Arc-Flash PPE Categories Explained

An arc-flash PPE category is a level, from 1 to 4, that stands for a set of protective clothing and equipment with a minimum arc rating. NFPA 70E offers two ways to decide what PPE a task needs. The incident energy analysis method calculates the energy at the working distance and requires PPE rated at or above it. The arc-flash PPE category method looks the task up in tables that assign a category, provided the equipment falls inside the table's limits. The same piece of equipment can use one method or the other, not both.

The four categories

Every category also includes non-arc-rated protection: hard hat, safety glasses, hearing protection, leather footwear, and gloves appropriate to the shock hazard. The table above is a summary. The full list is in NFPA 70E's PPE table, which is what a program should be built from.

The arc rating on a garment is its ATPV or E_BT, determined by testing to ASTM F1959. It's the incident energy the fabric can take before the wearer is likely to receive a second-degree burn, or before the fabric breaks open.

There is no category above 4. Where incident energy exceeds 40 cal/cm², NFPA 70E advises putting greater emphasis on de-energizing the equipment, because PPE alone becomes an increasingly thin margin.

The category method, and where it stops applying

The category tables list equipment types, each with conditions attached: a maximum available fault current, a maximum fault-clearing time and a minimum working distance. Inside those limits, the table gives a PPE category and an arc-flash boundary. Outside them, the table doesn't apply, and an incident energy analysis is required.

Working distance is 455 mm (18 in) for the low-voltage rows and 910 mm (36 in) for metal-clad switchgear. These rows are abridged from NFPA 70E-2021 Table 130.7(C)(15)(a), and table numbering and content have shifted between editions, so use the edition your program adopts.

The clearing-time limit of 0.03 seconds (two cycles) on most low-voltage rows is the one that catches people out. A panelboard fed from a breaker that takes a tenth of a second to clear is outside the table, however modest the fault current.

Worked example

A 480 V panelboard with 22.6 kA of available fault current, fed from a breaker that clears in 0.1 s. The fault current is inside the table's 25 kA limit, but the clearing time is not: 0.1 s is more than three times the 0.03 s limit. The category method can't be used, and the task needs an incident energy analysis.

As a screening illustration only, the Doughty equation for an arc in an enclosure, used by the site's arc-flash calculator, gives:

• At 0.1 s clearing: about 4.3 cal/cm² at 18 in, with an arc-flash boundary of about 3.6 ft
• At 0.2 s clearing: about 8.5 cal/cm², with a boundary of about 5.7 ft
• At 0.03 s clearing: about 1.3 cal/cm², with a boundary of about 1.6 ft

Incident energy is directly proportional to clearing time, so halving the time halves the energy. The boundary moves less, because energy falls off steeply with distance. Notice that at 0.03 s the screening figure sits well inside what the table's Category 2 and 3 ft boundary assume. The tables are deliberately conservative within their limits. A real label needs the IEEE 1584 method, with the actual arcing current and device curves.

What goes on the label

NFPA 70E requires equipment likely to be worked on while energized to carry an arc-flash label. The label states the nominal voltage and the arc-flash boundary, plus at least one of the following: the available incident energy and its working distance, the PPE category, the minimum arc rating of clothing, or a site-specific PPE level. It must not show both an incident energy and a PPE category. The risk assessment behind the label has to be reviewed at least every five years, and whenever the system changes in a way that could affect it.

PPE is the last line of defense

NFPA 70E ranks risk controls: elimination first, then substitution, engineering controls, awareness, administrative controls, and PPE last. Putting the equipment in an electrically safe work condition, de-energized, locked out and verified, eliminates the arc-flash hazard altogether. Where energized work can't be avoided, engineering controls reduce the incident energy before PPE is chosen:

• Faster clearing: energy-reducing maintenance switches, zone-selective interlocking, differential protection, and light-sensing arc-flash relays. The NEC requires an arc energy reduction method on circuit breakers rated or adjustable to 1,200 A or more (NEC 240.87).
• Distance: remote racking and remote switching take the worker out of the boundary entirely.
• Containment: arc-resistant switchgear directs arc energy away from the operator, within the limits of its accessibility type. See the switchgear compartments guide.

Common mistakes

• Using the category tables outside their limits. Check fault current and clearing time before reading the category.
• Assuming lower fault current means lower energy. Lower arcing current can put a device on a slower part of its curve and raise incident energy.
• Mixing methods on one piece of equipment. NFPA 70E allows one or the other.
• Letting labels go stale. A new transformer, changed settings or a different utility source can invalidate them. The impedance tolerance alone can move fault current by several percent.
• Treating PPE as the plan. De-energizing is the plan; PPE is for when it truly can't be done.

Frequently asked questions