Group 30 Electrical System—VNL, VNM
Design and Function
Fusible links
Fusible links are used to protect high-current circuits
against current overload when there is a short to ground.
The fusible link is a short length of wire that is smaller
in gauge than the wire in the protected circuit. In the
event of an overload the fusible link will melt, breaking the
circuit and preventing damage to the electrical system. If
a fusible link does open, special attention must be paid to
finding and repairing the cause.
Fusible links are used in two locations: two are at the
starter motor on the positive side feeding the cab main
power studs, and one is from ground on the starter motor
to engine ground. The fusible links on the positive side
are 10 gauge cables 120 mm (4.72 in.) in length. On the
ground side, it is an 8 gauge cable.
Diodes
Diodes are used on many of the vehicle’s circuits to
protect and isolate them from voltage surges, which can
occur when a circuit is turned off. Diodes allow voltage to
flow in one direction only, like a one-way check valve.
Circuit breakers
Circuit breakers are optional equipment. SAE Type 2
modified reset circuit breakers are the only type of circuit
breakers approved for use in VNL or VNM vehicles. They
may be used on accessory and ignition circuits only.
Circuit breakers protect a circuit from overload. When an
overload (high current flow) occurs in a circuit, a bimetallic
strip in the breaker is heated. This opens its contact,
temporarily breaking the circuit. When this bimetallic strip
cools down, it remakes the contact.
Type 2 circuit breakers are opened by current overload
and remain open as long as the power is on. A Type
2 circuit breaker keeps the bimetallic strip hot after
tripping by diverting a small amount of current through
a small coil of resistance wire. If power to the circuit
breaker is switched off long enough for the bimetallic
strip and resistance wire to cool down, the breaker will
automatically reset.
When any circuit breaker trips, it should be viewed as
an indication of a possible fault in the circuit. Every effort
should be made to identify and correct the fault if one
exists.
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