I work for a utility and they are telling me that all I need for Arc Flash rating is a cal. 4 for voltages under 1000v and cal. 8 for voltages above 1000 volts. Doesnt the rating change with clearing times of relays? I thought the lower voltages have higher arc flash hzards. I brought this up to safety members and they did not agree
Arc Flash
- Thread starter newt
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mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
THat's pretty scary advice indeed! Yes, generally speaking the lower voltage will have a higher incident energy for the same power rating. But you should be directed to meet the requirements of an arc-flash label on each piece of equipment, since there are so many factors that go into determination of the appropraite PPE for a given bus.
Their recommended 8 calorie PPE (HRC 2) may very well be appropriate for many of the systems above 1kV, but there still needs to be a determination made for each bus that looks at all of the potential sources and scenarios.
John M
Their recommended 8 calorie PPE (HRC 2) may very well be appropriate for many of the systems above 1kV, but there still needs to be a determination made for each bus that looks at all of the potential sources and scenarios.
John M
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Apples and Oranges
Apples and Oranges
Utility 'line' workers are covered under the NESC arc flash rules, which are different from those in NFPA70E.
Apples and Oranges
Utility 'line' workers are covered under the NESC arc flash rules, which are different from those in NFPA70E.
mayanees
Senior Member
- Location
- Westminster, MD
- Occupation
- Electrical Engineer and Master Electrician
Jim speaks the truth. That's likely NESC-compliant apparel for your application.
The fact remains that the lower voltages have higher incident energy levels when compared to like MV systems, but NESC rules apply, consistent with the OP.
John M
The fact remains that the lower voltages have higher incident energy levels when compared to like MV systems, but NESC rules apply, consistent with the OP.
John M
Thats to bad I know we have many cases were the arc flash hazard has to be much higher than 4 cal.(480 volt meter cans) why is it that utlilty can follow a standard that seems to be inadequate? Plus the fact that we are using wiggins meters and digitial volt meters with cat 3 rattings and lower.
A 1 Megawatt system will probably have a much higher arc flash energy at 480V than it will at 4160 Volts. That's easy to see - if the clearing times are the same, the 480V system can supply almost 10 times as much current into a short circuit. (At 480V, the circuit breaker or fuse rating would be almost 10x its value at 4160V).
However, for a utility, a higher voltage often means there are more Megawatts available. 4160 might feed a neighborhood 100KVA, while 12KV feeds a entire subdivision 1MW, and something higher feeds an entire town 10MW.
Therefore the higher voltage systems could easily have a higher arc flash energy.
However, for a utility, a higher voltage often means there are more Megawatts available. 4160 might feed a neighborhood 100KVA, while 12KV feeds a entire subdivision 1MW, and something higher feeds an entire town 10MW.
Therefore the higher voltage systems could easily have a higher arc flash energy.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Yes clearing time and available fault current are important.
And just like NFPA70E, there are foot notes which must be considered when using NESC table.
"These calculations are based on open air phase-to-ground arc. This table is not intended for phase-to-phase arcs
or enclosed arcs (arc in a box).
These calculations are based on a 15-in separation distance from the arc to the employee and arc gaps as follows:
1 to 15 kV = 2 in, 15.1 to 25 kV = 4 in, 25.1 to 36 kV = 6 in, 36.1 to 46 kV = 9 in. See IEEE Std 4-1995."
These calculations are based on a 15-in separation distance from the arc to the employee and arc gaps as follows:
1 to 15 kV = 2 in, 15.1 to 25 kV = 4 in, 25.1 to 36 kV = 6 in, 36.1 to 46 kV = 9 in. See IEEE Std 4-1995."
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