Difference between Fire & Gas System and ESD System ?
Difference is that an ESD is typically designed as normally energized (de-energize-to-trip) so that it is fail-safe. This way, if there is loss of power or connectivity between system components then the SIS will respond by tripping. This results in higher safety integrity, but it can result in increased spurious trips of the process.
For FGS, a spurious trip can have dangerous results. For example, initiating a water deluge system inside a building can cause damage to equipment and can be hazardous to personnel. Chemical fire suppression can be dangerous to personnel, and false alarms degrade the willingness to respond by plant personnel. For this reason, it is common to design a FGS as normally de-energized (NDE).
In a NDE design, the loop must be energized in order to initiate a trip of the FGS. This means that failures such as loss of power or connectivity between components are covert failures unless there is adequate diagnostics to detect the failures. In a NDE design, line monitoring is essential to detect open and short circuit failures in wiring between logic solver I/O and field devices.
Reference - emersonautomationexperts