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What happens when a single power suppy fails in a N+N configuration?

chuckk281
Trusted Contributor

What happens when a single power suppy fails in a N+N configuration?

Andrew wanted some explanation of the power supply redundancy:

 

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I think I know the answer - but I have not been able to confirm....

 

 

Have a customer with the following:

    2 x 3 phase power circuits - to the rack

    Each 3 phase power circuit is connected to a 3 phase PDU within the rack.

    The 3 phase PDU provide single phase 208volt power to the devices within the rack.

 

    The C7000 has a single phase power module - providing 6 x C19/C20 power inputs.

    There are two C7000 within the rack.

 

The C7000 is configured as follows:

    AC Redundant - "N+N".

    This provides a "bank A" (PSU 1 2 3) and a "bank B" (PSU 4 5 6) power supplies.

 

    6 identical power supplies in the C7000

 

    Connected left 3 power supplies to one of the two PDUs.

 

    Connected right 3 power supplies to the other PDUs.

 

    Dynamic power is disabled to ensure even power distribution between phases.

 

 

 

The question the customer has is what happens when a single power supplies fails.

 

Is 100% of the power drawn from the power supply bank with all good power supplies?

Bank A power supplies 1, 2 and 3 are good - 100% of load.

Bank B has a failed power supply - no load from any power supply.

 

Is the power evenly distributed to the remaining 2 good power supplies in the bank?

Bank A power supplies 1, 2 and 3 are good - 50% of load - split over 3 power supplies.

Bank B has a failed power supply - 50% load on remaining two power supplies.

 

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Monty had the answers:

 

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With enclosure Dynamic Power Savings disabled, all the power supplies installed share the enclosure load.

 

c7000 has a shared power design – all active power supplies ALWAYS share the enclosure load – independent of the OA Power Mode configuration.

 

We talk about A and B “sides” – but that only applies when the OA Power Mode is set to AC Redundant – and does not change to how power is shared.

The OA Power Mode only changes how the OA calculates Power Capacity and Power Available.

 

The question the customer has is what happens when a single power supplies fails.

All remaining power supplies share the enclosure load.

 

Is 100% of the power drawn from the power supply bank with all good power supplies?

               No, the load is distributed to all remaining power supplies.

 

Bank A power supplies 1, 2 and 3 are good - 100% of load.

               No, Power supplies 1, 2 and 3 handle 60% (3/5) of the load

 

Bank B has a failed power supply - no load from any power supply.

               No, Power supplies 4 and 5 handle 40% (2/5) of the load load for the scenario with a single power supply failure on the other “side”

 

Is the power evenly distributed to the remaining 2 good power supplies in the bank?

               No, all remaining power supplies share the enclosure load.

 

Bank A power supplies 1, 2 and 3 are good - 50% of load - split over 3 power supplies.

               No, Power supplies 1, 2 and 3 handle 60% (3/5) of the load for the scenario with a single power supply failure on the other “side”

 

Bank B has a failed power supply - 50% load on remaining two power supplies.

               No, Power supplies 4 and 5 handle 40% (2/5) of the load if power supply 6 fails.

 

 

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That was what Andrew needed. Other input?

1 REPLY
chuckk281
Trusted Contributor

Re: What happens when a single power suppy fails in a N+N configuration?

More info from Monty:

 

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When enclosure Dynamic Power Savings is enabled, pairs of power supplies (one from each ‘side’) are placed in standby to improve the power efficiency of the enclosure by shifting the enclosure load to all the active supplies.

 

The power supplies that are in standby do NOT share the enclosure load.  So in the examples below, placing a pair of power supplies across the two sides in standby would just result in shifting the enclosure power load to all remaining active power supplies.

 

My statement below is still accurate – “all active power supplies ALWAYS share the enclosure load”. 

 

The c-Class power supplies are designed to be most efficient near 50% utilization – to support the most popular AC Redundant input use case.

 

The c7000 OA design includes a dedicated enclosure power micro which makes real-time decisions to enable/disable pairs of power supplies based on all the power sensors to optimize enclosure power and ensure that rapid changes in power demand and any power supply or power input failure do not impact delivering power to the enclosure.

 

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