Fault masking is a phenomenon in which an unrecognized fault originated as a result of specific circumstances prevent the optimum functioning of the machine. Usually it occurs when a number of interlocking guards attached in a series connected to one circuit in a safety relay. Any fault due to a sort circuit in one of the pair of an interlock due to moisture or weld affect the guard witches, resulting in the opening of guard that is further detected by safety relay. The safety relay in turn shuts the associated faulty area of the machine and lock out because it registered the fault in the guard. However, the safety relay only sees the one of the channel open but not the other locking out the machine. While operator may close the guard the fault does not go away as it is registered in the safety relay and prevents any reset or restart. The operator may try to associate the faulty with successive guards by opening and closing the guards ignoring the fact that fault has been registered in the safety relay system. This undetected fault further reduces the functioning of the machine and any more faults can prove dangerous to the functioning of the machine.
This type of safety wiring is in use from a long time as it saves money that is spent on cabling and safety relays however it is the part of withdrawn standard EN 954-1 (EN 954-1 degraded category 4 to category 3) Category 3 still makes a part of standard EN ISO 13849-1 in which 6.2.6 clause conditioned the application of category 3 to specific requirements that is a single fault must not initiate the loss of complete safety function, or series of undetected faults must not lead to loss of safety function and or at least 60 percent of faults must be detected in diagnosis mechanism (DC= LOW). This ability to detect 60 per cent of dangerous faults can be impacted by fault masking decreasing the performance level.
Overcoming fault masking
The three available options as complying with the ISO 14119:2013 in an industrial setting are:
- Through Individual wiring
The fault masking can be prevented through individual wiring such that the interlocked guard switches should not attached in series or if in specific circumstances attached, their number should be limited whenever these use volt free contact technology. Other methods include switching the wires individually and to separate safety relays or introduction of individual inputs on safety controllers or accumulating small groups of wiring together.
- Using Self-monitoring interlocking devices
These devices are used when the switches are connected in series and with the help of self-monitoring transistorized outputs OSSDs can detect the faults within themselves.
- Safe distributed I/O systems
Using the devices that can distinguish between all the inputs is the working principle of the Safe Distributed I/O systems. The devices such as interlocking, including other i.e. light curtains, emergency stops, and two hand control are distributed across machine in a failsafe network. These are distributed throughout the machinery to test for faults.
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