Somewhat OT - can underpowered power supplies damage a system?

[Mikkel]

On 08/18/2010 04:18 PM, James Mckenzie wrote:
> suvayu ali <fatkasuvayu+linux at gmail.com> wrote:
>> Sent: Aug 18, 2010 1:31 PM
>> To: James Mckenzie <jjmckenzie51 at earthlink.net>, Community support for Fedora users <users at lists.fedoraproject.org>
>> Subject: Re: Somewhat OT - can underpowered power supplies damage a system?
>>
>> On 18 August 2010 09:08, James Mckenzie <jjmckenzie51 at earthlink.net> wrote:
>>> Maybe a brand name's power supply would have not shorted out, but then the mobo, drives and everything else
>>> would have received a 6000 volt shock. Â And yes, the UPS was brand name (highly rated BTW) but the short duration
>>> was so short that the system did not shut down.
>>>
>>
>> I think you are concluding incorrectly. A well made PSU ideally would
>> have blown the fuse, and saved both your PSU and your hardware. You
>> would only need to replace the PSU fuse to get back up and running. At
>> least that is my understanding.
>>
It depends on the level of over-voltage. When the fuse blows up,
instead of just melting the link inside, it indicates that that it
was subjected to a voltage well over its rating. The vaporised fuse
link also helped provide an arc path across where the fuse used to
be. Keep in mind that fuses protect from over current, but only
provide indirect protection from over voltage.

> What I'm describing is a Primary (6000/7200/13800) to Secondary
(220) transformer short. This means that the 220 lines for a short
duration was carrying the primary voltage. Yes, the fuse should have
blown, but in my case the fuse blew such velocity, the wire was
splattered on the glass casing. The internal transformer shorted out
due to the amount of current carried for that moment. Replacing the
fuse resulted in another splatter and what looked like an arc over.
In most countries there are load limiters placed on the secondary
side that prevent primary voltage from reaching your home (ever
wonder what those things on the power pole were.) Where I was living
at the time did not have these and a surge on the primary side
caused the short. Was interesting to see a ground mounted
transformer to this. This can also happen to an oil cooled
transformer if it overheats and the coolant is released (usually,
the secondary side is also disconnected when this happens.)
> 
> However, a really good UPS/PSU combination with an EXCELLENT
ground will drain off the overvoltage and the PSU will never see the
overvoltage. PSU's with blockers will also capture any overvoltage
and drain it through the earth ground.
> 
> This is why data centers use different UPS configurations than
> you have at home and industrial standards require less than .5 ohm
> (in some cases way less) to earth ground. There is an entire industry
build around this. Most homes have 'open' grounds and the people in
them are not aware. I've got to fix the ground in the house I live
in as the power strip I use is complaining of one.
> 
This is why you normally have a grounding grid under the data
center. A grounding grid is a series of ground rods with the
connecting cables making a grid connecting all the rods. The spacing
and length of the rods depend on the conductivity of the soil.
Getting 24' of ground rod to go straight down can be interesting -
they sometimes end up forming a U, depending on how many rocks you
hit. (8' sections coupled together as you drive each 8' section.)

Normally, all structural steal is also connected to the grounding
grid, as well as the electrical system. You want the entire building
to be at the same potential.

Now, when it comes to resident grounding, it gets interesting. The
testing to determine how many rods are needed, and the type of rod
you need, is usually inadequate. It is interesting when you check
out a grounding problem and find that the top foot or so of the
ground rods are missing, along with all the connecting wire. You
then discover the the jumper around the water meter is missing, and
some of the black iron pipe that used to run between the water meter
and the connection point for the electrical ground has been replaced
by PVC. They should never have relaxed the rule that required
grounding at the water meter.

Mikkel
-- 

  Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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