Muting in practice: when safety becomes a vulnerability

Designed to allow objects to pass through safety barriers in factories, muting can also introduce new safety risks if not implemented correctly.

Muting allows the safety function on machines or systems to be temporarily bypassed so that material can be fed in or out through protective devices such as light curtains or laser scanners without interrupting the production process. However, in doing so, personnel safety must be guaranteed at all times.

Muting applications often reach their limits in practice, however, allowing incorrect operation and manipulation. In such cases, alternative security concepts are required.

Unconscious risks

Various types of muting are used in industrial automation: whether they be 2-sensor, 4-sensor, time-controlled or sequence-controlled. The IEC 62046 international standard¹ regulates the requirements for entry and exit stations with muting and stipulates in particular that:

• muting must be activated via at least two independent bridging signals
• muting must offer protection against foreseeable incorrect operation or manipulation.
   

The standard thus defines clear requirements for the implementation of muting applications. In practice, however, it is not always fully complied with — either because the specific application requirements are not fully known and may therefore differ from real-life conditions, or because risky compromises are deliberately made to achieve high process stability. As a result, safety functions become less effective, and manipulation or incorrect operation are more likely.

For operators, this means an unconsciously increased liability risk and potentially serious consequences for employee safety.

Hazard 1: Safety gap due to pallet muting

In automated systems, meshed containers or other objects that are difficult for muting sensors to detect are often fed onto or discharged from pallets (Figure 1). The openings in the mesh structure prevent the muting sensors from generating a stable switching signal, making it impossible to mute the safety device. In practice, the pallet itself is sometimes used as a muting trigger (Figure 2) — a procedure that is not permitted, since a person could, for example, simply place an empty pallet in the safety device and deliberately disable the protective device.

Figure 1: A pallet cage is loaded onto a pallet.	Figure 2: Muting on pallet.

Solution 1: Smart process gating (SPG)

The safety gap caused by pallet muting can be reliably closed with smart process gating (SPG), in which the bridging function is activated by two independent control signals without external muting sensors. The gating on the safety light curtain is activated by:

• a control signal (CS) from the system control as the first (initiation) signal
• a protective field interruption (PFI) signal, triggered by the transported goods in the light curtain protective field, as a second (verification) signal.
   

The gating function is activated by the correct sequence of CS switching signal and protective field violation and is monitored by the light curtain. Shortly before the transported goods enter the protective field, the process control system (PLC) sends the CS switching signal to the safety light curtains. The timing must be set so that the distance between the transported goods and the protective field is less than 200 mm, preventing a person from passing through immediately before the goods.

If the transported goods enter the protective field within four seconds, the light curtain uses its own PFI signal and suppresses a safety shutdown. Gating ends either automatically immediately after the conveyed material has passed through and the protective field has been cleared, or by resetting the CS switching signal via the PLC.

This method enables a particularly compact and space-saving system design, as no additional muting sensors are required directly upstream or downstream.

Hazard 2: Safety gap due to incompletely loaded pallets

If a pallet is only partially loaded, or if the transported objects are significantly narrower than the conveyor system, a gap will occur during muting. A person can enter the danger zone through this gap without the safety function being triggered (Figure 3). To close the safety gap, the standard limits the maximum permissible clearance next to the transported goods to 200 mm. In real-world applications, however, passage gaps are often significantly larger, making it easy to bypass the protective device.

Figure 3: Excessive distance during muting means a person could enter the danger zone unnoticed alongside the transported goods.

Solution 2: Access guarding with dynamic format adaptation

The hazard caused by partially loaded pallets can be eliminated by means of access protection with dynamic format adjustment. Here, two safety laser scanners generate a joint, closed, vertical protective field. In addition, distance sensors installed on both sides of the conveyor belt detect the position and width of the goods on the pallet — or alternatively this can also be determined using the scanners’ integrated measuring function. The safety system uses this information to release a corresponding area in the protective field through which the transported goods can be conveyed without interruption (Figure 4).

Figure 4: Access guarding with dynamic format adaptation.

By doing it this way, access to the sides of the goods remains secured in accordance with standards. After the transported goods have passed through, the protective field is automatically closed again. If a person is walking or driving alongside them, this is also reliably detected. This safety solution’s innovative safety concept enables Performance Level d in accordance with ISO 13849-1².

<sup>1. International Electrotechnical Commission 2028, IEC 62046:2018: Safety of machinery — Application of protective equipment to detect the presence of persons.
2. International Standards Organization 2023, ISO 13849-1:2023 Safety of machinery — Safety-related parts of control systems Part 1: General principles for design.</sup>

Top image credit: iStock.com/Hramovnick