# Arc flash hazards and residential single phase systems

#### Becker Electric

##### Member
I need help understanding Nfpa 70e table 130.7(c)15(a)
Which states

“ panel boards, or other equipment rated 240 V and below
Parameters maximum of 25KA available fault current; maximum of 0.03 seconds (2cycles) fault clearing time; minimum working distance, 455 mm (18”)
Arc flash PPE category one”

What I’m failing to find is any information regarding single phase and am interpreting this as, since I don’t know how to calculate the available fault current I’m supposed to get an incident energy analysis, but as far as I know, there is no calculation for single phase incident energy. And so I am under the impression residential services that are single phase 240 are under 1.2 cal anyway and therefore category 1 PPE is not necessary (ofcourse insulated gloves and eye protection still should be worn).

So I’m interpreting this article as even in residential single phase 240v i need to be wearing minimum 4 Cal arc rated long sleeve shirt, pants, arc rated face shield, arc rated jacket, hardhat, safety glasses, hearing protection, if im working on live panel board (sub panel and or main) with 240 V and below or get an incident energy analysis for every house im taking the cover off of the main service panel and testing voltages.

I feel like I’m not understanding correctly.

#### David Castor

##### Senior Member
Arc-flash hazards at 240 V and below are much lower than for 480 V and above, but the risk is not zero. You're up against a fundamental limitation of using the Tables in NFPA 70E in lieu of doing the calculations. 25 kA of fault current would be quite a lot for a residential service. The 2 cycle clearing time assumes an upstream fuse or a breaker clearing on the magnetic trip. But unless you have that data, you are not supposed to use the Tables.

From a practical standpoint, consider that the NESC used by the utility on the other side of the meter requires PPE rated for 4 cal/cm2 for their meter techs at 240 V. This would include eye protection, face shield, long-sleeve shirt and pants rated at least 4 cal/cm2, and hard hat. Basically equivalent to NFPA 70E PPE Category #1.

You will have to make the decision, but I'd be comfortable working a 120/240 V residential panel as a PPE Category 1 situation. Working at 480 V is another world entirely.

#### Becker Electric

##### Member
Thank you for your reply. I appreciate your thoughts on the subject and believe it’s offered me some clarity on the situation.

#### Becker Electric

##### Member
Arc-flash hazards at 240 V and below are much lower than for 480 V and above, but the risk is not zero. You're up against a fundamental limitation of using the Tables in NFPA 70E in lieu of doing the calculations. 25 kA of fault current would be quite a lot for a residential service. The 2 cycle clearing time assumes an upstream fuse or a breaker clearing on the magnetic trip. But unless you have that data, you are not supposed to use the Tables.

From a practical standpoint, consider that the NESC used by the utility on the other side of the meter requires PPE rated for 4 cal/cm2 for their meter techs at 240 V. This would include eye protection, face shield, long-sleeve shirt and pants rated at least 4 cal/cm2, and hard hat. Basically equivalent to NFPA 70E PPE Category #1.

You will have to make the decision, but I'd be comfortable working a 120/240 V residential panel as a PPE Category 1 situation. Working at 480 V is another world entirely.
I forgot to ask, when you are talking about 240v below, generally speaking, being a lower arc flash hazard risk are you referring to single phase only or both single and three phase like in a high leg delta 208v.

#### David Castor

##### Senior Member
The arc-flash hazard at 240 V and below, whether single-phase or three-phase, is considerably lower than at higher voltages. It's more difficult for an arc to sustain itself. Three-phase systems will have higher incident energy than a single phase system, but both will be a lot less than a 480 V system with the same available fault current. Many companies label everything 240 V and below at 4 cal/cm2 for both single and three phase. There is definitely a hazard, but the right PPE can provide good protection.

#### Becker Electric

##### Member
The arc-flash hazard at 240 V and below, whether single-phase or three-phase, is considerably lower than at higher voltages. It's more difficult for an arc to sustain itself. Three-phase systems will have higher incident energy than a single phase system, but both will be a lot less than a 480 V system with the same available fault current. Many companies label everything 240 V and below at 4 cal/cm2 for both single and three phase. There is definitely a hazard, but the right PPE can provide good protection.
Thank you