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Relaying the Safety Message
Safety in the work place has become a topical issue with recent political moves to make individual directors culpable for serious injuries and fatalities in the workplace, with some even advocating jail sentences.
Accidents are very rarely unavoidable, and the key to their prevention is the analysis of all work place risk. This is the foundation of regulation EN 954-1 which stipulates machine users must carry out a machine risk assessment and implement adequate control systems. Safety switching circuits or electronic safety relays (ESR) are a key measure in guarding against accidents.
As a part of the risk assessment process a safety control system is selected to provide the best solution to reduce risk. A safety control system comprises input devices linked to an ESR and a machine actuator. An input device is a type of sensor or switch, for example: safety mat, emergency stop button, safety light curtain and proximity switch. Inputs may be connected to the ESR through a single or dual channel or as a dual channel with cross monitoring. The ESR then sends a command signal to enable or disable the moving parts of the machine. A typical application is a switch attached to the guard screen around a milling machine. If the guard screen is moved while the machine is operating this activates the switch, which then signals the relay to stop the machine.
EN 945-1 underpins the correct choice of safety control systems as it categorises them into how reliable they are in the event of a fault. Reliability is dependent on not only the correct selection of input and ESR but on how they're connected. The most reliable 'high category' equipment may be present in the safety control system, but if it has low category connection then the whole control system would be classified as low category. These categories are criteria for managers to assess the robustness of their control systems to deal with faults. If components are selected and safety procedures applied correctly then categories B and 1 are satisfied. The system must have the correct connection structure to attain a higher level of compliance, categories 2,3 and 4.
The type of connection and its safety category is outlined below:
- Single channel control circuit: The ESR is triggered through a single transducer contact or output. With this type of control circuit the safety circuit at most falls into safety category 2 of EN 954-1
- Dual channel control circuit: The ESR is triggered through two transducer contacts or outputs. With this type of control circuit the safety circuit at most falls into safety category 3.
- Dual channel control circuit with cross monitoring: The ESR with cross monitoring is triggered through two transducer contacts or outputs. With this type of control circuit the safety circuit falls into safety category 4. If faults occur the safety function always remains intact. Faults are detected at the proper time to prevent loss of the safety function.
The choice of emergency stop of the machine is determined by its risk assessment. In EN 60204-1 the stop function is divided into three categories; emergency stops must conform to category 0, category 1 or category 2.
Within a safety control system the ESR consists of a power section, electronic section and according to EN 60204-1 two back-up enabling and signalling paths. Its input is controlled by the electronic unit. In normal fault free operation, the relay's enabling paths are activated. In the event of the off button being activated or a fault (earth fault, wire break, cross fault) the enabling current paths are stopped immediately (stop category 0) or with a time delay (stop category 1) and the motor is disconnected from the power supply.
Moeller's ESRs meet the requirements of safety category 3 or 4 to EN 954-1. In emergency-stop applications the safety relays can switch several release current paths either for instantaneous or for time-delayed interruption of the power supply, once the emergency-stop button is actuated. If control cables are damaged in a way, for example, which may cause short circuits to exposed conductive parts or to earth, then the electronic safety relays ESR react accordingly. The ESR can distinguish different sorts of failures, they switch off immediately, preventing reconnection until the fault is rectified.
Safety relays rated for category 4 operation additionally feature a restart button monitoring function. This function ensures that the enable signal is only given after the restart button has been released. If the ESR is used to monitor safety guards, an automatic start function replacing the reset monitoring feature gives users also the opportunity to reduce cycle times within the production while maintaining a high safety level.
Contact mats are also a very effective measure to protect people entering a hazardous zone and can easily be monitored by safety relays. As soon as a person treads on the mat, it generates a short circuit between two conductors, which disconnects the machine or process from the power supply.
Special safety relays are available for two-hand controls. Two-hand circuits require simultaneous operation by both hands to initiate and maintain the operating status of a machine. As a result the operator is protected, because he can not reach the danger zone during hazardous procedures. The electronic safety relay monitors whether or not both buttons are operated within 0.5s of one another.
Risk analysis is the core to safety in machines, resulting in a safety circuit and procedures that ensure low-risk operation. The ESR is the 'brain' linking input devices to the hazardous moving parts of the machine so the correct choice of ESR is essential to the safety circuit's effectiveness. It is only with risk analysis to assess the requirements of the safety circuit together with a carefull understanding of safety legislation that machines can be designed and installed with safety maximised.
