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Have you/clients ever had an overcurrent device catastrophically fail when it interrupted a fault?
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 Post subject: Overdutied protective devices and catastrophic failures
PostPosted: Sun Jul 24, 2016 7:21 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1464
Location: Scottsdale, Arizona
People seem to love to watch videos of electrical explosions. One type of failure is when a protective device is applied beyond its interrupting rating and catastrophically fails (explodes) when it has to interrupt a short circuit. Although this does not happen for every overdutied device, it could happen. That is the main reason for performing a short circuit study - to identify these types of deficiencies so they can be corrected.

So with that intro, here is this week's question.

Have you/clients ever had an overcurrent device catastrophically fail when it interrupted a fault?
Yes
No


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 Post subject: Re: Overdutied protective devices and catastrophic failures
PostPosted: Sun Jul 24, 2016 7:33 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1464
Location: Scottsdale, Arizona
Saw class H fuses vaporize about 50 feet from me as a summer intern in the late 1970's. Left quite an impression during my early days. The electricians had just completed adding a fused disconnect switch to serve downstream equipment.

They energized the equipment in the next room with the switch already closed - "BOOM" there was a fault (they didn't bother to check the circuit), when they opened the switch, there wasn't much left of the fuses.

Should have used class R but they apparently didn't know any better and back then, I sure didn't know the difference. All the things we learn about regarding interrupting ratings, fuse classes, short circuit studies etc. are quite important. -especially when something goes wrong. Back then, (approaching 40 years ago) there wasn't as much attention given to what could happen.

After a failure, people would shrug their shoulders, and go find/fix the fault and replace the fuses (with the same type) just part of the "standard" procedure of the day.

We've come a long way!


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 Post subject: Re: Overdutied protective devices and catastrophic failures
PostPosted: Sun Jul 24, 2016 9:45 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
I had to answer yes, BUT I can't recall seeing an OVERDUTIED device fail. I've seen plenty of circuit breakers and fuses do what they are supposed to do and in particular with circuit breakers I've seen plenty do what they are not supposed to do, even while I'm standing there watching it. But not overdutied.

Which brings up a question of my own. Seems that most medium voltage expulsion fuses I've worked with (the Eaton/Cooper/S&C/LinePower series such as DBU, CMU, SMU, etc.) all have roughly a 12.5 kA interrupting rating in the 25 kV class which is somewhat common in the Southeast (Duke Power for instance uses loads of them). That's OK until you have at least a theoretical 20 kA+ rating. Now keep in mind that this is pretty much a theoretical thing. Expulsion fuses are MOSTLY an outdoor product (leaving aside for instance the SM-4, SM-5 types that can be used indoors with mufflers).

The thing is that when the thing works like it's supposed to (not overdutied) when it recognizes a fault, the boric acid inside heats up and it blows in half. You're left with two hangers and some charred remains. If it's overdutied as I understand it, it would explode. But that's the thing about expulsion fuses...what's the difference? They are designed to explode. Does that mean that it explodes "worse"? About 25% of the time you have to replace the fuse clips anyways because they are damaged in the process even on non-overdutied fuses. Rarely even the arm that the clips are hung on fails, too. So far the only thing I haven't seen destroyed is the body of the cutout.

So what happens? Does it just explode "worse"? Or is the 12.5 kA label really just a product of test lab results and the upper limit (if any) is unknown?

On a circuit breaker I can understand, as well as busbars. There are some extreme magnetic forces going on that are trying to force everything apart in the first place, by design. Generally speaking protective gaps in circuit breakers are small, and if you overdo it on a vacuum bottle the arcs coalesce in the bottle and the bottle fails. That all makes sense...but I'm having a hard time wrapping my head around what the consequence is on an expulsion fuse.

Granted this is all pretty much an academic exercise. A ground fault is already going to occur at 1.87 times less current by nature, and unless the grounding is unbelievably good, the impedance will drop the current enough so that it never sees anything over 12.5 kA. This means that the vast majority (.90%) of overhead line faults will not be overdutied. Furthermore, it only takes a couple thousand feet of line in the case I am concerned with to lower the short circuit current down to rated current, so from a mathematical point of view the likelihood that the fuse will ever see it is pretty small. But there is the obvious problem staring back from the short circuit study.

I'd even test it but throwing a piece of cable onto a line to cause a phase-to-phase fault that close to a protective device, knowing it is not designed to handle it...seems a bit risky to me.


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 Post subject: Re: Overdutied protective devices and catastrophic failures
PostPosted: Mon Jul 25, 2016 6:47 am 

Joined: Fri Jul 10, 2009 10:14 am
Posts: 27
Location: West Central, OH
A 22KAIC rated MCC that was connected to a 39KSCA system. Larger motor on system shorted on a restart. The fireball actually went out the upper end of the MCC unit. Only took 57 days to restore the MCC unit and restart the system. The site had the electrical study completed but had not taken the time to resolve the findings. Needless to say, they fast tracked a project to resolve the other issues at the site.


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 Post subject: Re: Overdutied protective devices and catastrophic failures
PostPosted: Fri Jul 29, 2016 10:16 am 
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Joined: Tue Jul 03, 2007 5:00 pm
Posts: 45
Location: Michigan
Never saw a device actually explode but I had to deal with cleaning up the mess. It wasn't too bad other than the outage. Needed to clean things up a bit, fix the problem and replace the breaker. Would not have wanted to be standing close when it happened. It was an old plant before anyone really seemed to care as much about breaker ratings.


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