[Gllug] UPS & line filtering

[Chris Bell]

On Fri 10 Feb, Alain Williams wrote:
> 
> On Fri, Feb 10, 2006 at 11:57:11AM +0000, t.clarke wrote:
> > I am no electrical expert,  but as far as I am aware all 'commercial' UPSs
> > include protection against spikes and bown-outs as well as straight-forward
> > loss of supply.  Home UPS design I guess will be somewhat less rugged, given
> > the cheap price.   However, I would also guess that severe spikes are much less
> > common in a domestic situation than in a commercial one, since the spikes
> > (I presume) are generated by large inductive loads being switched on/off nearby.
> 
> That is the point - they protect against brown_outs/power_loss, but not all
> protect against spikes. I read an article on the subject some time ago that
> convinced me that it is not that clear cut and that what a UPS manufacturer
> advertises is often not quite true or is overkill (== costs more than needed).
> 
> That is why I asked the list - looking for real knowledge/experience.
> 
   Most spike filters comprise a common-mode filter, with both live and
neutral wound together on a transformer/choke core, plus shunt capacitors
and voltage dependant resistors. The voltage dependant resistors are
designed to short out voltage spikes. Computer equipment power supplies
normally include both types of filter at their input, and can cope with most
normal spikes.
   Computer power supplies are usually switch mode, and the input current
depends on the input voltage. If the mains voltage drops, the input current
increases so that the input power remains constant for a given load. If the
voltage drops for a relatively long time there is a chance that the
increased current might blow the input fuse.
   There is a different problem if the incoming mains supply comes from a
generator. If the generator slows slightly, mains frequency transformers
start to look like dead shorts and the fuse can blow.
   If you decide to install a UPS, it is normal to specify a UPS with a
greater-than-expected power rating. The battery will slowly deteriorate, and
will require replacement at intervals, and a higher specification will allow
a longer backup time. When the mains supply does fail, some UPS units
maintain approximately the same voltage as normal mains, others only supply
sufficient voltage for a switch-mode computer PSU which can compensate for
the reduced drive, perhaps rather less than 200 volts instead of 240.
   My protection against lightning strikes is a heavy earth cable direct to
an earth spike from my TV aerial pole (a lightning conductor reduces the
chance of a direct lightning strike by providing a continuous discharge
current), plus additional common-mode filtering on mains, telephone, and TV
downlead cables. Commercial units are available with gas discharge
over-voltage protection connected to an earthing terminal on the case.

-- 
Chris Bell

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