|
Machine Safety Applications FAQs
|
 |
Q: All Banner Safety Light Screens, SI Series Safety Switches and Safety Relay Modules are approved by OSHA. |
|
FALSE: OSHA does not grant approvals to any type of device (safety or otherwise). Thus, Banner's safety devices are not OSHA approved, and neither are those of any manufacturer. Banner does, however, design all of our safety devices to be in compliance with the requirements OSHA lays down for personnel safety devices.
|
|
Q: OK, OSHA doesn't approve anything, but all Banner safety products are CE approved. |
|
FALSE: As in the case of OSHA, CE does not directly approve products. Throughout the European Union, companies (called Notified Bodies) exist to test a manufacturer's device to applicable European standards. These companies issue a Certificate of Conformity when a tested unit passes their tests. Often the tests they use are drawn from a wide variety of applicable standards, so this "approval" is not necessarily to a single standard.
|
|
Q: I've heard that I can't run my Safety Light Screen through my programmable controller. Is that right?
|
|
TRUE: A programmable controller (PLC) does not contain the redundant circuitry on either the input side or the output side to properly monitor the functioning of a safety circuit. The ANSI B11.1 standard in Annex B4, very clearly states that the use of a plc to control a safety circuit should be avoided.
|
|
Q: Just a minute, I just spoke to a salesman who claimed he sells a safety PLC. That means he's lying, right? |
|
FALSE: Currently there are a handful of vendors who offer a device known as a Safety PLC. These devices typically cost several times more than a standard plc, and typically do contain redundant circuitry to properly monitor a safety circuit.
|
|
Q: It's OK to use my old Mini-Array emitter and receiver to guard my machine as long as I buy a new control box for them, right? |
|
FALSE: Other than the housing, there is not a single similarity between Banner's measurement curtains (such as the BEAM-ARRAY and MINI-ARRAY) and Banner's Safety Light Screen products (such as MACHINE-GUARD, MINI-SCREEN, and MICRO-SCREEN). Many people falsely believe that the only difference is the control box when, in fact, all internal circuitry and even the optical devices (LEDs and phototransistors) change between the products.
|
|
Q: PSDI (Presence Sensing Device Initiation) saves a significant amount of time during the manufacturing process versus using a safety light screen and a cycle start button. |
|
FALSE: All the studies (typically performed in Europe) that talk about the time saving nature of PSDI compare it to two-hand control systems. With a two-hand control system (depending on press speed) you often have a physical separation of several feet between the two push buttons (or other actuators) and the point of operation of the machine. In addition, the machine will run only when the operator holds both buttons down. This means that the operator has to spend time walking back and forth between the machine and the buttons, and that he/she is unable to stage the next part during the machine cycle. Using a safety light screen to guard the point of operation allows the operator to stage the next part at anytime during the machine cycle because he/she is no longer tied to the buttons. In most cases, the cycle time difference between PSDI and a safety light curtain with a cycle start button is well under a second.
|
|
Q: I did the separation distance calculation and it says I should mount the light screen 14.5 inches away from the pinch point. The frame of the machine only sticks out about 7 inches, so it's OK to put it there, right? |
|
FALSE: The separation distance calculation looks at the total time required by the safety system to completely halt the hazardous motion of the machinery and compares it to the size of object which can be reliably detected by the safety light screen. Based on that calculation, a distance is generated which is the minimum safe distance from the point of operation for the safety light screen to be positioned. If the safety light screen is placed closer than this distance there is a chance that I can move my hand through the screen into the dangerous motion faster than the machine can stop, creating a very real possibility of injury.
|
|
Q: I need to jog my machine during a process set-up. There's no problem jumpering the safety light screen out is there? |
|
FALSE: Any time the safety light screen system in intentionally disabled you have created an unsafe condition. In many cases, there may be an even greater potential for injury than normal. Over time, the operator will come to rely on the safety light screen protecting him/her, and while (when jumpered out) the screen is physically still present, it no longer has the ability to stop the dangerous motion.
|
|
Q: I need to guard two areas on one machine, it's OK to use two separate sets of emitters and receivers with one control box, right? |
|
TRUE: As long as you select the right control box. Banner offers Dual-DIN controllers for the MINI-SCREEN family. In addition, the MULTI-SCREEN family of controllers allows you to use two sets of sticks from either the Machine-Guard family and/or the Mini-Screen family.
|
|
Q: A safety light screen system with a fast response time will always be able to be mounted closer to the point of operation than a unit with a slower response time. |
|
FALSE: The issue of response time is not the sole determining factor in determining the separation distance in a safety light screen system. The other critical issue is referred to as MODS (or Minimum Object Detection Size). This is a rating of the smallest sized object which will be reliably sensed by a given safety light screen. {The test rods shipped with safety light screen systems represent this MODS.} In the final analysis a safety light screen system with a very fast response time, but a very large MODS, may need to be mounted much further from the point of operation than another system with a slow response time, but very small MODS.
|
|
Q: OK, got it, so besides response time I also need to look at beam spacing. |
|
FALSE: This is a very misunderstood area by many people, but beam spacing and MODS are not the same thing. Beam spacing is, well, the space between adjacent beams in a safety light screen system. MODS defines the smallest thing that system can see, and takes into account factors such as the diameter of the beams and excess gain (or optical power) available in the system. Often the MODS of a system is defined as an object large enough to block two consecutive beams. Thus, as the object moves through the screen, it will always block at least one beam.
|
|
Q: I've heard that using two safety switches on each door is safer than using a single switch, but if it is really a safety switch it doesn't really matter. |
|
FALSE: It does matter. Often, the statement will be made that a safety switch with two safety rated contacts gives you all the redundancy you need to be safe. One problem with this thinking is most easily seen in the case of a large door or gate on your machine. In the case of a large door, it is fairly easy to imagine a situation where one corner can be pried open (or partially open) while the opposite corner remains firmly shut. Potentially, this could create a point of entry to the dangerous motion that is unguarded. In the case of smaller doors or openings, two switches still represent a higher degree of reliability than a single switch with two safety rated contacts. The design and construction of a safety switch will use a single actuator and plunger arrangement. Thus, although the switch can provide redundancy in the contacts themselves, it is only through the use of multiple switches that complete redundancy can be achieved.
|
|
Q: My employees are well trained, they know better than to reach over or around a safety light screen. Besides it's cheaper to buy a short screen, and not mess around with mounting all those guards along side the screen. |
|
TRUE & FALSE: It is true that many employees are well trained in today's factory environment, but quality training is only part of the story. The problem may well not be someone thinking in terms of intentionally defeating the safety devices you install. A significant number of accidents occur when a machine operator reacts to a situation without stopping to think, in other words, because of a reflex action. A common example is seeing an item we’ve just set down begin to tip over. Our normal reflex action is to catch it before it falls. Very common in industry is the operator who gets into a flow with his/her machine. The sequence of feeding and removing parts becomes automatic as the machine cycles. It is when the normal flow is interrupted that even an experienced, trained operator is most likely to have a reflex action take over. In an instant of time an improperly sized light screen can be defeated and an injury can take place before the operator even realizes something has gone very wrong.
|
|
For answers to your machine safety questions, contact a Banner Applications Engineer:
e-mail: sensors@bannerengineering.com
on-line request form page
telephone: 1-800-809-7043 |
