Arc Flash Hazard Analysis / Risk Assessment

 

1. What are the steps involved in Arc Flash Analysis?

A. Arc Flash Hazard Analysis or Risk Assessment is a study conducted by trained safety experts to analyze electric equipment and power systems in order to predict the amount of incident energy from an arc flash.

According to IEEE std 1584, there are 9 steps involved in this Arc Flash Analysis, which we are going to discuss here.

Step 1: Collect the system’s installation data: The largest effort in an arc-flash hazard study is collecting the data from the site.

Start by reviewing the one-line diagrams and electric equipment, site, and layout arrangement with people who are familiar with the site. The diagrams have to be updated to show the current system configuration and orientation before the arc-flash study starts. The one-line diagrams must have all alternate feeds. If SLDs are not ready, prepare them.

            Once the diagrams are completed in the basic electric system scheme, enter the data needed for the short circuit analysis. The study must take into account all the sources, along with utilities, and power generators, and for motors of 37 kW and larger than that contribute energy to short circuits. The SLDs should represent all the transformers, transmission lines, distribution networks, system grounding, current limiting reactors and other current-limiting devices, voltage correction capacitors, disconnectors, switchgear, motor control centers (MCCs), panel boards/switchboards including protective coordinating devices, fused load interrupter switches including fuse varieties and sizes, feeders and branch networks, also the motors down to the 600 V or 400 V range, and transformers supplying instrument power. No need to consider the equipment range below 240V unless and until it involves at least one 120 kVA or larger low impedance transformer in its immediate power supply.

            From the utility collect the values of power angle or X/R ratio and the fault current MVA. Most of the utilities readily supply the data on the existed fault level and power angle values at the point of service. When the data is not given, Public Utility Commission can be requested the respective utilities to furnish this information as much as realistic.

            Note down the name late details of all the equipment. Typically this would include voltage/voltage ranges or tap settings, ampacity, kilowatt or kilovolt amperes, momentary or interrupting current rating, impedance or transient/ sub transient reactance data, etc. Next note conductor and cable data along with its installation for all electrical circuits between the utility and the distribution and control equipment.

                        Finally, the data from the protective devices and the transformers i.e., current transformer, voltage transformer, or control power transformer must be collected. All this data should present on nameplate details or available at time-current curves. Otherwise, it should be mentioned in recent maintenance test reports or in specifications. In any case, the user should verify old data is still up-to-date by checking with the owner’s representative and, if necessary, by checking in the field. In some cases the ratings of fuses installed and protective device coordination settings can be determined by Field Inspection Method.

Step 2: Determining modes of operation of the system:

For a simple radial distribution system there is only a normal mode of operation, whereas in a complex system there may be different modes of operations that exist, which are as follows.

— May be more number of utility feeders are in working state.

— Tie breaker in the secondary side bus of the Utility interface substation may be in the open or closed state.

— One or more primary feeders may exist in a Unit substation.

— Unit substation having two transformers whose secondary tie is in open or closed condition.

— MCC (Motor Control Center) are with one or two feeders, in which one or both feeders may be energized.

— Connected generators may run in parallel along with the utility supply or in standby condition.

Here, It is very important to obtain the existed short-circuit current for all the modes of operation that provides both the maximum and the minimum available short-circuit currents.

 

Step 3: Determining bolted fault currents

Input all the data from the one-line diagrams and the data collection effort results in a short-circuit analysis. Commercially available software can run thousands of buses at a time and allow easy switching between different modes of operation. The simple calculator included in this IEEE 1584 standard can calculate bolted fault currents of the radial systems up to 600 V. Find the symmetrical RMS value of the bolted fault current and X/R ratio at each and every point, for all the locations where people work by taking each of these points as a bus. For all the modes no need to run all the buses because bolted fault currents of some buses will not be affected by all the modes. Consider an example, if we connect secondaries of transformers together, then primary side fault energy may not increase.

            It is necessary to include all the cables because the error on the high side may not increase safety always, it may decrease it. Reduced fault currents may exist longer time than currents of higher magnitudes as mentioned in protective-device coordination (TCC) curves.

 Step 4: Determining the arc fault currents

The arc fault current at the required point and the magnitude of that fault current passing from the first upstream protective device to downstream must be selected.

This arc fault current magnitude mainly depends upon the bolted fault current. The magnitude of this bolted fault current in the protective device can be obtained from the short-circuit analysis by looking at a one-bus-away run. This will separate faults that occur in a normal feeder, alternate feeder, and downstream motors.

Then the arc fault currents can be calculated. The obtained arc fault current magnitude will be lower than that of the bolted fault current magnitude due to arc impedance, mainly for applications < 1 KV. For medium-voltage levels, the bolted fault current is still greater than the arc current, and it should be calculated.

Step 5: Finding the coordinating device characteristics and the duration of the arc existence

Updated system TCC curves can be obtained from the field survey. If not found, it is better to create them with the help of commercially available software easily. Otherwise, for a simple study, we can use protective device characteristics, which were present in data from the manufacturer.

            TCC curves of fuses obtained from the manual may contain both clearing time and melting times. If so, use the clearing time. If they provide only the average melting time, add 15% of that, i.e., up to 0.03 seconds, and 10%. The manufacturer’s TCC curves include both tripping time and clearing time in case of circuit breakers having integral trip units.

For circuit breakers operating with relays, the curves show only the relay operating time in the time-delay region. The recommended circuit breaker operating times were shown in the following table. Opening times for the mentioned circuit breakers can be verified by the manufacturer’s data.

  Table 1—Power circuit breaker operating times

Type and Rating of the Circuit Breaker

Opening time at 60 Hz

(cycles)

Opening time

(seconds)

Low voltage (molded case)

(< 1000 V) (integral trip)

1.5

0.025

Low voltage (insulated case)

 (< 1000 V) power circuit breaker

(integral trip or relay operated)

3.0

0.050

Medium voltage (1–35 kV)

5.0

0.080

Some high voltage(> 35 kV)

8.0

0.130

 In a limited number of cases, this data is incorporated into the model and TCC curves are not required. Different types of current-limiting fuses were tested to find the effect of current limiting action on incident energy and results were included in the model. A generalized solution will be provided for some circuit breakers with integral trip units, and it is part of the model. For a circuit breaker, it is implemented only when the arc current is in the instantaneous or highest level trip range.

 Step 6: Documentation of the system voltages and equipment classes

As shown in the following table-2, document the system voltage and the class of equipment for each and every bus. This will allow the application of equations based on standard classes of equipment and bus-to-bus gaps as shown in the following table.

                 Table 2—Classes of equipment and typical bus gaps

Classes of equipment

Typical bus gaps (mm)

15 kV switchgear

150-152

5 kV switchgear

102-104

Low-voltage switchgear

30-32

Low-voltage MCCs and panel boards

24-25

Cable

12-13

Other

Not Required

 Step 7: Select the working distances

Arc-flash protection always depends only on the incident energy on the person’s face and body at the working distance, but not on the incident energy on the person’s hands or arms. Based on the percentage of the person’s skin which is burned the degree of injury classified. As the head and body forms a large percentage of total skin and injury to these areas becomes much more life threatening than the burns on the extremities. The typical working distances were mentioned in the following table. (Table-3)

Table 3—Classes of equipment and typical working distances

Classes of equipment

 Typical working distance (mm)

15 kV switchgear

910

5 kV switchgear

910

Low-voltage switchgear

610

Low-voltage MCCs and panel boards

455

Cable

455

Other

To be determined in field

 Typical working distance is defined as “ the sum of the distance between the worker standing in front of the equipment, and from the front of the equipment to the potential arc source inside the equipment”.

 Step 8: Determine the incident energy for all equipment

Any good software for calculating incident energy must be selected. In some cases, the equations in the models, are embedded in the software program or worksheet. In some programs, the problem is solved only at one bus at a time; whereas some programs offer the facility that, hundreds or thousands of buses can be solved simultaneously. After getting incident energy, generate Arc Flash Labels.

                           
Step 9: Determining the flash-protection boundary for all the equipment:


To find the Arc flash-protection boundary, the equations for finding incident energy can be solved for the distance from the arc source at which a second-degree burn could occur.

 The incident energy must be set at the minimum value beyond which a second-degree burn could occur. The programs include the flash-protection boundary based on incident energy of 5.0 J/cm2

2. What are the standards we follow in Arc Flash Risk Assessment?

A.  No individual code or standard defines all the requirements for arc-flash safety. Arc flash safety is based on the cumulative requirements found in several codes and standards, which include OSHA, NFPA70E, IEEE1584, NFPA70, NESC, which will be discussed as follows.

1. OSHA: Occupational Safety and Health Administration

Federal laws and regulations address a variety of safety issues, which include electrical safety requirements which are necessary to safeguard the employees in the workplace. This OSHA addresses electrical safety requirements for all employee workplaces, along with commercial office and industrial environments. The points we should notice include:

 Section 29 CFR 1910.335: It focuses on what employees to be provided with, and to use appropriate personal protective (PPE) equipment when working in areas where there are potential electrical hazards, such as arc flash hazards.

 • OSHA’s General Duty Clause: It states that employers should provide a place of employment that is free from recognized hazards, which include arc flash hazards.

 OSHA also allows the states to develop and operate their own safety and health programs through an OSHA-approved state plan. These state plans must be as effective as the federal OSHA program. States often adopt some standards similar to OSHA but should enforce stricter safety requirements. Employers should be aware of and stay updated on any additional local electrical safety requirements if they operate in a state with an OSHA-approved plan.

2. NFPA 70E: Electrical Standard for safety in the Workplace


National Fire Protection Association (NFPA) is the standard mainly meant to provide a safe working place for employees whose job responsibilities involve interaction with electrical equipment and systems with potential exposure to energized electrical equipment. NFPA 70E is intended to be used by employers, employees, and OSHA, and addresses electrical safety requirements for all employee workplaces, along with commercial offices and industries.

NFPA 70E gives specific requirements for selecting Personal Protective Equipment (PPE) and warning the workers regarding equipment’s potential electrical hazards. Employers are advised to adopt and follow NFPA 70E standard and to comply with OSHA’s general duty clause. Refer the Article 90 of NFPA 70E for a clear description of what the standard does and does not cover.

 • NFPA 70E Article 130.5 enforces the employers to conduct an arc-flash risk assessment for all the electrical equipment. The purpose of performing this arc-flash risk assessment is to notify all the arc-flash hazards involved in the workplace, so that the employees can be aware of the associated hazards of the equipment and provided appropriate PPE. One of its requirements is to provide arc-flash labels i.e., danger and warning labels for electrical equipment indicating system voltage, arc-flash boundary, and other information to determine the required PPT.

  • NFPA 70E provides two methods for the employees to determine the minimum PPE arc rating required at electrical equipment based on the “incident energy analysis method” or also called the “arc flash PPE categories method.” To obtain the available incident energy at the equipment, this method performs an arc flash study, in compliance with IEEE 1584.

   


 
This Arc Flash PPE categories method is to determine the equipment’s Arc Flash PPE Category, which is previously known as the Hazard/ Risk Category (HRC), this will tell you the PPE required for the task. As described in the tables, if a task is not listed, or the exact conditions are not met, this method is useless, and an incident energy analysis will be required. For this reason, it is better to follow the Incident Energy Analysis method instead of Arc Flash PPE method to generate arc flash labels for all equipment.

In NFPA 70E 2015 edition, unlike previous editions of NFPE, here employers are not permitted to list out a PPE Category on an arc flash label if the label is provided with the available incident energy.

This NFPA 70E is a standard adopted only by employers not by all local jurisdictions. Though NFPA 70E is not officially followed by OSHA, employers may utilize NFPA 70E in compliance with OSHA.

 3. IEEE 1584: A performance Guide for Arc-Flash Hazard Calculations

It is a standard which is having methods and procedures to calculate the amount of Arc Flash Incident energy to which the employees may expose. The results of this incident energy calculation are used to suggest the appropriate PPT according to NFPA 70E. Calculating the existed incident energy needs the data about short circuit and over current protection settings because without performing short circuit and protective device coordination studies, we cannot perform arc flash hazard analysis. The arc flash hazard analysis needs the field data to cross verify with the facility’s electrical distribution and overcurrent protective device settings.

 4. NFPA 70 or NEC:

NFPA 70 also called National Electric Code (NEC) is a standard deal with safety of persons, property or equipment from electrical installation hazards. In Article 110.16 of NEC, it is stated that electrical equipment that is likely to require interaction while energized shall be field-marked to warn workers of potential electric arc-flash hazards. NEC-2017 version extended the article 110.16 requiring more stringent arc-flash labeling requirements for service equipment of rating 1,200 amps or more, essentially requiring the equipment to be assessed for arc-flash risk. According to NFPA 70E OR IEEE1584, if an arc flash label is provided, then no need for this NEC labeling. This NEC is regionally adopted and enforced by states and local municipalities; whereas a new arc-flash labeling system will be enforced by local jurisdictions that have adopted NEC 2017.

 NESC: ANSI C2 National Electrical Safety Code

The National Electrical Safety Code (NESC) focuses mainly on hazards related to electric power utility systems and requirements for protecting the employees from related hazards. And it extends its scope to the point of service where it is transferred to the on-site wiring system (where the scope of NFPA 70E begins).

 

 

 

 

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