Nomex® Helps Protect against Heat of an Electric Arc
Heat from an electric arc is an important hazard that workers need to be protected against.
Personal Protective Equipment (PPE) for protection against the thermal effect of an electric arc is subject to norm IEC 61482-2.
What is an electric arc?
An electric arc is a continuous electric discharge of high current between conductors, generating very bright light and intensive heat. The arc blast presents a very serious hazard (potentially fatal) because of the risk of severe burn injuries caused by intensive heat. The electric arc also generates other potentially severe hazards such as very intense light, a high pressure wave, noise and toxic fumes.
The DuPont™ Arc-Guide
DuPont has developed the DuPont™ Arc-Guide, a web-based service designed to help the user assess and manage the arc flash hazard within the framework of European legislation. It uses a unique European approach to risk assessment based on four Ps: Prediction, Prevention, Protection and Publishing.
The DuPont™ Arc-Guide helps the user predict the level of hazard and apply prevention techniques before selecting the appropriate Personal Protective Equipment. It also provides the user with guidance on publishing risk assessment results and on training workers to safely carry out their tasks.
Within the Prediction section, the DuPont™ Arc-Guide helps the user calculate the incident energy (in units of energy per surface area) to which a worker may be exposed when operating at a certain distance on or close to energised equipment.
In order to provide adequate protection, the arc rating of the garment must be higher than the potential incident heat energy calculated.
Quantifying the arc rating of a protective garment
1. The arc rating is most commonly quantified by the Arc Thermal Performance Value (ATPV) determined according to IEC/EN 61482-1-1/Method A. The ATPV represents the maximum incident thermal energy in units of energy per surface area (e.g., kJ/m2 or cal/cm²) that a fabric can support before the wearer will suffer second-degree burns.
The break-open threshold energy (or EBT) is another fabric characteristic. It represents the highest incident energy exposure value on a fabric where the garments do not exhibit break-open. The formation of holes in the fabric caused by break-open would allow heat or flames to pass through.
The arc rating of a clothing fabric is equal to its ATVP or its EBT (in cases where the EBT is lower than the ATPV or no ATPV can be determined).
The arc rating of a garment is equal to the arc rating of its constituent fabric, if the garment passes the visual performance criteria of IEC 61482-2 when exposed to an incident energy slightly higher than the arc rating of the constituent clothing fabric according to IEC/EN 61482-1-2/Method B.
2. The box test according to IEC/EN 61482-1-2 is another way to measure the protective performance of clothing against the thermal effects of an electric arc. In this test, the fabric is exposed to an electric arc severely confined and directed by a very specific box with a very specific electrode arrangement.
There are two Classes: test conditions for Class 1 try to simulate typical exposure conditions for a short-circuit current of 4kA protected by devices limiting the duration of the arc to 0.5 seconds in confined space, and of 7kA respectively for Class 2.
The key criterion for a fabric to pass the box test Class 1 or Class 2 is that the heat transferred through the fabric during Class 1 or Class 2 testing does not cause second-degree burns.
The arc rating of a garment is equal to the Class of its constituent fabric, if the garment passes the visual performance criteria of IEC 61482-2 when exposed to the garment test. The test must be applied under the test conditions of the Class of the constituent clothing fabric according to IEC/EN 61482-1-2.
Electric arc protective clothing can receive IEC 61482-2 certification if one of the following requirements are met:
- The arc rating of the clothing must be at least 4cal/cm2.
- The clothing has to pass at least the arc box test Class 1.
This test equipment developed by DuPont measures the ATPV and the EBT value of protective fabrics and garments in simulated open electric arc incidents according to IEC/EN 61482-1-1. It is capable of exposing the fabrics or garments to incident energies of up to 70 cal/cm².
Arc-Man® can test fabrics on panels, measuring the ATPV of the fabric system (in cal/cm²) so that it can be compared to the incident energy calculated in the end user’s risk assessment. Arc-Man® can also perform complementary garment tests on a torso to show the effect of a garment’s design and integrity of manufacture, allowing a prediction of the garment’s behaviour (e.g., of the closure system, the pockets, etc.).
The box test according to IEC 61482-1-2 (equivalent to EN 61482-1-2) can also be performed on the DuPont™ Arc-Man® testing facility.
Electric arc test results and recommendations
- The arc rating of a fabric not only depends on its weight, but to a large extent also on other characteristics such as fibres and yarns constructions and their combinations.
- DuPont recommends that all layers of arc protective clothing in a multi-layer system should be made of inherently flame-resistant materials.
- Inappropriate underwear could ignite or melt, thus having an adverse effect on protection and aggravating the outcome of an electric arc incident.