Home main breaker arc flash?? Can it happen

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Old 08-07-08, 11:46 AM
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Home main breaker arc flash?? Can it happen

A household main breaker panel with 150 amps
Can an arc flash happen when shutting off the main breaker?
What is in the breaker that disipates the energy?
 
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Old 08-07-08, 12:18 PM
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A boss of mine always said to stand to one side of the panel (and head turned away) when throwing a breaker. He claimed the panels were designed to focus the explosion straight out. Don't know for sure if that is true but on a 480 did once see a panel cover travel straight out more then 20feet. Why take a chance on 240. Play it safe stand to one side and wear a heavy glove (burn protection not shock). Also when working in the panel remember to keep one hand in your pocket.
 
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Old 08-07-08, 12:46 PM
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I suppose in the sense of all things that are possible it could happen. However, OSHA does not consider a panel of that size or voltage to be an arc flash hazard. The only PPE requirement is natural fiber clothing (which applies to any work were exposure to heat or fire is a possibility), work gloves and safety glasses.
 
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Old 08-07-08, 01:28 PM
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"IEEE says that certain voltages and ampacities aren't considered arc-flash hazards. Systems rated below 240V-to-ground and below about 400A (125kVA) fit in this category. For all practical purposes, these systems can be considered Category 0. As such, residential electrical systems, small commercial 208/120V services, and the secondary of a 480-208/120V step-down transformer (112.5kVA or less) are defined as presenting little or no arc-flash hazard."

- http://ecmweb.com/mag/electric_keys_understanding_nfpa/

However there are still significant risks in residential panelboards - for example to your eyes from the splattering of wiring and panel components - especially in service equipment were there is a possibility of creating a short circuit at the SECs ahead of the of the service disconnect - though I'm only observing panels at inspections, not servicing them, I always wear eye protection over my standard prescription glasses before removing a deadfront.
 

Last edited by Michael Thomas; 08-07-08 at 02:25 PM.
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Old 08-07-08, 11:10 PM
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Most case on the 240 volt or less system on resdentail area the arc flash is pretty low unless you have bolted fault { true short circuit } and any bolted fault before the main breaker like example meter socket that have no fuse inside and if it was shorted out it will work like uncontrolled welder.

{ the short circuit on meter socket can happend due the POCO secondary circuit don't have fuse on them only on primary side (high voltage ) and they don't useally blow fuse not very fast at all.}

Just keep in your mind the arc flash can happend at any voltage even low voltage system like battery in automotive system can make arc flash as well.,,

Merci,Marc
 
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Old 08-08-08, 12:02 AM
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Maybe I am missing something?! Is this problem happening to you or you are just asking about the possibilities? Breakers of any size In my career will produce a arc if under the right or for that matter wrong conditions. The right conditions may be a large load associated with that circuit that would produce this effect. A short circuit will also show these symptoms. A previous gentleman eluded to staying away from the front of the panel. I remember throwing my first 2000 amp disconnect for a Rite-Aid I had wired and the spring loaded throw and sound I will never forget. No problems, but am sensing maybe the same fear I had when I was young in this businesss about worst case problems.
 
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Old 08-08-08, 06:08 AM
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Just asking the possibilities
I was showing my son how to shut off the main breaker in an emergency and started to wonder if I was putting him at risk.
I work as an operator in a power plant with 600v and 4.16 kv and 13.8kv breakers so I am comfortable throwing the switch.
But it did make me wonder if a small "blast" could come from the panel.
My conclusion is that it is done every day somewhere by someone. I will teach my son to turn off all the load breakers first and then use a glove and proper technique to throw the main switch.
Does this sound safe?
 
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Old 08-08-08, 07:24 AM
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Originally Posted by frankiee View Post
Just asking the possibilities
I was showing my son how to shut off the main breaker in an emergency and started to wonder if I was putting him at risk.
I work as an operator in a power plant with 600v and 4.16 kv and 13.8kv breakers so I am comfortable throwing the switch.
But it did make me wonder if a small "blast" could come from the panel.
My conclusion is that it is done every day somewhere by someone. I will teach my son to turn off all the load breakers first and then use a glove and proper technique to throw the main switch.
Does this sound safe?
IMO, with the deadfront in place your major concern (to the extent you ought to have one) is not related to arc flash, it's related to the possibility of an energized panel (that is, the metal exterior of the panel is energized).

This can occur for an number of reasons, for example because an incorrect fastener has been used to attach the deadfront and has damaged the insulation of a wire behind the deadfront and inside the panel:



Probably a quarter of the panels inspect have at least one sheet-metal or drywall screw installed, however in my experience an energized panel is a very uncommon.

Still, if you wanted to check you could buy a inexpensive non-contact tester, it will light or beep (depending on the type) when held just above the surface of on energized panel.
 

Last edited by Michael Thomas; 08-08-08 at 09:21 AM.
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