Steps to reproduce:

1. Create a host and apply  Mellanox by SNMP template 
2. Remove power cable from a Mellanox PSU 

Result:
No problems triggered
Expected:
A problem is triggered

Cause:

The item prototype in PSU discovery refers to an OID which does not change its value when a power loss occurs. It seems always reporting the value: enabled (2).

• Item prototype name, key and oid:
  sensor.psu.statusentStateOper.{#SNMPINDEX}  
  {{1.3.6.1.2.1.131.1.1.1.3.{#SNMPINDEX}
  }}
• Trigger prototype expression:
  count(/Mellanox SNMP/sensor.psu.statusentStateOper.{#SNMPINDEX},#1,"eq","{$PSU.STATUS.CRIT}")=1

Proposed solution

As Mellanox support team suggested, use entStateAlarm oid:

• Item key and oid:
  sensor.psu.statusentStateAlarm.{#SNMPINDEX}
  {{1.3.6.1.2.1.131.1.1.1.5.{#SNMPINDEX}
  }}
• Trigger prototype:
  last(/Mellanox SNMP/sensor.psu.statusentStateAlarm.{#SNMPINDEX})<>0

Additional note

In our environment we kept both items and triggers, to be safe.