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Re: OT : ebuyer UPS Recall

--- In ukha_d@xxxxxxx, "Mark A. Ryder" <mark@m...> wrote:
> I found a design-flaw in this UPS....the hard way.
> My understanding is that there is a flaw in the design of the
> that switches in when mains is restored in these UPS's.   Not
> whether it is the reason for the recall though.
> When power is cut and restored within a couple of seconds, the
> component seems to apply inverter power and mains power to the
> output.   I have both the 650 and 1000 model.  (It was the 1000
> that fried)
> Luckily for me, the fault occurred when I was standing next to
> rather than the units tucked away in the loft.    I had just
> installed a new rack for some new network data storage in the
> and the UPS was installed to provide clean power and cover any
> of mains power.    I switched-off the wrong plug by mistake (the
> supplying the UPS) and switched it back on within a couple of
> seconds.    As soon as power was switched back in, the UPS
> sizzling, crackling and emitting a rather disturbing plume of
> smoke.   The quickest thing I could do was pull the mains lead
> the back of the unit, but it was still frying.  Nothing cut-out,
> fuses or anything!     In desperation I pressed the button on the
> front of the UPS and it still worked and shut the thing off.
> It made me seriously consider the wisdom of trusting these units
> a remote/hidden install in the loft space.
> Mark
> Mark Andrew Ryder

A UPS is one of the few devices that when unplugged from the mains
can still output mains voltages.  Even if switched off at the
control panel as well as disconnected from the mains, they should be
still regarded as live.  You need to disconnect the batteries as
well as unplug the UPS; particularly if shipping a faulty unit back
to the supplier.  You don't want the parcel catching fire!  ;-)

When the is a power cut, the UPS draws current from the batteries.
The battery current can be as much as 20 times the current being
drawn by the load from the UPS.  E.G. if your load draws 0.5 amps at
240Vac and the UPS has a single 12V battery then must draw 10 amps
at 12V to supply the load.  In practice it will be a little more
than this because nothing is 100% efficient.
Sometimes the inverter switching circuitry fails and a bank of FETs
(transistors) are short circuit across the batteries.  As a lead
acid battery can supply hundreds of amps, then the FETs burn or
explode.  On small UPSs you get burning, etc.  On big UPSs it is
more akin to a box of fireworks going off!  ;-)

In a week or twos time I will be selling a job lot of small UPSs.
I'll advertise them on here as a bulk purchase.


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