Odd power PDU problem

  • Thread starter Thread starter stevergfrancis
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stevergfrancis

QN: do I have a bad PDU?

Symptoms:
3 servers, each connecting to 2 PDU's via dual power supplies.
Each PDU was showing 6.3 A load. (15 A max.)

When moving all 3 outlets off PDU so it could be physically moved, PDU
B tripped its overload breaker.

The nameplate load of the 3 servers is > 15A, but the combination of 2
x 6.3 (measured Amps) is not.

So, it seems either:
- PDU B is bad
- the measurements of 6.3 A on each circuit is incorrect
- there is a power draw spike when servers switch from 2 power supplies
to 1.

Does anyone know if the latter case is true?
Or have other theories?
Thanks
 
QN: do I have a bad PDU?

Symptoms:
3 servers, each connecting to 2 PDU's via dual power supplies.
Each PDU was showing 6.3 A load. (15 A max.)

When moving all 3 outlets off PDU so it could be physically moved, PDU
B tripped its overload breaker.

The nameplate load of the 3 servers is > 15A, but the combination of 2
x 6.3 (measured Amps) is not.

So, it seems either:
- PDU B is bad
- the measurements of 6.3 A on each circuit is incorrect
- there is a power draw spike when servers switch from 2 power supplies
to 1.

Does anyone know if the latter case is true?
Or have other theories?
Thanks
Click to expand...

There are some pictures here, of what the current consumption of
a computer power supply looks like. They show an oscilloscope
trace for PFC (power factor corrected) and non-PFC supplies. I know
my clamp-on ammeter has a lot of trouble giving a correct readout
on my non-PFC ATX supplies, for a computer in sleep, and it could
be the ammeter on the PDU is not measuring what you think it is.

http://www.xbitlabs.com/articles/other/display/psu-methodology_8.html

Power factor is why there is a difference between the "watts" and
"VA" rating of equipment. The first article that came up in a
search for ("power factor", watts, VA) was this.

http://ecom.mysylvania.com/miniapps/lightingcenter/PDFs/faq0002-0297.pdf

It could be the PDU display shows "watts", and the breaker is
seeing (2x6.3)/0.7 = 17.8A ?

Paul
 
Paul said:
(e-mail address removed) wrote:
.... snip ...

There are some pictures here, of what the current consumption of
a computer power supply looks like. They show an oscilloscope
trace for PFC (power factor corrected) and non-PFC supplies. I know
my clamp-on ammeter has a lot of trouble giving a correct readout
on my non-PFC ATX supplies, for a computer in sleep, and it could
be the ammeter on the PDU is not measuring what you think it is.

http://www.xbitlabs.com/articles/other/display/psu-methodology_8.html

Power factor is why there is a difference between the "watts" and
"VA" rating of equipment. The first article that came up in a
search for ("power factor", watts, VA) was this.

http://ecom.mysylvania.com/miniapps/lightingcenter/PDFs/faq0002-0297.pdf

It could be the PDU display shows "watts", and the breaker is
seeing (2x6.3)/0.7 = 17.8A ?
Click to expand...

There is no such thing as power factor for DC, which is what the
output of a power supply is. You can talk about it on the input
(mains) circuit. Power factors involve the phasing, which in turn
requires AC. Thus, on the DC side, watts = V * A.

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