How To Choose the Right Thermal Circuit Breakers
When selecting circuit protection devices for trucks, off-road equipment and other mobile equipment, it’s important to have a complete understanding of the system to be protected. Don’t rush or minimize the process when planning your circuit protection.
The Heat Factor
In high-amp vehicle applications (typically ranging from 50-200 amps) ambient heat can contribute to nuisance tripping of circuit breakers, especially with under-the-hood applications. For example, a 100-amp rated circuit breaker might trip at 80 amps due to ambient heat. Thus, it’s important to build the heat factor into your specifications. Using a slightly higher-amp circuit breaker than the minimum required can offset the likelihood of nuisance tripping and reduce wear on the breaker.
7 Easy Steps
Mechanical Products, a leading manufacturer of thermal circuit protection devices, recommends an easy seven-step process for selecting high-amp thermal overcurrent protection. Here are some highlights.
- Determine which devices, components and circuits need to be protected and why. Keep the dynamics of circuit protection in mind. To avoid nuisance trips from start-up inrush and harmless power surges, include a margin of tolerance between the steady state current of the circuit and the rating of the protector. In general, the recommended margin for fuses is 25%; for circuit protectors, 15-20%. For more details see the Overcurrent Protection Selection technical brief by Mechanical Products.
- Understand how potentially damaging overcurrents and natural inrush currents and surges can develop in your devices. Systems typically spike when you turn on the ignition, for example. Today’s vehicle chassis are built to ensure safe current flow between the alternator, ignition, and battery, but variables can be introduced when you start adding additional circuits. For more information, see Overcurrent: Eliminate the Hazards.
- Determine where a current interruption device should be placed. Depending on placement, you will need to consider the type and size of automotive wire or cable, and ambient temperatures. Will the circuit be under the hood where temperatures are high, or somewhere else on the vehicle? Also consider surface and panel mounting options. Surface mounted breakers are convenient when adding additional circuits, but typically leave more exposed wires. Panel mounting is often subject to space limitations but enables wiring to be tucked away and out of sight. Series 17 Circuit Breakers from Mechanical Products come in panel-mountable options such as the ones shown at right.
- Calculate the magnitude and duration of the potential fault currents of the circuity and components you’ve identified. Determine maximum voltage requirements of the protective devices you will use.
Also consider what kind of environmental elements the circuit protection device will be exposed to. If your circuit protection device is going to be subject to harsh environmental elements, you might want to consider weather-proof or marine rated components. For more information about environmental considerations, see Waytek’s Guide to IP Codes for Vehicle Electrical Components.
- List the supplementary requirements for the protective device. What will it be connected to? Possible items include an auxiliary switch for an alarm circuit, power distribution devices, or a battery separator or isolator. Also factor in environmental considerations, electrical trip time, relay trip time, and color identification if needed.
- Determine the regulatory requirements. Depending on your application, you might be required to use circuit protection devices that are SAE rated or UL rated.
- Choose a circuit protection device that meets all the requirements of your application.
The Importance of Documentation
No matter which brand of circuit protection you choose, make sure the manufacturer provides supporting documents and testing data so that you can be confident in the quality and reliability of the device.
For additional resources see A Basic Guide to Circuit Breaker Types and Simplifying Circuit Protection and Power Distribution.
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