is possible that feathers, grease and dirt are building up on the shackles, causing resistance to increase. To prevent this, shackles must be regularly cleaned throughout the day. Ideally, just before the shackles return to the live-bird shackling station, the shackles must pass through a wet cleaning system to remove feathers, dirt and any severed feet retained in the shackle. If the cleaning system fails to effectively remove severed feet, shackling staff must remove them manually before loading a conscious bird into that shackle; otherwise the quality of the stun is likely to be compromised. If shacklers continually have to remove severed feet, then the cleaning system should be repaired, or replaced with a more effective model.

Shackle washing serves an additional purpose. Although the birds’ legs are touching the shackles, wide variation in resistance still occurs. Shackles must always be dripping wet when birds are hung into them because the water should help to form a better contact between the leg and the metal, reducing the variation in resistance and therefore may improve the immediacy of stunning.

If empty shackles are wetted prior to birds being loaded into them, spraying occupied shackles with water at the leg-shackle interface, just prior to birds entering the waterbath is unlikely to have a significant effect on resistance and is therefore not necessary. If occupied shackles are sprayed with water at the leg-shackle interface, just prior to the entrance to the waterbath, then it is critical that sprays do not: a) disturb birds on the shackle line, especially as they enter the waterbath (eg the spray should not get water in birds’ eyes); b) cause birds to receive pre-stun electrical shocks; or c) wet birds’ plumage and bodies too much because this may create a shunt and current may flow over the exterior of the head and body instead of through the brain and heart. It is important to use saline water for all these purposes because this increases conductivity, relative to clean water.

The accumulation of a type of scale on shackles (Figure 12), most likely an electrolytic build-up of biological material such as grease/fat from the birds’ legs, can sometimes dramatically increase the resistance within a circuit and can make the difference between effective and ineffective stuns. Therefore, as well as being cleaned with water and wire brushes, the shackles (and any other scale-susceptible equipment, eg electrodes) must be cleaned with an acid at least once a week to help prevent the build-up of scale. Monitoring equipment for scale is critical and a shackle must be immediately replaced if it cannot be descaled sufficiently.

Figure 12. Brown scale on a leg-shackle contact point. Scale must be removed from all contact points if stunning is to be effective. Image: Paul Berry Technical Ltd.

High-voltage electrical parameters are advantageous for poultry welfare at slaughter (read the section ‘The effect of electricity on an animal’). If high-voltage electrical parameters are used, with time, carbon can build up on the shackles, resulting in poor conductivity between the legs and the shackle. The shackles must therefore be regularly cleaned in an acid bath to restore normal electrical contact.

Continuous contact between the shackles and the earth bar

The second electrode, ie the earthed rubbing bar, must be in firm and constant contact with each metal shackle in which a bird is restrained, for the entire duration that each bird’s head is in contact with the electrified water or wet plate. This includes when the shackles move rapidly, ie when birds swing off an entry ramp into the water, or during disturbance (eg if birds flap as they enter the water they can pull their shackle, and the shackles of other birds, away from the earthed rubbing bar). Similarly, make sure occupied shackles do not overlap onto occupied or unoccupied shackles, because this may reduce or alter the flow of current through a bird. If a shackle does not have continuous contact with the earthed rubbing bar, a bird will receive an intermittent flow of current which is also likely to be below the required minimum amplitude. Such poor current flow is unlikely to effectively stun a bird, particularly if it occurs at the entrance to a waterbath, in which case it may simply cause pre-stun shocks in an otherwise-conscious animal. Repeated applications of electricity may also have an adverse effect on carcass quality.

To ensure physical contact is continuously maintained between the shackles and the earthed rubbing bar, the bar should push against the shackles and gravity and the weight of the birds will keep the shackles against the earthed rubbing bar.  Two earthed rubbing bars, in very close proximity, can trap the shackles between them, prevent shackles overlapping and ensure each shackle is always in contact with at least one bar. The earthed rubbing bar(s) and the shackles must be monitored for signs of wear at their respective pressure points and must be replaced when contact is no longer effectively made.

Please refer to the HSA Guidance Notes No.7 on ‘Electrical Waterbath Stunning of Poultry’ for additional detail and illustrations of how to secure continuous contact between the shackles and the earth bar. 

Next: Summary - optimising current flow