Talking back between our readers
When David Loucks of Eaton wrote an article on selective coordination, he didn’t have to wait a month for a reaction from Plant Engineering readers. The reaction came the next day, via the Talkback feature at www.PlantEngineering.com
“In his article,” the reader wrote, “Mr. Loucks indicates that even when fault current falls within the instantaneous trip range of two circuit breakers exposed to the same magnitude of fault current, the breaker closest to the fault could be the first breaker to open, as the breaker closest to the fault would likely be the smaller (lower amp frame and trip) of the two breakers and would have lower mass to overcome allowing it to start opening first and introducing arc impedance to the faulted circuit, thus reducing the fault current to a level lower than the instantaneous pickup of the upstream (and slower) breaker.
“My first question is once the instantaneous trip has been triggered on any circuit breaker, is this not the point of ‘no return,’ meaning that the breaker will trip, regardless of what happens to the fault current over time?”
And the reader added this thought: “Relying on the idea of a downstream breaker acting faster than a larger upstream breaker in the instantaneous trip range of both breakers to prevent the upstream breaker from tripping instantaneously sounds like something that would need testing to confirm such operation and not something that can be guaranteed.”
At PlantEngineering.com, Loucks was able to respond with a complete explanation, and a conversation had begun.
“Regarding the instantaneous tripping issue, yes, if in fact both breakers were in the process of opening, then you would have a point of no return; and while the smaller breaker might open first, the upstream breaker would still trip eventually,” wrote Loucks. “But what I was talking about was not the breaker contacts opening, but rather the tripping mechanism within the breaker that tells the breaker when to trip.
“The larger breaker will have a larger, heavier tripping mechanism that must be accelerated with a force and/or distance greater than what would be required on a smaller breaker. When that smaller breaker tripping mechanism operates a few milliseconds before the big one, you add arc impedance to the circuit.
“And I agree about your testing comment. You couldn’t guarantee that this would work in all cases. Another interesting reason that could explain why we don’t hear about selective coordination issues, at least from properly sized and set breakers, is this %%MDASSML%% the 2005 code mandates selective coordination on emergency systems.”
The Talkback feature at www.PlantEngineering.com is a great way to contact story authors and editors. It’s also a great way to build a network of plant managers. That kind of dialogue is important today as plant managers around the world work to solve common problems on the plant floor.