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Will you begin using the instantaneous to reduce arc flash energy for breakers 1200A and greater?
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 Post subject: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Sun Oct 16, 2016 7:18 am 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1468
Location: Scottsdale, Arizona
The NEC requires that where a circuit breaker’s highest continuous current trip setting is rated or can be adjusted to 1200 Amps or higher, a method for reducing the arc energy must be provided.

The 2014 Edition of the NEC provided a list of methods which included: zone-selective interlocking, differential relaying, energy-reducing maintenance switching and energy-reducing active mitigation systems or an approved equivalent means. The device’s instantaneous trip function was excluded from the list.

The 2017 Edition of the NEC now permits the use of the device’s instantaneous trip unit or instantaneous override as long as there is sufficient arcing short circuit current for it to trip. IEEE 1584 – IEEE Guide for Performing Arc-Flash Hazard Calculations is referenced in this section as a method for calculating the arcing short circuit current.

Since the National Electrical Code (NEC) is a U.S. based standard, this requirement may not be applicable to those in other countries. It would be interesting to hear the different views.

With that long introduction, here is this week’s question:

Will you/your clients begin using the instantaneous to reduce arc flash energy for breakers 1200A and greater?

Yes
No
Doesn’t apply
We were using it anyway/already


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Oct 17, 2016 3:34 am 
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Joined: Fri Jul 08, 2016 10:01 am
Posts: 227
Location: Indiana
The State I'm in is still under the 2008 NEC so I don't have a lot of experience with this, however, I have used the maintenance mode function a couple of times. It appears to me you can accomplish the same thing by changing the settings on the breaker like you would for coordination purposes and just adjust the instantaneous setting to the lowest setting. This is more cumbersome though than "flipping the switch".

I think I would rather have the maintenance mode switch than have to change settings. It's easy to change the settings on some breakers but some it is not so intuitive and some you have to take covers of to get to the dials or keypad. If you're on a college campus like me with dozens of buildings and gear by every manufacturer it would be easiest just to have the maintenance mode switch than have to remember how to change settings on many different breakers.

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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Oct 17, 2016 8:34 am 
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Joined: Fri Apr 15, 2011 7:43 am
Posts: 177
Location: Colorado
The problem with low voltage breakers is there is not enough flexibility to dis-able the instantaneous setting (or set it high enough) so there is not a mis-coordination between the main and feeder breakers.

Many engineers do not understand protection and simply will add these new "arc flash" breakers without understanding where or when to apply instantaneous settings. The plant is then left with an electric system that will trip the main breaker which may result in a dangerous situation.

I have worked with "arc flash" breakers that have an arc reduction setting (lower instantaneous) but do not have high enough setting to remove the instantaneous. Not so great for controlled shut down situations. I have not run across any breakers that are programmable, enough, to work properly without going to a switchgear style breaker with a relay. They probably are out there but I just have not seen one


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Oct 17, 2016 9:25 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Some of the electronic trip units are pretty good and some are horrendous. I believe the GE Spectrum RMS ones have quite a bit of adjustability but the old Digitrips (particularly Westinghouse) are terrible and might as well not be adjustable at all given the narrow range the work in. There are electronic trip units available at least with ABB and GE that work pretty good. Also if you are retrofitting, it is very hard to beat the URC Arc Pro and Arc Pro 2. The situation is very different with Cutler Hammer and Square D breakers...they are unimpressive to say the least. Also watch out on pricing. An electronic relay basically just uses CT's and a microprocessor controller to fire a trip pulse. Starting at around 400 A, they SHOULD be less expensive than their electro/thermal/mechanical counterparts but a few manufacturers gouge the customer very badly on their electronic trip units even though the manufacturer's costs drop significantly.


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Tue Oct 25, 2016 11:17 am 

Joined: Mon Feb 23, 2015 10:19 am
Posts: 7
I work at a paper mill where they utilize secondary main power breakers on each of their unit substation transformers. The main breaker trip units are usually capable of higher than a 1200A setting.
We normally set the instantaneous setting of these main breakers up out of the way and use the short time settings to clear for bus faults thereby coordinating with the feeder breaker trip units where instantaneous settings are employed.

Is this NEC article now outlawing this practice?


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Oct 31, 2016 7:37 am 
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Joined: Thu Oct 18, 2007 7:10 pm
Posts: 261
Location: NW USA
bubba wrote:
I work at a paper mill where they utilize secondary main power breakers on each of their unit substation transformers. The main breaker trip units are usually capable of higher than a 1200A setting.
We normally set the instantaneous setting of these main breakers up out of the way and use the short time settings to clear for bus faults thereby coordinating with the feeder breaker trip units where instantaneous settings are employed.

Is this NEC article now outlawing this practice?


I suspect the NEC was written in anticipation of large commercial building service entrances, but now it creeps into your field of work too.

If you need to work on the bus energized and there is excess arc flash exposure, you will probably have to set instantaneous at some pickup level to make this workable, which might have been your procedure previously anyway (we've done that at a refinery now for several years without mishap). The likelihood of a high magnitude fault is pretty low, and in the event of such a catastrophic event, shutting down the bus might be justified.


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Oct 31, 2016 8:10 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 511
Location: Wisconsin
bubba wrote:
I work at a paper mill where they utilize secondary main power breakers on each of their unit substation transformers. The main breaker trip units are usually capable of higher than a 1200A setting.
We normally set the instantaneous setting of these main breakers up out of the way and use the short time settings to clear for bus faults thereby coordinating with the feeder breaker trip units where instantaneous settings are employed.

Is this NEC article now outlawing this practice?


The NEC says the breaker needs to have an 'instantaneous trip' function, it does not tell you how it needs to be used nor what minimum level is required.
For the most part this section pretty much just prohibits devices with only 'short time' settings (i.e. LS).


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 Post subject: Re: 2017 NEC 240.87 Instantaneous Trip
PostPosted: Mon Nov 07, 2016 8:37 am 

Joined: Mon Feb 23, 2015 10:19 am
Posts: 7
The wording of the 240.87 (B).5 is:

"An instantaneous trip setting that is less than the available arcing current".

This is pretty clear.
Jeff Hill


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