The bus plunge into the Yangtze River in Wanzhou, Chongqing in early November drew widespread attention. According to the police's account of the incident, the accident was caused by a violent physical altercation between a passenger and the driver, which led to the loss of control of the vehicle and ultimately resulted in the tragedy.
If the passenger had remained calm, and if the driver had been mindful of safety, perhaps the tragedy could have been averted. However, relying on people to control their behavior in time to prevent accidents is clearly unreliable. Adding protective barriers to the driver's seat is perhaps the most direct and effective solution currently available.
How should we take safety precautions?
Whether in daily life or industrial production, it is essential to conduct thorough risk assessments and take necessary measures, such as installing appropriate protective equipment and designing safety control systems, to mitigate risks.
What may seem like simple security measures actually involve a great deal of knowledge.
Figure 1. Protective devices improve production safety.
"Fixed protection? Portable protection?"
Protection, as the name suggests, is used to protect people from harm caused by machines. In some cases, protective measures can also protect the machine's production process from human interference. For example, preventing people near the machine from arbitrarily interrupting time-sensitive production processes.
For newly purchased machinery in a factory, should fixed or movable protective measures be installed? The main determining factor is the frequency of personnel entering and exiting the danger zone.
If entry into the hazardous area is only required during maintenance and process correction phases, and the removal and replacement of protective devices are relatively easy, then installing fixed protective devices is suitable.
If the production process requires intervention and the foreseeable frequency of personnel entry and exit is higher than once a week, or if the removal and replacement of fixed protective devices are difficult, then movable protective devices should be installed.
Different types of protective equipment are suitable for different situations and have different costs. Appropriate protective equipment should be selected according to the site conditions.
Figure 2, Safety door protection device
Safety control of protective devices
According to the assessment, if it is determined that a movable protective device is to be installed, then the next step is to determine the appropriate interlock safety level (SIL or PLr) according to EN62061 or ENISO13849-1, and then design and verify the control system.
Sensors, such as safety interlock switches with protective locking mechanisms, serve as inputs to the control system to detect the position of the protective devices. The protective locking function determines the location based on the detection results, allowing the protective device to open only after the dangerous action has ceased. The protective device can be linked to an interlocking system; when a safety door is opened, the interlocking system ensures that dangerous machine functions stop and effectively prevents accidental restarts.
It is important to note that if a risk assessment confirms a risk of personnel entrapment, the safety door's locking device must include an escape release function to prevent the door from being inaccessible in the event of a power failure or other malfunction. During a power outage, if the safety door is closed and the control system cannot issue an unlocking command, a person in the danger zone can manually open the locking device. However, this requires that the operator be trained and understand how to operate the mechanical release function.
Fault shielding
When using sensors to monitor mobile protective devices, a key question is whether a higher number of sensors connected in series with diagnostic and evaluation equipment will result in higher diagnostic coverage.
Consider the following case:
1. Three safety door sensors are connected in series to an evaluation device. Initially, all safety doors are closed, the relay output is "on", and the machine starts running.
2. A short circuit occurs in the switch circuit between the safety door on the left and the N/C contact. In this case, the fault will not be detected, and the machine will continue to run.
3. The safety door on the left is opened, sending a signal to the relay. During the signal switching comparison, the relay detects an inconsistency and switches to a fault state: the safety door closes, and the machine cannot be restarted.
4. If the rightmost safety door is opened, the relay detects that the condition is normal, the fault state is reset, the safety doors close sequentially, and the machine is ready to start. However, the short circuit fault still exists.
The above describes undetectable faults in the safety circuit, technically known as fault masking. If the number of faults in the safety circuit increases, the entire safety gate system will fail.
Mechanical switches and magnetic proximity switches must take this type of fault shielding into account. Due to the limited possibility of fault shielding, the maximum diagnostic coverage (DC) that the switch can achieve is also limited, which affects safety functions and the performance level PL that can be achieved.
A pair of interlocking switches independently connected to a safety controller can achieve a maximum diagnostic coverage of 99%. According to ISO/TR24119, the maximum diagnostic coverage of a group of switches connected in series depends on the number of switches and their operating frequency.
Only switches with internal diagnostic circuits and OSSD outputs based on RFID technology can be free from the effects of fault shielding. For example, if safety function requirements reach PLe level, switches with integrated fault detection technology must be used to avoid fault shielding. The DC and PL of a safety circuit composed of series-connected safety switches will also remain unaffected.
As a safety ambassador in the field of automation, Pilz is committed to bringing users the most suitable solutions.
Pilz China
Sales hotline: 4006400650 Website: www.pilz.com.cn
