Whether for safety, security, regulatory or other reasons, access control makes sure that people are, if not always in the right place, never in the wrong one. Fencing, locks, interlocks, walls, gates and guards are all examples of products that help ensure safety. Clearly, the optimal access control solution will differ between machine guarding and restrictions into a hazardous or toxic materials plant building, for example, but the premise is the same: only fully authorised, certified or trained personnel can operate the machinery or enter the area.
ISO14119: Safety of machinery - interlocking devices associated with guards, is a good starting point, according to Jason Reed, account manager at Pilz Automation Technology, an expert in this field. He says: “While the standard is not a legal [requirement], it shows that you are in line with publications like the Machinery Directive or the Supply of Machinery (Safety) Regulations. For our customers it’s about making sure they select the right type of interlock for the application, taking into account important factors such as time delay: when the switch connection is broken, will inertia keep the machine running, or do you have to wait for a reduction in temperature before gaining access, for example?”
He adds: “Another factor is motivation to defeat [the interlock]. Back in the old days, operators may have had a spare tongue actuator or even a screwdriver to ‘trick’ the machine into thinking the guard was closed. If there is a motivation to defeat, how can you overcome it?”
RFID SWITCHES
Pilz today offers solutions based on technologies such as RFID, which can be coded to different levels depending on factors such as the motivation to defeat. The two parts of the RFID switch are unique as a pair, so even if a machine user had a spare actuator it would not be a match for the interlock.
“All of these factors influence product selection and are discussed in the ISO14119 standard,” says Reed. “Whether you’re a machine builder or an end user looking to update or retrofit your system, the standard sets out the factors that influence product selection.”
Common interlocks are broken down into four different types in the standard: Type 1 - interlocking device with mechanically actuated position switch with un-coded actuator (such as hinged interlocks); Type 2 - interlocking device with mechanically actuated position switch with coded actuator (like tongue-actuated position switches); Type 3 - interlocking device with non-contact actuated position switch with un-coded actuator (such as proximity switches); and Type 4 - interlocking device with non-contact actuated position switch with coded actuator (like RFID tag- actuated position switches). At present, track key interlocks are a variant of Type 2, but they will have their own type in a new version of the standard currently at final comments stage.
SAFE CONTROLS
Other standards also come into play, such as ISO13849: Safety-related parts of control systems. Here, it is necessary to determine the performance level for the particular safety component, which will subsequently have an influence on interlock selection, not just the type but its architecture.
Explains Reed: “If there was an accident, is it likely to cause a minor or major injury? What frequency of access is required? And what is the possibility of avoidance? Depending on how you answer these questions will determine a performance level from A to E. The higher the performance level, the higher cost will likely be to implement the components and architecture required.”
With a traditional tongue-actuated interlock, there exists what is classed as the potential for a single-point mechanical failure on the actuator. As a result, the standard allows a fault exclusion claim if it is possible to over-engineer it mechanically. “In such a case, we can provide a different handle that is mechanically over-engineered so that the actuator will never fail: the handle will always fail before the actuator,” says Reed. “We can therefore claim a fault exclusion on that, allowing us to reduce the performance level from C to D.”
Another good reference standard is ISO14118: Safety of machinery, which covers unsafe start-up scenarios.
“For example, assume I have a standard RFID non-contact interlock on a gate. When I open the gate it would stop the machine, but if someone closes the gate behind me, the machine will start again, so we need to address that possibility,” explains Reed.
KEY TO SUCCESS
“In modern systems, you might even need to control who has access to the machine,” says Reed. “We have a technology where keys are unique to individuals, granting various authorised permissions for different machine types depending on training levels. You can restrict who can do what and when, and even introduce time-stamping that allows you to record and document each individual to ensure they are trained or re-certified every 12 months, for example. If not, the time stamp on the key will prevent access.”
So, do any statutory requirements exist for inspection and testing of existing controls?
“That all comes down to your performance levels, which also depend on your test requirements,” says Reed. “With modern safety controllers you can embed that into the real-time clock of the safety relay. You could actually carry out a function test on an interlock every week, for example. If the safety relay does not see completion of the test, it would not allow the machine to operate.”
BOX: Machine safety and access controls that work together
There are many ways to ensure the safety of personnel in manufacturing facilities, including the use of trapped key interlocks and access control locking modules. Trapped keys are a safety device used to limit who enters certain areas and how. They are based on the simple and practical premise of ‘a key cannot be in two places at once’.
“A term to discuss here is ‘sequential access control application’, also known as sequence of operation,” says Will Cole, automation specialist at Horizon Solutions, a US-based distributor of electrical, automation, industrial, energy and safety solutions. “This will indicate how people on the plant floor should de-energise the machine, how they use the keys, and then how they return the keys and restart the machine. This process is a central element of overall plant and personnel safety.”
Among the solutions available from Horizon is the Rockwell 440T family of rotary trapped key interlock switches (pictured above left) and the Rockwell 442G multifunctional access box (MAB; pictured above).
“Although both are individual safety devices in their own right, they can work in tandem to control how employees enter a hazardous area; you can deploy both on the same door in the same area,” says Cole.
Rockwell Automation’s ‘SAFETY-AT186A-EN-P’ publication from June 2021, available via www.is.gd/efoyaz, guides readers through how the 442G MAB locks and monitors access to the hazardous area.
“If the door is opened or unlocked, or a fault is detected in the safety function, I/O safety modules de-energise and monitor a redundant pair of 100S-C contactors,” says Cole. “Sample project files are included to help you implement this safety function.”