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 Post subject: Insulated Bus and Arc Flash
PostPosted: Wed Mar 20, 2013 3:29 pm 
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I was recently asked about if insulating the bus in a switchgear or motor control center would have any affects on arc flash mitigation and or reduction of electrical accidents? Would this reduce three phase faults? Would the gear still have the ability to achieve such an arc? So thought I would throw this out to the forum. I appreciate your responses.


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PostPosted: Wed Mar 20, 2013 4:05 pm 
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EPRI has done some work on this with overhead lines. Sometimes the arc stops when it hits an insulating cover - not quite the same thing but same idea. If a bus was insulated, it would be difficult for the arc to begin. All that being said, I believe the answer is Yes, it helps but No, you can't take credit for it when thinking about "what if". Any others have ideas about this?

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PostPosted: Wed Mar 20, 2013 4:11 pm 
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Yes and no. It may reduce the likelihood of a failure, and shift around the types of failures. For instance vault mounted gear which is solidly insulated is extremely well protected. But when a fault occurs, insulation alone will not change the amount of energy released. If the likelihood of a fault is low enough that it is acceptable then there is no need for protection if we accept the risk. This is the difference between looking at the hazard (how bad can it get) and likelihood (what's the chance), both of which determine risk.


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PostPosted: Mon Mar 25, 2013 10:33 am 
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I know of two papers discussing insulated bus bars:

Staged Tests Increase Awareness of Arc-Flash Hazards in Electrical Equipment
(IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 36, NO. 2, MARCH/APRIL 2000)

and

The Behavior of Arcing Faults in Low Voltage Switchboards
H. Bruce Land, III


The first paper states that insulated bus bars extinguished the arc and minimized the damage.

The second paper however, by H. Bruce Land, III concluded that:
"The conclusion of the insulation testing is that insulation will not extinguish a moving arc. As insulation becomes involved in the arcing it will enhance the stability of the arc and will increase damage. Thus while insulation can reduce the probability of the occurrence of an arc from one cause, if an arc should occur the insulation will cause the arc to be more stable and the damage to be more extensive. These conclusions from the laboratory testing have now been confirmed by actual arcing faults in 450 and 4160 VAC switchboards, constructed with and without bus insulation, that have been deployed on Navy ships."


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PostPosted: Mon Mar 25, 2013 11:54 am 
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I am not aware of a standard or guide that allows you to take credit in terms of incident energy values or PPE in the case there is an event. However, if during a hazard assesment you believe there is enough insulation and isolation to change the nature of the potential hazzard I believe that can be taken into account. If you look at a potential hazard as a combination of "exposure", "potential severity" and "probability of an event during the exposure, with that level of severity", then having a well insulated system addresses the "probability" aspect of this analysis and may make a difference from that perspective. However, once you have incident energy calculations for an exposure the insulation/isolation should be looked at from the perspective of is there, or is there not, a distinct probability of an incident. If there is, then the PPE and other precautions should be dictated by the expected severity, i e. incident energy expected. A partial reduction in probability may make the situation better as far as probability of an event, but it should not, normally, reduce the precautions taken. Nevertheless, it is good to consider doing whatever can be done to improve the probability of an event not happening. Not an EHS expert, but this would be my interpretation of the current literature and discussions I have heard on the subject.


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