Chapter 3.2
Fixed guards prevent access to hazards and must be strong, only tool-removable, and not stay in place after fasteners are removed. Below we explain when, where and which guard to use in different types of situations.
Webinar - From risk assessment to choice of guarding
Webinar - Key rules for access to guarded areas from international standards based on the reason for access and its frequency
Selection and positioning of a protective fence
Hazard area
Determine the position of the hazard area.
Height
Determine the maximum height of the hazard area. In this case 1600 mm.
Horizontal distance
Determine the horizontal distance of the hazard zone to the fence.
In this case 400 mm.
Select correct fence height from table in the standard
EN ISO 13857, Table 2, adapted to Axelent’s fence height range (see table above).
In this case, the correct minimum fence height is 2200 mm.
Ground clearance
Check if reaching around the guard into a hazard zone is possible.
Ground clearance
If a hazard area is near the ground, it must be at least 1100 mm away, otherwise the ground clearance must be closed, e. g. with Axelent kick plate.
Part ejection or spillage
Can parts be ejected or hazardous liquids be spilled?
Part ejection or spillage
If the parts are smaller than the selected mesh pitch or liquid can be spilled, consider using
plastic or sheet metal panels.
Movable guards for frequent access
The rules for guards are based on (EN) ISO 14120.
If a hazard area rarely needs to be accessed (typically less than once per week), one will use so called fixed guards. A fixed guard is a cover preventing access and holding the hazard inside the machine. It is considered part of the machinery, not a type of optional accessory.
Three basic rules apply to fixed guards:
Since fixed guards are not monitored, they must only be removed when the machine is stopped safely. The standards require that they are removed under a “safe system of work” only. That is a formal procedure laid down in writing.
The procedure should include at least the following information:
When the requirement for so called “no-loss fasteners” first appeared, there was much discussion about it. But their use has meanwhile been clarified by comments in (EN) ISO 14120.
No-loss fasteners need to be used when:
No-loss fasteners are not needed when guards need to be removed only for major overhaul or transport to a new installation site. Axelent’s X-Guard system always meets the requirement, since it does not feature removable fasteners, but one needs the X-key to remove panels.
The international standard requires a minimum height of 140 cm. Lower fences are easy to climb over and thus not allowed. In most cases, fences will be a lot higher.
Proper height depends on two dimensions
When these dimensions are known, you can read the required height of the fence from the table below which has been taken from (EN) ISO 13857. The same values are given in ANSI B11.19 Annex E and CSA ISO 13857.
Note: There are also minimum distances that must be kept from the inside of a mesh panel to the hazard zone.
Mesh pitch - Safety distance
Up to a slot height of 20 mm, the safety distance is ≥ 120 mm.
The same applies to square openings that are no bigger than 30 x 30 mm.
However, if the slot is more than 20 mm high and 30 to 65 mm wide a greater safety distance of 200 mm is needed. If you need to go closer to the hazard for lack of space, use solid panelling.
Standard mesh pitches offered by Axelent:
If a hazard area needs to be accessed frequently typically more than once per week, movable guards should be used.
There are two main requirements for them:
This is called “interlocking” because it “locks” movement of the drive(s) of the machinery depending on the condition of the guard. Most movable interlocking guards come in the shape of hinged or sliding doors, flaps and lids.
The condition of doors is monitored by so called guard or safety switches the proper word in standards is “interlocking device”. Essentially there are three forms of these quite narrow rules apply to their use. For more information, see chapter 3.6 Rules for safety switches.
Sometimes machinery will need some time to stop when a guard is opened.
If the machine needs some time to stop after the guard door is opened. People could still reach moving parts, which is dangerous. To prevent that movable guards can be fitted with door switches that prevent opening during operation. The machine will first have to be stopped, then the door can be unlocked. This is called “guard locking” and is discussed in section 3.4
Some designers will set the control up so the movement immediately starts (again) when a door is closed. But in general, this is not allowed.
The limits for use of such guards are quite strict:
To save time or move big heavy guards, electric, pneumatic or even hydraulic drives are sometimes employed to power guards.
Powered moving guards may present a crushing or shearing hazard. If that is the case, the guard must be fitted with additional safety equipment. Additionally, force/speed of moving guards must be kept within tight limits.
Without any additional safety devices a force of 75 N and the energy of 4 J may not be exceeded by a closing guard. This is to prevent broken limbs or even more severe injury, but 7.5 kg on your finger will still hurt.
For this reason so called “trip bars” are used on powered guards. A trip bar is a type of rail or rubber fin that detects body parts in the way of the moving guard. The force actuated by the moving guard trips this device and the guard will stop moving or even return to its open position. The trip force is limited to 150 N and 10 J.
As alternatives to a trip bar one may use counter weights or another form of force reduction (slip-type coupling, pressure reduction valves, torque limiters).
