Lessons · Electrical · arc flash
Arc flash: the explosion that needs no contact
When a fault jumps across air, the arc is hotter than the surface of the sun for a fraction of a second and throws molten metal and a pressure wave; the hazard is measured in calories per square centimetre at your working distance, and 1.2 cal/cm² is enough for a second-degree burn.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
A dropped screwdriver across a live bus in a 480 V panel. The person is not touching anything. The label on the panel door says how bad it can be at the distance your hands would be, and what clothing rating you need. Reading it is the difference between a startle and a burn unit.
How to think about it
The first control is not PPE: it is not working live. De-energize, lock out, verify. When the work truly must be live, read the label: incident energy in cal/cm², the arc-flash boundary, the PPE category. Your clothing's arc rating must be at least the incident energy, and everyone inside the boundary wears it.
Worked example
Label: incident energy 6.2 cal/cm² at 18 inWhat you would receive at working distance if the bus faulted.
6.2 / 1.2 = 5.2 times the burn thresholdBare skin at 18 inches is a serious burn.
PPE arc rating 8 cal/cm² ≥ 6.2 cal/cm²The suit is rated above the hazard. This is the minimum, not a target.
Arc-flash boundary on the label: 42 inInside that distance, without rated clothing, an arc gives a second-degree burn. The boundary is where the bystander stands.
Your turn
A label says 4.0 cal/cm². Write the check on a suit rated 8 cal/cm².
8 cal/cm² ≥ cal/cm²: the suit is enough
Solve one, graded on the server
The trap
Counting a face shield and gloves as arc-flash protection while wearing a polyester shirt. Synthetic fabric melts onto skin. If the label calls for an arc rating, it means every layer from the skin out.