(1) Tests for intrinsic safety will be conducted under the
general concepts of "intrinsically safe" as defined in Subpart A of this
part. Further tests or requirements may be added at any time if features of
construction or use or both indicate them to be necessary. Some tests
included in these requirements may be omitted on the basis of previous
(2) Intrinsically safe circuits and/or components will be
subjected to tests consisting of making and breaking the intrinsically safe
circuit under conditions judged to simulate the most hazardous probable
faults or malfunctions. Tests will be made in the most easily ignitable
mixture of methane or natural gas and air. The method of making and breaking
the circuit may be varied to meet a particular condition.
(3) Those components which affect intrinsic safety must
meet the following requirements:
(i) Current limiting components shall consist of two
equivalent devices each of which singly will provide intrinsic safety. They
shall not be operated at more than 50 percent of their ratings.
(ii) Components of reliable construction shall be used
and they shall be so mounted as to provide protection against shock and
vibration in normal use.
(iii) Semiconductors shall be amply sized. Rectifiers
and transistors shall be operated at not more than two-thirds of their rated
current and permissible peak inverse voltage. Zener diodes shall be operated
at not more than one-half of their rated current and shall short under
(iv) Electrolytic capacitors shall be operated at not
more than two-thirds of their rated voltage. They shall be designed to
withstand a test voltage of 1,500 volts.
(4) Intrinsically safe circuits shall be so designed that
after failure of a single component, and subsequent failures resulting from
this first failure, the circuit will remain intrinsically safe.
(5) The circuit will be considered as intrinsically safe
if in the course of testing no ignitions occur.
(b) Complete intrinsically safe equipment powered by low energy
(1) Short-circuit tests shall be conducted on batteries at
normal operating temperature. Tests may be made on batteries at elevated
temperature if such tests are deemed necessary.
(2) Resistance devices for limiting short-circuit current
shall be an integral part of the battery, or installed as close to the
battery terminal as practicable.
(3) Transistors of battery-operated equipment may be
subjected to thermal "run-away" tests to determine that they will not ignite
an explosive atmosphere.
(4) A minimum of 1,000 make-break sparks will be produced
in each test for direct current circuits with consideration given to reversed
(5) Tests on batteries shall include series and/or
parallel combinations of twice the normal battery complement, and the effect
of capacitance and inductance, added to that normally present in the
(6) No ignition shall occur when approximately 1/2-inch
of a single wire strand representative of the wire used in the equipment or
device is shorted across the intrinsically safe circuit.
(7) Consideration shall be given to insure against
accidental reversal of polarity.
(c) Line-powered equipment and devices:
(1) Line-powered equipment shall meet all applicable
provisions specified for battery-powered equipment.
(2) Nonintrinsically safe components supplying power for
intrinsically safe circuits shall be housed in explosion-proof enclosures and
be provided with energy limiting components in the enclosure.
(3) Wiring for nonintrinsically safe circuits shall not be
intermingled with wiring for intrinsically safe circuits.
(4) Transformers that supply power for intrinsically safe
circuits shall have the primary and secondary windings physically separated.
They shall be designed to withstand a test voltage of 1,500 volts when rated
125 volts or less and 2,500 volts when rated more than 125 volts.
(5) The line voltage shall be increased to 120 percent of
nominal rated voltage to cover power line voltage variations.
(6) In investigations of alternating current circuits a
minimum of 5,000 make-break sparks will be produced in each test.
(d) The design of intrinsically safe circuits shall preclude
extraneous voltages caused by insufficient isolation or inductive coupling.
The investigation shall determine the effect of ground faults where
(e) Identification markings: Circuits and components of intrinsically
safe equipment and devices shall be adequately identified by marking or
labeling. Battery-powered equipment shall be marked to indicate the
manufacturer, type designation, ratings, and size of batteries used.