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 Post subject: LV Panelboards
PostPosted: Wed Jun 17, 2015 2:06 pm 

Joined: Mon Mar 29, 2010 8:31 am
Posts: 24
Location: Jonesboro, AR
I have (3) 480VAC panel boards in my facility that are protected by 400 Amp RK5 fuses and lie several hundred feet from the main gear and the overcurrent protection. With the small amount of fault current provided by the utility the melting time of these fuses is 1.5-3 seconds at the available arcing current at both full arching current and 15% reduction when taken from the panel location. From what I can determine this puts these 3 panels around 20 cal/cm^2 in incident energy. 2 of these panels reside in small rooms (one office, one small electric utility room) with the arc flash boundary around 9ft. The size of the rooms and other equipment that is in the rooms may impede the use of the 2s rule. So what are acceptable ways to reduce the opening time to reduce the incident energy? I'd like to get down to @ 8 cal/cm^2 so that we don't have to dress up in the moon suit to prove the panel is dead.

Is sizing the fuse to the current load an acceptable practice? One of these panels is lighting for the office space and not subject to change very much. Would sizing a fuse for 125% of the current load be an acceptable way to reduce the incident energy?


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 Post subject: Re: LV Panelboards
PostPosted: Thu Jun 18, 2015 5:39 am 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 525
Location: Wisconsin
Have you performed a risk analysis to determine if a hazard actually exists?
Small panelboards containing molded case breakers are not known for 'exploding into fireballs' when they have been applied on the correct voltage and with the appropriate SCCR.

Absolutely, you can reduce the size of your fuses if that will help and the loading allows it.
Have you looked into Class J fuses (if the switch will accept them) or tried different manufacturers?


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 Post subject: Re: LV Panelboards
PostPosted: Fri Jun 19, 2015 4:23 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 832
Location: Rutland, VT
If the fuse holder allows it, you might want to look at a A6D fuse. I have had some success with reducing IE by using that fuse.

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 Post subject: Re: LV Panelboards
PostPosted: Fri Jun 19, 2015 11:21 am 
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Joined: Tue Oct 26, 2010 9:08 am
Posts: 2174
Location: North Carolina
Class RK5 fuses are really intended for motor loads. The reason for the current limiting function is to allow the motor to start without tripping the fuse. It's not really intended to handle resistive (lighting) loads but with several manufacturers if you order a fused disconnect switch without specifying the fuse class, that's what you get.

Further, class RK5 is really just describing a fuse package these days as is the Class J fuses. There are standards on fuses that specify an outside limit in terms of melt time but you can freely melt faster. That's what the A6D fuses are about...giving you Class J/RK1 performance in an RK5 package. If you order a new disconnect, I'd encourage giving serious consideration to using Class J fuses. They are smaller, cheaper, and faster than Class RK5's, although with only resistive loads again the current limiting function is probably doing a disservice by decreasing the current which INCREASES trip time and arc flash in many ccases.

When sizing fuses, be careful of the downstream load. The recommended sizing is indeed 125% of the maximum current allowed for lighting loads. It is 175% of motor loads (for 3 phase motor loads) but that's only a maximum...quite often you can go down to as low as 125% depending on the starting time of the motor without causing nuisance tripping. However in either case this only reduces arc flash downstream of the fused disconnect. If you want to address it at the disconnect, you need to go upstream of it.

You might also be better off with a circuit breaker in either case. Especially microprocessor circuit breakers are much more precise than fuses. At high (short circuit) conditions, fuses win every time since even the fastest breakers rarely open any faster than 1 cycle for small molded case breakers while fuses open at 1/4 cycle (4 ms). But under less than short circuit condiitons wuch as what you are describing, breakers win every time because the opening rate is almost a constant of between 1 and 4 cycles (depending on the size of the breaker) and trip units can be as fast or slow as you need. Setting a breaker's instantaneous trip point to just below the arcing current (the 15% lower case) allows you to trip in under 100 ms whereas with a fuse by the time you get fast tripping, the current rating at the slow end may be so low that it nuisance trips under normal operating conditions. And with some microprocessor breakers you can have selective trip functions so that for instance you can flip a switch ("maintenance switch") to turn on instantaneous tripping. This will interfere with coordination during normal operating conditions but dramatically reduces arc flash if used during maintenance activities.


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 Post subject: Re: LV Panelboards
PostPosted: Sun Jun 28, 2015 7:58 am 

Joined: Mon Jun 09, 2014 9:19 am
Posts: 43
We have had good luck lowering our hazards at our facilities with Class J or RK1 fuses. Another option is to replace the fused disconnect with a circuit breaker disconnect. You can get a current limiting circuit breaker that may lower the hazard to less than 4cal/cm^2. Whatever you decide to try, you can play around with the model in your software and try different fuse options to see what works best for your facilities. If you have an outside engineering firm helping you, most will work with you to find the best option.


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