Providing Safety
USP in the Constructeur – Unique Safety Products

Unique Safety Products in the Constructeur

Unique Safety Products is featured in the latest issue of the trade magazine the Constructeur (November 2021). De Constructeur is the magazine for mechanical engineers and designers. The article discusses, for example, the protection of hazardous areas with Trapped Key Interlocks.

USP in the Constructeur

Safeguarding dangerous zones

constructeur cover

What is a Trapped Key Interlock system? How do you increase the safety level of an existing machine with a Trapped Key system? And what is entrapment and how can you prevent it? These are some of the questions that will be discussed in greater detail in this article. Also discover more about the latest developments in machine safety, such as RFID key tags. And find out which different industries we visit and advise. We do not only visit the machine industry, but also zoos and amusement parks.

Are you interested in USP’s original (Dutch) article in the Constructeur about safeguarding hazardous areas? You can find the article on page 14 of the latest issue of the Constructeur.

Safeguarding hazardous areas with trapped key interlocks

Safety switches and mechanical interlocking systems are often associated with access control to dangerous machineries such as presses, mixers and robots. However, the range of applications is much wider, such as access to maintenance areas of train tracks or roller coasters. And what about enclosures at the zoo? Rob Voordendag of Unique Safety Products enjoys solving various safety problems.

“Every time we come across different industries and processes, we look for an appropriate safety solution,” says Rob Voordendag. ‘Providing Safety’ is what we do,” adds colleague Robert van de Ven. “Our mission is that our customers’ employees can work in a safe environment and return home in one piece. We solve machine safety issues for customers through advice and products.”

“We increasingly do so in cooperation with safety expert, Nick de With of Fusacon. It is enormously inspiring to solve the various complicated safety issues for customers, where product knowledge, industry leading practices and legislation and regulations come together.” Voordendag: “Together with De With, we worked out and implemented the machine safety aspects for maintenance and inspection on the moving parts of the lift bridge at the new Botlek Bridge, for example.”

Trapped key interlocks
According to the Machinery Directive, dangerous machinery and processes must be safely shielded from the user. But how do you do this according to the law and regulations, and what is possible? Are you going to protect an existing machine or is the protection for a new machine? And how much do you spend? Moreover, there are various options for security. In any case, carrying out a risk assessment is essential to determine the level and type of protection required. Voordendag explains that the trapped key interlock system in particular is increasingly being used to secure dangerous areas or to simply increase the safety level of older installations. The supplier Fortress Interlocks started with this type of system 50 years ago in the chemical and petrochemical industry. This eventually evolved into the manufacturing industry.

A trapped key interlock (key transfer system) consists of at least two parts; a mechanical door interlock and a safety key switch that ensure that guards/access doors or hatches remain closed until the machine is stopped. The transmission of coded keys from the energy isolating section to the access point obliges operators to follow a predetermined operating sequence. The fact that the door safety devices are fully mechanical saves on installation costs and wiring.

In addition, a trapped key system is more resistant to harsh industrial conditions (such as vibration, humidity and dust) and is less likely to fail than an electrical solution such as a traditional wired safety switch.

Robert van de Ven “Our mission is that our customers’ employees can work in a safe environment and come home in one piece.”

How does it work in practice? You switch off the control power of an installation with a key switch equipped with safety contacts, then you unlock an encrypted trapped key that gives you access to a zone/door behind which a potentially dangerous machine is located. It is a coercive system, in fact it is the same as a safety switch; when you open a door the machine behind it is already stopped. And if that machine has an exit time, you have to make a request before you can enter. With a trapped key system, it works no differently, only you first switch off at a place near the control. You get the encrypted trapped key free, then you can use that key to open a door lock to the danger zone. As long as that door remains open, this key remains locked and the machine cannot be started. You can expand it to include multiple power sources, multiple access doors, which allows you to build a complete forced-key system. To maintain the integrity and safety of a key system, the use of unique key codes is essential. The trapped key systems of Fortress Interlocks have their own developed and patented key system with more than 200,000 unique key codes. Despite this enormous amount of key codes, they are also stored at the end-user level to prevent duplicates from being issued.

The advantage of the trapped key interlock system is that it can be used fairly simply to safeguard an existing machine (retrofit) or to increase the safety level. Often, these machines have outdated PLCs or only have safety relays in them. If you are going to protect that by putting a safety switch on each door, you also need safe I/Os on all doors to wire back to your PLC. Often a new safety PLC is needed to meet all the safety requirements. Everything must be redundant. This is a costly affair. With a trapped key system (disconnection by means of a safety key switch) this is much simpler. It can often also be connected to the existing control system. There is only one place where you need to check the electrical system: you switch off the main current or the control current (with a safety relay). Then one (or more) key(s) is/are released mechanically to unlock one or more access points. In this way, PLe or SIL 3 – the highest level of security – can be achieved by a mainly mechanical control system in which only two safety contacts are required from the control system.

Rob Voordendag: “In fact, the more automated the system, the easier it is to protect.

Entrapment hazard
“Because of an accident with a palletizer, we received a request from a manufacturer a while back. They were looking for a permanent solution to safeguard the robot cell,” says Voordendag. It was about a safety device that would prevent the robot from starting up as long as there is someone inside the cell. You can have a safety switch on the access door so that you can only enter when the robot is really stationary, but this does not prevent a colleague behind you from closing the door and starting up the robot again. Especially if there is no detection that someone is inside. You can install an emergency release on the inside so that you can always get out again. But that is no use if you are trapped or injured by the robot, it is just treating the symptoms.

Sensors everywhere
There are actually three options for solving this common problem. The first is to install safe sensors everywhere that detect movement. This can be done, for example, with safe laser scanners (particularly suitable for clean environments) or radar scanners (work well in industrial environments but have limited control). The machine can then only be started if the sensors do not detect anyone. This becomes difficult and costly when dealing with a complex environment with multiple machines and all existing fixed objects and corners. Where do you have to put all the sensors to detect every point in the zone? The safety level is up to SIL 2/PLd. However, a palletizer quickly falls into the highest class of necessary protection (SIL3/PLe) and then this solution is not sufficient. Then you end up with the trapped key system which gives the highest protection.

With integrated safety key
A second option is to equip the entrance doors with a safety switch with an extracted key. This safety key has to be extracted. You make a request on the door, the robot stands still, then you have to turn the key and take it out otherwise you can’t open the door. You take the key inside. The door can never be locked as long as the operator has the key with him and the robot cannot be restarted. Voordendag: “We call this system key-in-pocket or personal safety. By carrying the key with you, you guarantee your own safety. The only disadvantage is that if someone enters after the person with the key, the second person will not be detected. The first person could go out and start the system again. Solution is a second key, but you can’t have the key taken out twice and up to how many people do you go? This can be covered by including in your procedure that the second person hangs a security padlock on the lock mechanism so that the key cannot be entered. The key-in-pocket system is widely used for confined spaces where full body access is required during maintenance or cleaning.”

Retrofit Interlock blocking device
The third option is when you already have a security switch on the door and you want to add the functionality of a mandatory removable key. It is a fully mechanical system and can be applied to any door type. It ensures that you cannot open the door unless you take the key with you. The door cannot be closed (and thus the safety contacts are not switched) behind you as long as you have taken out the key.

RFID key
The latest development in the key area are the RFID key tags that work in combination with ProfiNet-safety or CIP-safety controlled safety switch. These are a kind of access passes with a unique code. Each employee gets a unique code on his RFID key that is stored in the database of the supplied controller. With the RFID key, the employee can open doors. Authorised people can access certain rooms and not others. In this way, it is possible to check via the PLC and the database whether people are present in those rooms and when everyone has logged off so that the machine can be switched on again. It is also possible to gain more insight into the productivity, how long someone has been inside and what he did to solve the problem. For example, the cleaning team can then be given access at a different time.

The Zoo
“We don’t only come into the machine industry,” says Voordendag. “But for example also in zoos. With a trapped key system on the entrance doors of animal enclosures, you ensure that employees are forced to close cages behind them. Then you know for sure that the enclosures are closed and the animals cannot escape through the doors. We also work in amusement parks. For example, during maintenance on a roller coaster. (Then you don’t want carts to be able to drive through the technical area during maintenance. Or in case of the metro or train, certain sections of the track have to be shielded during maintenance. This ensures that maintenance staff can work safely and that no metro can pass by.

Manual operations
“Safeguarding manual operations alongside dangerous machinery can sometimes be very difficult,” says Voordendag. “Customers want to comply with laws and regulations but they also need to be able to continue production. The regulations leave little room for doing this safely without having to shut everything down every time, which would delay production. For example, the manual import of products. Or at slaughterhouses, for example, the boning of a carcass. This is done by hand but on a moving belt and then it goes into a chopping machine right next to it. Separation between man and machine is more difficult the more interaction there is between man and machine. The more difficult it is to comply with laws and regulations. You work in a process that is in progress. You can put up a light screen but when you go through it, the machine stops and so does the work. “You can think up very complex systems with radar sensors that detect whether people are present in certain dangerous places. But if it’s not just about crossing a line but also depends on the movements the machine makes, you have to determine when and which movements pose a risk. Try programming that. Whatever you come up with, the systems are not watertight. Authorised access is another possibility: then only people who have been sufficiently trained are allowed in. In fact, the more automated the system, the easier it is to secure, as in the production of cars, for example. That happens almost autonomously, without any intervention by employees.

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