# Disconnect arcing



## ELECTRICTECH (Nov 15, 2015)

I'm new to this site and happy to become a member.

So I had a Quarry electrician ask me which is more dangerous, the opening of a 480 disconnect or the closing of a 480 disconnect. He explained that the opening is more dangerous due to the plasma and the ionization of the air. I believe if there is a load then yes the opening of the disconnect will draw an arc and could be greater of the two evils but only if there is a load.
Otherwise they would react the same if there is no load. 

Not sure if I am correct hence the reason I'm putting this question to the members,

Thanks
Marc


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## JohnDeere (Nov 10, 2015)

I wouldn't open a disconnect under heavy load and I wouldn't close a disconnect into a known short. Other than that I regard disconnects as a piece of equipment that will operate safely when used as intended. 


Sent from my iPad using Tapatalk


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## Big John (May 23, 2010)

You do proper equipment maintenance and neither one will be dangerous.

That said, there can be more uncertainty when closing a disco because of the possibility of closing into a fault. 

Opening pretty much only risks mechanical failure.


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## wildleg (Apr 12, 2009)

you can look up the cut sheets for a lot of modern safety switches (light and heavy duty) online and see the descriptions for safe operation under load. 

However, I have seen a lot of older stuff, especially older buckets, that I would not want to be near closing or opening under load. It's certainly not best practice.


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## John Valdes (May 17, 2007)

Would a quarry electrician be different than a regular electrician?
I mean I have worked in quarry's before and I even worked for a company called "Quarry Electric" out of Hollywood Fl.
I'm just curious as to why you used the term "quarry electrician"?


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## Big John (May 23, 2010)

Probably because jurisdiction falls under MSHA so there's a bit of a special world to comply with.


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## CADPoint (Jul 5, 2007)

Welcome to the Forum!

I personally don't know the exact answer to your question as you asked it.
My thoughts are that infact you are trying to understand more of the physics
verses a safe work practices. 

In most cases standard work safety is handled for most electricians by OSHA.

In you case MSHA - Mine Safety and Health Administration should cover
you rules of applying and using electricity.

www.msha.gov

Since both agencies are under the Department of Labor, I don't think there is any differences and that you aren't allowed to use and apply electricity as your stateing. 

I usually don't have to turn off gear under load and also don't turn much large equipment on either. What I have seen done in the field is the use of hooks and loops on poles or even rope to power up equipment. People also always stand off to the side of these devices including breakers as well.

You can read more about Arcing and/or Arcing and Sparking, and also
Arc Flashing > Understanding Arcing Equipment.

Enjoy


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## cuba_pete (Dec 8, 2011)

Opening a 480 with even a small trafo as a load will still produce enough of a spike to trip upstream breakers, even through conditioners.

Been there, done that.


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## emtnut (Mar 1, 2015)

It's been almost 20yrs since I worked in mv.
From what this old head remembers, the old air breakers had a slightly higher possibility of arcing on opening. On closing, if an arc started, since the breaker was closing, there was less chance of O2 depletion. On opening, the arc would build up, and burn more O2, feed the arc, and so on !
Not referencing open/close on a fault


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## JRaef (Mar 23, 2009)

ELECTRICTECH said:


> I'm new to this site and happy to become a member.
> 
> So I had a Quarry electrician ask me which is more dangerous, the opening of a 480 disconnect or the closing of a 480 disconnect. He explained that the opening is more dangerous due to the plasma and the ionization of the air. I believe if there is a load then yes the opening of the disconnect will draw an arc and could be greater of the two evils but only if there is a load.
> Otherwise they would react the same if there is no load.
> ...


The problem with a discussion like this among electricians is that there is no simple universal right answer. As you already noticed, you have to qualify it: Load or No Load? Fault or No Fault? etc. etc. etc.

Aside from that, and speaking ONLY of non-fault but loaded conditions, why is arcing under normal circumstances inherently evil or dangerous? When opened under load, ALL switches produce an arc, and plasma, and ionization. The smaller the load and the lower the voltage, the smaller the arc, but it is always there. The trick is containing it _safely_. 

All safety switches are, by definition, a SAFE way to deal with the normal arcing. The REASON they are called "Safety Switches" is exactly that; before that, people used open knife switches, and the arc that was drawn on opening under load would often burn the operator.








So mfrs came up with the idea of enclosing the switches inside of a strong enclosure (the first were cast iron) and called them "Safety Switches".

And by the way, there is an arc when you CLOSE a switch as well. Once the contact surfaces get close enough to where the dielectric of the air gap is low enough, the current jumps the gap, albeit for just a fraction of a second. But again, it is contained SAFELY.

Safety switches, if listed under UL 98 (not all are), must be tested as "Load Break / Fault Make" duty. That means they can SAFELY break a load up to their continuous (or HP) rating, and they can SAFELY close into a fault up to their short circuit rating. But you will notice that a non-fused safety switch will have a much much lower fault current rating than a fused version of the exact same switch. i.e. 10kA vs 200kA. That's because on the fused switch, they are relying on the clearing time of the fuse.


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## ELECTRICK2 (Feb 21, 2015)

cuba_pete said:


> Opening a 480 with even a small trafo as a load will still produce enough of a spike to trip upstream breakers, even through conditioners.
> 
> Been there, done that.


What's a trafo?
Prolly a stupid question but....


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## JRaef (Mar 23, 2009)

Trafo = common British/European shortening of "transformer".


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## Big John (May 23, 2010)

I swear years ago I saw reasonably modern LV disconnects (manufactured in the 70s or later) that actually said they weren't for transformer duty.

Only time I've ever run into that.


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