By Ken Cybart

Circuit protection devices seem straightforward. Sometimes it's tempting to procrastinate and not worry about circuit protection until the design is nearly complete. This is usually a mistake. The end of the design process is not the time to discover there isn't enough room for the breaker, or that the environmental conditions are unsuitable for a standard circuit breaker.

Leaving circuit protection until the end of your design can cause delays, increase costs, and unfortunately, can contribute to redesigns. That's because choosing the right circuit protection isn't as easy as it seems. Let's take a look at some of the more common errors designers make in choosing circuit breakers, and explain at what stage in the design cycle it is prudent to begin considering circuit protection.

The first common error is to not leave room for the breaker. This is not an electrical error, but a failure to coordinate the design with other functions in your organization. The electrical designers must inform the mechanical designers of the required characteristics. For example, circuit protection built into a panel should have preliminary specifications prior to passing the design to the mechanical engineer. Physical characteristics such as number of poles, splash guards, frame size, and actuation type all affect the amount of space a panel builder will need.

Another common error is the failure to choose combination components. Circuit breakers are now offered with features such as switches, relays, power inlets, LEDs, and alarm indications, to name a few. Determining which functions can be handled by the circuit breaker early on in the design process will determine the size of the complete design. Properly choosing combination components can save space, installation time, reduce cost, and increase the overall reliability of the design.

Choosing circuit breakers, such as the E-T-A 8345, is a vital part of electrical product engineering and should be done early in the design process.

If the circuit breaker will be on the front panel and act as the power switch, the designer must make sure the breaker is intended for use as an on/off switch. Some breakers are designed for this function, but others are not, and will wear out mechanically if used inappropriately. EN 60934 approved S-type breakers can be used as on/off switches, but some manufacturers do not use this terminology, so check the manufacturers data sheets for the individual circuit breaker selected.

It is also important to consider the external environment in which the circuit breaker will be used. Where corrosive atmospheres or salt spray are present, panel seals are available that can protect breakers from hazards ranging from splashing to full immersion. In some cases, optional features such as water splash covers and reset button seals can change the physical dimensions of the product. The decision to have these optional features should be determined in the design phase. In addition, optional features need to be accounted for in the bill of materials.

Designers must also realize that the trip time of a thermal breaker can be affected by operating temperature. Typically, this characteristic is beneficial because the breaker tracks the requirements of the equipment as the temperature fluctuates. But if the breaker is in the highest and hottest part of the equipment it may need derating. A general rule, at least for rack-mounted equipment, is "the lower the better." Place the circuit breaker in the lowest part of the design for the most stable temperature if the design is prone to hot spots.

Another common error involves not leaving enough spacing between thermal breakers. As mentioned before, thermal breakers are sensitive to ambient temperature; they also generate their own heat. Leave at least one millimeter of space between breakers to prevent heat build up. If the breakers must touch each other, derate them to 80 percent of their normal amperage rating. In addition, do not ignoring mounting orientation. Thermal breakers are unaffected by mounting orientation, but many breakers with a magnetic component are meant to be mounted right side up on a vertical panel. Trip times for magnetic circuit breakers can be affected by mounting position because the movement of the breaker's tripping solenoid can be affected by gravity. In the situation where the circuit breakers mounting position is not stable, such as an airplane, consider selecting thermal circuit protection.

Also consider internal resistance and voltage drop-in low-voltage circuits. While all circuit breakers have some electrical resistance, thermal breakers tend to have more resistance at low current ratings because they dissipate some power to operate. Thermal-magnetic breakers tend to have even more resistance because of the magnetic coil in series with the thermal element. The lower the current rating of the breaker, the greater the resistance will be, and in some low-voltage circuits the voltage drop across the breaker may not be tolerable. There may also be cases in which the heat produced by a thermal breaker may be a problem. Determine whether this characteristic can be detrimental to your design. Often, the features and reliability of thermal circuit breakers outweigh the affects of internal resistance on the design.

Circuit breaker selection is multi-step process. The best advice is to make a preliminary breaker selection early enough so the unit can be accounted for in the mechanical design; detailed breaker specifications will develop as the design proceeds. Specifying the right breaker at the right point in the design process can lead to a more robust design, avoid redesigns, decrease development costs and add value to a product.

Ken Cybart is senior applications engineer at E-T-A Circuit Breakers. He can be reached at ken.cybart@e-t-a.com or 847-827-7600.
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