When attempting to safeguard a piece of automation equipment using electrical safeguarding devices such as light curtains, area scanners and safety mats, careful consideration must be taken to ensure they are not just placed in an arbitrary location that is convenient to install or mount. Many factors must be considered when performing a safety distance calculation.
As machinery and processes continue to become faster and more complex the need to be aware of, and continue to strive for improved safety measures has never been more at the forefront of electrical control system design then it is today. Due to this ever growing trend, the industry in turn has evolved to keep pace by marketing products and protective devices that aim to put the safety and well-being of the people or persons operating machinery at the forefront of the design process.
Applicable Standards In North America
In this article we will examine two protective devices, namely, light curtains and safety mats and perform example safety distance calculations as outlined in the American National Standards Institute, ANSI B11.19-2010 Performance Requirements for Safeguarding and the Canadian Standards Association CSA Z432 – Safeguarding Of Machinery.
These standards outline the requirements for the design, manufacture (including re-manufacture and rebuilding), installation, maintenance, operation, and safeguarding of industrial equipment to prevent injuries and accidents and enhance the safety of personnel who operate, assemble, and maintain machinery.
Safety Distance Calculation Example: Mechanical Power Press
A mechanical power press cannot come to an immediate stop in any scenario. There is an accumulation of delays in the safety system from the moment a stop is asserted to the time when the press actually comes to a complete stop. These delays can include: safeguarding device detection time, Safety PLC processing time, network delays, relay dropout time, just to name a few. These worst case reaction times must be considered when performing a proper safety distance calculation.
If the design engineer neglects these “built-in” delays and places, for example, a light curtain device to close too the point-of-operation, the results can be fatal! Additionally, some machines may require a coast-down time because the process is operating so quickly that asserting an immediate stop could damage the equipment.
Safety Distance Calculation – A Closer Look
The minimum safety distance of any device is dependent on the time required to process the “Stop” command and how far the operator can penetrate the detection zone before being detected. The formula used in Canada and the US is as follows,
US CAN : Ds = K × (Ts+ Tc+ Tr+ Tbm )+ Dpf
Ds – is the minimum safe distance from the danger zone.
K – is a speed constant 63 inch/s (1600 mm/sec).
T – is the overall stopping time of the system.
This time can be broken down into its incremental parts (Ts+ Tc+ Tr+ Tbm) for easier analysis,
Ts – the worst stopping time of the machine or equipment.
Tc – the worst stopping time of the control system.
Tr – the response time of the safeguarding device used, including its interface.
Tbm – the additional stopping time allowed by the brake monitor before it detects stop-time deterioration.
Dpf – the depth penetration factor is the maximum travel towards the hazard before detection by the safeguarding device.
For a normal approach to a light curtain with a resolution ≤ 40 mm, the ANSI and Canadian standards used are,
Dpf=3.4 ×(Object Sensitivity-6.875 mm)
With safety mats, the safe distance must take into account the operators pace and stride. Assuming the operator is walking and the safety mats are mounted on the floor, the operator’s first step onto the mat has a depth penetration factor of 1200 mm or 48 in.
If the operator must step up onto a platform, then the depth penetration factor can be reduced by a factor of 40% of the height of the step.
Safety Distance Calculation – Performed
Now that we have an understanding of safety distances let’s perform an actual safety distance calculation. We will compute the safety distance calculation required to deploy one of, a light curtain protective barrier (light curtain), or a safety mat protective barrier.
To determine the stopping time of the machine (Ts), we would typically take 10 stop time measurements, calculate the mean, then add 3 standard deviations. This calculated value, or the highest of the 10 measured values, should be used as the value of Ts in the formula.
Another important element, highlighted above, is the depth penetration factor (Dpf). This refers to how far into the protective zone an object (or person) can travel before it is detected by the safeguarding device. This calculation, as depicted in the previous section, is dependent on the mounting, orientation and resolution (if a light curtain) of the device.
The overall stopping time of the system, T, with the exception of , the response time of the safeguarding device used, has been determined from the device data sheets and documentation as follows:
Safety PLC (single fault max) 82 ms
Relay Dropout Time (max) 20 ms
Max Brake Stop Time 215 ms
Total Response Time (minus Tr) 317 ms
I will use a value of T = 0.317 sec + Tr for all further calculations.
If employing a light curtain with a resolution of 14 mm and a response time of 20 ms, the calculation then becomes,
Dpf = 3.4(14mm-6.875mm) = 24.2mm ≈ 1 inch
Ds = K × (0.317+ Tr ) + Dpf
= 63 × (0.317 + 0.2) + 1
= 63 × (0.517) + 1
≈ 34 in.(864 mm.)
Therefore the placement of the light curtain must be 34 inches or 864 mm away from the nearest point of the hazard on the press.
If employing a safety mat with a response time of 20 ms, the calculation then becomes,
Dpf=1200 mm = 48 inches
Ds = K × (0.317+ Tr ) + Dpf
= 63 × (0.317+ 0.2) + 48
= 63 × (0.517) + 48
≈ 81 inches (2057 mm)
Safety Distance Calculation – Conclusion
While a safety mat provides the most cost effective solution and ease of installation, based on the safety distance calculation above it would place the operator at an impracticable distance away from the press. Therefore, the best solution will be a 14 mm resolution light curtain or area scanner. This will allow the operator to get as close to the press as possible while maintaining a safe distance from the press.
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