Arc flash suits are the last line of defense for electrical workers exposed to the intense thermal energy of an arc flash event. While the total arc rating (ATPV or EBT) of a suit is widely discussed, the concept of the breakopen threshold is equally critical but often misunderstood. This article explains what breakopen thresholds are, why they matter, and how they determine the real-world effectiveness of protective clothing.
The breakopen threshold refers to the point at which the fabric of an arc flash suit is compromised by thermal energy. In ASTM F1959 standard testing, this threshold is defined as the incident energy level (in cal/cm²) at which the material begins to break open—meaning it forms holes or tears large enough to allow direct thermal energy to reach the wearer’s skin. Unlike the arc thermal performance value (ATPV), which identifies the energy level below which the fabric prevents a 50% probability of second-degree burn, the breakopen threshold (EBT) focuses solely on mechanical fabric integrity.
Why does this matter? Consider a scenario where a suit has an ATPV of 8 cal/cm² but an EBT of only 6 cal/cm². In an arc flash with 7 cal/cm² of incident energy, the fabric may not cause a second-degree burn directly, but it could break open. Once broken, the suit no longer provides a barrier. The incoming heat can then cause severe burns in micro-seconds. Therefore, the breakopen threshold is the practical limit of protection—it ensures the suit stays physically intact, which is a prerequisite for thermal insulation.
The breakopen threshold becomes the defining safety parameter when the fabric fails structurally before reaching its ATPV rating. This is common in lighter-weight or less-durable materials. For end-users, selecting an arc flash suit with a high breakopen threshold is vital. It provides a safety margin: even if the incident energy is slightly higher than expected, the suit remains whole, giving the worker critical extra seconds to escape. OSHA and NFPA 70E standards require that arc-rated clothing is labeled with the lower of ATPV or EBT. This means the listed arc rating for many suits is actually the breakopen threshold if it is lower.
The function of breakopen thresholds extends into material science. Manufacturers test specific fabric weaves, coatings, and layers to delay breakopen. For instance, high-tenacity aramid blends with moisture barriers tend to have higher EBT values. The breakopen threshold also influences the design of multi-layer systems: the outer shell must resist melting and hole formation, while inner layers must prevent heat transfer. In real-world applications, breakopen thresholds are why a heavy-duty 40 cal/cm² suit is far more reliable than a lightweight 8 cal/cm² suit for a high-hazard task.
In conclusion, the arc flash suit breakopen threshold is not a secondary parameter—it is the gatekeeper of survival. It dictates whether the suit remains an effective barrier or becomes a hazard itself. For safety professionals, understanding this function means moving beyond simple ATPV numbers. They must evaluate the breakopen threshold to ensure that the suit will stay intact when it matters most. Only by integrating both thermal protective value and fabric integrity can a truly safe arc flash protection program be achieved.