There are processes or equipment that are often very difficult to start up, either initially or after a shutdown if they are tripped on “low” conditions.
To avoid this difficulty, some interlocks have bypasses that will avoid the low trip contact until the equipment or unit is running, and then clears itself so that the trip action is now in place, i.e. when an abnormal low condition arises, the system will trip.
This is often used on compressor start-up, when low speed will trip the unit. Sometimes, a time-delay action is used, where a pre-determined time is permitted to allow the process to obtain its operating level. These bypass can also be called as Start-up Bypass Interlock.
Bypasses may also be necessary in order to test components on-line without tripping protected equipment. However, indiscriminate bypass of an interlock system will compromise the protection function of the system.
A strict protocol should be followed when bypassing of interlock is attempted. Usually only authorized personnel are permitted to bypass any interlock. This can be achieved either using a key or manually operated switch. When a bypass is initiated, an alarm will be triggered.