# Alarm Trip Delay

The Alarm Trip Delay is the length of time a channel's reading must meet the channel's “Alarm Violation Criteria,” before the channel goes into UNACKNOWLEDGED ALARM State and begins dialing.

- The default value is 2 seconds for all input channels.
- Between the time from the onset of a violation until the trip delay expires, if a channel's reading is announced during a report, the extra Tag Word Alert will be appended to the channel status message.
- If the channel value ceases meeting the Alarm Violation Criteria before the Alarm Trip Delay expires, no alarm will occur.

Note: The finite time required to complete each scan of the network effects how quickly each channel's reading will be updated. Therefore, unless the alarm trip delay is much longer than the scan time, the effective delay will be substantially affected by the scan time.

For example, suppose the scan time is 100 seconds and the Alarm Trip Delay is set to 50 seconds. If a violation persists for just ten seconds at the address location (SNA) which the channel is monitoring, there is a 90% chance that it will not be detected during scanning. If by chance it is detected, then the channel value will reflect the violation for the next 100 seconds, and the alarm will be tripped despite the condition having persisted for much less than the Alarm Trip Delay setting.

- In practical terms, this means that it is important to arrange for the PLC to maintain violation values for at least as long as the networks scan time in order to assure detection.
- It also means that the Alarm Trip Delay setting will not be the primary determinant of how long a violation must persist at the SNA before it trips an alarm unless the Alarm Trip Delay setting is substantially longer than the scan time.

For more information on the "granularity" of readings imposed by scan time, see Scan Time and Channel Latency.

There are two ways to change the Alarm Trip Delay for channels:

1. Common Alarm Trip Delay -- Common for all channels (does not include power failure or communications failure alarms).
2. Alarm Trip Delay -- Configuration for each individual channel.

Note: Changing the Common Alarm Trip Delay setting overrides any previously set individual Channel Violation Trip Delays. If you wish to establish a different default trip delay, as well as different individual channel trip delays, configure the common trip delay first, then configure the differing individual channel trip delays.

If you are getting a lot of nuisance alarms, with a call saying, "ALARM NOW NORMAL," you may want to consider increasing the trip delay. A good example of this would be the power fail trip delay. In some areas, it is very common to have short periods of power failure. These may not be of particular concern, so setting the power failure trip delay to a larger value could save unnecessary phone calls.

In addition to Alarm Trip Delay and Power Failure Alarm Trip Delay, another important alarm delay setting is the Communications Alarm Delay which determines how long an interval is allowed between scans of the Network before a Communications Failure Alarm is tripped.