Loop Mode (2): Energized to Safe

Energized to safe mode works as contrary to that of de-energized to safe. In energized to safe mode, no electrical current flows through the instrument loop during plant normal condition. Loop will be energized during plant upset.

For illustration, see below picture of a simple unit consisting push button (PB) and solenoid valve controlling a deluge valve (XV). During plant normal condition (no fire) the push button contact is at open position so that no current flows within the loop. Similarly, the solenoid valve of deluge valve is not being energized by electrical power. This condition drives the deluge valve to close position, prevents the fire water from ring main passing through the deluge valve.


If there is a fire in deluge coverage area, someone would activate the push button. Once activated, the switch within push button changes state to close position and enables electrical current flows within the loop. This current will signal the control system and execute the predetermined action i.e. to open deluge valve by energizing the solenoid valve loop. Therefore, once the solenoid valve energized, it will make the deluge valve to open, allowing water flows through to extinguish fire.

This mode is also commonly applied for ESD push button for shutting down a plant.

The purpose of having energized to safe system in such system is to avoid misoperation of final element (activation of deluge valve on above example) due to instrument failure or unintended instrument cable disconnection.

Imagine if this system applies de-energized to safe mode instead. Even only one failure on solenoid valve cable or system side fault e.g. I/O card, will cause the deluge to open automatically. Not only will this cause the equipment including surrounding instruments to wet, but this will also be dangerous as the equipment within the area is still not electrically isolated.
Worse would happen in ESD push button, as it will cause a plant shutdown that results in production loss.

Further discussion of line monitoring in energized to safe mode will be in next post.

Additional Fact: Deluge valve is also activated by confirmed fire signal generated by flame detector, smoke detector or heat detector.

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Loop Mode (1): De-Energized to Safe

Electric based Instrument loop on plant operate in two modes: either energized to safe or de-energized to safe. De-energized to safe mode, also known as fail safe system means that during plant normal condition, there will be electrical current flows through the instrument loop. When trip/shutdown is required, the loop will be de-energized.

See illustration on below picture of a simple unit consisting Pressure Switch High High and SDV. During plant normal condition (pressure is lower than HiHi setting) the switch contact is closed and enables current flow within the loop. Likewise, the solenoid of SDV is energized by electrical power to allow air supply stroking the SDV and then forcing it in open position.

As long as the control system receiving current signal of pressure switch loop, plant is considered as normal. If the pressure increases and reaches HiHi setting, pressure switch will be open and there will be no current flows, hence alarming plant control system.The predetermined executive action shall then be taken to put system in safe condition i.e. de-energize the SDV’s solenoid to make the main valve close.

The reason de-energized to safe mode to be implemented in such system is to make it in safer state (in above case by closing the main valve) when either one of the following conditions occurs:
- Unintended instrument cable disconnection (either pressure switch or solenoid valve)
- Electrical black out

Energized to safe system will be discussed in other post (energized to safe system).

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