MIL-DTL-15109D
3.2 General requirements. Resistors and rheostats shall conform to MIL-E-917 and NEMA ICS 1 as specified
herein.
3.2.1 Tolerance. The total resistance of resistors and rheostats shall not differ from the design value by more than
10 percent. A variation of 20 percent will be permitted from the design values of any step or section.
3.3 Characteristics. Resistors and rheostats shall have the following characteristics, as specified (see 6.2).
3.3.1 Voltage. Resistors and rheostats shall be designed for a maximum circuit voltage of 150, 300 or 600 volts
alternating current (ac) root mean square (rms) or direct current (dc) (see 6.2).
3.3.2 Mounting. Resistor assemblies and rheostats shall be constructed for bulkhead, deck, or switchboard
mounting, as specified (see 6.2). Individual resistor units shall be furnished with mounting brackets, mounting bolts
for single-end mounting, or without mounting hardware, as specified (see 6.2).
3.3.3 Duty. Unless otherwise specified (see 6.2), resistors and rheostats shall be constructed for continuous duty.
If other duty, in the case of resistor applications, is required, NEMA ICS 1 standards for the specified duty shall apply
(see 6.2).
3.3.4 Ambient temperature. Unless otherwise specified (see 6.2), resistors and rheostats shall be constructed for
a 50 degrees Celsius ( C) ambient. Where the normal temperature of the surrounding atmosphere exceeds 50 C,
the rating shall be based on the specified ambient (see 6.2).
3.3.5 Enclosures. Enclosures shall be open or dripproof, as specified (see 6.2). Enclosures shall be furnished for
resistors only when specified (see 6.2). If not specified, dripproof enclosures shall be furnished for bulkhead mounted
rheostats and open enclosures for switchboard mounted rheostats. Dripproof enclosures shall meet requirements of
NEMA ICS 1 when inclined to any angle not greater than 45 degrees from the normal position and conform to the
requirements of 3.5.4.
3.3.6 Temperature rise. Temperature rises shall not exceed the values specified in NEMA ICS 1.
3.4 Construction.
3.4.1 Internal connections. Connections that are embedded in resistors and rheostats shall be soldered, welded,
or brazed. Low melting solders that would be adversely affected by the operating temperature of the resistor or
rheostat shall not be used. Connections shall be mechanically secure and electrically continuous. External
connections to the resistors or rheostat shall be so that the heat from the resistor will not adversely affect the
connection.
3.4.2 Rheostats. Rheostats shall be constructed to ensure reliable operation under severe service. Springs shall
not be stressed near or beyond their fatigue limits. Springs shall not carry current. Exposed metallic parts, other than
terminals of plate type rheostats, shall be grounded. Two-terminal type rheostats shall be so connected internally
that chatter under vibration or shock will not cause an open circuit. Rheostats shall be so arranged that speed,
voltage, or other function being controlled is increased will clockwise rotation of the handwheel. Dual purpose
rheostats shall increase speed with clockwise rotation and increase voltage with counter-clockwise rotation.
3.4.2.1 Brushes. The sliding brushes of plate type rheostats shall be easily renewable and so constructed as not
to be blocked by slight projections on the fixed contact buttons.
3.4.2.2 Remote operation. When remote operation is required, a motor-driven operating mechanism shall be
provided consisting of the motor, gearing, limit switches, necessary shafting, and connections. An emergency
manual operating feature shall be provided.
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