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Effectively guarding a large work cell

December 1, 2007

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We have a large work cell area that contains several machine operations that run automatically. What is the best way to guard such an area?

Answered by Russ Wood, application engineering manager, Omron Scientific Technologies Inc., Fremont, CA.

When many people think of machine guarding, the first thing they consider is protecting the point of operation and sensing the presence of the operator's hand or arm. However, industrial processes quite frequently include automated work cells with machines performing repetitive tasks. The safety goal in these situations can simply be to keep the machines from operating while someone is present in the area.

Guarding methods include the use of presence-sensing optical safety devices such as light curtains and scanners; pressure-sensitive safety mats; and fixed, interlocked barrier guards. Whatever method is chosen, it is important to accurately determine the safe mounting distance for the guarding devices. The safe mounting distance ensures that once the safety system is activated, the machines have time to stop before personnel can reach the hazardous area. Additionally, personnel should not be able to reach over, under or around the guard and reach hazardous portions of the machines. In the United States, formulas to compute this distance are presented within OSHA standards and also by ANSI. The formulas offered by ANSI include more variables, result in more conservative safe mounting distances and should be used whenever possible.

Presence-sensing safety devices such as safety light curtains can be used to guard the perimeter and can be configured so that once a person walks through the vertical plane of light into the hazardous space, a stop signal is sent to the guarded machines. This stop signal is maintained until the light curtain is reset from a station outside and with a view of the entire guarded area. A keyed reset usually is recommended to provide an additional level of security over the reset process.

Pressure-sensitive safety mats and laser-based area scanners also can be used to guard an area. Mats are placed on the floor and should cover an area defined by the safe mounting distance formula. Also, a properly installed mat system will include perimeter trim to keep the mats from being inadvertently moved or repositioned. Laser-based scanners employing "time of flight" technology allow a user to program the specific shape of a hazardous area and can have both a warning zone and protection zone. The former is used to sound an alarm or notify personnel that they are approaching a hazardous area, while the latter will send a stop signal to the machine. Safety light curtains also can be installed horizontally, rather than the more common vertical installation, to cover a defined area.

All four of the methods mentioned above can be useful in areas with enough space to allow for safe mounting. However, automated work cells frequently are crammed into the space available, making safe mounting difficult or impossible. This leads to an emphasis on another, more basic guarding method – fixed and interlocked barrier guards.

Interlocked guards provide a physical barrier through which an employee cannot pass. When properly designed and installed, these guards effectively limit access to a hazardous area. Emphasis should be placed on the selection of materials for the guard (acrylic panels, wire mesh, etc.) so that the size of the openings does not allow access to the hazardous point. Tamper-resistant safety interlock switches are combined with barriers so that when the barrier is opened, a stop signal is set to the machines. Note that with standard interlock switches, safe mounting distance formulas must be followed so that the machines can come to a complete stop before the operator reaches the hazard. However, some safety interlocks incorporate solenoid-powered locking devices which, when used in conjunction with a timer or stop-motion sensor, hold an interlocked gate closed until the machines are in a safe state, allowing the guards to be mounted somewhat closer to the machines and saving floor space.

Open areas present challenges not present in point-of-operation guarding. To be safe, select well-tried components and always remember to consider the safe mounting distance.



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