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 Post subject: High Energy Situation
PostPosted: Wed Oct 13, 2010 12:19 pm 
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Joined: Mon Apr 13, 2009 11:54 am
Posts: 8
Location: Midland, Tx
I am doing a study and have some across question. I have 1000kVA transformer bank (primary 12470V secondary 480V) that feeds into two 750A Square D Fused Safety Switches. Incident energy on the primary side of the switch is 73. The primary of the transformers is protected by fused cutouts. I have looked at changing out the fuses in the cutout but was not able to bring it down below 40. I am able to put in a Recloser that could bring it down to a category 3. Does anyone have any other sugestions? And just so I understand when we talk about arc flash boundaries and PPE this refers to someone working on the equipment (IE opening or closing the switch). This equipment is outside and anyone can walk around it.


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PostPosted: Wed Oct 13, 2010 1:19 pm 
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Joined: Wed Jul 07, 2010 9:26 pm
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Location: Toronto, ON, Canada
What about the secondary protection? primary protection you cannot do much about reducing the size due to the inrush current, it's typically better to have it a bit overfused, but the secondary protection of the transformer (on the secondary side, downstream, load side), you could play around with the fuse size and type, if that did not work, you can look into retrofitting the fuse with an LSI trip unit circuit breaker, and adjust instantaneous settings in order to bring down arc flash levels,
if LSI trip unit upgrade is too expensive, you can look into IR window installation for the IR scans, and perhaps remote switching and racking, and working de-energized


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PostPosted: Wed Oct 13, 2010 1:25 pm 
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Location: Charlotte, NC
You can use a remote switch actuator to operate the switch outside the AFB.


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PostPosted: Wed Oct 13, 2010 1:38 pm 
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Joined: Mon Apr 13, 2009 11:54 am
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Location: Midland, Tx
Okay maybe this is more of my lack of understanding then. The safety switch one-line representation is a switch followed by a fuse. The arc potential is at the point where the switch would break contact which is before the fuse. Changing that fuse size would not help the rating on the switch. So if i put another protective device (ie circuit breaker) before then dont I create the same situation at the point where the circuit breaker trips open? I ask this cause after the fuse, the category is a 2.


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PostPosted: Wed Oct 13, 2010 3:16 pm 
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Location: Charlotte, NC
mahan is refering (I assume) the the 12,470 switch and fuses, you can install a mini MV VCB in place of those fuses with current (or light) sensing on the secondary side of the transfromer, which will reduce you Ei on your 480V switch dramatically. We have done this retrofit for a few companies but it is very expensive.


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PostPosted: Thu Oct 14, 2010 7:27 am 
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Joined: Wed May 07, 2008 5:00 pm
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Location: Rutland, VT
From your description I assume that the secondary side of the transformer feeds directly into the fused disconnect switches.

I would think that the least expensive option would be a remote operating device. What is the IE at at distance of 4 to 6 feet? Could the switch be operated at a distance using just a hotstick? How many times is this switch operated in a year? If it is daily, the cost justification could be made to replace it with a device designed to operate remotely.

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Barry Donovan, P.E.
www.workplacesafetysolutions.com


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