In the event of primary power failure, the following critical circuits must
be automatically connected to the standby power supply and restored to normal
operation within ten seconds after interruption of the primary supply.
(1)
All life critical electric circuits.
(2)
Inside chamber lighting circuits.
(3)
Outside chamber lighting circuits near critical controls and
indicators.
(4)
Chamber atmosphere monitoring and control circuits.
(5)
Fire protection and alarm circuits.
(6) Communication circuits.
A sound-powered telephone should be
furnished as a backup.
Section 3.
WIRING
1.
WIRING OUTSIDE CHAMBERS. All wiring outside the chambers must be in
accordance with DM-4, Electrical Engineering (Reference (1)) and NFPA 70,
National Electrical Code (Reference (5)). Wire must be routed through cable
trays, raceways, or conduit of steel or aluminum.
2.
WIRING INSIDE CHAMBERS. Electric wiring inside hyperbaric chambers is a
potential ignition source which should not be present. When electric power
must be used in the chamber, single-phase AC or DC with potentials not
exceeding 120 volts may be used. Consideration should be given to the use of
low voltage DC systems (6, 12, 24, 48 volts).
a.
Intrinsically Safe Circuits. When electric circuits must be
designed for and installed in the chamber, they should be intrinsically safe
in that they are incapable of producing a spark or causing ignition.
The fundamental idea of intrinsic safety is to limit the energy which
circuits can draw from a power supply, even under fault conditions, to levels
which are incapable of causing ignition in a hazardous atmosphere. The bulk
of intrinsic safety research has been directed toward use in explosive
industrial atmospheres such as methane and hydrogen in air at one atmosphere.
However, the basic approaches should be applicable to hyperbaric conditions
once minimum ignition energies are determined. Recent references on
intrinsic safety include Redding, R.J., Intrinsic Safety (see Reference (15)
and Magison, E.C., Electrical Instruments in Hazardous Locations (see
Reference (16)).
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