Sunday, July 21, 2013

Safe Distance and Stop Time Measurement

There are many engineered controls utilized in industry, these include light curtains, area scanners and two hand controls.  An important consideration in the performance requirements of engineered controls is the safe distance.  This is the minimum distance that the device must be mounted from the hazard in order to be effectively providing protection to operators and bystanders.  The safe distance is dependent on the time taken to process the stop command, and how far an operator can penetrate the detection zone before detection The formula used to calculate safe distance (as referenced in CSA Z432-04 Safeguarding of Machinery) is:


Ds =K x (Ts + Tc + Tr + Tbm) + Dpf



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

K              = speed constant: 1.6 m/s (63 in/s) 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


Tr            = response time for the safeguarding device, including its interface


Tbm        = additional stopping time allowed by a brake monitor (if installed)


Dpf          = maximum travel towards the hazard within the presence sensing device field that may

                    occur  before a stop is signalled.


A critical tool for calculating safe distance is the stop time measurement device.   If the stopping time for the equipment and its control system are unknown, the stop time device can be used to measure it.  The machine measures the time it takes the machine to stop after a signal is given.  It is mainly used on stroking or reciprocating machines (punch presses and press brakes).  Accessories can be attached to also use it with rotating equipment.  If the machine has a presence sensing device, such as a light curtain, the plane of light can be interrupted by a flag which will provide the stopping time.



Stop Time Demo



Ensure that your engineered controls provide the protection they have been designed for!






Posted by Kristin Petaski at 7:04 PM 0 Comments
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