Start up bypass is facilities provided for selected inputs, which are unhealthy prior to process plant start up. The example of inputs which requires start-up bypass is Low-Low process variable that designed to trip the system such as Pressure Low-Low, Level Low-Low of transmitter reading or switch actuation
During activation of start-up bypass of any inputs, only shutdown trip which is inhibited. Input status shall remain indicate instrument reading. Start-up bypass is activated manually by means of protected key-switch from the control room and shall have clear indication in the operator station that start-up bypass is being activated.
Start-up bypass is de-activated automatically once the input is confirmed healthy (input has passed low-low trip setting) or after pre-determined time or manually by removing the key-switch.
Start-up bypass term is often interchangeable with start-up inhibit