Page 235 - Uninterruptible Power Supplies

Notes on Systems Installation
Notes on Systems Installation 233

Under certain adverse conditions, such filters will exacerbate the wave-
form problem.
The static UPS system may be classed as a comparatively high
impedance power source and thus adequate sub-subject protection
should be allowed for, usually in the form of high speed fuses or circuit
breakers. The many designs of rotary systems may have up to 14 to 15
times FLC to allow for subcircuit protection, so the problem is some-
what eased.
The autonomy period of the battery will have to be considered. As a
guide most systems have 10-, 15-, or 30-min autonomy but there are
some notable exceptions. Oil/chemical production, power generation,
communications, and defense systems may have very long autonomy
periods even up to 12 to 24 h, but these are the exceptions. The choice
of 10 to 15 min autonomy has been made from the following considera-
tions: It was considered feasible to shut down the computer in an
orderly fashion if the prime mover failed to provide power. Since it will
take at least 20 to 30 s for the detection of prime mover failure, is there
sufficient time to react to such a condition and then perform an orderly
shut down?
The alternative of employing wet cells should be envisaged and the
following points should be taken into account. Stands, usually of steel
construction these days, should be provided with a heavy plastic
sheathing or a high-quality paint to inhibit chemical attack. The bat-
tery isolator should be immediately adjacent to the battery stand as
cable from the isolator to the actual battery is clearly unprotected.
Battery terminals are to be insulated usually with a close-fitting plas-
tic cap. If cells are to be operated at relatively high ambient tempera-
tures say above 30°C then the specific gravity of the electrolyte in the
cells should be reduced (refer to the manufacturer).
Clearly, such cells will evolve gas when charging, in particular hydro-
gen, and a 4 percent admixture of hydrogen with air is an explosive con-
dition. Most advice is to ensure that the maximum hydrogen content in
the environment is no greater than 1 percent. The production of hydro-
gen is basically a function of plate area, and charging current.
A simple formula to calculate gas emission is as follows:

Number of cells charging current 0.0004194 cubic meters.
Knowing the overall volume of the room, it is then easy to calculate the
number of air changes per hour to ensure a low concentration of hydro-
gen gas. Such ventilation should preferably be by fans employing
flameproof motors. The room should use flooring capable of withstand-
ing acids, walls should be painted in a corrosion-proof light-colored
paint, and lighting fixtures should be vapor proof.

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