What is meant by control system is failsafe-enrgized?

 

This is when a relay is permanently energised, so if you encounter a fault, the relay will open as opposed to the opposite where a fault occurs and you can't energise the relay due to a broken wire of loss of power, etc.

 

Design of a device which needs to fail safely under abnornal conditions is called "Fail safe Design".

Shutdown application normally energised circuit will be used and on failure of power supply or any actuating medium (say air) the final control element will fail safely.

E.g.:
Normal condition: Emergency Shutdown valve (ESDV) - Open and will close on abnormal condition.

If the circuit is de-energised due to loss of power, then the solenoid valve which is energised under normal conditions will be de-energised and ESDV will close.

Also on loss of instrument air, ESDV will close.

Hope this clarifies.

Regards,
GB

 

It is worth mentioning that not every signal in a fail-safe system should have a normal state of "1". For example:

A valve "open" command from HMI to a fail-safe system should be a momentary "1". But the valve "close" command to a fail-safe system should be an unambigous monentary "1" also.

or

A motor "start" command from HMI to a fail-safe system should be a momentary "1". But the motor "stop" command to a fail-safe system should be an unambigous monentary "1" also.

Also, bypassing an shutdown condition to repair the transmitter or switch is abnormal. But the "bypass" command should be a maintained "1".

To summarize, the failsafe-energized principle applies mostly to hard inputs/outputs.

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Why can't the stop signal be an unambigous momentary zero?

Most of us probably do things the way you have described them, but I am curious why you think it is so critical to do it the way you have indicated.