In event of a malfunction, all manufacturing processes require some means of advising the operating personnel that the plant, or the process or both are moving toward a condition where either the product specification, product quality, or safety of plant or personnel could be affected. Notice that in the foregoing statement we have said “moving toward”; this is very important. The reason for raising an alarm prior to reaching an unwanted state is to enable safe corrective or evasive action to be taken in order to avoid the aftermath of not being prepared, or of learning about a malfunction when it is already too late to correct it.