Posts Tagged ‘power circuit breakers’

Image of Circuit Breaker Main and Arcing Contacts

July 9th, 2012 Comments off


Circuit Breaker Main and Arcing Contacts with Copper Splatter

Circuit Breaker Main and Arcing Contacts with Copper Splatter

When discussing circuit breakers, we like to have good images to show the difference between the main contacts and the arcing contacts. When a circuit breaker opens, the main contacts should open partially first before the arcing contacts start to open. There should be no arcing damage to the main contacts because the arc interruption takes place between the arcing contacts. This protects the current carrying surfaces of the main contacts so there is minimum contact resistance at the main contacts. Good contact surface means no overheating. For the same reason, when the circuit breaker closes, the arcing contacts close first, suffering any arcing damage. After the arcing contacts are closed, the main contacts close. All this keeps the main contacts in good condition. The arcing contacts are enclosed in something called an arc chute that extends and separates the arc until it is extinguished when the circuit breaker opens. When the arc chutes are contaminated with dirt or high humidity or interrupt very high loads or fault currents, the arc chutes take a beating. Whether you’re talking about Square D circuit breakers, Cutler Hammer circuit breakers, Siemens, Westinghouse or GE General Electric circuit breakers, molded case circuit breakers, power circuit breakers, or medium voltage air circuit breakers, the function of the arc chutes is the same, take a beating to protect the main breakers. The attached image shows the damage to the stationary and movable arcing contacts and to the arc runner that extends into the arc chutes to extend and divide the arc until it is extinguished. The main contacts, the 16 curved segments across the middle of the image, are in good shape. No arcing damage what so ever. This image is from a medium voltage air circuit breaker.

Circuit Breaker Large Over Current Time Delays

December 29th, 2010 4 comments

In MIDWEST’s training classes for qualified personnel, there is a segment where we explain the long time delay range within which a Square D 1000 amp circuit breaker should trip due to an overload. This information is received with anything from amazement to skepticism to outright disbelief, even though we show the Square D circuit breaker characteristic trip curve.  The overload time delay information is not restricted to Square D circuit breakers. It’s the same with Cutler Hammer, GE General Electric, Siemens, ITE, Westinghouse, Merlin Gerin, or Federal Pacific circuit breakers.


In our training example we use an old Square D 1000 amp MA type circuit breaker.  If we tested this circuit breaker at 3000 amps, that’s 300%, the minimum to maximum trip range is about 45 seconds to 340 seconds. It might trip in 45 seconds or it might not trip for 340 seconds.  This is an old thermo-magnetic circuit breaker, which typically works by heating a bi-metal in the over current trip device. Many newer breakers use electronic over current devices which have more repeatable overload time delay test results.


The illusion is that these Square D, Cutler Hammer, Westinghouse circuit breakers are designed to directly protect people. They are not. The breakers protect the equipment connected to them and they protect the electrical system. They are designed for the characteristics of the equipment connected, such that connected equipment will not be damaged by an overload or fault. This is a basic limited explanation. So, when you think of molded case circuit breakers, power circuit breakers or air circuit breakers, it’s important to know these breakers don’t just trip right at the breaker trip device rating.       

Free 3000 Amp Circuit Breaker

April 23rd, 2010 Comments off

A free 3000 amp circuit breaker! This sounded too good to be true when MIDWEST’s Switchgear Shop made the announcement.  And, of course, they were just being wise guys. It was another opportunity to spin up the Engineers a little, and it worked. What they meant was the breaker was shipped loose on the pallet, in other words free. It seems MIDWEST was shipped an old 3000 amp power circuit breaker for maintenance and testing.  The breaker was properly wrapped, boxed, and strapped to a sturdy piece of plywood.  The plywood was on a sturdy pallet. Looked like someone did a good job shipping this fairly large power circuit breaker. 


But, the breaker was free, loose on the pallet, because the plywood was not fastened to the pallet. It could just slid around, slid right off the pallet. We’ve seen a lot of different techniques used to ship old and new circuit breakers. Some good ideas and some bad. In this case, someone went all out, but still missed the mark.  Fortunately there was no damage and we added this information to our training file.

High Current Testing Old Circuit Breakers at Low Voltage

October 20th, 2009 Comments off

MIDWEST was testing power circuit breakers for a large manufacturer.  The electrician working with us thought we were joking when we said we were putting 6000 amps through his 2000 amp main circuit breakers.  We told him to grab the bus from the test set to the breaker and he could feel the vibration and heat from 6000 amps.  He could hear the loud hum of the test set and the vibration of the old circuit breaker used in their foundry.  He wouldn’t touch it until we touched the bus first.  He had that strange look of “I see it, but I just don’t believe it.”  We told him the output of the high current test set was going into a direct short and the output voltage was only a couple volts.  We had him measure the output voltage with a voltmeter.  Then he understood what was happening.  It doesn’t take much voltage to push a lot of current through a dead short.