Dual-powered data centers provide concurrent maintainability and high availability when coupled with dual-powered devices. However, proper distribution planning is still required to achieve the ultimate goal of zero downtime.
In a dual system, power is typically supplied to the cabinet utilizing a whip, a flexible conduit with an attached outlet on one end that is hardwired into a power distribution unit (PDU) or breaker panel.
These whips are typically sized in increments of 10 amps such as 20 amp, 30 amp, etc. In the data center industry, the standard allowable load on a whip is eighty percent (80%) of the breaker rating. For example, a 20 amp A-B whip pair would be limited to 16 amps.
Setting up A-B Power to Prevent Breaker Trip on Failover
A server cabinet contains eight servers, each with dual power supplies consuming 2 amps per server at full running load. If a 120 volt, 20 amp A-B power whip pair is delivered to the cabinet the load will be distributed as follows:
Power Circuit A – 8 amps (with both A-B circuits active)
Power Circuit B – 8 amps (with both A-B circuits active)
Total power draw for the A and B circuits is 16 amps. If power circuit B fails or must be de-energized for maintenance, the A power circuit in all eight servers will be required to deliver twice the power to the server – for a total input load of 16 amps on a 20 amp power circuit.
Remember…8 servers, each server needs 2 amps and only one power supply is now energized per server. This is within the 80% breaker rating design criteria, so this example is within the design specification.
When a breaker is tripped during failover, it’s usually caused by doubling the number of servers without accounting for power load. The 20 amp A circuit is loaded to 16 amps and the B circuit is also loaded to 16 amps.
If power circuit B fails or must be de-energized for maintenance, the A power circuit in all 16 servers will be required to deliver twice the power. A total input load of 32 amps on a 20 amp power circuit. In this scenario, the breaker will trip and the servers will go down.
Preventing breaker trip during failover requires understanding your specific power load and accounting for the balance in your dual power design.
What stories can you share where this has happened to you or a colleague?