Principles, Standards and Implementation

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Protective Measures and Complementary Equipment

Non-Contact Interlock Switches

Some of the latest functional safety standards focus on the need for complete fault tolerance as part of the requirements for device that is being used for high risk levels (e.g. SIL 3 or PLe). Because, in theory, mechanically actuated switches have single points of failure (e.g. the tongue actuator) even though they have two electrical switching channels. This means that dual channel non-contact switches may be preferred in these cases because they do not generally have the single mechanical failure points.

For non-contact interlocks, no physical contact (under normal conditions) takes place between the switch and actuator. Therefore positive mode operation cannot be used as the way of ensuring the switching action, and we need to use other methods to achieve equivalent performance.

Redundancy

Just as described in the section on tongue interlock switches, a high level of safety can be provided by non-contact devices designed with component duplication (or redundancy). In case of a failure of one component there is another one ready to perform the safety function and also a monitoring function to detect that first failure. In some cases it can be an advantage to design devices with components that have the same function but different failure mechanisms. This is referred to as diverse redundancy. A typical example is the use of one normally open contact and one normally closed contact.

Oriented Failure Mode

With simple devices we can use components with an oriented failure mode as explained in ISO 12100-2. This means using components in which the predominant failure mode is known in advance and always the same. The device is designed so that anything likely to cause a failure will also cause the device to switch off.

An example of a device using this technique is a magnetically actuated non-contact interlock switch. The contacts are connected with an internal non-reset overcurrent protection device. Any overcurrent situation in the circuit being switched will result in an open circuit at the protection device that is designed to operate at a current well below that which could endanger the safety-related contacts.

Due to the use of special components, the safety-critical fault likely to occur would be a welding of the reed contacts due to excessive current being applied to the switch as illustrated in Figure 68. This is prevented by the non-reset overcurrent protection device. There is a large margin of safety between the rating of this device and the reed contacts. Because it is non-reset, the switch should be protected by a suitably rated external fuse. The Allen-Bradley Guardmaster Ferrogard interlocks use this technique.

Figure 68: Simple Magnetic Operated Noncontact Interlock

Non-contact devices are designed with smooth enclosures and are fully sealed, making them ideal for food and beverage applications as they have no dirt traps and can be pressure cleaned. They are extremely easy to apply and have a considerable operating tolerance so they can accept some guard wear or distortion and still function properly.

One important consideration when applying non-contact switches is their sensing range and tolerance to misalignment. Each product family has an operating curve showing sensing range and tolerance to misalignment, as shown in Figure 69.

Figure 69: Non-Contact Operating Curve

Another important consideration for applying non-contact switches is the direction of approach of the actuator, as shown in Figure 70. The coding techniques determine which approaches are acceptable.

Figure 70: Approach of Actuator Affects Performance

Defeatability—Non-Contact Interlock Switches

It is important that the switch is only operated by its intended actuator. This means that ordinary proximity devices which sense ferrous metal are not appropriate. The switch should be operated by an "active" actuator.

When protection against defeatability by simple tools (a screwdriver, pliers, wire, coin, or a single magnet) is deemed necessary by the risk assessment, the noncoded actuation types must be installed so that they cannot be accessed while the guard is open. An example of this is shown in Figure 71. They should also be installed where they are not subjected to extraneous interference by magnetic/electric fields.

Figure 71: Sliding Guard Protects Access to Sensor

A high security against defeat can be achieved by using a coded actuator and sensor. For magnetically actuated and coded devices the actuator incorporates multiple magnets arranged to create multiple specific magnetic fields. The sensor has multiple reed switches specifically arranged to operate only with the specific magnetic fields of the actuator. Unique coding is generally not feasible using magnetic coding techniques. Unique coding, where an individual actuator is “tuned” to an individual sensor.

The reed switches used with magnetically coded switches are often small. To avoid the risk of welded contacts some switches use one normally open contact and one normally closed contact as outputs. This is based on the premise that you cannot weld an open contact. The logic device or control unit must be compatible with the N.C. + N.O. circuit arrangement and must also provide overcurrent protection. The Allen-Bradley Guardmaster Sipha interlocks use the coded magnetic technique.

RFID Non-Contact Interlock Switches

Non-contact interlock switches based on RFID (Radio Frequency Identification) technology can provide a very high level of security against defeat by “simple” tools. This technology can also be used to provide devices with unique coding for applications where security is paramount.

The use of RFID technique has many other important advantages. It is suitable for use with high integrity circuit architectures such as Category 4 or SIL 3.

It can be incorporated into devices with fully sealed IP69K enclosures manufactured from plastic or stainless steel.

When RFID technology is used for coding, and inductive technology for sensing, a large sensing range and tolerance to misalignment can be achieved, typically 15…25 mm. This means that these devices can provide very stable and reliable service combined with high levels of integrity and security over a wide range of industrial safety applications.

The Allen-Bradley Guardmaster SensaGuard interlocks use the RFID technique.