This article explains the meaning of the alarm " PEM Invalid AC Configuration " which may arise on the chassis and ways to mitigate the issue.

The below alarm will be set on the device:

user@host> show chassis alarms no-forwarding

2 alarms currently active
Alarm time               Class  Description
2023-08-11 23:53:47 UTC  Minor  PEM Invalid AC Configuration

Error in Syslog:

Aug 12 00:00:43.765 2023  host alarmd[13278]: Alarm set: Pwr supply id=318767205, color=RED, class=CHASSIS, reason=PEM Invalid AC Configuration
Aug 12 00:00:43.765 2023  host craftd[8324]:  Major alarm set, PEM Invalid AC Configuration

user@host> show chassis power

PEM 0:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 58 W (zone 0, 1 A at 58 V, 2% of capacity)
 

PEM 1:
State: Empty
Input: Absent
 

PEM 2:
State: Online
AC input: OK (1 feed expected, 1 feed connected)
Capacity: 2050 W (maximum 2050 W)
DC output: 114 W (zone 0, 2 A at 57 V, 5% of capacity)
 

PEM 3:
State: Empty
Input: Absent
 

System:
Zone 0:
Capacity: 4100 W (maximum 4100 W)
Allocated power: 480 W (3620 W remaining)
Actual usage: 172 W
Total system capacity: 4100 W (maximum 4100 W)
Total remaining power: 3620 W

user@host> show chassis hardware


Hardware inventory:
Item Version Part number Serial number Description
Chassis AAABBBCCCDDD MX480

PEM 0 Rev 01 740-063046 AAABBBCCCDDD PS 1.4-2.52kW; 90-264V AC in
PEM 2 Rev 01 740-063046 AAABBBCCCDDD PS 1.4-2.52kW; 90-264V AC in. >>> only 2 PEM found

AC Power Supply Configurations:
 

The MX high-capacity and normal-capacity power supplies each support either of the following AC power configurations:
 

• In the low-line (110 V) AC power configuration, the MX240 router contains either two AC power supplies (nonredundant), located horizontally at the rear of the chassis in slots PEM0 and PEM1 (left to right); or four AC power supplies (redundant), located in slots PEM0 through PEM3 (left to right). The low-line configuration requires two power supplies, and the third and fourth power supplies provide redundancy. Each AC power supply provides power to all components in the router. When two power supplies are present, they share power almost equally within a fully populated system. If one power supply in a redundant configuration fails or is removed, the remaining power supplies assume the entire electrical load without interruption. Two power supplies provide the maximum configuration with full power for as long as the router is operational.
   

• In the high-line (220 V) AC power configuration, the MX240 router contains one or two AC power supplies, located horizontally at the rear of the chassis in slots PEM0 and PEM2 (left to right). The high-line configuration requires one power supply, with the second power supply providing redundancy. Each AC power supply provides power to all components in the router. When two power supplies are present, they share power almost equally within a fully populated system. If one power supply fails or is removed, the remaining power supply assumes the entire electrical load without interruption. One power supply can provide the maximum configuration with full power for as long as the router is operational.
   

Resolution:
 

• MX480 has 2 zones (0 and 1) and there should be one PEM in each zone (MAX 2 supported) for router to be functional.
• In this case we have inserted both the PEM's in zone 0 which is triggering the alarm.
• When two AC power supplies are installed, insert one power supply into an odd-numbered slot and the other power supply into an even-numbered slot."
• Please try to move each PEM to respective zones and check if the alarm is cleared 

NOTE: PEM slot  0  and 2,  1 and 3 are redundant to each other. 

If the issue doesn’t  get resolved after the above steps, please contact your technical support representative to investigate the issue further  JTAC Support