Page 115 - Uninterruptible Power Supplies

Additional Information Relating to the Standby Supply Installation
Additional Information Relating to the Standby Supply Installation 113

of the materials of which they are constructed, in general the heavier
the material the greater the reduction.
Knowing the sound pressure level within the room, the sound
reduction index of the materials used in its construction, and the
acoustic properties of any louvers or other sound attenuating items,
it is possible to calculate the sound pressure level at defined positions
outside of the room. The assessment of the resulting sound pressure
levels at remote locations takes into account the distance (the inverse
square law applies), and for distances greater than a few hundred
meters, molecular absorption adds to the attenuation. There is likely
to be additional attenuation due to screens and barriers such as walls,
but it is safer to regard this as a bonus rather than take it into
account.

Local Authority Requirements
In the United Kingdom the local authority will have its own rules for
noise limitation, and in mixed residential and industrial areas these
will have been devised to minimize complaints from local residents.
Standby generating plant runs only occasionally and is less likely to
cause complaints than continuously running plant. A method for rating
industrial noise in mixed areas is described in British Standard 4142.
The method is to compare the new noise level at specific sites with the
existing background noise level over a 24-h period, and three guide-
lines are given:

1. If the new source is 10 dB above the existing noise, complaints are
likely.
2. If the new source is 5 dB above the existing noise, it is of marginal
significance.
3. If the new source is more than 10 dB below the existing noise, com-
plaints are unlikely.

The British standard also includes the warning that noise assess-
ment is a skilled operation and should be undertaken only by persons
who are competent in the procedures.

Safe Working Procedures
Access to the generating set should be restricted to personnel who are
authorized to work on or near it. If the set is located within an engine
room, the room should be locked unless access has been authorized and
a key issued to allow maintenance, repair, or cleaning. If the set is

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