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Rack PDU Circuit Breaker Operation and Expectations - Outlet LED and messaging/alarms

Issue

Why do the outlet LEDs remain on after one or more circuit breakers trip on a Switched Rack PDU?

Why don't any managed Rack PDUs display a message or alarm when one or more circuit breakers trip?


Product Line
  • Rack Power Distribution (PDU)
    • AP8XXX series Rack PDU
    • AP78XX Metered Rack PDU
    • AP79XX Switched Rack PDU

Environment
  • All serial numbers
  • Switched Rack PDU models with circuit breakers 
  • All managed Rack PDU models with circuit breakers

Cause

Switched Rack PDU (and Switching with Metering By Outlet) models are the only model type that have an LED on each PDU outlet to indicate if the outlet is on or off according to the on board Network Management Card configuration. The relays that "switch" the outlets only switch one side of the outlets. The LED indicates that the coil of relay is energized. There are no circuits to measure the output voltage of the circuit breakers. Thus. when a circuit breaker trips, the coil of the particular outlet relay can still be energized.

Managed Rack PDUs in general do not have the capability to monitor the output voltage of the circuit breaker. Therefore, the user does not receive an alarm when a circuit breaker trips. The closest option intended to assist with monitoring for tripped breakers or a lower than normal load is the low load threshold. The user can configure threshold so that if a low on a bank or entire PDU goes below a certain limit, such as in the event an on board breaker trips and all attached equipment to the bank or PDU drops (and the PDU still has input power), the low load threshold will be triggered and notify the user via the configured notification methods.


Resolution

Outlet LED behavior on Switched PDU models is explained above and is working as designed. 

The user can consider configuring low load thresholds on all managed PDU models in an effort to understand when load current goes below a particular threshold, like in the case of an on board breaker trip. Please note, for very small loads around 1 amp, low load thresholds may not be helpful based on the metering accuracy documented in knowledge base ID FA156074.

Further detail on low load threshold benefits:
Note: The Rack PDU generates an alarm when any bank exceeds its rated value. However, if a circuit breaker trips, there is no definitive indication that the circuit breaker is open, other than that the current for that bank will drop. Set the Low Load Warning to 1 amp for these reasons:
  • The default setting for the Low Load Warning is 0 amps. This effectively disables the warning. With a setting of 0 amps for the Low Load Warning, the web interface will not indicate that a circuit breaker may have tripped.
  • A 1 amp detection threshold for the Low Load Warning for Bank Load Management will help to indicate that a circuit breaker may have tripped.
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