Safety Articles

Response to student Arc Flash Questions

May 27, 2014 | Uncategorized

I regularly conduct NFPA 70E training courses. Listed below are two questions that may enhance your understanding of electric arcs with respect to receptacles as well as residential homes:

1. Is there an electric arc flash hazard from a 120 volt electrical receptacle?
Response: A single phase 120V circuit would not really sustain an arc due to low fault current. The clearing time is very fast. That is why IEEE states not to calculate or be concerned on single phase circuits below 240V if fed from sources less than 125KVA.

2. What is typical short circuit current going from transformer to a residential home?
Response: Most transformers feeding homes are 25-50KVA single phase- 120/240V. These are generally low impedance transformers, but by the time the wiring etc. is considered, the fault current is in the 3000A range AT THE PANEL. The small #12 wire drops this quickly for branch circuits. Again, this is generally single phase, so the arc does not sustain past ½ cycle at this current and voltage level. This can still give a loud BANG like on a dryer circuit etc. if a plug comes apart. But the thing to remember is you are dealing with what would cause a second degree burn 18” away from the item basically instantly.

Jon