Specifying a Safety Mat System
By Joe Lazzara, Scientific Technologies Inc.
Often overlooked, frequently neglected, and always walked on, safety mats offer a convenient means of protecting workers against exposure to several potential hazards of a manufacturing environment. Like any other safety device, they must be properly selected, installed, and maintained to provide a reliable and safe method of machine guarding. Let’s examine some of these critical issues when selecting a safety mat system.
Presence sensing safety mats are often used to guard access to automated manufacturing cells, robots and machinery. They can be used in conjunction with other types of guards, such as fencing or safety light curtains, especially when protecting access to a large machine or work cell perimeter. Comparing safety mats to other guarding solutions, such as sliding gates, mechanical barriers or pull back restraints, safety mats offer operators freedom of movement and flexibility, as well as ease of access for machine maintenance, setup and material flow.
When it comes to specifying a presence sensing safety mat system, what you don’t know can hurt you. Three factors that must be considered for the proper selection and installation of safety mats include:
- The distance from the point of hazardous operation to the sensing edge of the mat furthest from the danger zone (the safety distance)
- The minimum size of the detection zone
- The type of perimeter trim to be used.
Safety Distance Calculation
The first and by far the most important consideration is the calculation of the safety distance. There is a minimum mat size that should be placed between a worker and a hazardous motion. Many users will "eyeball" the application, look at the area where a machine operator would stand and say, "that looks like it needs a 24-inch wide mat." It may not be enough.
In standard B11.19 the American National Standards Institute (ANSI) states that, "The safety mat device shall be located such that the operator cannot reach the recognized hazard before hazardous motion of the machine has ceased."
Unfortunately, the ANSI standard stops there. In order to figure out how much mat you need between a machine and a worker, it’s necessary to refer to the standard EN 999. This standard provides a formula for determining the correct mat size for a specific hazard. The formula is similar in nature to another calculation, which some readers may be familiar – the mounting distance formula for a safety light curtain.
The mat formula reads as follows:
S = (63 in./sec. x T) + (47.2 in. – 0.4H)
Where:
S = The minimum distance from the danger zone to the detection zone in inches. This calculation assumes that the approach of a person toward a dangerous area is at walking speeds.
T = t1 + t2
T = The overall system stopping time.
t1 = The maximum time between the actuation of the safety mat and the controller output relays being in the de-energized state.
t2 = The response time of the machine being guarded. The time required to stop the machine or remove the risk after receiving the output signal from the mat system.
H = The distance above the reference plane (floor) in inches. When mats are mounted on the floor, H = 0.
Example:
As an example, let’s say the stopping time of the guarded machine is 0.300 seconds. The response time of the mat system is 0.025 seconds. The safety mat is to be mounted at the reference plane (floor).
S = (63 in./sec. x T) + (47.2 in. – 0.4H)
or
S = (63 in./sec. x (t1 + t2)) + (47.2 in. – 0.4H)
Therefore:
S = (63 in./sec. x (0.300 sec. + 0.025 sec.)) + (47.2 in. – (0.4H x 0.0in.))
S = (63 in./sec. x 0.325 sec.) + (47.2 in. – 0.0 in.)
S = 20.5 in. + 47.2 in.
S = 67.7 in. or 1720 mm
This calculation specifies that the distance starting at the mat edge furthest from the hazardous location should be at least 67.7 inches. This also implies that the floor area, from a distance of 67.7 inches to the hazardous location must be guarded by mats or other means, such as fencing, to prevent any undetected access to the hazardous location. Using this formula will usually result in a larger safety mat specification than most people would estimate.
Minimum Detection Zone
The distance a person walks is generally a factor of the walking speed and the stride length. As we have already seen, the positioning of safety equipment, determined by these calculations assumes a person is walking. The stride length affects the minimum size of the mat detection zone. The EN 999 standard has determined that this dimension is equal to 750 mm or 29.5 inches. Thus, the minimum size of a safety mat, measured toward the direction of movement should be at least 29.5 inches, and is identified as "C" in the figure.
Safety Mat Mounting Trim
ANSI standard B11.19 also states that, "Whenever possible, the mat should be fixed in place in such a manner so as to prevent easy relocation or removal by the operator or other unauthorized personnel."
Perimeter trim can help with this requirement, but users need beware that not all perimeter trim is the same. Three of the most optimum types of trim include two-part perimeter ramp trim, blunt trim, and two-part joining trim.
Two-part perimeter ramp trim holds mat in place and simplifies installation by providing an aluminum base with channels for running cables, and a snap-on PVC cover. Blunt trim is used where a mat needs to be secured in place, but the edge being secured does not present a trip hazard. Two-part joining trim is used to create an active area between two adjacent mats.
Next time, we will take a further look at selecting and installing safety mats by examining the international safety mat standard, IEC 1760. Until next month, be safe out there!
Lazzara on Automation Safeguarding is a monthly column written for Safetyonline.com and Plantautomation.com
Joe Lazzara is president and CEO of Scientific Technologies Inc. (STI, Fremont, CA), the largest provider of automation safeguarding solutions in North America. Lazzara began his career with Hewlett Packard in 1973 where he had responsibility for safety and environmental issues for one of HP’s largest divisions. He joined STI in 1981 as vice president and became president in 1989 and CEO in 1993. Lazzara received a bachelor’s of environmental engineering degree from Purdue University and an MBA from Santa Clara University.
