3.11.2 Circuit protection. The electrical system shall have circuit breakers. The circuit
breakers shall conform to SAE J553 and shall be located near the wiring connector socket,
readily accessible for service. Switches and circuit protection devices installed on the semitrailer
shall be of permanent vaporproof design or enclosed in vaporproof boxes, except circuit breakers
located at the front of the semitrailer may be installed in a weatherproof box away from tank
vents or openings.
3.11.3 Independent 24-volt dc system. When specified (see 6.2), a 24-volt dc lighting
system independent of the 12-volt dc lighting system shall be provided for use with a towing
vehicle equipped with a nominal 24-volt dc system. Separate wiring and bulbs shall be provided
for the 24-volt dc system. The system shall conform to 3.11 through 3.11.3. The 24-volt dc
system shall conform to the Federal Motor Carrier Safety Regulations referenced herein, except
the 24-volt dc stop and turn lamps shall be combined.
3.11.4 Interconnected 24-volt dc system. When specified (see 6.2), capability of operating
the 12-volt dc system (see 3.11) from a towing vehicle equipped with a nominal 24-volt dc
electrical system shall be provided.
3.11.5 Receptacle 24-volt dc. When either an independent or interconnected 24-volt dc
system is specified, the front of the semitrailer shall also be equipped with a 12-contact
receptacle and cover conforming to MS 75021, part number MS 75021-1. The receptacle shall
be located in accordance with SAE J702. The 24-volt dc, 12-contact receptacle shall be
connected to the lights as follows:
Contact B. Connect to left-hand turn signal and stop lamp (yellow).
Contact D. Connect to ground (white).
Contact E. Connect to clearance, side marker, identification and tail lamps (black and
Contact J. Connect to right-hand turn signal and stop lamp (green).
Contact L. Connect to ground (white).
The remaining contacts shall not be connected. Circuits B and J on tactical trucks are
combination stop and turn indicator circuits. On the interconnected 24-volt dc system, the
normal 12-volt dc turn signal lights will function both as turn signals and stop lights, and the
normal 12-volt dc stop lights will not be operational when the semitrailer is connected to a
towing vehicle with a 24-volt dc power supply. Because of this condition, the stop light (red)
circuit is not connected to the 24-volt, 12-contact receptacle.
3.11.6 Resistors. When an interconnected 24-volt dc system is specified (see 3.11.4), the
12-contact receptacle shall be provided with resistance to each circuit to reduce the voltage of the
tactical (military design) towing vehicle from a nominal 28-volts dc to 12-volts dc. Each circuit