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Custom Alarms

A custom alarm defines its condition as a JavaScript expression. The alarm is ON while the expression returns a truthy value and OFF when it returns a falsy one.

Analog and digital alarms watch a single variable. A custom alarm is for when the condition depends on several variables, or on a calculation.

The fields every alarm carries (name, groupId, delay, part, isActive, …) are listed on Alarm Management.

The custom alarm form — the condition field

Writing the condition

Section titled “Writing the condition”
// Alarm when power and temperature are high together
var power = ins.getVariableValue("ActivePower_kW").value;
var temp  = ins.getVariableValue("Temperature_C").value;
return power > 500 && temp > 70;

The expression runs on the group’s scanTimeInMillis period, so it should not do heavy work — it is re-evaluated on every scan.

When to use one

Section titled “When to use one”
SituationWhy
The condition depends on more than two variablesA digital alarm combines at most two
The comparison is between two variablestank1 > tank2 rather than a fixed limit
A derived quantity is being watchedEfficiency, differential pressure, a ratio
The condition mixes a threshold with logic“Temperature high and pump running”

If the condition is just one variable against a fixed threshold, prefer an analog alarm: the thresholds are visible as fields, deadband and deviation come ready-made, and the condition is evaluated without running a script every scan.

Likewise, combining two boolean signals with AND/OR, or inverting one, needs no custom alarm — a digital alarm offers both as fields.

Things to watch

Section titled “Things to watch”

For writing expressions and the methods available: Expressions · Alarm API

Section titled “Related”