arc flash: frequently asked questions

An arc flash is a sudden release of electrical energy through the air, caused when current travels between conductors or from a conductor to earth. It produces an explosive burst of extreme heat, intense light, and pressure. Temperatures can exceed 19,000 °C, vaporising metal and igniting clothing instantly. It is one of the most serious hazards for anyone working on or near live electrical equipment.

Arc flash is typically caused by a fault where current diverts from its intended path. Common triggers include equipment failure (loose connections, worn insulation, aged switchgear), human error (accidental contact with live parts, uninsulated tools, skipped lockout/tagout procedures), poor system design (insufficient conductor spacing, incorrect protective device coordination), and environmental factors (dust, moisture, rodent ingress). Read more about what causes arc flash.

An arc flash can produce temperatures exceeding 19,000 °C (approximately 35,000 °F)—nearly four times hotter than the surface of the sun. At these temperatures, metal components vaporise in milliseconds and non-arc-rated clothing can ignite or melt onto the skin. This extreme heat is why arc-rated PPE is essential for anyone working within the arc flash boundary.

Arc flash and arc blast are two different consequences of the same electrical fault. Arc flash is the intense thermal energy—extreme heat and light—that causes burns. Arc blast is the explosive pressure wave created by rapidly expanding air and vaporised metal, which can reach supersonic speeds, throw workers off their feet, and send shrapnel across the area. Both occur simultaneously, making arc events extremely dangerous.

Electric shock occurs when current passes through the body, disrupting heart rhythm and causing muscle contractions. Arc flash does not necessarily involve current flowing through the body, it is an uncontrolled energy release through the air, causing thermal burns, blast injuries, and flying debris. The protective measures differ: shock protection uses insulated tools and rubber gloves, while arc flash protection requires arc-rated PPE. A comprehensive electrical safety programme must address both hazards.

Arc flash incidents can cause severe thermal burns, burns from ignited or melted clothing, blast injuries (ruptured eardrums, concussion), eye damage from intense UV light, permanent hearing loss from sound levels exceeding 160 dB, inhalation injuries from vaporised metal and toxic fumes, and impact injuries from molten metal or shrapnel. These injuries can be life-changing or fatal. Wearing correctly rated arc flash clothing significantly reduces the severity of harm.

A typical arc flash lasts only a fraction of a second, often just milliseconds. Devastating injuries can occur before a worker has any time to react, which is why protective clothing must be in place before work begins. Properly coordinated circuit breakers and fuses can limit arc duration and reduce total incident energy.

Arc flash incidents are more common than many realise. In the UK, the HSE reports numerous electrical injuries each year, many involving arc flash. The most at-risk industries include power generation, industrial electrical maintenance, construction, petrochemicals, rail, and manufacturing. Many incidents are preventable through proper risk assessment and correctly specified arc flash PPE.

Yes. The combination of extreme heat, explosive pressure, toxic fumes, and projectile shrapnel makes arc flash one of the most lethal workplace hazards in the electrical industry. Even survivable incidents often result in life-changing injuries—severe burns, permanent hearing or vision loss, and long-term respiratory damage. Arc-rated PPE is designed to reduce burn severity to a survivable level, which is why it must be properly specified through a thorough arc flash risk assessment.

Any industry where workers interact with live or potentially live electrical equipment carries arc flash risk. The most affected sectors include power generation and distribution, industrial electrical maintenance, petrochemicals, rail and transport, construction, renewables and EV infrastructure (particularly DC systems), and data centres. Alsico designs arc flash protective clothing for all of these environments.

There is no safe voltage threshold. Arc flash incidents have occurred at voltages as low as 230 V in the UK. Even equipment operating at 400–600 V can produce dangerous incident energy levels. The severity depends not only on voltage but on available fault current, arc duration, and working distance.

An arc flash should not occur on equipment that has been correctly de-energised, isolated, and confirmed dead through approved lockout/tagout procedures. However, incidents have occurred on equipment believed to be safe due to incorrect isolation, back-feeding, residual energy in capacitors, or procedural failures. Workers should wear appropriate arc flash protective clothing during the entire de-energisation and proving-dead process, until the equipment is confirmed safe.

An arc rating measures a fabric's ability to protect the wearer from the thermal energy of an arc flash, expressed in cal/cm². It represents the maximum incident energy the material can resist before a second-degree burn would be expected. Only garments specifically tested and assigned an arc rating should be used in arc flash environments—a "flame resistant" label alone is not sufficient.

ATPV is the incident energy (cal/cm²) at which there is a 50% probability of sufficient heat transferring through the fabric to cause a second-degree burn. It is determined through Open Arc testing (IEC 61482-1-1 / ASTM F1959) by measuring thermal energy transfer against the Stoll Curve. When both ATPV and EBT are tested, the lower value becomes the garment's arc rating.

EBT is the incident energy (cal/cm²) at which there is a 50% probability of the fabric physically breaking open—developing holes or tears that expose the skin to the arc. It is determined during the same Open Arc test as ATPV. Fabrics where EBT is lower than ATPV are typically more insulative than they are strong—they resist heat transfer well but may rupture before enough energy passes through to cause a burn.

Both are determined during the Open Arc test but measure different failure points. ATPV measures the energy at which there is a 50% chance of a second-degree burn through the fabric (heat transfer failure). EBT measures the energy at which there is a 50% chance of the fabric physically breaking open (structural failure). The lower value becomes the garment's arc rating, as it represents the first point at which protection could fail.

ELIM is the maximum incident energy below which there is no recorded data point showing a second-degree burn or fabric break-open—a 0% probability of failure. ATPV, by contrast, is based on a 50% probability of a burn. ELIM therefore offers a significantly more conservative safety threshold.

ELIM represents the energy threshold at which a garment demonstrated complete protection in testing—zero probability of a second-degree burn or fabric break-open. ATPV, by contrast, accepts a 50/50 chance of a burn at the rated energy level. The European regulatory view is that this 50% risk may conflict with the EU PPE Directive's requirement that PPE should not impose harm.

The Stoll Curve is a scientifically established model predicting the onset of second-degree burns based on thermal energy exposure over time. During Open Arc testing, sensors behind the fabric measure heat transfer, which is plotted against the Stoll Curve. ATPV is the energy level at which transferred heat reaches the Stoll Curve with 50% probability. ELIM uses only values below the Stoll Curve, giving a 0% burn probability.

The Open Arc Test (IEC 61482-1-1 / ASTM F1959) exposes fabric to an open electrical arc in a controlled laboratory setting. Sensors behind the fabric measure the thermal energy that passes through, producing quantitative values, ATPV, EBT, and ELIM, in cal/cm². These allow safety managers to match PPE precisely to incident energy calculations from the risk assessment.

The Box Test (IEC 61482-1-2) confines an electric arc inside a plaster box with a specific electrode arrangement, directing a constrained exposure onto the fabric. The garment is assessed on whether it prevents flame spread, does not break open, and does not ignite. It assigns a pass/fail classification: Class 1 (4 kA, 0.5s) or Class 2 (7 kA, 0.5s). Unlike the Open Arc Test, it does not produce a numerical energy rating.

LOI measures the minimum oxygen concentration (as a percentage) needed to sustain combustion of a material. Since normal air contains approximately 21% oxygen, materials with an LOI above 21% will not sustain burning in normal conditions. Cotton has an LOI around 18% (burns readily), while modacrylic fibres sit at 29–32% and aramids at 28–30% (inherently flame resistant). LOI is useful for comparing fibre flammability but is not a substitute for full arc flash testing.

Incident energy is measured in calories per square centimetre (cal/cm²), quantifying the thermal energy reaching a given surface area at a specified distance from an arc flash. On unprotected skin, 1.2 cal/cm² is generally the threshold for a second-degree burn, which is why the arc flash boundary is defined at this level. Incident energy is calculated using system data (fault current, arc duration, working distance) and used to select arc flash clothing with an arc rating exceeding the predicted exposure.

Category 1 requires a single layer of arc-rated clothing with a minimum rating of 4 cal/cm²: a long-sleeve shirt and trousers or a coverall, an arc-rated face shield (with balaclava) or arc flash hood, hard hat, safety glasses, ear canal inserts, and heavy-duty leather gloves. Arc-rated jacket, rainwear, or parka as needed. All exposed skin must be covered by arc-rated material.

Category 2 requires arc-rated clothing with a minimum rating of 8 cal/cm²: a long-sleeve shirt and trousers or a coverall, an arc flash hood or arc-rated face shield with balaclava, hard hat, safety glasses, ear canal inserts, and heavy-duty leather gloves. This is the most commonly used category in industrial settings.

Category 3 requires multiple layers of arc-rated clothing with a minimum system rating of 25 cal/cm²: jacket and trousers or coverall (usually as a tested layered combination), an arc flash suit hood rated to 25 cal/cm², rubber insulating gloves with leather protectors or arc-rated gloves, hard hat and safety glasses under the hood, and ear canal inserts. All components must be compatible and collectively rated.

Category 4 is the highest level, requiring a full arc flash suit system rated to 40 cal/cm²: multiple layers of arc-rated clothing (shirt, trousers, coverall, suit jacket, suit trousers) tested as a complete system, an arc flash suit hood, arc-rated gloves, hard hat, safety glasses, ear canal inserts, and leather footwear. If incident energy exceeds 40 cal/cm², the task is considered too dangerous for PPE alone—the hazard must be reduced first.

The incident energy analysis method uses engineering calculations (typically IEEE 1584) to determine exact incident energy at each piece of equipment, then selects PPE with an arc rating exceeding that value. It is more precise. The PPE category method uses predefined NFPA 70E tables that assign a category (1–4) based on equipment type and voltage. It is simpler but relies on generalised assumptions.

Below 1.2 cal/cm², arc-rated clothing is generally not required—this threshold is where unprotected skin is unlikely to sustain a second-degree burn. However, workers should still wear long sleeves and trousers in non-melting natural fibres (never synthetics), safety glasses, and heavy-duty leather gloves. Shock hazards still exist below this level and remain statistically more deadly. For any task above 1.2 cal/cm², properly rated arc flash clothing must be worn.

Retired garments must never be donated, resold, or re-entered into use. Options include manufacturer take-back schemes (alsico's aRX initiative collects end-of-life garments for fibre-to-fibre recycling), controlled disposal through proper waste channels after clearly decommissioning the garment (cutting or marking to prevent reuse), and energy-from-waste facilities where fibre recycling is unavailable. The most important step is ensuring retired garments are permanently removed from circulation.

Arc flash garments use specialist fibre blends (modacrylic, aramid, cotton, antistatic) that standard textile recycling facilities cannot typically process. However, fibre-to-fibre recycling technologies are advancing. Alsico contributes through the aRX initiative, collecting end-of-life garments for recycling in cooperation with fibre-to-fibre programmes. In the meantime, responsible end-of-life management includes proper disposal, energy-from-waste where fibre recycling is unavailable and working with suppliers who have established take-back programmes.

The environmental footprint spans fibre production (energy-intensive for modacrylic and aramid), dyeing and finishing (water and energy consumption), garment assembly, transport, and end-of-life disposal. Longer-lasting garments reduce total impact—inherent FR fabrics outlast treated alternatives since protection doesn't degrade. alsico minimises impact through efficient production, sustainable fabric technologies, recycled and bio-based fibres where possible, and end-of-life collection through the aRX programme. Discover our sustainability approach.

Choose inherent FR fabrics (permanent protection means fewer replacements), prioritise garment durability (reinforced construction lasts longer), specify inclusive sizing (proper fit reduces stress-related wear), work with manufacturers demonstrating transparent sustainability practices and ethical labour conditions, and ask about end-of-life take-back or recycling schemes. alsico integrates sustainability into every aspect, from ALSIFLEX® technology (two-thirds less CO₂ than comparable fabrics) to our end-of-life garment collection.

Inherent FR fabrics are already more sustainable than treated alternatives—permanent protection means longer garment life and less waste. Alsico also invests in efficient dyeing and finishing (lower temperatures, reduced water use), lighter-weight protective fabrics with lower environmental footprint, and fabrics designed without elastane for easier end-of-life recycling. Alsico's Kibo fabric, powered by ALSIPRO technology, delivers multi-hazard protection with reduced environmental impact. Explore our innovation.