CD Automation srl
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14.3.4
Action of alarm output.
(also indicated as "Alarm Action")
When an alarm status is associated with the state of a physical output (ex. relay)
Quando allo stato di un allarme è associato lo stato di un uscita fisica (es. relè) it’s necessary to define the
relationship between the alarm state and relay state.
Two action are possibile:
1) Direct action: out ON when the alarm is ON (ex.: excited relay when the alarm is ON)
2) Riverse action: Out OFF when the alarm is ON (ex.: un-excited relay when the alarm is ON)
Direct action is the most commonly used but you must remember that the reverse action allows to have an
alarm signal when the instrument does not work or is off(because the ON state of alarm is indicated by the
OFF state of output, when the instrument is switched off or not working the output will be definitely OFF).
In the case of relay output , the status of Output may not be sufficient to describe the state of the contact. In
fact, when the output relay is provided with changeover contacts it is clear that the choice of the normally
closed contact (NC) or normally open (NO) allows for equal status of the relay to reverse the state of the
contact.
However remains the considerations valid regarding the possibility of having an alarm signal when the
instrument is switched off or not working (reverse action).
14.3.5
Resetting of an Alarm
The alarms we have seen are based on the concept that when the measure falls in the OFF area the alarm
also goes OFF
automatically.
In these cases it is said that the alarm is equipped with automatic reset.
In some cases it is preferable that the alarm still remains in the ON condition even after the measure is back
in the OFF zone, the alarm will return to the OFF condition only after a physical action (pressing a key or
other). In this case we say that the alarm has a manual reset.
The reason of this choice is due to the level of danger of the anomaly reported from the alarm, an
overcurrent can damage the system and cause fire or dangerous situations for the users
Forcing the user to perform an action also ensures that we take note of the report and eliminate the cause of
the problem before resetting the system.
Alarms with manual reset may have different behaviors depending on different situations but, generally, one
can identify two families of behavior:
1)
Alarm with unconditioned reset
Are those alarms that, when performed manually reset, set the alarm to OFF condition even if the alarm
condition is still present.
A typical example are the alarms that drive the sirens, once the user reaches the machinery switch off the
siren and then performs the necessary actions to remove the alarm condition. To get a further alarm signal is
necessary that the measure goes to the area OFF and back in the ON area.
