3.8 US specifications for safety distance calculation
 The U.S. Code of Federal Regulations, Volume 29, Part 1910, Subpart 0 defines the calculation of the minimum safety distance of a protective device. OSHA 1910.217 requires that with the installation of a Safety Light Curtain a minimum distance, which corresponds with the prescribed distance of a guard, is observed in every case (see OSHA 1910.217, table 0-10). If the safety distance calculation results in a greater value, this must be used.
ANSI B11.19-2003 calculation formula for the minimum safety distance for AOPD with right-angle approach (point of operation guarding):
The following calculation formula applies for applications of optoelectronic protective devices with approach direction of body parts in an angle of 30° to 90° to the protective field level (see Approach of the body part from a right-angle to the protective field level):
Ds = Hs x (Ts + Tc + Tr + Tbm) + Dpf
| DS |
The minimum safety distance in inches or millimeters between the next danger zone and detection point (protective field)
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| Hs |
Hand speed (approach speed of body parts or bodies) in inches/s or millimeters/s. ANSI B11.19-2003 provides hand speeds of 63 ‑ 100 inch/s. 63 inches/s is frequently calculated, which equals 1600 mm/s.
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Elements of the entire stop time of the machine:
| Ts |
Stopping time of the machine measured at the last control element in s
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| Tc |
Response time of the machine control system in s (note: Ts + Tc are usually measured together with a stopping time measuring device)
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| Tr |
Response time of the protective device (incl. interface module) in s
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| Tbm |
Additional response time for the brake wear and tear which is not detected by the tracking monitoring of the brakes. If the machine does not have a brake monitoring unit, approx. 20% of the measured tracking time (Ts + Tc), or a factor in accordance with the specifications of the machine manufacturer must be added as a reference value for the brake wear and tear.
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| Dpf |
Penetration factor in inches or millimeters. This additionally added distance is based on the fact that, depending on the resolution of the protective device, a body part can get a certain distance closer to points of operation before it is detected by the protective device.
Dpf (inches) = 3.4 x (resolution – 0.276), result > 0
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| Resolution |
Dpf (mm) |
Dpf (inches) |
| 14 mm |
24
|
0.9 |
| 20 mm |
44 |
1.7 |
| 30 mm |
78 |
3.1 |
Calculation formula for the minimum safety distance for AOPD with parallel approach (danger zone guarding):
The following calculation formula applies for applications of optoelectronic protective devices with approach direction of body parts parallel or in an angle up to 30° to the protective field level: The formula is derived from the ANSI formula and based on the principles of EN 999. With protective devices arranged in this way the safety distance from the point of operation is measured from the furthest away protective field boundary, as the detection of the body part begins here (see Body part approach parallel or up to max 30° to the protective field level).
Ds = Hs x (Ts + Tc + Tr + Tbm) + DH
DH = 1200 mm – (0.4 x H)
| DH |
Additional distance for lower limbs. DH always at least ≥ 850 mm (arm length)
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| H |
Height of protective field above reference plane (floor).
Permissible installation heights H of a protective device with resolution d [mm]:
15 x (d – 50) ≤ H ≤ 1000 mm
Required resolution d of a protective device with installation height H:
d [mm] ≤ H / 15 + 50 mm
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Attention:
If H is greater than 300 mm (12 inch) the danger exists of room to crawl under. This must be taken into account with the risk assessment.
Calculation formula for the minimum safety distance of Multiple Light Beam Safety Devices for access guarding:
If the risk assessment determines that a detection of the penetration of the entire body is sufficient, the following calculation formula must be applied (see Safety distance and beam heights of Multiple Light Beam Safety Devices for access guarding). The additional 850 mm to be added on corresponds with the arm length:
Ds = Hs x (Ts + Tc + Tr + Tbm) + DH
DH = 850 mm
Attention:
This type of arrangement of the protective device allows an operator to be between the sensor and the point of operation without being detected after crossing the protective device. A start/restart interlock function that prevents the machine from starting is provided in every case. The command device (reset button) must be positioned so that the entire danger zone can be seen and it cannot be operated from the danger zone.
With the risk assessment and selection of the appropriate protective device, a possible getting around, e.g. crawling under the lowest beam, reaching over the highest beam, reaching through or climbing through two beams must be taken into account.
| Number of beams of the protective device |
Height of the beams above reference plane |
| 4 |
300, 600, 900, 1200 mm
|
| 3 |
300, 700, 1100 mm |
| 2 |
400, 900 mm |
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