FAQ The alarm threshold of IM400 is set to 1Kohm, why the alarm dropout at more than 1.2-1.3Kohm?

[ChenLiang]

With IM10/20 range and IM400 range an hysteresis principle is apply with threshold alarm and prealarm levels

the principle is :

alarm is activated when default is at the threshold value; alarm is desactivated when default is 20% higher than threshold value. This is to avoid on/off alarm switching when a default is closed to the threshold

The same principle exists with old range XM/XML

[Madleen_Laclef]

An application note was just written explaining this hysteresis principle.

The purpose of this characteristic of the product is to ensure that if the insulation level fluctuates around the alarm threshold, the customer does not get many flickering alarms.

Notice that the competition IMDs (Bender...) use the same principle.