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Legrand PDU User Guide 515
In the threshold settings, the Deassertion Hysteresis field determines a new level to trigger the "deassertion"
action.
This function is similar to a thermostat, which instructs the air conditioner to turn on the cooling system when
the temperature exceeds a pre-determined level. "Deassertion Hysteresis" instructs the Legrand PDU to
de-assert the worse state for a sensor only when that sensor's reading reaches the pre-determined
"deassertion" level.
For upper thresholds, this "deassertion" level is a decrease against each threshold. For lower thresholds, this
level is an increase to each threshold. The absolute value of the decrease/increase is exactly the hysteresis
value.
For example, if Deassertion Hysteresis = 2, then the deassertion level of each threshold is either "+2" or "-2"
as illustrated below.
Threshold value
Deassertion value
Upper Critical = 33
Deassertion level = 31
33 - 2 = 31
Upper Warning = 25
Deassertion level = 23
25 - 2 = 23
Lower Critical = 10
Deassertion level = 12
10 + 2 = 12
Lower Warning = 18
Deassertion level = 20
18 + 2 = 20
To use each threshold as the "deassertion" level instead of determining a new level, set the Deassertion
Hysteresis to 0 (zero).
Note: The difference between Upper Warning and Lower Warning must be at least "two times" of the
deassertion value.
How "Deassertion Hysteresis" is helpful:
If you have created an event rule that instructs the Legrand PDU to send notifications for deassertion events,
setting the "Deassertion Hysteresis" is helpful for eliminating a number of notifications that you may receive
in case a sensor's readings fluctuate around a certain threshold.
Deassertion Hysteresis Example for Temperature Sensors
Assumption:
Upper Warning threshold is enabled.
Upper Warning = 20 (degrees Celsius)
Deassertion Hysteresis = 3 (degrees Celsius)
"Deassertion" level = 20-3 = 17 (degrees Celsius)
When the Legrand PDU detects that a temperature sensor's reading drops below 20 degrees Celsius,
moving from the "above upper warning" range to the "normal" range, either of the following may occur:
If the temperature falls between 20 and 17 degrees Celsius, the Legrand PDU does NOT perform the
"deassertion" action.
If the temperature drops to 17 degrees Celsius or lower, the Legrand PDU performs the "deassertion"
action to announce the end of the "above upper warning" state.
