California Department of Transportation
 

3. OVERLOAD PROTECTION (DC and AC CIRCUITS)

3.1 Location

  1. Each overcurrent device shall be readily accessible and labeled. The labels shall include the circuit designation and the current rating for each circuit.
  2. Each overcurrent device shall not be exposed to mechanical damage.
  3. Each overcurrent device shall not be near any easily ignitable material or where explosive gas or vapor may accumulate unless ignition protected.
  4. Each overcurrent device shall not damage adjacent items.
  5. Each overcurrent device shall be positioned for easy accessibility.

3.2 Fuses

Where fuses are used, it is recommended that an extra fuse of each type and rating shall be supplied or attached to the applicable unit(s) of the equipment. Panel-mounted fuse holders shall permit replacement of fuses without use of tools. (Recommended fuses are mini blade type, ATO/ATC type, maxi fuse type and ignition proof ANL/CNL type).

3.3 Re-settable Circuit Protectors

Circuit breakers or other re-settable devices shall be used to protect critical circuits, or where predictable overloads or surges occur because of peculiar equipment functions or operator effects which are unavoidable. (Recommended circuit breakers are, mini blade type, ATC type, Maxi type, Series 180 Hi-Amp type, Series 174 flat pak, Series 120 short stop and Hydraulic Magnetic type A-series, B-series, and C-series).

3.4 Conductor Carrying Capacity

Conductors shall be protected in accordance with their current carrying capacities. If the allowable current carrying capacity does not correspond to a standard device size, the next larger overcurrent device may be used provided it does not exceed 125% of the conductor current carrying capacity.

3.5 Fusible Link

Fusible links are not recommended but if used shall not be in the driver’s compartment.

3.6 Voltage Rating

The voltage rating of each circuit breaker or fuse shall not be less than the nominal circuit voltage of the circuit it is protecting.

3.7 Storage Battery Main Fuse

Each positive output conductor from a storage battery shall have a manually reset, trip-free circuit breaker or fuse, except those circuits as describe in CFR Title 49 section 393.31 "Overload Protective Devices". It is preferred to make this connection at the starter motor relay’s positive terminal, not at the battery’s positive terminal.

  1. The circuit breaker or fuse shall be as close as practical to the battery.
  2. If it is physically impractical to place the circuit breaker or fuse at the battery, then it may be placed within 72 inches (measured along the conductor) as long as the conductor is contained throughout its entire length in loom and stays within an enclosure such as a junction box, control box, enclosed panel or engine compartment. Exceptions require prior written approval from Caltrans, Division of Equipment, Quality Assurance at 916-227-9709.

3.8 Current Overload Protection

No overcurrent protective device shall be connected in series with any conductor which is grounded at the power source unless the device simultaneously opens all load conductors in the circuit and no pole operates independently. Protective devices for wired-in equipment shall be connected to the load side of the equipment power switch (main circuit power disconnect). For portable equipment, a separable connector or the attachment plug and receptacle shall serve as the main circuit power disconnect and the protective device should be on the load side of the equipment on-off switch.

3.9 Power Source Branch Fuse

  1. A manually reset, trip-free circuit breaker or fuse shall be placed at the source of power for each branch circuit or conductor except those circuits as describe in CFR Title 49 section 393.31 "Overload Protective Devices". Fuses in branch circuits shall open before the fuses in the primary or main circuit.
  2. If it is physically impractical to place the circuit breaker or fuse at the source of power, it may be placed within seven inches of the source of power for each circuit or conductor measured along the conductor.
  3. If it is physically impractical to place the circuit breaker or fuse at or within seven inches of the source of power, then it may be placed within 40 inches (measured along the conductor) of the source of power for each circuit or conductor, so long as the conductor is contained throughout its entire distance in split loom and stays within an enclosure such as a junction box, control box, enclosed panel or engine compartment.

Battery connection schematic showing location of fuses and main frame ground

3.10 General Circuit Breakers

  1. Circuit breakers shall give a visual indication when tripped. Holding the switching device closed on an overload shall not prevent tripping of a breaker (except Type 1).
  2. All circuit breakers for use on 120 or 240 V AC 60 Hz shall have thread studs or screw terminals for attaching ring terminals.
  3. Circuit breakers shall be designed for mobile equipment applications. Circuit breakers designed for stationary operation, e.g., house, building, stationary equipment shall not be used.
  4. For AC applications, circuit breakers shall be Hydraulic Magnetic type, UL recognized or UL listed or, if ignition protected, UL 1500. (e.g. Carling technologies A-series, B-series, C-series.)
  5. The use of standard Type 1 (automatic reset) or Type 2 (non cycling) circuit breakers require prior written approval. Call Caltrans, Division of Equipment, Quality Assurance Branch at 916-227-9709.

3.11 Manual operation

  1. Circuit breakers, if required, shall be capable of being manually operated to the ON and OFF positions.
  2. Circuit breakers shall not be used as ON-OFF switches unless such breakers have been specifically designed and tested for that type of service.

3.12 Position Identification

Circuit breakers shall have easily identified ON, OFF and TRIPPED positions except that the TRIPPED position may be the same as the OFF position with no differentiation between OFF and TRIPPED being required.

Last updated 01/14/04