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Resolve to Exercise Yearly
Even breakers need to move
Paul Sharke, Senior Technical Editor -- Design News, January 9, 2006
As you're coming down off your treadmill this month, vowing to keep a resolution to get in shape, you might want to give each of your circuit breakers a little flip, too. That's the recommendation at this website, anyway http://rbi.ims.ca/4911-531.
Talking with Cooper Bussmann Sales VP Larry Woolums at the recent Rockwell Automation Fair had me thinking about fuses and circuit breakers more than I have for some time. I asked Woolums why fuses are still specified for motor circuit protection when breakers are available that can do the same job.
A fuse turns out to be a highly-engineered product, he said, designed to melt as a certain current passes through it. It has no moving parts.
Bussmann's line of low peak fuses are designed to limit the current that can move past the fuse before it breaks. An example from the company's handbook, available at http://rbi.ims.ca/4911-532, illustrates the various currents that could get by in 50,000 symmetrical amp fault.
Bussmann's concern is worker safety, of course. The company worries about the burns a worker could sustain while standing in front of an opened electrical box as a short is happening. Several photos show the explosive effect of 22,000 available amps shorting through a 10 hp IEC motor starter that's protected by Type 1 protection. Type 1 motor starter protection prohibits any danger to persons but allows for component damage. A parenthetical, "with enclosure door closed," explains how the components might be damaged but not a person.
Ordinarily, an enclosure would be shut and anybody near it shielded from any explosion. But I know of at least one situation where a worker would be in front of a live motor starter with the door opened. That's during a thermography inspection, a routine of many maintenance programs.
Type 2 motor starter protection permits no component damage and, of course, no danger to persons—again, parenthetically, "with enclosure door closed." To my pedestrian ears, though, that sounds like "no boom allowed." If I'm a thermographer standing in front of an open panel when a short comes down I know which type I'd want installed.
Current limiting molded case circuit breakers are available that will trip the fault and limit the current that gets through, but the Bussmann book says they are three to four times as costly as regular MCCBs and irregular maintenance can extend the time they need to clear a fault. Standard circuit breakers that use fuses as limiters are better, as are some of the company's dual element, time delay fuses, it says. Bussmann believes its low peak fuses are the best choice.
I'm sure the circuit breaker makers have plenty of counter arguments. As Ken Cybart of E-T-A once wrote in these pages http://rbi.ims.ca/4911-533, " 'Fit for further use' breakers must survive a three-cycle short circuit test and continue to provide overload and short circuit protection."
Nothing about engineering is ever clear cut. The codes themselves leave the choice of providing Type 1 or Type 2 motor starter protection to the specifiers.
The debate this sparks goes to the very essence of engineering, the choices that engineers must make to balance safety and reliability, cost and convenience, while protecting people and equipment and waiting for human error.
I'm confident all the engineers reading this exercise their breakers regularly.
Reach Paul at paul.sharke@reedbusiness.com.
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