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Arc-Flash Overview

Sunday, July 1st, 2012

This page provides an overview of how arc-flash calculations work in Design Master Electrical 8.2. The goal of arc-flash calculations is to calculate the incident energy at a specific distance from the equipment during an arc-flash event. The calculated energy is used to determine how to keep workers safe while working on the equipment live.

The personal protection equipment (PPE) required for the person working on the equipment is based upon the incident energy at the working distance from the equipment. The arc-flash-protection boundary is the distance unprotected works must keep between themselves and the equipment.

In both cases, the calculated energy values will limit the burns on the works to second-degree.

There are three commands you will use when performing arc-flash calculations.

The DM Electrical->One-Line Devices->Set Arc-Flash Values command is used to specify the additional information about panels required to perform arc-flash calculations. Most of the work related to arc-flash is done using this command.

The DM Electrical->One-Line Schedules->Insert Arc-Flash Schedule command will print a schedule on the drawing documenting the arc-flash calculations.

The DM Electrical->One-Line Schedules->Create Arc-Flash Stickers command will create stickers for each of your panels and display them in your web browser. You can then print them using a color printer and sticker paper and attach them to your panels in the field.

Set Arc-Flash Values

When you run the DM Electrical->One-Line Devices->Set Arc-Flash Values command, the following dialog box is opened. It’s big! Click on the image to see a full-size version, or run the command in Design Master Electrical.

Arc-Flash Dialog Box

Each one-line device is listed as a row in the grid. The bold columns are calculated for you by the software. You are responsible for filling in the other values.

While you are using the dialog box, you can hover your mouse over a column title to see a tooltip that describes the information that goes in the column.

Each of the columns in the grid are described below.

Device: The one-line device. Devices are listed in a tree based upon which devices are fed from other devices.

Calculate Arc-Flash: Whether or not arc-flash is calculated for the device. Devices that are not calculated will not be displayed in the arc-flash schedule on the drawing and will not have stickers created.

Because it is difficult to sustain an arc-flash below 208V, devices with voltages less than 208V do not need arc-flash calculations performed. You have to manually uncheck this box for devices below 208V.

Voltage: The voltage of the device. The voltage determines the arc-flash calculation method that is used. The voltage is set when you create the one-line device.

Equipment Type: The equipment type from IEEE Std 1584-2002 Table 4. The type of equipment selected sets a distance ‘x’ factor. The ‘x’ factor is used as an exponent in the calculation. Higher ‘x’ values result in higher incident energy values.

The following list displays the ‘x’ factor for each type of equipment for voltages between 208V and 1,000V.

  • Switchgear: 1.473
  • MCC : 1.641
  • Panels: 1.641
  • Open air: 2.0
  • Cable: 2.0

For voltages below 208V and above 1000V, consult IEEE Std 1584-2002. The ‘x’ values are slightly different, but the relative orderings are the same (switchgear is the lowest, open air and cable are the highest).

Configuration Type: Whether the device is enclosed or not. Enclosed devices reflect the arc-flash toward the worker, resulting in larger incident energy values. Most panels and other devices inside buildings should be set to Box.

  • Open: no enclosure, lower incident energy
  • Box: enclosed equipment, higher incident energy

Grounding: The type of grounding on the device. Grounded devices have lower incident energy values.

  • Grounded: lower incident energy
  • Ungrounded and high resistance grounded systems: higher incident energy

Gap Between Conductors: The gap between the conductors. The most accurate calculation can be used when the gap between conductors is between 13 mm and 152 mm.  Within that range, the smaller the gap, the larger the incident energy.

If possible, set the gap based the manufacturer’s specifications. Typical gaps between conductors for devices between 208V and 1,000V are listed in IEEE Std 1584-2002 Table 4.

  • MCC: 25 mm
  • Panels: 25 mm
  • Switchgear: 32 mm
  • Open air: 10-40 mm
  • Cable: 13 mm

For voltages below 208V and above 1000V, consult IEEE Std 1584-2002.

Working Distance: The distance from the possible arc point to the person working on the device. It includes  both the distance from the device to the worker and the distance from the front of the equipment inside to where the arc flash would occur.

The incident energy will be calculated at this distance from the device. The farther away from the device, the lower the incident energy. Any part of the body closer to the device than this distance will be exposed to higher incident energy than is calculated.

If possible, set the working distance based upon the actual dimensions of the equipment. Typical working distances are given in IEEE Std 1584-2002 Table 3.

  • Low-voltage switchgear: 610 mm
  • Low-voltage MCCs and panelboards: 455 mm
  • Cable: 455 mm
  • Other: To be determined in field

Bolted Fault Current: The bolted fault current is the fault at the equipment. This value is calculated based upon the other settings in your project. All the arc-flash calculations assume the equipment and the fault are three-phase. For single-phase equipment, the calculations provide conservative results.

Arcing Current, 100% & 85%: The predicted three-phase arcing current used to determine the operating time for the protective devices. For voltages less than 15,000V, the 100% column is calculated based on the values to the left in the grid. The 85% column is based upon the 100% column.

For voltages greater than 15,000V, the arcing current is equal to the fault current.

Arcing Time, 100% and 85%: The duration of the arc-flash has a significant impact on the incident energy. The longer you are exposed to the flash, the more intense the burn.

The arcing time is based upon the time-current curve for the specific breaker you are using. These values are provided by the manufacturer. You typically can find them on their website.

You are responsible for entering two values, one for the arcing time at 100% of the arcing current, and the other for the arcing time at 85% of the current.

The incident energy is a function of the arcing current and the duration. Lower arcing currents that take longer to close the breaker can result in higher incident energy values than high arcing currents. To account for this, incident energy is calculated at 100% of the arcing current value and at 85% of the arcing current value. The greater of the two incident energy values is used to determine the PPE required for the device.

Fuses must be handled differently from breakers. The time-current curves for fuses may include both melting and clearing times. Use the clearing time if it is listed. If only the melting time is listed, for greater than 0.3 seconds add 10%, otherwise add 15% to the time.

If the arcing fault current is above the total clearing time at the bottom of the curve (0.01 seconds), use 0.01 seconds.

NFPA 70E, Appendix D, 7.3 recommends limiting the duration to 2 seconds. The idea is that if the person is exposed to an arc-flash for that long, they will move away from the danger.

Incident Energy, 100% and 85%: The total incident energy is calculated based upon the arcing current, duration, and working distance.

Required PPE: Personal Protection Equipment (PPE) rating required to work on the device. The rating is based upon the larger of the two calculated incident energy values. NFPA 70E 2012 Table 130.7(C)(16) lists the minimum arc rating for clothing in cal/cm². All of the calculations are done in J/cm². Both values are listed below.

  • PPE 0: 0-5 J/cm² (0-1.2 cal/cm²)
  • PPE 1: 4-16.7 J/cm² (1.2-4 cal/cm²)
  • PPE 2: 16.7-33.5 J/cm² (4-8 cal/cm²)
  • PPE 3: 33.5-104.6 J/cm² (8-25 cal/cm²)
  • PPE 4: 104.6-209.2 J/cm² (25-40 cal/cm²)
  • PPE X: > 209.2 J/cm² (> 40 cal/cm²)

Arc-Flash-Protection Boundary Energy: The arc-flash-protection boundary is the approach limit for unprotected workers.

You can specify the energy value. This is the amount of energy that will be delivered at the boundary distance. The default energy value of 5.0 J/cm² will limit burns to an unprotected worker to second degree during an arc-flash incident.

Arc-Flash-Protection Boundary Distance: The boundary distance is calculated based upon the specified Arc-Flash Protection Boundary Energy value.

Insert Arc-Flash Schedule

The DM Electrical->One-Line Schedules->Insert Arc-Flash Schedule command will insert a schedule on the drawing displaying the results of the arc-flash calculation. The schedule can include all of the information in the grid in the Set Arc-Flash Values command. The default schedule displays only a most relevant fields.

The dialog used to insert the schedule works like all the other schedule insertion dialog boxes in Design Master Electrical.

Arc-Flash Schedule

Create Arc-Flash Stickers

The DM Electrical->One-Line Schedules->Create Arc-Flash Stickers command will create a sticker for each one-line device that has Calculate Arc-Flash checked using the Set Arc-Flash Values command.

If the incident energy at the device is less than 40 J / cm2, a warning label will be printed. If the incident energy is greater than 40 J / cm2, a danger label will be printed.

The sticker also will display the shock protection information for the device. Shock protection is a function of the voltage of the device. The values are based upon NFPA 70E-2012, Table 130.4(C)(a).

Examples of the two types of stickers created are shown below.

Arc-Flash Warning Label

Arc-Flash Danger Label

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