JBD wrote:
The 'exception' for these small transformers was not removed, as much as it was modified. The new requirement is to follow the IEEE 1584 standard (which is where the 125kVA from in the first place).
Because no study is being performed, some companies use the 70E task tables (considering the current and time restrictions) and label single phase equipment based on the 'worst case' task. For <240V this would be voltage measurements as HRC=1.
Other companies assume that modeling these as if they were 3-phase will provide a conservative result.
Lets take a different tack and those with field investigation (actual arc flash cases) experience please answer the following:
1. How many have seen an arc flash that has done damge to the torso at 208V. I have seen one in twenty years of arc flash investigations. I have personally investigated close to twenty that have been 480 volt or higher. The one that was 208 volts, the contractor had left overcurrent protection out of a 400A circuit.
2. How many have EVER seen an arc flash that has done damage to the torso on a 120/240 volt system.
3. Is it even possible to sustain an arc at 120/240 since you will always have a voltage zero, unlike a three phase system. If you look at the testing PGE) that has been done at 208 volts it takes an extremely high fault current to sustain an arc and that is typically a fault current you do not see at 120/240, then factor in the voiltage zero that an arc must fight at 120/240.
4. If we have to quantify a 120/240 volt circuit does anyone think that we will next have to quantify a 120V/20A branch circuit?
