It is a common practice to lockout and/or tagout control devices. In a nuclear power plant’s control room with its many analog controls, you might (for example) see red covers over specific switches. or you might see a lockable handle on a control panel twisted to the off position and it takes an operator key to twist it back on.
The purpose of doing this is informational rather than functional. It tells the control room operators at a glance that the controls for System X are not available, and therefore, someone is working on System X. Of course in any power plant, work on System X is already known to the shift operator or whoever is in charge.
You can think of these control device locks or tags being similar to an “Out of Service” sign for an elevator. The elevator maintenance technician isn’t relying on people to keep themselves from pushing the call button while he’s in the elevator shaft. He’s got that whole system locked down, and he’s just letting people know that elevator isn’t available for use.
Signs can easily be removed, even inadvertently. Locks for controls aren’t designed for lockout/tagout, and barriers to defeating them are correspondingly low. You can’t accidentally bump the lock on a circuit breaker and inadvertently cause it to fall off unnoticed.
In a plant with a modern touchscreen control system, you can’t hang these physical locks or tags on virtual controls. A system like this may have a notification means that serve a similar purpose. For example, the graphic for Caustic Tank B will be rendered in red, and the control logic will gray out the corresponding onscreen controls. As with the physical locks or tags, this doesn’t actually lockout the equipment. These locks and/or tags are not isolating devices in the lockout/tagout process because they don’t remove any source(s) of energy.
NFPA 70E requires that locks and tags are installed only on circuit disconnecting means [120.2(E)]. That’s because only circuit disconnecting means can remove electrical energy from the circuit.
NFPA 70E also prohibits using control devices such as pushbuttons or selector switches as “the primary isolating device”. In many plants, the supervising operator will not permit controls to be locked or indicated out of service until having verified that lockout/tagout is complete.
Regardless of the policies at whatever facility you are working, always communicate with the operators so everyone understands what’s going on. But don’t rely on that communication to serve as protection in lieu of lockout/tagout.
Controls don’t remove the source of energy from the circuit; they just direct where it should go (and thus shock hazard is still present if only the controls are locked). The purpose of a lockout/tagout is to remove sources of energy. And it may be energy on which controls have no effect. For example, locking out the controls of a hydraulic press will have no effect on whether the ram can drop onto you. To prevent that, you must remove the kinetic energy by lowering the ram to the lowest practical position and/or use “for the purpose” blocks to isolate any remaining kinetic energy.
