An example of one approach to an intrinsically safe circuit is shown in
Figure 9-2. The DC power supply is equipped with:
A grounded screen between primary and secondary windings.
A current-limiting resistor in series with the load.
A voltage-limiting Zener diode across the load.
This circuit is much safer than an unprotected supply since steps have been
taken to maintain isolation and to limit the energy which can be drawn even
if the load is accidentally short circuited.
Explosion-Proof Circuits. Explosion-proof circuits of mineral
insulated wire or rigid metallic conduit and electric equipment sealed within
explosion-proof enclosures are acceptable but not as desirable as
intrinsically safe circuits. The idea of containing an explosion within a
sealed enclosure which will not fracture is good, but maintaining the
integrity of the seals in the wide range of pressures and pressure
differentials in the chamber is a big problem. Explosion-proofing wiring
practices are contained in NFPA 70, the National Electrical Code (Reference
(5)). Hyperbaric atmospheres are Class (1), Division (1), Group C
atmospheres as defined by the Code. Explosion proofing increases the bulk,
weight, and cost of the circuit components.
Termination. All wires to be routed into the chamber must
terminate on a terminal board before entering the chamber. All wires routed
within the chamber and carrying current greater than 50 ma shall be
hard-wired, soldered, or terminated on terminal boards. Wires carrying low
signal voltages may be installed in removable connectors of the screw type to
prevent accidental parting and arcing.
Raceway. All power conductors inside the chamber must be
installed in rigid metallic conduit. For research facilities where many
tests are conducted and a wide variety of leads for signals and
instrumentation are required, the designer should give consideration to the
installation of terminal boards with a variety of connectors. The tester or
experimenter can attach the leads to the terminal board for the transmission
of data to equipment outside the chamber. This eliminates the experimenter
having to locate a penetration wherever one is available, All connections
should be of the locking type to prevent accidental disconnection.
Separation of Power. All power cables, low-voltage cables, and
signal cables must enter the chamber through separate penetrations.
Cable Insulation. Insulation on all cables installed in chambers
must be nontoxic and nonflammable in oxygen-enriched environments. Tests
have shown that Teflon-coated wire and Kaptan polymide film over FEP
insulation displays these characteristics.