Cutting cost on design is a good thing unless your design goes up in smoke. As OEMs compete in an increasingly low-cost global marketplace, many engineers are considering inexpensive circuit breakers or fuses, instead of full-featured, high-quality circuit breakers for equipment.
The wrong decision can have broad consequences. I know someone who threw out two relatively new microwave ovens because the circuit protection did not reset after overheating. He switched to another brand. Something similar happened to me. I opened the back of my microwave and found a fuse that had blown as a result of aging. As an electrical engineer I knew what do; typical consumers are left fuming.
OEMs need to consider the whole customer experience. Imagine the cost to one equipment manufacturer who flew a support technician all the way to Alaska only to learn that the "failure" was caused by a fuse that the customer could not find.
The place to start is component quality. Consider how the circuit protection device is manufactured and tested. Unlike circuit breakers, the individual fuse you buy cannot be tested without destroying itself. And, as fuses age, their trip characteristics change.
Fuses also introduce uncertainty into the design after it leaves the factory. A user, tired of repeatedly replacing the same fuse, puts in one with a higher rating. (The 5A fuse blows all the time? I'll put in a 10A fuse). This might solve a temporary problem, but the outcome could result in melted wiring, or even a fire.
Circuit breakers, on the other hand, can be manually reset. They can be individually tested, and they resist aging. What's more, the equipment manufacturer maintains control over circuit protection.
Just specifying a circuit breaker isn't enough. You must know what the manufacturer's data sheet is really telling you. The interrupting capacity rating is the maximum current that the device can interrupt safely without exploding or causing a fire. Some circuit breakers can survive certain short circuits and still operate. Other can't.
Many engineers don't understand that UL 1077 recognized circuit breakers can be tested to a variety of standards, which means they offer different levels of protection. The circuit breakers can be overload tested at 1.5 times their rating (for general use) or six times their rating (for across the line motor starting). "Fit for further use" breakers must survive a three-cycle short-circuit test and continue to provide overload and short circuit protection. You may have to call the manufacturer to find out how the circuit breaker was tested before you know the true measure of its integrity.
Quality doesn't always cost more. Often the higher cost of a circuit breaker can be re-couped by selecting a unit that combines protection with a switch, relay, or illumination. Justify a quality breaker to the bean counters by including features such as auxiliary contacts or undervoltage release that add value to the customer. What's more, planning ahead for quality protection produces customer satisfaction and loyalty and saves costly redesign that alone could pay for the small cost of doing it right.
The sacred role of the design engineer is to balance cost reduction against safety and function. Now, more than ever, we engineers have to stick up for our designs.
Cybart can be reached at ken.cybart@e-t-a.com.
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