Interlock, interlocking defeat

Log Count Rate High or Interlock Defeated" alarm occurs at 10 cpm, or if Interlock switch is placed in "Defeat" position Cannot withdraw control elements when fission counter is moving. 

A defeat mechanism to bypass the door interlock may also be necessary for the purpose of testing. 

One day the inevitable happened. Having defeated the interlock, an operator opened a drain valve in error instead of a transfer valve. 

A control system designed to automatically change the operating conditions such that the COP is no longer outside the Never-Exceed-Limit (N-E-L). The system may shutdown the operation or it may change conditions such that operation continues in a different mode. The Safety Interlock System must be independent from the normal control system to avoid "common mode" failure where one failure defeats both the normal control system and the Safety Interlock System. 

A safety interlock control function must be separate from the BPCS. Its function is not on-spec product but the prevention of a catastrophic event that would result in human injury or death or damage to equipment. Safety interlocks are usually hardwired to make it difficult to bypass or defeat them. This is done because there have been past occurrences of a unit engineer or a process technician jumping to the conclusion that an alarm was faulty and there was no problem. They tried to go around the interlock to shut off the alarm or prevent process interference. Safety interlocks must not be bypassed without written approval. 

An audible or visible warning device is needed if a required interlock is bypassed or defeated. The warning system should be fail safe, i.e., if it became inoperative the unit should be inoperable. 

A switch used on guard doors that is used to detect if the door is opened while the machine is running, and uses a coded actuator to prevent intentional defeat. Safety interlock switches use positive opening contacts, which ensure that the closed switching contact is forced open when the guard is opened, without reliance upon spring action. 

Never operate any system with interlocks that have been defeated or bypassed... 

The residual weakness of the proposed more detailed method is the non-inclusion of the duration of access, and consideration of the probability of defeat or misuse. Above all the graphical method should have been developed to cover a range of safeguarding applications not just interlocked guards. 

Each protective device should have an individual door over the front, or common door for dual-mounted units, equipped with one or more voidable interlocks that prevent the door from being opened when the switches are in the on position, unless the interlock is purposely defeated by activation of the defeater mechanisms. All breakers or fusible switches should be removable from the front of the MCC without disturbing adjacent units. The MCC should have space or provision for future units. 

Modifications in the reactor protection system to enable defeating interlocks or overriding protective trips... 

As a general principle, safety-related control systems such as interlocks should be well designed, be of simple construction, and be sufficiently robust for the application. Furthermore, it should not be possible easily to defeat interlocking devices, yet where appropriate it should be possible for authorised persons to override them for tests or other necessary purposes. 

In general terms, interlocks must not be easily defeatable. Figure 13.5 shows a limit switch used in the interlocked cover of a test jig in which a component to be tested is first inserted in the jig and becomes automatically connected to the power supply via the test contacts. Closing the lid allows the limit switch to complete the control circuit, which energises the coil of the contactor leading to the contactor s contacts closing and the test contacts... 

A SRO has defeated the LCRM control drive withdrawal interlock. 

Log Count Rate alarm at 10E06 cpm or if interlock switch is placed in defeat position... 

Except for certain R D uses, these lasers are typically embedded in the equipment and have interlocked cabinets that enclose the beam path and the point where the beam contacts the target. This makes the lasers Class 1 during normal operations, but they are considered Class 3b or 4 lasers when the safety interlocks are defeated. The most common operation where it is necessary to remove the beam enclosures and defeat the interlocks is during alignment of the laser beam. 

The functional safety plays a role only in the inherent safety of the machine functions and in the interlocking of the enclosure. With the third step a tolerable risk can be achieved, if the instructions for the intended application are followed by the user. Otherwise, only step 1 provides a risk reduction and an orange of even red risk range may put the operator in jeopardy, e.g. when defeating safeguards. 

Figure 15.2 Electrophoresis apparatus. Improper use of this equipment or attempts to defeat the electrical interlocks has resulted in fatal shocks.

---