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 Post subject: Buss Duct Safety?
PostPosted: Mon Feb 10, 2014 7:32 am 

Joined: Fri Feb 07, 2014 12:23 pm
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Hello! This is my first post on this blog and I am fairly new to the safety field. Electricity is not an area that I have much knowledge, but I do know it is some dangerous stuff. The problem we have is that our main buss duct and main switch is all category four. We are working on a way to lessen the buss duct but till then we how should our maintenance department going about shutting down either the buss or the main switch. This question came up because the maintenance supervisor said he would rather have his guys disconnect the bus plug to change a fuse (the buss duct will still be live), rather then shutting down the main switch. His reasoning is the buss duct is the lesser of the two evils. Please advise on how to protect the employees.

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PostPosted: Mon Feb 10, 2014 12:03 pm 
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Location: North Carolina
Bus plugs are definitely not the lesser of two evils for two reasons.

1. The bus plug in the designs that I have used does not have a mechanism such as an overhung cam to force the energized contacts apart rapidly (milliseconds). Thus it is most similar in design to the stabs on MCC buckets and draw-out switchgear cells. The type of design that does not result in rapid mechanical interruption has frequent failures. Draw out switchgear for instance is 80% more likely to develop an arc flash than bolted-in equipment with the exact same circuit breakers. While circuit breakers and disconnect switches can and do fail, the failure rates are almost an order of magnitude less than opening and closing "stabs", whether switchgear, MCC buckets, or bus duct. Thus you are drastically increasing the likelihood of an arc flash if you switch from opening and closing contacts that have spring or magnetic separation and closing mechanisms to those that don't have any kind of "rapid" contact separation mechanism.
2. In many cases as you move further away from the power source, the resistance causes a decrease in current. This would seem to be safer but there is a catch to it. Also, the fault current and thus the time it takes the breaker or fuse to trip is longer, which usually (but not always) results in higher arcing energy released. This is not always the case such as if you are operating in the instantaneous region of the breaker or if the fault current is so low that it reaches the cutoff on the arcing time (typically set to 2 seconds). But for most realistic cases where arc flash is considered to be an issue, increasing the distance also typically increases the hazard. Now this effect is typically small and makes only a slight difference in the incident energy. Frequently from a very practical perspective the difference is nonexistant. The difference between say 14 and 15 cal/cm^2 is not important...a 25 or 40 cal/cm^2 multilayer arc flash suit is still required. But if you are already borderline it can be the difference between say 1 and 1.5 cal (nonmeltable clothing vs. arc flash PPE).

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PostPosted: Mon Feb 17, 2014 7:37 am 
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Joined: Fri Dec 06, 2013 9:30 am
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Location: Pittsburgh, PA
You really need to have an experienced engineer evaluate your options to modify the system to decrease the arc flash incident energy. One potential solution might be to shunt trip your main switch with an instantaneous "maintenance mode" relay that could be switched into the circuit only when maintenance is being performed. Idealy, this relay would have an instantaneous setting low enough to trip for an arcing fault at the far end of the buss duct but high enough not to nuisance trip when a motor starts or for a fault further downstream in your system (i.e., non-selective coordination). It may even be possible to have this setting active during normal operation, which would be preferred, but your system would have to be evaluated before an optimal recommendation can be made. I have performed many evaluations of this type. Feel free to message me if you would like to discuss further.

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PostPosted: Mon Feb 17, 2014 12:09 pm 

Joined: Wed Feb 01, 2012 10:44 am
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It seems to me that you have two situations run together. The first situation is to de-energize the main switch, which would then de-energize the entire bus duct, and the second is to replace fuses in a bus plug. For the first situation, I would assume there is no other way to de-energize the main bus unless the main switch is opened (assuming you have a fused switch and not a breaker?). Opening this switch with proper PPE would be the correct way to de-energize the entire bus duct. This would be the safest way to add/remove/move a bus plug.

However, to just replace the fuses in a bus plug, you do not need to open the main switch. The bus plug should have a disconnect to de-energize the fuses in the bus plug. Proper PPE should still be worn, but the fuses would be isolated from the main bus by the bus plug's disconnect. If in a high ceiling area, a hookstick can be used to further isolate the worker from the bus plug during the switch opening procedure.

In days past, bus plugs were added/moved/removed from energized main buses. Energized bus plug work can still be done today, but the risk and liability are increased, a higher skill set is necessary for the electrical workers, etc. The variables are more than I can address here.

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