Arc Blast Field Marking

Article 110.16 of the 2014 NEC requires all switchboards, meter enclosures*, panel boards, industrial control panels, and motor control centers that are likely to be subject to examination, adjustment, servicing or maintenance while energized be field marked to warn qualified personnel of potential electric arc flash hazards.
Implementing This Regulation

This article will require the field electrician to place appropriate warning signs on all equipment that falls into this category.

I have included several links on this page that should help you to better understand the requirement of this regulation and to comply with the intent of the code.

* Meter enclosures was added in the 2005 NEC.
Arc Flash Hazards

We should take a better look at arc flash hazards and the intent of 110.16 before we tackle the actual calculations required to meet the intent of the code.

Click on the icon below. After reading the tutorial, close the window to return to this page.
Boundary Calculations

It is suggested that the warning sign be marked with the "Flash Hazard Boundary".
The flash hazard boundary is the minimum distance from the equipment that an individual can be located during an arc flash event and sustain only curable burns. This is assuming they are not wearing protective equipment.

The calculation for this distance seems complicated at first, but it is actually quite simple. Due to the liability involved, we will not make calculations. We will determine protection levels using the tables in NFPA 70E and the Tables provided in our Corporate Safety Manual. If a calculation is required, please contact the Director of Safety and Loss Prevention. We will then determine the manner in which we will comply with the calculation.  We have included explanations of the calculation so that you can better understand how the protection is determined in the tables.

Four pieces of information are used  to make this calculation and if a calculation is required you may be asked to provide this information. 
1. The bolted fault MVA at the point involved
2. The voltage of the system
3. The percentage of impedance rating of the transformer supplying the
4.  The fault clearing time of the over current protection for the equipment.
Bolted Fault MVA at the point involved

This term usually confuses most people because it is so rarely used and because we are used to dealing with volt-amp ratings in kVA not MVA.

To determine the bolted MVA you will need the kVA rating of the transformer supplying the equipment.

Then you will simply divide this kVA by 1000.  This will give you the MVA rating.

For example:
A panelboard is fed by a 480/208 three-phase 45 kVA transformer.
To calculate the bolted fault MVA at the disconnect
simply divide 45 by 1000.
45/1000=.045 MVA

Voltage of the system

This is simply the voltage of the system. You should use the actual measured voltage if it is known. Use the nominal voltage if a voltage measurement is impractical. It should be the actual line voltage at the point involved. 

The voltage for the panelboard listed above will be 208 V.
We do not have the ability to take a voltage reading so we will use the nominal voltage.
Percent of Impedance of the Transformer

All transformers must have a nameplate and the nameplate must have an impedance rating. This impedance rating is expressed as a percentage. You will need to check the nameplate of the transformer supplying the equipment and write down the impedance rating.  We will always enter this number as a straight number. Do not convert it to it's decimal equivalent.

Let's suppose that the transformer supplying the panelboard had a Z% of 2.5 %.
We would consider this rating to be 2.5. 
Not .025 which would be the decimal equivalent of 2.5%
Fault Clearing Time

This is the time that will elapse before the overcurrent protective device (circuit breaker or fuse) will trip and interrupt the power to the equipment in the event of a fault current event.

We tend to think of this time as instantaneous but it is not.  The formula for determining this time period is the number cycles required to trip the device divided by the frequency of the circuit.  The frequency of the circuit will normally be 60 Hz. The number of cycles needed to trip the device can be obtained from the manufacturer and is sometimes stamped on the device.

Most standard over current devices require 6 cycles. Most current-limiting devices require 2 cycles. Most arc-fault interrupting devices require 1 cycle.

We are using standard non-time delay fuses to over current protect our panelboard.  The cycles required to trip the device is 6. 
At 60 Hz the electrical arc will last for .1 seconds before the overcurrent device interrupts the power to the circuit.
The Sign

It is recommended that the sign on the equipment warning of the flash hazard contain the following information:

1. Flash Hazard Boundary
2. cal/cm2 Flash Hazard at 18"
3. PPE Level recommendation
4. Shock Hazard
5. Limited Approach Boundary
6. Restricted Approach Boundary
7. Prohibited Approach Boundary

It should be noted that NEC Art.110.16 requires only that the equipment be marked "Warning. Arc Flash Hazard."  The information above is not required by code but, if provided, it will help the individuals that must service the equipment evaluate the hazards of working on energized parts and also help ensure the safety of non qualified personnel that work in the area near the equipment.

This is determined using NFPA 70E after performing a hazard risk analysis. The hazard risk analysis and selection of PPE shall only be done by qualified personnel who have receive adequate training in these procedures.
Shock Hazard

This is simply the nominal voltage when the cover is removed or the energized parts are exposed.
Limited Approach, Restricted Approach, Prohibited Approach

All of these distances are determined by using Table 2-1.3.4 from NFPA 70E. I have not included the entire table but I have included the distances for each of these for nominal voltages of 51-300 and 301-750 volts.  For further information consult NFPA 70E

51-300 Volts                                    
Limited Approach Boundary - 3'6"
Restricted Approach Boundary - Avoid Contact
Prohibited Approach Boundary - Avoid Contact

Limited Approach Boundary - 3' 6"
Restricted Approach Boundary - 1' 0"
Prohibited Approach Boundary - 0' 1"

Limited Approach Boundary: A shock protection boundary to be crossed by only qualified persons (at a distance from a live part) which is not to be crossed by unqualified persons unless escorted by a qualified person.

Restricted Approach Boundary: A shock protection boundary to be crossed by only qualified persons which, due to it's proximity to a shock hazard requires the use of shock protection techniques and equipment when crossed.

Prohibited Approach Boundary: A shock protection boundary to be crossed by only qualified personnel which when crossed by a body part or object requires the same protection as if direct contact is made with a live part.
NFPA 70E and Arc Blast Module Test

Answer all of the questions and fill out the required contact information. An email with the test results will be emailed to E Light Electric employees or to the job site Safety Manager for non E Light Electric employees within 2 working days from receipt of your submission.
A Certificate of Completion will be issued for scores of 80% or higher.
1. Which article of the NEC requires that panelboards and motor control centers be marked with warnings concerning the hazards of arc flash potential?
2. The NEC requires that the approach boundaries be included in the marking for panelboards and motor control centers.
3. To determine the Fault Clearing Time, the number of cycles required to clear a fault is divided by the _____________________ of the circuit.
4. To convert kVA ratings into MVA ratings, the kVA rating must be ________________.
5. It is recommended that Arc Blast Warning signs include the following information: Flash Hazard Boundary, Cal/cm2 Flash Hazard at 18", __________________________________, Limited Approach Boundary, Restricted Approach Boundary and Prohibited Approach Boundary.
6. The percent of the impedance of the transformer is determined by ____________.
7. A shock protection boundary to be crossed by only qualified personnel which when crossed by a body part or object requires the same protection as if direct contact is made with a live part is a definition of______.
8. The limited approach boundary for a 208V, three-phase panel is __
9. A shock protection boundary to be crossed by only qualified persons, which is not to be crossed by unqualified persons, unless escorted by a qualified person is a definition of __________________________.
10. If a calculation of the Calories or boundaries is needed we should gather all the information and conduct the calculation.
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NEC, NFPA 70E, The Code and 2015 Code are references to the National Electric Code. The National Electric Code is published by the National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02269. The National Electric Code and NEC are registered trademarks of the National Fire Protection Association.
NFPA 70E is also published by the NFPA and copyrights are held for both documents.
Thank you for visiting our Arc Blast Safety Web Site. I hope this information helps to contribute to the safety of the electrical industry and to the electricians that provide safe electrical systems for all of us to use. We also express our gratitude to the NFPA and all the persons that contribute to the codes and standards that make our homes, businesses and recreation sites a safer place to be. Please verify all information and do adequate research before developing electrical safety policies and working on energized parts. The information in this page is but a small portion of the available information. You may want to also visit the NFPA web site and Cooper Bussmann Web Site for excellent information.

The information presented on this web page is intended for the internal use of E Light Electric Services and their employees. E Light Electric Services takes no responsibility for the information contained herein or its application or use by any other person or organization.  Note this module is based on the 2015 version of NFPA 70E. Click here to learn more about the 2015 version of NFPA 70E.

Ted "Smitty" Smith
Training Department
NFPA 70E and Arc Blast Module
11. Which of the following is not a hazard of electricity.
12. All work is considered to be energized work unless a ______________________ has been reached.
13. In the box below, write the three steps that must be accomplished to reach an electrically safe work condition.
14. In the box below, explain a situation in the past where you have worked on a live circuit and explain how you would handle that situation now.
15. Any task requiring work to be performed on electrical equipment or systems where it has been determined that interrupting the electrical power to that equipment or system will cause greater hazard to persons or property is the definition of a ____________________
16. Equipment and circuitry shall be considered to be de-energized and in an electrically safe work condition if all of the following steps have been successfully completed:
All sources of potential power have been identified.
All sources of potential power have been locked out and tagged according to company lock out and tag out procedures
All circuitry and equipment has been tested to ensure that it is de-energized and no voltage is present. 

17. For the purposes of this policy energized shall mean that equipment or wiring is a source of or connected to electrical energy in excess of _______volts. Any equipment or wiring that has not been placed in an electrically safe work condition shall be considered to be energized.
18. Removing the panel cover from a 480V distribution panel and leaving the dead front in place while performing circuit testing with a multimeter is classified as a HRC _________________ task.
19. An apprentice is working on a junction box that contains 120v circuitry, the junction box is fed from a panel board that is not energized but the feeders for the panel board are terminated at both distribution point and the panel. The panel is locked out by the supervisors lock only. The apprentice when questioned does not know the exact circuit numbers of the circuits in the junction box. Is this considered energized work?
20.. Energized Work Permit (Standard) must be reviewed and approved by the project manager responsible for the work, the Vice President of Operations or the Area Manager responsible for the work and by the Director of Education and Loss Prevention. In an emergency situation an approval may be obtained by voice communication. This is limited to extreme conditions
21.. Before any work can be done on energized electrical equipment, a work permit must be completed and approved. E Light Electric Services has a Standard Work Permit and a Troubleshooting and Testing Work Permit. The Troubleshooting and Testing Work Permit may only be used for specific tasks involving troubleshooting and testing. Once a problem has been identified and it is determined that further energized work will be necessary to repair the problem a Standard Work Permit will need to be completed and approved
22. All potential hazards and risks shall be recorded on the work permit, including but not limited to the following:
Arc Blast
Uncontrolled shut down of system
Potential damage to equipment
Potential Damage to Personnel

23.  Barricades shall be used in conjunction with safety signs where it is necessary to prevent or limit employee access to work areas exposing employees to un-insulated energized conductors or circuit parts. 
24. Employees involved in energized work shall be informed of emergency contact numbers for medical and fire personnel and shall be briefed on how to direct emergency responders to the work site and work area should they be required.
E Light Electric Services, Inc.
Excellence in Electrical
361 Inverness Drive South, Suite B, Englewood, CO 80112
(303) 754-0001
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The table below is used to determine the minimum level of protection that is required to perform energized work for E Light Electric Services, Inc. This table is developed by using the requirements of NFPA 70E and includes most of the common tasks that we would be required to do. A complete listing of the tables can also be found in NFPA 70E. If you have any questions or need further explanation of this table please contract the Director of Safety and Loss Prevention directly at 303-550-5292
This page was last updated: July 19, 2017
A. Energized Task
B. Critical Task
C. Important Task
D. None of the above
A. True
B. False
A. 12
B. 50
C. 120
D. 220
A. 1
B. 2
C. 2*
D. 3
A. Yes
B. No
A. True
B. False
A. True
B. False
A. True
B. False
A. True
B. False
A. True
B. False
A. 110.14
C. 220.20
D. 110.16
A. True
B. False
A. current
B. power factor
C. bolted fault current
D. frequency
A. divided by 1,000
B. divided by 100
C.multiplied by 1,000
D. multipled by 100
A. Contact number for responsible person
B. Name of the installing electrician
C. Recommended PPE Level
D. Hazard Risk Catagory
A. reading the nameplate of the main service transformer
B. dividing the actual power of the transfomer by the true power the transformer
C. finding the square root of the sum of the resistance squared and the reactance squared
D. reading the nameplate of the transformer supplying the panelboard or equipment in question
A. limited approach boundary
B. restricted approach boundary
C. prohibited approach boundary
D. none of the above
A. 4'
B. 2'
C. 3'6"
D. 1'
A. limited approach boundary
B. prohibited approach boundary
C. restricted approach boundary
D. none of the above
A. True
B. False
A. shock
B. burns
C. arc blast
D. ventricular fibrillation
A. locked out condition
B. electrically safe work condition
C. de-enregized and cautious condition
D. tagged out condition