Minimal Safe Distance Calculation

The Minimal Safe Distance Calculation shall guarantee that a person is not able to reach a dangerous machine part before the machine movement has stopped.

This is calculated with the formula as called for from the ANSI B11.19-2003 Performance Criteria for the Design, Construction, Care and Operation of Safeguarding Standard.

Ds = [K x (Ts + Tc + Tr + Tspm)] + Dpf

Where:

Ds = minimum safe distance between the safeguarding device and the hazard.

K = speed constant: 1.6 m/sec (63 inches/sec) minimum based on the movement being the hand/arm only and the body being stationary.

Ts = worst stopping time of the machine/equipment.

Tc = worst stopping time of the control system.

Note: Ts + Tc are usually measured together with the JOKAB SAFETY Stop Time Analyzer.

Tr = response time of the safeguarding device including its interface.

Tspm = the additional stopping time, in seconds, allowed by the stopping performance monitor before it detects stop time deterioration.

Dpf = maximum travel towards the hazard within the presence sensing safeguarding devices (PSSD) field that may occur before a stop is signaled. Depth penetration factors will change depending on the type of device and application.

Dpf in mm = 3.4 x (Os - 6.875mm), but not less than 0mm

Dpf in inches = 3.4 x (Os -0.275"), but not less than 0"

Where:

Os = minimum object sensitivity or resolution.

Note: for horizontal detection fields, the Dpf is fixed at 1.2 m (48")

Example 1:

With the aid of the JOKAB SAFETY Stop Time Analyzer, a mechanical power press has a measured stopping time (Ts + Tc) of 325 ms. This includes both the stopping time of the machine as well as the stopping time of the control circuit. The response time of the Focus Safety Light Curtain F4-35-900 with 35 mm (1.38") resolution and 900 mm (35.43”) protective height is 20 ms (Tr). The stop time break monitor is set for 400 ms.

Ds = [K x (Ts + Tc + Tr + Tspm)] + Dpf

Ds = [1.6m/s x (325ms + 20ms + 75ms)] + 95.63mm

Ds = [1600mm/s x 420ms] + 95.63mm

Ds = 672mm + 95.63mm

Ds = 767.63mm

Ds = [63inch/s x (325ms + 20ms + 75ms)] + 3.76"

Ds = [63inch/s x 420ms] + 3.76"

Ds = 26.46" + 3.76"

Ds = 30.22"

Note: In this example the safety light curtain must be mounted no closer than 767.63mm (30.22") from the hazardous pinch point.

Example 2:

With the aid of the JOKAB SAFETY Stop Time Analyzer, a robotic loader has a measured stopping time (Ts + Tc) of 175 ms. This includes both the stopping time of the machine as well as the stopping time of the control circuit.

The response time of the horizontally mounted Focus Safety Light Curtain with 35mm (1.38") resolution is 24ms. The depth penetration factor is fixed at 1200mm (48").

[K x (Ts + Tc + Tr)] + Dpf

Ds = [1.6m/s x (175ms + 24ms)] + 1200mm

Ds = [1600mm/s x 199ms] + 1200mm

Ds = 318.4mm + 1200mm

Ds = 1518.4mm

Ds = [63inch/s x (175ms + 24ms)] + 48"

Ds = [63inch/s x 199ms] + 48"

Ds = 12.54" + 48"

Ds = 60.54"

Note: In this example the horizontal safety light curtain must be at least 1518.4 mm (60.54”) from the hazardous pinch point.

K = The maximum speed at which an individual can approach the hazard, expressed in inches per second.

To quote ANSI B11.19-2003: "The factor K is the speed constant and includes hand and body movements of an individual approaching a hazard area. The following factors should be considered when determining K: a) Hand and arm movement; b) Twisting of the body or shoulder, or bending at the waist; c) Walking or running.

One of the accepted values for K is the hand speed constant (it is usually considered as the horizontal motion of the hand and arm while seated). Its common value is 63 in./s although other values (typically higher) are also used. The hand speed constant does not include other body movements, which can affect the actual approach speed. Consideration of the above factors should be included when determining the speed constant for a given application."

Notes on Mounting

Vertical Safety Light Curtains

A vertically mounted safety light curtain that is used as the primary safety device must be mounted so that the bottom beam is no higher than 300mm (12") from the ground. Mounting heights above 300mm (12") may require supplemental safeguarding to prevent crawling or ducking under the horizontal light curtain. The top beam must be no lower than 900mm for reach over applications and 1200mm (48") for reach through applications. Mounting heights lower than this will require additional safeguarding.

Horizontal Safety Light Curtains

A horizontal safety light curtain that is used as the primary safety device must be mounted no higher than 300mm (12") from the ground. Mounting heights above 300mm (12") may require supplemental safeguarding to prevent crawling or ducking under the horizontal light curtain. The minimal length the safety light curtain can be 1200mm (48") long.

The formula below is for mounting different resolutions from a horizontal surface.

Height = 15 x (Os - 50mm)
Height = 15 x (Os - 2")

Vertical Perimeter Safety Light Grids

A vertically mounted safety light grid must be mounted such that the bottom beam is no higher than 300mm (12") from the ground. If the light grid is of only 2 beams then the second beam must be located 900mm (36") from the ground. For a 3 or 4 beam system the top beam must be 1200mm (48") above the ground.