MIL-DTL-23928G
4.5.4 Short circuit. Samples shall be subjected to short circuit testing to values of short circuit current of 100,000
amperes for AQB panels and 50,000 amperes for ALB panels (average of the maximum root mean square (rms)
asymmetrical of the three phases). The source shall be initially at the rated voltage of the panel to be tested (see
3.5.3). The short circuit tests shall be conducted to determine that the structural arrangements provide adequate
bracing under short circuit conditions, and if the electrical clearance distances employed within the panel design are
adequate under conditions where ionized gases are emitted from the circuit breakers. The circuit shall consist of an
AQB-LF400 back-up circuit breaker, and AQB or ALB panel with circuit breakers, and a short circuiting breaker on the
load side of one of the panel breakers. The test circuit shall have a power factor not greater than 15 percent (X/R
ration 6.6 or greater) with X and R in series connection. During the test, the AQB of ALB panel breaker shall be
closed, then the AQB LF400 back-up breaker, and finally the short circuiting breaker. It is permissible for the AQB
and ALB circuit breaker subjected to more than its interrupting capacity to sustain damage during this test, but there
shall be no damage to the panel or circuit breaker mounting foundation.
4.5.5 Vibration. The sample panel shall be energized and subjected to type I vibration tests up to and including the
frequency range of 33 hertz (Hz) in accordance with MIL-STD-167-1. One-half of the installed circuit breakers shall
be closed, the remaining breakers shall be in the open position. An oscillograph shall be used to check the closed
breakers for excessive contact bounce and to check the open breakers for momentary closures. Panel mounting
shall be as specified for bulkhead mounted equipment in accordance with MIL-STD-167-1. The use of resilient
mounting shall not be permitted. Acceptability criteria shall be as specified (see 3.5.4).
4.5.6 Salt atmosphere. When specified by the application of the panels (see 6.2g and 3.5.5), the sample shall be
subjected to a salt fog test in accordance with MIL-STD-810. The equipment shall not be energized during the test.
Upon completion, the sample shall be inspected for compliance with the requirements of 3.5.5.
4.5.7 Dielectric strength. Each sample shall be subjected to dielectric testing (see 3.5.6). The sample shall be
subjected for 1 minute to a dielectric test voltage, the effective potential of which is twice the panel rated voltage plus
1,000 volts (except low voltage panels such as 24-30 volts for which twice rated voltage plus 500 volts is applicable),
applied between points of opposite polarity and between live parts and ground. The frequency of the voltage shall be
60 Hz rms ac and shall approximate a true sine wave.
4.5.8 Shock. The sample shall be subjected to H.I. shock tests in accordance with MIL-S-901 for grade A,
class I lightweight, type A equipment (see 3.5.7). One half of the installed circuit breakers shall be closed, the
remaining breakers shall be in the open position. Main and auxiliary contacts shall be monitored by oscillograph, FM
tape recorder, or suitable digital data acquisition system. All equipment shall have a minimum frequency response
and sampling rate of 2 KHz. Digital signal processing of acquired data shall be capable of producing suitable
waveform plots with 0.5 millisecond minimum resolution, and for analysis purposes shall be able to expand contact
events with graphical editing to determine time increments of the individual sample points. Monitoring shall include all
main contacts and, if applicable, at least one normally open and one normally closed auxiliary contact. Panel
mounting shall be in accordance with test fixture 4A of MIL-S-901. Fuses (or other indicators which provide a lasting
indication) shall be connected so to detect any momentary short in between live parts or live parts and ground. The
test shall be conducted with all equipment and cables in the panel energized with no load. The sample being tested
shall not be reconditioned or adjusted during the testing. The panel being tested shall be considered as failing the
test if functional or structural parts are loosened or damage is evident (see 3.5.7).
4.5.9 Interlock bars. The sample shall be subjected to tests of the interlock bars to demonstrate proper function
and alignment of parts. The bars shall properly permit both circuit breakers to be open or only one to be closed with
a minimum time delay of 2.5 seconds from initiating the trip of one circuit breaker before the second breaker can be
closed. Improper operation occurs when the bars permit two circuits to be closed simultaneously causing short
circuits between isolated generators. The tests shall demonstrate and verify the following:
a.
The proper interlock bar is installed in relation to the orientation of the circuit breakers.
b.
The interlock bar is properly aligned with the circuit breaker handles. This shall be checked after the
interlock bar and circuit breaker face plate are removed and reassembled.
c.
The proper fit of parts exists by attempting purposeful misalignment; loosen hardware and ascertain if
interlock bars and face plates can be held out of tolerance when bolts are retightened (see 4.5).
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