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 Post subject: Arc Flash Reduction
PostPosted: Tue Dec 13, 2011 6:00 am 

Joined: Tue Feb 17, 2009 11:04 am
Posts: 37
At my facility, I have a 2000 amp main breaker (1960’s vintage) with a digitrip on it. The breaker feeds 480 volts into 4(500mn) per phase cables. These cables are then separated into two separate power cables using different buses. Each cable is 2 (350mn) per phase. After the large box with the buses, the cables go into a disconnect, one for each set of power cables. Each power cable goes into an AJAX power supply with a 550 amp breaker on it, about 20 feet away. Currently, the arc flash at the disconnects is calculated to be 82cal/cm2. The reason it is so high is because the time is 2 seconds due to the fact the digitrip may not blow. What would be the best way to reduce the arc flash hazard to protect the safety of the operators? Operators need to use the disconnects for lock and tag, when they are working on a furnace. The box is always closed and no work is being done inside the box.
Currently, we have the operators using a hot stick at 4’ from the box with a 40 cal suit to protect them. The calculated arc flash at 36” is 32cal/cm2.


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PostPosted: Tue Dec 13, 2011 7:51 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 525
Location: Wisconsin
Have you considered rebuilding or replacing the 2000A breaker? A properly maintained breaker with an Instantaneous setting should provide protection to your disconnects.


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PostPosted: Tue Dec 13, 2011 8:21 am 
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Joined: Mon Jun 02, 2008 11:58 am
Posts: 1103
Location: Charlotte, NC
Retrofit the breaker with an AC PRO and quick trip (Arc flash reduction switch), we do these retrofits everyday.


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PostPosted: Tue Dec 13, 2011 2:39 pm 
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Location: North Carolina
Vica1ME wrote:
At my facility, I have a 2000 amp main breaker (1960’s vintage) with a digitrip on it. The breaker feeds 480 volts into 4(500mn) per phase cables. These cables are then separated into two separate power cables using different buses. Each cable is 2 (350mn) per phase. After the large box with the buses, the cables go into a disconnect, one for each set of power cables. Each power cable goes into an AJAX power supply with a 550 amp breaker on it, about 20 feet away. Currently, the arc flash at the disconnects is calculated to be 82cal/cm2. The reason it is so high is because the time is 2 seconds due to the fact the digitrip may not blow. What would be the best way to reduce the arc flash hazard to protect the safety of the operators? Operators need to use the disconnects for lock and tag, when they are working on a furnace. The box is always closed and no work is being done inside the box.
Currently, we have the operators using a hot stick at 4’ from the box with a 40 cal suit to protect them. The calculated arc flash at 36” is 32cal/cm2.


I suggest you look at IEEE Gold Book and make an estimate of the potential failure rate of the circuit breaker itself. The new NFPA 70E-2012 also speaks to the potential failure rates. If the failure rate is acceptably low, then there may be no reason to mess with the PPE in the first place.

Second, digitrips stink. Your range of adjustability is horrendous. I'd retrofit with either an AC Pro as mentioned or if you want the ultimate in flexibility, to an SEL 751A. You may also want to consider just getting rid of the entire circuit breaker because the opening speed of that particular mechanism is going to be a major limiting factor. A nice Toshiba or if it's low voltage enough, a Joslyn Clark vaccuum breaker might be possible to retrofit in the same space if the current is truly low enough. Otherwise, you'd have to go to an SF6 retrofit if you wanted a better/faster/smaller unit.

I'm not sure why you are modelling the concept of the Digitrip failing by the way. Electronic trip units, even old Digitrips, are vastly more reliable than dash pots and electromechanical designs. I've had plenty of foundry experience and at this point I'm completely sold on electronic trip units as far as reliability goes.

Final option with your arrangement is to put in cable protectors. Basically you can get a fuse small enough to fit inside the existing enclosures that bolts onto the existing lugs. This gives you fuse speeds (4-5 ms) and current limiting for a relatively low cost. Just ignore the intended purpose of the fuse (cable protection in multiple conductor/phase scenario with redundancy) and instead use it as simply a fuse. In this case you want the unit to current limit/trip ONLY in the event of a high current short circuit. Then the existing circuit breaker takes care of everything else.


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PostPosted: Wed Dec 14, 2011 12:50 pm 

Joined: Wed Dec 14, 2011 11:09 am
Posts: 2
Hi,
I am the person with this problem. Vica1ME posted it for me.
Who can install the AC Pro arc reduction switch or similar technology in Cleveland, OH? What is the advantage of this over in line fuses or breakers?


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PostPosted: Wed Dec 14, 2011 2:28 pm 
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graphguy wrote:
Hi,
I am the person with this problem. Vica1ME posted it for me.
Who can install the AC Pro arc reduction switch or similar technology in Cleveland, OH? What is the advantage of this over in line fuses or breakers?


You can install them yourself or pretty much any local circuit breaker maintenance shop is probably familiar with them and can do it. The kit that it comes with is outstanding in this regard as it is very comprehensive (comes with shims, detailed instructions, mounting brackets, etc.).

Let's be clear what we are talking about though. An AC Pro trip unit is just that...a trip unit. It is sold by Utility Relay Company. You can get full documentation on their web site. What you do is take an old circuit breaker, clean it up, install new CT's and a microprocessor trip relay onto it. This replaces your old trip unit, including Digitrips.

The advantage of this particular trip unit is that it is very fast relative to electromechanical technology, much more adjustable than the Digitrip (very useful for difficult coordination problems), very easy to set up, the display doesn't go bad over the years like the Digitrips do, and it comes with pretty much every "bell and whistle" as a complete system. For instance it has Modbus communication built in if you want to connect it to your SCADA system for remote monitoring and diagnostics.

The underlying physical breaker mechanism remains the same though. So whatever the mechanical opening/closing time is, and any mechanical problems with it, remains unchanged.

Compared to an electromechanical trip unit or a fuse, it is much more accurate and usually a good deal faster than the original trip unit. Compared to the Digitrip it's probably about the same...you are simply limited by circuit breaker speed. Compared to a fuse which trips in 1/4 cycle, your limiting factor is the speed of the breaker itself. You are trading very good adjustability and capability to coordinate with the loss in speed. There are also some obvious heat dissipation advantages of electronic trip units (basically, no heaters) vs. both fuses and electromechanical trip units.

The "maintenance switch" thing is something of a gimmick that the Digitrip can't do but most modern trip relays can do. You can have multiple trip curves which are push button selectable. So many plants are putting instantaneous trips on a selector switch. With instantaneous tripping off (or delayed...basically an ST-only curve), you get your normal, well coordinated system. With the switch on, you ruin coordination but set the breaker to trip as fast as possible with a very low setting so that in the event that an arcing fault occurs, the breaker goes into instantaneous trip mode and opens as quickly as possible. Often this is the difference between <4 cal/cm^2 and 30+ cal/cm^2.


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PostPosted: Thu Dec 15, 2011 6:01 am 

Joined: Wed Dec 14, 2011 11:09 am
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Thanks for the help.


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PostPosted: Thu Dec 15, 2011 7:42 am 
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Joined: Mon Jun 02, 2008 11:58 am
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Location: Charlotte, NC
PaulEngr wrote:
You can install them yourself or pretty much any local circuit breaker maintenance shop is probably familiar with them and can do it. The kit that it comes with is outstanding in this regard as it is very comprehensive (comes with shims, detailed instructions, mounting brackets, etc.).
Easy enough to do your self but most places don't have the test equipment to test it after installation.

PaulEngr wrote:
The advantage of this particular trip unit is that it is very fast relative to electromechanical technology, much more adjustable than the Digitrip (very useful for difficult coordination problems), very easy to set up, the display doesn't go bad over the years like the Digitrips do, and it comes with pretty much every "bell and whistle" as a complete system. For instance it has Modbus communication built in if you want to connect it to your SCADA system for remote monitoring and diagnostics.
AC PRO has a lot of bells and whistles (Every function you can think of that you can turn on or off and never a need for changing rating plugs) but communications does not come with the AC PRO, that is an upgrade and a pretty big adder if you need it.

The underlying physical breaker mechanism remains the same though. So whatever the mechanical opening/closing time is, and any mechanical problems with it, remains unchanged.

PaulEngr wrote:
The "maintenance switch" thing is something of a gimmick that the Digitrip can't do but most modern trip relays can do. You can have multiple trip curves which are push button selectable. So many plants are putting instantaneous trips on a selector switch. With instantaneous tripping off (or delayed...basically an ST-only curve), you get your normal, well coordinated system. With the switch on, you ruin coordination but set the breaker to trip as fast as possible with a very low setting so that in the event that an arcing fault occurs, the breaker goes into instantaneous trip mode and opens as quickly as possible. Often this is the difference between <4 cal/cm^2 and 30+ cal/cm^2.


There is an excellent PPT on the URC website that explains how this works (I wrote it for them many years ago :) )


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