MIL-PRF-17773 Switches, Bus Transfer, Electric Power Automatic And ManualThis specification covers the Automatic Bus Transfer Switch \(ABT\) \(see 6.4.1\) and the Manual Bus Transfer Switch \(MBT\) \(see 6.4.13\), herein referred to as ABT, MBT, or switch\(es\), for use on surface and submarine Naval vessels. Unless otherwise specified herein, this specification applies to both ABTs and MBTs. Requirements for Static or Solid State ABTs \(SABTs\) are not defined in this specification. MIL-PRF-32150 specifies requirements for SABTs.

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MIL-PRF-17773C(SH)

3.4.10 Auxiliary switches. Unless otherwise specified (see 6.2), ABTs over 100 amps shall be provided with

one double pole, double throw (DPDT) auxiliary switch, and ABTs 100 amps and under shall be provided with one

single pole, double throw (SPDT) auxiliary switch for remote indication of the supply line connected to the load

circuit or for a function limiting device, or both. The auxiliary switch contacts shall be wired to a terminal board in

the ABT for connection to the remote equipment. Auxiliary switches shall have a minimum rated continuous

current of 5 amps and a rated voltage of 500 VAC and 250 VDC and meet the endurance requirements as specified

in Method II, Table X.

3.4.11 Switching mechanisms. Switching mechanisms of bus transfer switches shall use contactors, circuit

breakers, or rotary snap switches as specified in Table VI.

TABLE VI. Circuit breaker types and contactor sizes for bus transfer switches.

Manual

Automatic

Type 1/

Amperes (A)

Above 600 3/

Greater than 250

AQB

ACB

25 to 600 2/

250 or less

AQB

Transfer contactors

Contactor relays

25 to 600

Transfer contactors

15 to 540

(sizes 0 to 6)

Rotary switch,

10 to 60

Type SR with off

Notes:

1/ For specific sizes of AQB circuit breakers available refer to MIL-DTL-17361.

For specific sizes of ACB circuit breakers available refer to MIL-C-17587.

For specific sizes of contactor relays available refer to MIL-DTL-2212.

For specific sizes of rotary switches available refer to MIL-DTL-15291.

2/ Standard sizes of transfer switches utilizing transfer type contactors shall be 25, 50, 100, 150, 250, 300, 400, and

600 A.

3/ Sizes above 600 A shall be as specified. (see 6.2)

3.4.12 Contactors. Contactors shall be actuated by magnetic coils or motor-driven cams and shall be

mechanically held.

3.4.13 External control cable connections. Terminal boards or studs shall be provided for external control

connections if more than 12 external control wires are required. Terminal boards and studs shall be accessible

from the front of the enclosure with the front panel open, and the control wire connection points shall be

accessible and identified.

3.4.14 Grounding. Grounding shall be in accordance with MIL-STD-1310.

3.4.14.1 Chassis grounding. All external parts capable of electrical conduction shall be at ground potential at

all times in accordance with MIL-STD-1310. Each chassis within the enclosure shall be electrically bonded to

minimize electromagnetic interference (EMI). The DC resistance measured from the conductive frame of any

assembly receiving primary power and the unit electrical bond point shall not exceed 0.1 ohm.

3.4.14.2 Signal grounding. A single point ground (SPG) system shall be used within the ABT and shall be DC

isolated from the chassis/safety ground. To achieve this SPG system, all signal circuits and secondary DC power

returns shall be referenced to a single point within the unit. The signal shall be DC isolated from the chassis ground.

3.4.14.3 Line-to-ground impedance. The insulation resistance or impedance to ground shall be measured with

all operating components attached and connected in the ABT's normal operational condition with no internal wiring

disconnected. Insulation resistance of ABTs shall be not less than 10 megohms as specified in MIL-E-917.