5.3 Reducing or Eliminating Pass-Through Hazards
A
pass-through
hazard is associated with applications where personnel may pass through a safeguard, such as the S4B
Safety Light Curtain (which issues a stop command to remove the hazard), and then continues into the guarded area. This is
common in access and perimeter guarding applications. Subsequently, their presence is no longer detected, and the related
danger becomes the unexpected start or restart of the machine while personnel are within the guarded area.
In the use of light screens, a pass-through hazard typically results from large safety distances calculated from long stopping
times, large minimum object sensitivities, reach-over, reach-through, or other installation considerations. A pass-through
hazard can be generated with as little as 75 mm (3 in) between the sensing field and the machine frame or hard (fixed)
guarding.
Eliminate or reduce pass-through hazards whenever possible. While it is recommended to eliminate the pass-through hazard
altogether, this may not be possible due to machine layout, machine capabilities, or other application considerations.
One solution is to ensure that personnel are continually sensed while within the hazardous area. This can be accomplished
by using supplemental safeguarding, such as described by the safety requirements in ANSI B11.19 or other appropriate
standards.
An alternative method is to ensure that after the safeguarding device is tripped, the corresponding safety monitoring device
latches and requires a deliberate manual action to reset. This method of safeguarding relies upon the location of the reset
switch as well as safe work practices and procedures to prevent an unexpected start or restart of the guarded machine. The
S4B Safety Light Curtain does not provide a configurable Manual Start/Restart (Latch Output) function. For these
applications, this function must be implemented in the external safety monitoring device.
WARNING:
•
Use of the Banner device for Access or Perimeter Guarding
•
Failure to observe this warning could result in serious injury or death.
•
If a Banner device is installed in an application that results in a pass-through hazard (for example,
perimeter guarding), either the Banner device System or the Machine Primary Control Elements
(MPCEs) of the guarded machine must cause a Latched response following an interruption of the
defined area. The reset of this Latched condition may only be achieved by actuating a reset switch
that is separate from the normal means of machine cycle initiation. Lockout/Tagout procedures per
ANSI Z244.1 may be required, or additional safeguarding, as described by ANSI B11.19 safety
requirements or other appropriate standards, must be used if a passthrough hazard can not be
eliminated or reduced to an acceptable level of risk.
5.4 Supplemental Safeguarding
As described in
Calculating the Safety Distance (Minimum
on page 15, position the S4B such that an
individual cannot reach through the defined area and access
the hazard point before the machine has stopped.
Additionally, the hazard cannot be accessible by reaching
around, under, or over the defined area. To accomplish this,
supplemental guarding (mechanical barriers, such as
screens or bars), as described by ANSI B11.19 safety
requirements or other appropriate standards, must be
installed. Access will then be possible only through the
defined area of the S4B System or through other
safeguarding that prevents access to the hazard.
The mechanical barriers used for this purpose are typically
called "hard (fixed) guarding"; there must be no gaps
between the hard (fixed) guarding and the defined area. Any
openings in the hard (fixed) guarding must comply with the
safe opening requirements of ANSI B11.19 or other
appropriate standard.
Figure 4. An example of supplemental safeguarding
Hard (fixed) Guarding
Reset Switch
Conveyor
Opening
Area
Guarding
Robot
Turn-
Table
Hard (fixed)
Guarding
Area
Guarding
Safety Light Curtain/Screen
This is an example of supplemental safeguarding inside a robotic work cell. The S4B, in conjunction with the hard (fixed)
guarding, is the primary safeguard. Supplemental safeguarding (such as a horizontal-mounted safety light screen as an area
guard) is required in areas that cannot be viewed from the reset switch (for example, behind the robot and the conveyor).
S4B Safety Light Curtain
18
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