MIL-DTL-17361G(SH)
3.5.16.1 Thermal magnetic trip devices. Unless otherwise specified (see 3.1), circuit breakers shall have
thermal magnetic trip devices combined in a readily replaceable trip unit assembly. For 250 ampere frame sizes and
larger, the trip unit assemblies from the same manufacturer shall be interchangeable for ampere ratings within a
particular frame size.
3.5.16.1.1 Long-time delay trip element. Tripping characteristics of the long-time delay (LTD) for thermal-
magnetic devices shall be based on the current flowing through all poles simultaneously in series and shall be as
specified (see 3.1).
3.5.16.1.2 Instantaneous trip device. The instantaneous trip device shall have no intentional time delay (see
6.4.13 shall be provided with trip calibrations as specified (see 3.1). Instantaneous tripping characteristics shall be
based on the current flowing through any one pole. Devices having adjustable instantaneous calibrations shall
comply with the settings as specified (see 3.1 and 6.2). When the instantaneous trip setting is not specified on an
order, circuit breakers shall be shipped with instantaneous at low setting. Accessibility to the adjustable trip unit
(see 6.4.29) settings shall be from the front of the circuit breakers or by removal of the circuit breaker molded cover.
3.5.16.2 Electronic trip devices. Circuit breakers with electronic trip devices shall incorporate LTD, short-time
delay (STD), and instantaneous trip features as specified (see 3.1 and 6.2). The circuit breaker continuous current
rating shall be set by means of a rating plug (adjustable or interchangeable) or adjustable switch. A means for
adjustment shall be accessible from the front of the circuit breaker without having to remove the circuit breaker
cover. For circuit breakers with fuse units, removal of the fuse unit to adjust the settings is acceptable. A test plug
or test jack shall be provided to facilitate the connection of a portable test set. When adjustable settings are not
specified on an order, circuit breakers shall be shipped with all adjustable settings in their LOW or MIN position.
3.5.16.2.1 Long-time delay trip element. Tripping characteristics of the LTD shall be based on single-phase
current flowing through any one pole and also based on three-phase current flowing through all poles
simultaneously.
3.5.16.2.2 Long-time thermal memory. Circuit breakers shall respond to successive long-time over-currents by
tripping in shorter time periods. A thermal memory function shall provide for over-temperature protection of load
circuits against the effects of repeated overload conditions. The amount of time delay reduction shall be
approximately inverse to the amount of time elapsed. When the overload condition returns to normal, the thermal
memory shall reset back to the normal LTD setting, provided that another overload is not experienced.
3.5.16.2.3 Instantaneous trip device. The instantaneous trip device shall have no intentional time delay (see
6.4.13). Instantaneous tripping characteristics shall be based on the current flowing through any one pole.
3.5.16.2.4 Short-time delay trip element. STD tripping characteristics shall be as specified based on the current
flowing through any one pole.
3.5.17 Current limiting devices.
3.5.17.1 Current limiting fuses. When specified (see 3.1), circuit breakers shall include internally mounted
plug-in current limiting fuses to provide short circuit protection at high current values (see Figure 2). Fuses shall be
in accordance with MIL-PRF-15160.
3.5.17.1.1 Function of current limiting fuses. Fuses mounted in a circuit breaker shall be readily installed or
replaced in the field. In addition to interrupting the circuit, each fuse shall provide a mechanical force to trip the
circuit breaker mechanism. The blowing of the various fuse types under short circuit conditions shall occur within
the root mean square (rms) asymmetrical ampere values specified (see 3.1).
3.5.17.1.2 Fused circuit breaker characteristics. Fused circuit breakers shall have the following characteristics:
a. Time-current characteristics of the fuses and the circuit breaker shall be coordinated to prevent unnecessary
blowing of the fuses and to provide protection to the circuit breaker without damage.
b. The circuit breaker shall safely and satisfactorily clear faults up to the current values at which the fuses will
blow (see 3.5.17.1.1). During the circuit breaker performance the fuses shall not blow, be damaged, or change their
characteristics.
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