Checkweigher and Packaging Line Machine Safety: Guards, Interlocks or Light Curtains?

1. Map the real hazard zones before selecting a device

A checkweigher is rarely an isolated box. A working line can include two or three conveyor sections, transfer rollers, a weigh belt, a pusher, swing arm, flap, drop section or powered roller reject, a reject bin and an interface to filling, sealing, labelling, cartoning or palletising equipment. The assessment therefore has to cover the complete line interface and the work people actually perform.

Separate normal production from product sampling, reject recovery, clearing a jam, cleaning, changeover, fault finding and maintenance. A position that is not approached during automatic running may become the most frequent hand-access point when a reject bin fills or a carton stops in front of a pusher.

2. Use the risk-reduction hierarchy before choosing a safeguard

ISO 12100 treats hazard identification, risk estimation, risk evaluation and risk reduction as the starting point. First ask whether the design can remove or reduce the hazard: lower a reject mechanism's force or speed, remove an accessible nip, reduce an opening, move a service point outside the danger zone or change the layout. Use guards or protective devices for the remaining risk. Warning labels, training and personal protective equipment do not replace a feasible engineering control.

1. 1
   Define the machine boundary

   Include upstream and downstream machines, all operating modes, personnel, material flow, utilities and reasonably foreseeable misuse.

2. 2
   Identify hazards by task

   Review production, loading, reject recovery, jam clearing, cleaning, changeover, setup and maintenance separately.

3. 3
   Reduce risk at the source

   Change force, speed, geometry, openings or layout before choosing a fixed guard, interlocked gate or electro-sensitive device.

4. 4
   Design the whole safety function

   Specify sensing or interlocking, safety logic, final switching or drive function, reset, diagnostics and fault response.

5. 5
   Measure and validate

   Test stopping, faults, reach-around, restart and isolation under real speed, load and adverse conditions, then retain the evidence.

3. Fixed guard, interlocked gate, guard locking or safety light curtain?

These methods are not mutually exclusive. A line may use fixed guarding around drives, an interlocked or guard-locked service door, a safety light curtain at a necessary open transfer and presence sensing where whole-body access is possible.

4. When is a safety light curtain suitable on a packaging line?

A safety light curtain can be appropriate at an open feed or discharge point, a reject station that must remain accessible, or an automated cell where a physical door would interrupt frequent access. The essential condition is that the hazardous motion can reach the safe state defined by the risk assessment before a person can reach the hazard.

• Consider it where the opening must remain available and the complete machine can stop reliably within the validated separation distance.
• Do not rely on it alone where the primary risk is a flying object, blade, hot surface, spray, pressure release or motion that continues too long after detection.
• Add further measures if a person can pass completely through the light curtain, stand behind it or bypass it from above, below or either side.
• If product must pass through the protective field, use a separately designed and validated muting function where permitted. If an approved blanking function is used, follow the device manufacturer's constraints and apply the required compensating measures. Never bypass or permanently suppress beams as an ad hoc workaround.

5. Specify the safety light curtain from the application, not the brochure headline

For project discussions, DAIDISIKE DQA and DQC families can be treated as light-curtain candidates, while DQSA is an area or perimeter detection concept rather than a physical guard. Before either light-curtain family is proposed, verify the exact ordered model's documented device type, output architecture, response time, claimed PL or SIL suitability and certificate scope; a family name is not evidence of those properties. A non-contact door monitor such as DX-C1 reports guard position but does not hold a door closed. A guard-locking device such as DX-W2 is the relevant type to evaluate when residual danger remains after a stop request. None of these component names, by itself, proves the performance level of the complete machine.

6. A light curtain only works as part of a complete safety function

When the protective field is interrupted, the device outputs change state. A safety relay or safety PLC evaluates the channels and faults. Final switching elements or a validated drive safety function then bring the hazardous motion to the safe state. The sensor, logic and final element must be designed and validated as one chain.

A standard PLC may receive status and diagnostic signals, but reading a light-curtain signal does not by itself establish a safety function. The safety-related stop must be implemented by an architecture suitable for the required risk reduction and verified for broken wires, short circuits, contactor or drive faults, response time and restart behaviour.

7. Separation distance, reset and restart are different engineering questions

Do not copy a universal light-curtain distance from a web page

The separation distance depends on the applicable approach parameters, detection capability, light-curtain response, safety-logic response, final-element response, measured machine stopping time and application uncertainty. ISO 13855:2024 addresses positioning methodology. Without a stopping-time measurement on the actual machine and operating condition, no fixed distance can be declared safe for every checkweigher or packaging line.

A clear protective field is not the same as permission to restart

Clearing the field only means the detection zone is no longer interrupted. The risk assessment and control design must decide whether a manual reset is needed, where the reset control is located, whether a separate start command is required and whether automatic restart is permissible. Reset should not itself initiate hazardous motion.

Whole-body access creates a stand-behind problem

If a person can pass through the light curtain and remain inside the safeguarded space, an outside operator may see a clear field while someone is still at risk. Evaluate additional presence detection, controlled reset, escape release, visibility and energy isolation instead of treating a clear light curtain as proof that the area is empty.

8. Jam clearing and cleaning require hazardous-energy control

A stopped pusher can still contain pneumatic pressure. A flap, lift or diverted product can move under gravity. Conveyors can restart through automatic sequencing, remote commands or upstream and downstream line logic. When jam clearing, entry, guard removal, cleaning or maintenance exposes personnel to unexpected energisation or stored energy, apply the site-specific hazardous-energy-control procedure, including isolation and verification, under the rules applicable at the installation site.

• Define who is authorised to stop, isolate, lock, verify and restore the machine; do not rely on a verbal warning.
• For pneumatic reject mechanisms, address the isolation valve, residual pressure and any part that can fall or spring back.
• Confirm that upstream or downstream equipment cannot restart the section through line interlocking while a jam is being cleared.
• Do not return a bypassed or removed safeguard to automatic production until it has been restored and functionally checked.
• Put clearing and cleaning steps into the handover documents and training record, not only on a caution sticker.

9. Installation and acceptance checklist

This checklist helps a project team find omissions. It does not replace a model-specific risk assessment, safety-control design or validation by competent personnel under the laws and standards applicable at the installation site.