Trystan Casey
Cal/cm², or calories per square centimetre, is a unit used to measure incident energy, or the amount of heat energy released during an arc flash. An arc flash is a release of extremely high-temperature heat energy from an electric arc, which can result in the ignition of flammable materials and serious burns to human tissue. The higher the cal/cm², the greater the heat energy.
| Characteristics | Values |
|---|---|
| What does CAL mean? | Calories per square centimeter (cal/cm2) |
| What is incident energy? | A certain amount of heat energy expelled during an arc flash |
| What is an arc flash? | A massive amount of extremely high-temperature heat energy released nearly instantaneously from an electric arc resulting in the ignition of flammable materials and serious burns to human tissue |
| What is an electric arc? | When current passes through air (normally an insulator), which develops a conductive plasma cloud |
| How is incident energy measured? | In calories per square centimeter (cal/cm2) or joules/cm2 |
| What is the formula for cal? | The amount of energy needed to raise the temperature of 1 cubic centimeter (cm3) or gram of water by 1° Celsius |
| What is the onset of second-degree burns? | 1.2 calories per centimeter squared per second |
| What is a limited approach boundary? | An approach limit at a distance from an exposed live part within which a shock hazard exists |
| What is a flash protection boundary? | An approach limit at a distance from exposed live parts within which a person could receive a second-degree burn if an electrical arc flash were to occur |
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What You'll Learn
Cal/cm² is a unit to measure incident energy
Cal/cm², or calories per square centimetre, is a unit used to measure incident energy. Incident energy is the amount of thermal energy impressed on a surface at a certain distance from an electrical arc event. In other words, it is the amount of heat energy transferred across a surface area. The higher the cal/cm², the greater the heat energy.
Cal/cm² is used to determine the potential level of exposure during an arc flash event. An arc flash is a release of incident energy, which can result in the ignition of flammable materials and serious burns to human tissue. The onset of second-degree burns may occur at 1.2 cal/cm² per second. This is equivalent to holding a finger over the flame of a cigarette lighter for one second.
To protect against arc flash events, workers can wear personal protective equipment (PPE) with a calorie rating. The calorie rating of PPE is given in cal/cm² and indicates the amount of incident energy the garment can protect against. The higher the cal/cm² rating, the greater the protection offered by the PPE.
It is important to note that the cal/cm² rating of PPE should be equal to or greater than the expected level of incident energy exposure. If the cal/cm² rating of the PPE is lower than the expected incident energy, additional layers of arc-rated garments may be required to increase the protection level.
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Calories/cm² is a measurement of heat energy transferred across an area
Calories/cm² (cal/cm²) is a unit used to measure incident energy, which is the amount of heat energy transferred across a certain area. It is a measure of the amount of thermal energy impressed on a surface at a certain distance from the source, which is generated during an electrical arc event. An electrical arc occurs when current passes through the air, which normally acts as an insulator, and develops a conductive plasma cloud. This is often observed during welding processes.
The cal/cm² unit is used to quantify the amount of energy delivered to a point at a particular distance from an arc flash. This value is important for determining the appropriate protective clothing required to safeguard against potential arc flash hazards. The higher the cal/cm² value, the greater the heat energy, and consequently, the higher the level of protection needed.
In the context of electrical safety, cal/cm² is used to assess the potential level of exposure during an arc flash event. This information is crucial for selecting the appropriate personal protective equipment (PPE) to safeguard workers. The PPE must have an ATPV (Arc Thermal Performance Value) or EBT (Energy Breakopen Threshold) value that meets or exceeds the cal/cm² number associated with the specific hazard.
It is important to note that the calorie, as a unit of energy, has different definitions depending on the context. In the International Table (IT) calorie, commonly used in engineering, 1 calorie is defined as 1/860 of an international watt-hour, which is equal to approximately 4.2 joules. In thermochemistry, the thermochemical calorie is used, which is equal to 4.184 joules. In nutrition, the kilocalorie or kilogram calorie is often used, where 1 kilocalorie is equal to 1000 calories.
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Cal/cm² is used to select the proper PPE for workers
Cal/cm² is a unit used to measure incident energy, which is the amount of heat energy expressed during an arc flash. An arc flash is a massive amount of extremely high-temperature heat energy released from an electric arc, which can result in the ignition of flammable materials and serious burns to human tissue. The higher the cal/cm², the greater the heat energy.
The NFPA 70E committee changed their terminology from hazard risk category (HRC) levels to PPE levels in the 2015 revision of this standard. This was because people unintentionally used the calorie rating of an HRC as a protection level of PPE. Instead, the calorie rating is a way of determining the potential level of exposure during an arc flash event, which then allows for the selection of the proper PPE to protect the worker.
The PPE manufacturer is required to obtain and identify arc ratings in their products, which represent the amount of incident energy (in cal/cm²) on a material or multilayered system of materials that results in a 50% probability of causing second-degree skin burns. For example, PPE Category 1 represents the lowest level of Arc-Rated PPE required, with a minimum arc rating of 4 cal/cm². PPE Category 2 requires a minimum arc rating of 8 cal/cm², and so on.
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Higher the cal/cm², the greater the heat energy
Cal/cm² is a unit used to measure incident energy, which is the amount of heat energy expelled during an arc flash. An arc flash is a massive release of extremely high-temperature heat energy from an electric arc, which can result in the ignition of flammable materials and serious burns to human tissue.
Cal/cm² is a measure of the amount of energy that can be delivered to a point at a particular distance from an arc flash. This is also known as the calories per square centimetre rating and is displayed on the equipment. The higher the number, the greater the heat energy. For example, a typical cotton shirt will burst into flames at around 5 cal/cm², while 1.2 cal/cm² per second can cause second-degree burns.
The cal/cm² number is important for determining the appropriate personal protective equipment (PPE) to be used in a given situation. The PPE must have an ATPV or EBT value that is at least equal to or greater than the cal/cm² number. For example, a flash suit, which covers the entire body except for the hands and feet, is typically rated at 40 cal/cm². If the ATPV or EBT value of the PPE is lower than the cal/cm² number, additional layers of arc-rated garments may be required to increase the protection level.
It is important to note that incident energy is not just a measure of the amount of heat energy, but also includes other factors such as the distance from the source and the duration of exposure. These factors are crucial in understanding the potential dangers of incident energy and arc flash events and in determining the appropriate safety protocols and PPE for electrical workers.
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Cal/cm² is displayed on equipment, while ATPV or EBT is on garment labels
Cal/cm² is a unit used to measure incident energy, which is the amount of heat energy expelled during an arc flash. It is a measurement of the heat energy transferred over a certain area. In other words, it is the amount of energy that can be delivered to a point at a particular distance from an arc flash. The higher the cal/cm², the greater the heat energy.
ATPV and EBT, on the other hand, are values given to flame-resistant (FR) fabrics. They indicate how the fabric will perform when exposed to incident energy and what level of protection it can provide during an arc flash incident. ATPV stands for Arc Thermal Performance Value, which is the point at which 1.2 cal/cm² of incident energy is transferred through the fabric, resulting in a second-degree burn. EBT stands for Energy Breakopen Threshold, which is the point at which the fabric breaks open before the onset of a second-degree burn.
The distinction between Cal/cm² and ATPV or EBT is important because they serve different purposes. Cal/cm² is displayed on equipment and is related to incident energy, while ATPV or EBT is displayed on garment labels and indicates the level of protection provided by the fabric. This information is crucial for workers' safety, as it helps them select the appropriate personal protective equipment (PPE) for their tasks.
To ensure a successful arc-rated garment program, employers should train their employees on the Cal/cm² and ATPV or EBT values. Workers need to understand the level of protection they require to perform their tasks safely. If the ATPV or EBT value of the garment is not equal to or greater than the Cal/cm², the worker must take additional protective measures, such as adding another layer of arc-rated garments.
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Frequently asked questions
CAL is short for Calories per Centimeter Squared (cal/cm2), which is a unit used to measure incident energy. It refers to the amount of thermal energy impressed on a surface at a certain distance from an electrical arc event.
Incident energy is the amount of heat energy expelled during an arc flash. It is expressed in cal/cm2, with a higher cal/cm2 value indicating greater heat energy.
Cal/cm2 is used to determine the potential level of exposure to incident energy during an arc flash event. This information is then used to select the appropriate PPE to protect workers from arc flash hazards. For example, a worker might say, "I need an 8 cal-rated shirt," with the rating indicating the level of protection provided by the PPE.
