In a word, speed. As dpc said, a vacuum breaker is designed for interrupting short circuits, thus it needs to be fast. Vaccum breakers are also useful in loads that must switch frequently or rapidly such as in a generator application, an industrial arc furnace, or a recloser.
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
A load breaker is just a switch, for example 3 or 4 poles, for turning on and off electrical loads. There is no protection against overcurrent in this device. cool
Well, to be fair, an LBS is typically used with fuses, so there would be SC protection for the system. But of course, a VCB will open all 3 phases in the event of a fault, a fuse only clears the phase(s) with the fault and thus risks single phasing.
But there is an argument to be made for safety as well. A vacuum bottle, while very good at interrupting current, is considered by many people to not be suitable for isolation of a circuit after a fault has occurred. After a fault, small amounts of conductive material can deposit on the inside of the vacuum bottle and track measurable potential across the very small gap that exists. If someone sees that the VCB is open, they may mistakenly believe that they are isolated from the line source when in fact the leakage may be substantial, and lethal. A Load break Switch, with all of it's faults, is at least a very large air gap when it comes to circuit isolation.
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"If someone sees that the VCB is open, they may mistakenly believe that they are isolated from the line source when in fact the leakage may be substantial, and lethal"
Absolutely....We use VCB's all of the time, but always include a visible means of disconnect ahead of the unit for safety.
Alan
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Any cicruit breaker should not be used for isolation purposes by simply opening it. Only with the breaker racked into the disconnect postiion should the circuit be considered isolated. That is the design and what is required to meet the "visible verification" requirement of OSHA LOTO procedures.
Functionality wise VCB>LBS
LBS can kill the load current within its rating and may close on to a fault within its withstand rating.
VCB can kill the fault current within its interrupting rating.
Be careful don't be confused by the kA rating of a LBS, it is just a pass through withstand.
Good point, I forget to include that when I am asked (which happens more often than I would like to admit). Since the LBS would not be the fault interrupt device, why would the mfr design it to do so?
I had never thought about that until I saw video of a witness test at KEMA for a 5kV controller I worked on that used an LBS as the disconnect / isolator ahead of a vacuum contactor. The final test was of the withstand capability and they replaced the fuses with bus bars, then bolted them together to ground. They then create the fault but they weren't supposed to try to open the switch. The test technician didn't read that and did it anyway (from a distance with a motorized operator they had attached for the test); the fireworks display was spectacular.
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