Why light curtains alone do not always provide sufficient protection
Light curtains provide an effective method of protecting operators from moving or hazardous machine parts without the need for physical guarding.
In industrial machine safety applications, safety light curtains are widely used because they allow machines to remain accessible for operation, maintenance, and material infeed while maintaining a high level of protection. However, in certain situations, light curtains alone may not provide sufficient risk reduction and must be complemented by additional safeguarding measures.
In many applications, light curtains form an important part of the safety concept. They detect when a person or object enters a protected zone and ensure that the machine stops before hazardous motion can occur.
However, a light curtain is not sufficient as the sole safeguarding measure in all situations. Depending on the machine, access to the hazard zone, and the way operators interact with the installation, additional safety measures may be required. In particular, risks can arise when a person remains inside the protected zone while the machine can be restarted.
In this article we examine in more detail:
- How safety light curtains work
- When they are effective as a safeguarding measure
- What their limitations are
- Why entrapment hazards can occur
- How additional solutions such as trapped-key, presence safeguarding or interlock blocking devices can contribute to a safer machine layout
What is a safety light curtain and how does it work?
A safety light curtain is an opto-electronic protective device used to monitor access to a hazardous area. The system typically consists of two main components:
- a transmitter
- a receiver
The transmitter emits a series of infrared light beams toward the receiver. Together, these beams form a detection field. As long as all beams are received without interruption, the system considers the situation safe. When a person or object interrupts one or more beams, the receiver immediately detects the interruption. A safety signal is then sent to the machine control system, causing the machine to stop or a hazardous movement to be interrupted.
Detection zone and resolution
The resolution of a light curtain determines which objects can be detected. Depending on the type of light curtain, the system may be designed to detect:
- fingers
- hands
- a full body
For applications where operators need to place their hands inside a machine area, a high-resolution light curtain is often used. For larger access openings to a machine cell, a light curtain designed to detect the full body may be applied.
Typical applications
Light curtains are used in many industrial sectors, for example in:
packaging machines
assembly lines
robot cells
palletizers
conveyor systems
In these applications, light curtains enable operators to work safely near moving machine parts without unnecessarily restricting productivity.
When light curtains are effective as machine safeguarding
In many situations, light curtains are a highly effective method of safeguarding machinery. Particularly in applications where operators require frequent access to a machine, they offer clear advantages compared with physical guarding.
Access without physical barriers
An important advantage of light curtains is that they do not create a physical barrier. Operators do not need to open doors or remove guards before accessing a machine. As soon as a person interrupts the detection field, the machine automatically stops.
This makes light curtains particularly suitable for processes in which:
- materials are manually fed into the machine
- operators regularly intervene in the process
- rapid access to the machine is required
Flexibility in production environments
Light curtains also provide flexibility in machine design because they do not require a physical structure. They can be integrated relatively easily into existing installations. For many manufacturers, a light curtain therefore represents a practical solution to ensure both safety and productivity.
Part of a safety concept
It is important to emphasize that light curtains are usually implemented as part of a broader safety concept. They are often combined with other measures such as:
- safety control systems
- emergency stop systems
- physical guarding
The effectiveness of a light curtain therefore always depends on how it is integrated into the overall safety design of a machine.
The limitations of light curtains
Although light curtains are effective in many situations, they also have several inherent limitations. These limitations mainly relate to the way light curtains detect intrusions and how operators can access the hazard zone.
Bypassing the light curtain
A light curtain only monitors the area where the light beams are present. If an operator can reach the hazard zone via another route, the safeguarding can be bypassed. This may occur, for example, when access is possible:
- from the side of a machine
- above the light curtain
- below the detection field
For this reason, openings around the light curtain often need to be covered with physical guarding.
Safety distance and stopping distance
A second important factor is the safety distance between the light curtain and the hazardous parts of the machine. When a light beam is interrupted, the machine must have sufficient time to come to a complete stop before a person can reach the hazard point.
There is a defined method (in standards such as EN ISO 13855) for calculating the distance between the light curtain and the hazard point. The distance is determined by factors such as:
- the response time of the light curtain
- the stopping time of the machine
- the speed at which a person can move
If this distance is not calculated correctly, an operator may be able to reach the hazard point before the machine stops.
Open machine cells and multiple access points
In larger installations, such as robot cells or palletizers, a machine may have multiple access points. In such situations, a light curtain often monitors only one side of the installation. This means that additional safeguarding measures may be required to protect the entire hazard zone.
The risk of entrapment and unexpected restart
A risk that often receives less attention when using light curtains is the risk of entrapment. This risk arises when an operator is inside the hazard zone while the machine can be restarted.
A typical scenario may look as follows:
- an operator interrupts the light curtain
- the machine stops
- the operator enters the machine cell
- the light curtain becomes clear again
- the machine can be restarted
In this situation, the operator may still be inside the machine cell while the safety system no longer detects any presence.
Unexpected machine restart
If a machine is restarted while someone is still inside the hazard zone, this can lead to dangerous situations. This risk is particularly present in installations where operators regularly need to enter the machine cell for:
- maintenance
- inspection
- troubleshooting
Preventing unexpected restart is therefore an important part of a well-designed machine safety concept.
How trapped-key safeguarding makes light curtains safer
To reduce the risk of entrapment, light curtains are sometimes combined with trapped-key safeguarding. This type of safeguarding ensures that a machine cannot be restarted while an operator is still inside the protected zone.
Mechanical blocking of the light curtain
Trapped-key systems use a mechanical key principle. When an operator enters the protected zone, a safety key is released. The operator takes this key into the machine cell. At the same time, a mechanical system ensures that the light curtain remains interrupted until the key is returned. As a result, the machine cannot be restarted while the key remains inside the machine cell.
Operation of the BRAVO entrapment protection
- When using safetylight curtains for full-body detection, there is a risk of an unintended machine start when the operators are in the safeguarded area (entrapment hazard).
- The Bravo (PBL) interlock blocking device is fully mechanical and can be easily retro-fitted with only 2 fixings on or next to the light curtain’s receiver.
- By turning the key, the blocking plate comes in front of the light curtain and blocks the beams. The key can only be removed when the blocking plate blocks the light curtain.
- The operator takes the released key into the safeguarded area as a personal protection against an unexpected restart. As long as the key is removed, the blocking plate is locked.
- When more operators need to enter the safeguarded area, safety padlocks can be placed around the lockable dust cover to prevent key entry and thus a restart.
As long as the key is not returned to the system, the light curtain remains interrupted. This prevents the machine from restarting. This mechanical principle helps ensure that a machine cannot be restarted while an operator is still present in the hazard zone.
When should light curtains be combined with additional trapped-key safeguarding?
In many installations, it is advisable to combine light curtains with additional safety measures. This is particularly the case when operators are able to fully enter the hazard zone.
- Robot cells
- Robot installations are often large enough for an operator to enter. In such situations, a light curtain alone may not provide sufficient protection against entrapment hazards.
- PPalletizers and automated installations
- Palletizers and automated logistics systems often contain multiple moving components and large working areas. Operators may need to enter these zones for maintenance or troubleshooting. In such installations, a combination of safeguarding measures may be required.
- Maintenance and interventions
- During maintenance work, operators are often located directly inside the hazard zone of a machine. In these situations, it is essential that the machine cannot start unexpectedly. By combining light curtains with additional safeguarding solutions, a more robust machine safety concept can be achieved.
Veelgestelde vragen over lichtschermen en mechanische insluitingsbeveiliging
A safety light curtain is an opto-electronic safety device used to monitor access to a hazardous machine area. The system consists of a transmitter and a receiver that create a field of infrared light beams. When a person or object interrupts these beams, the system sends a signal to the machine control system, causing the machine to stop or a hazardous movement to be interrupted.
Light curtains are particularly effective in applications where operators require frequent access to a machine, such as packaging machines, assembly lines, or conveyor systems. They allow fast access without physical barriers such as doors or safety fences.
A light curtain only monitors the area where the light beams are present. If an operator can reach the hazard zone via another route, for example from the side of the machine, above the light curtain, or below the detection field, the safeguarding can be bypassed. In addition, the safety distance between the light curtain and hazardous machine parts must be calculated correctly.
The safety distance between a light curtain and a hazardous machine component is calculated according to standards such as EN ISO 13855. This distance takes into account the response time of the light curtain, the stopping time of the machine, and the speed at which a person can reach the hazard zone.
An entrapment hazard occurs when an operator is inside a machine zone while the machine can be restarted. This can happen, for example, when someone passes through a light curtain, the machine stops, but the light curtain is cleared again while the operator is still present in the hazard zone.
Additional safety measures are often required when operators can fully enter a machine cell, for example in robot cells, palletizers, or large automated installations. In these situations, solutions such as trapped-key systems can help prevent a machine from restarting while someone is still inside the hazard zone.
