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New Member

Why two power suplies from a 6509 would fail

The customer had 2 power suplies failures at the same time. In the log the message was the following:

%C6KPWR-4-PSMISMATCH : power supplies rated outputs do not match.

We needed to replace both power suplies, but the customer just cannot accept that two power suplies can have a problem simultaneously.

Does someone can help about the customer question?


Re: Why two power suplies from a 6509 would fail


The explanation of the message is:" This message indicates that the two power supplies in the system have different output ratings". "For more efficient load sharing, use power supplies with the same output rating."

Look at

I hope this helps.

Best regards.


New Member

Re: Why two power suplies from a 6509 would fail

I found this information in the manuals, but as matter of the fact, the two power suplies were damaged. The customer just cannot accept that a system that is supposed to have a high availability may have a failure, and would like a better explanation why the 2 power suplies had a problem simoultenously.


Re: Why two power suplies from a 6509 would fail

There are many reasons why powersupplys will get offline at the same time.

Since we have no other information than what you have written here (wich basically is none) I will treat it as a general question.

Why powersupplys in general can break down.

1) Poor incoming current.

The current incoming is not even and can have pretty serious spikes in it.

equipment connected to the same powersource will experience the same power inconcistencies. fx if someone is starting a bigger electrical engine of some sort, fx an elevator. Then that will cause a dip and then a spike. Brownouts and blackouts are genereal faliure scenes since what happens is that many equipments try to start att the same time, and that causes a big spike wich can break powersupplies.

To counter this, one uses UPS or a powerfilter.

2) Manifacturing errors.

If the 2 powersupply's are made at the same factory at the same time it can have the same faults due to design or manufactoring details

FX a shipment of condensatorsr is bought but they have a flaw in them. or the design of the powersupply is erroneous.

then the same flaw will be in both powersupplies.

To counter this you want different batches when it comes to critical equipment that backs eachuther up.

3) share load.

If you have two powersupplies that are online at say 40% load each and one breaks down the other has to take over the load that the first one have taken care of, and that strain can couse a cathastrophic faliure and the unit breaks both powersupplies due to the suden increase of strain.

To counter this one can use several powersupplies so the strain is lesser on the remaining ones if one fails.

4) share overload

If you have 2 powersupplies wich both are at 60% then if one breaks down there will be 120% taken out of the one that still works, and that will most likely break both of them or cause the unit itself to fail due to low input.

To counter this, check that you have enough powersuplies to loose one or two.

5) Power outage

This doesnt realy break the powersupplies, but will caouse the unit to fail. it could be due to fires aor justmechanical damages

To counter this one uses 2 different powersources. with 2 different non interconnecting ways from the powersource to the power user.

This are just some of the more common faliures.

good luck

New Member

Re: Why two power suplies from a 6509 would fail

Hello Hobbe,

Reading your message, I would exclude the reasons 3, 4 and 5. If it is the reason 2, Cisco should be involved to tell us, if there is any problem reported. The reason 1 would be up to the customer, in this case I'm going to recomend him to analyse their environment.

You say that you need more information to analyse this problem, what information would be necessary?