Why you do not want nuisance alarms.  Alarm blocking provides the installer and user the ability to avoid nuisance alarms when a particular piece of machinery is running or stopped.  An example is when a hydraulic pump is switched off, and the hydraulic oil pressure drops.  It becomes a nuisance alarm to receive a low hydraulic oil pressure alarm each time, the operator switches the pump off.  An alarm unit  or the entire alarm system must provide some form of feature to disable/enable these alarms through a separate input, that being an analog or switched input.
Continuing on the hydraulic pump example, the pressure can be measured by a low pressure switch or a pressure sensor input the alarm unit or system.  So when the pressure drops below a defined level the alarm monitor will alert user to the faulty condition.  However when the operator enters the control room, and turns the knob to switch the pump on, the alarm monitoring can be activated this way, and again when it is switched off the alarm monitoring is deactivated.  Another example is a fuel flow switch on a diesel engine, that can be used to block for low oil pressure alarm.
The next part of the installation is a small delay in activation of the alarm by either an external OFF timer or as a part of the alarm system.  Then the operator turns the pump on, it takes the motor 5 or 10 seconds before normal pressure has been reached.  This time period needs to be accounted for too so the alarm is not activated as soon as the pump is switched on.
The best practice is not to block alarms that are unrelated.  I have seen when all pump alarms are blocked, and not just low pressure.  There is no need to block high hydraulic oil temperature or low hydraulic oil tank levels.
If you find this useful, please let us know, by contacting us on info@selcousa.com
Anders Jakobsen
SELCO USA, INC.