5-Attachment 5 REQ-0002500 - Arc Flash.pdf

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CITY OF DETROIT, MICHIGAN

DETROIT WATER AND SEWERAGE

DEPARTMENT

CONTRACT NO. CS-1340

TASK 29

SOUTHWEST WATER TREATMENT PLANT

Arc Flash Study

Prepared By:

ARCADIS G&M of Michigan, LLC

November, 2010 i

Table of Contents

1. Executive Summary 1

2. Introduction and Purpose 3

3. Hazards and Risks of Arcing Faults 5

3.1 General 5

3.2 Limitations of the Human Body 6

3.3 Definitions for Thermal Measurements and Limits 7

3.4 Working Distance 7

3.5 Arc Flash Boundary 7

3.6 Risk 8

3.7 Classification of Arc Flash Hazard/Risk 8

4. Electrical Approach Boundaries 9

4.1 Limited Approach Boundary 9

4.2 Restricted Approach Boundary 9

4.3 Prohibited Approach Boundary 10

5. System Modeling 11

6. Electrical System Overview 12

6.1 Contributions 12

6.1.1 24 kV Utility Service 12

6.1.2 4.8 kV Generators 12

6.1.3 Motor Contribution 12

6.1.4 Contribution from Passive Devices 13

6.2 Protective Devices 13

6.3 System Impedances 14

6.3.1 Transformers 14

6.3.2 Conductors 15

ii

6.3.3 Electrical Loads 15

6.4 The Electrical Distribution System 15

6.4.1 24 kV System 15

6.4.2 4.8 kV Medium Voltage Switchgear 16

6.4.3 Generator Switchgear 16

6.4.4 Low Voltage Unit Substations 16

6.4.5 Motor Control Centers 16

6.4.6 Low Voltage Systems Excluded from the Arc Flash Labeling Process 17

6.5 Modeling Scenarios 20

6.5.1 Scenario No. 1 – Minimum Fault Contribution 20

6.5.2 Scenario No. 2 – Maximum Fault Contribution 21

6.5.3 Scenario No. 3 – Generator Fault Contribution 21

6.5.4 Scenario No. 4 – Generators and Utility in Parallel 22

7. Arc Flash Calculations 24

8. Analysis of Calculation Results 26

8.1 24 kV Services 26

8.2 10 MVA Transformer Secondary Side 26

8.3 Medium Voltage Switching Operations 27

8.4 Medium Voltage Motors 28

8.5 Unit Substations 29

8.6 Low Voltage Distribution Systems 30

9. Personal Protective Equipment 32

10. Equipment Labeling 36

11. Electrical Safety Regulations and References 38 iii

Tables

Table 1 – Hazard/Risk Categories 8

Table 2 – Incident Energy Levels for Branch Devices 19

Table 3 – Recommended PPE for Hazard/Risk Categories 32

Table 4 – Rubber Insulating Glove Ratings 34

A1 Air Gap Distances (IEEE 1584) A3

A2 Working Distances A7 iv

Appendices

A Arc Flash Modeling Assumptions

B Utility Data

C One Line Diagrams

D Arc Flash Calculations - Scenario 1 (Minimum Fault Conditions)

E Arc Flash Calculations - Scenario 2 (Maximum Fault Conditions)

F Arc Flash Calculations - Scenario 3 (Emergency Generators Only)

G Arc Flash Calculations - Scenario 4 (Utility paralleled with Generators)

H Arc Flash Calculations - Comparison of 4 Scenarios

I Evaulation of Arc Flash Hazards at End Field Devices

J Sample Work Permit

K Labels

L Field Data

M PPE Checklist

NOTE:

Refer to Appendix H for a list of electrical equipment which has been identified and field labeled as an arc flash hazard. Appendix H contains a table which lists

LABELED EQUIPMENT and the corresponding arc flash incident energy.

Appendix H also includes a list of UNLABELED EQUIPMENT. The electrical equipment in this list has been evaluated, and in the event of a fault, the incident energy presented to the worker will be less than 1.2 cal/cm at normal working distance. This equipment is categorized as HRC 0, and arc flash PPE (beyond the minimum HRC 0 PPE) is not required.

Electrical workers should always wear HRC 0 PPE (safety glasses and non-meltable clothing) whenever working on live equipment with voltages which exceed 50 volts.

Southwest Water

Treatment Plant

1. Executive Summary

The intent of this study is to provide calculations and information related to arc flash hazards for the Southwest Water Treatment Plant] located at 14700 Moran Road in

Allen Park, Michigan. This study identifies incident energy and arc flash boundaries for all major electrical equipment so that appropriate Personal Protective Equipment

(PPE) can be identified for plant personnel when these workers are exposed to live equipment.

Section 2 provides a brief introduction to NFPA 70E-2009 and OSHA regulations related to arc flash hazards. Work should be conducted on de-energized equipment whenever possible. If work must be performed on equipment in an energized state, appropriate PPE must be worn. A work permit process must be followed prior to performing work on energized equipment. NFPA 70E, Article 130 and NFPA 70E, Annexes E through N provide useful information for preparing an electrical safety program for addressing arc flash hazards. A sample work permit is also provided in

Appendix J.

Section 3 describes the hazards and risks associated with arc flash and arc blast.

Injury risks include burns, shrapnel, hearing and vision damage, and loss of life. Table

1 of this section describes the five Hazard/Risk Categories (HRC) associated with arc flash, and the incident energy levels associated with each category. These

Hazard/Risk Categories are the basis for defining appropriate PPE.

Section 4 provides information on the approach boundaries which must be established around a work area when energized parts are exposed. Three boundaries must be established for the prevention of shock and electrocution, and a fourth boundary must be established for arc flash. These boundaries are for the protection of the worker and for the safety of other people who may be in the general area when live work is being performed. The shock hazard boundaries are established based on the information provided in NFPA 70E-2009, Article 130, Table 130.2(C).

Section 5 provides a brief overview on the modeling software used to model the electrical distribution system at this facility. EDSA Paladin® DesignBase™ 3.0 software has been used for modeling and for all calculations provided in this report. Additional information on assumptions used in the modeling of the system is provided in

Appendix A. DTE Energy utility information utilized in this study is provided in

Appendix B.

Section 6 describes the electrical system at the water treatment plant and is provided as a supplement to the one line diagrams provided in Appendix C.

Section 7 provides an overview of the arc flash calculations data produced from this arc flash study, including an overview of the information presented in the appendices of the report.

Section 8 provides an analysis of the results of the arc flash calculations. The analysis is broken down to reflect the one-line diagrams and models used in this study.

Problem areas have been identified and proposed solutions are recommended.

Significant items noted during the development of this report are as follows:

Several medium voltage motor nameplates displayed rated voltages which are less than the connected voltage. The nameplates are expected to be in error, but this issue should be investigated. Refer to Section 8.4 of the report for more details.

Three of the four unit substation main circuit breakers have short time pickup settings which are too high, which results in excessive arc flash energy.

These settings should be reduced. Refer to Section 8.5 of the report for recommended settings.

Two outgoing unit substation feeder circuit breakers have short time pickup settings which are too high, which results in excessive arc flash energy.

These settings should be reduced. Refer to Section 8.6 of the report for recommended settings

The motor control center MCC 2B-1 is located in a corrosive area, but the enclosure is not rated for this environment. This is a violation of NEC 110.11, and the situation should be corrected

The two original 1960 motor control centers located in the filter galleries are badly corroded, and should be replaced.

The application of PPE for arc flash protection is described in Section 9. Various levels of PPE are recommended based on the HRC rating of the electrical equipment.

Section 10 provides information on equipment labeling, and Section 11 lists additional reference sources which can be used for development of an arc flash safety program.

2. Introduction and Purpose

The primary purpose of this document is to provide arc flash calculations, appropriate labeling, and information needed in order to comply with the following recommended and required safety practices:

• Standard NFPA 70E-2009, Article 130: This standard provides details and methods related to the analysis of arc flash hazards for electrical systems, detailing the arc flash incident energy, boundary distances, and PPE requirements.

• OSHA 29 CFR Subpart S, 1910.132 (d): This section of the OSHA regulations defines the criteria for Hazard Assessment and PPE identification, and defines the employer’s responsibilities for assessing the workplace to determine if hazards are present, or likely to be present. The employer is required to identify those hazards which necessitate PPE.

• OSHA 29 CFR Subpart S,1910.335 – Safeguards for Personal Protection:

This section of the OSHA regulations discusses Personal Protective

Equipment. The regulations note that employees working in areas where there are potential electrical hazards shall be provided with, and shall use, electrical protective equipment that is appropriate for the specific parts of the body.

• MIOSHA General Industry Part 33, Personal Protective Equipment: Rule

3308(1) requires the employer to assess the workplace to determine if hazards are present which necessitate the use of personal protective equipment. If present, then the employer must take steps to ensure the appropriate use of

PPE.

In addition to the analysis and calculations provided in this report, NFPA 70E-2009 also recommends that the following be included as part of an overall electrical safety program:

• Provide an electrical safety program with defined responsibilities.

• Purchase and provide appropriate electrical and arc flash PPE for workers.

• Train workers on the hazards of arc flash and arc blast.

• Provide appropriate tools to allow work to be completed safely.

• Provide warning labels on equipment to advise workers of the risk of arc flash and shock hazards.

The data and information provided in this report can be used as a basis for developing strategies that have the goal of minimizing burn injuries. These strategies include requiring specific levels of PPE when working on live equipment and encouraging the practice of working on de-energized equipment whenever possible.

The data provided in this analysis is not intended to imply that workers be allowed to perform work on exposed energized equipment or circuit parts. It must be emphasized that the industry-recommended method of minimizing electrical injuries and fatalities is to ensure that equipment is de-energized and in an electrically safe work condition (i.e.

– locked out and tagged out).

Work intentionally performed on or near energized equipment or circuits is limited by standards and regulations. OSHA 29 CFR Subpart S,1910.333 limits the situations in which work is performed on or near equipment or circuits that are (or may be) energized.

• “Live parts to which an employee may be exposed shall be de-energized before the employee works on or near them unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations.”

3. Hazards and Risks of Arcing Faults

3.1 General

An electrical fault in a power distribution system results from unintended contact between two conducting surfaces at different voltage potentials, or a breakdown in insulation between the two surfaces. The two surfaces may both be energized

(creating a line-to-line fault) or one of the two surfaces may be grounded (creating a line-to-ground fault). The cause of the fault can include any of the following conditions:

• Deteriorating insulation

• Improper spacing between energized components

• Corrosion or build-up of energized parts

• Over-voltage conditions

• Accidental dropping of tools or equipment across energized surfaces.

An arcing fault is a bridge of electrical current across an air gap which results in the release of a sudden burst of electrical heat energy. This sudden burst of light and heat energy is referred to as an arc flash. The fault current through the resistance of an air gap is similar to electrical current passing through the filament of a light bulb, but on a much larger scale. Heat and light are produced from the resistance of the material (air) as the current passes through it. However, under the circumstances of an arcing fault, almost half of the available energy from the electrical power source can be converted into thermal radiation. Temperatures as high as 35,000 o F can be produced from an arcing fault. This heat can cause severe burns and can ignite the clothing of personnel in the vicinity of the arc flash. Fatal burns are possible at distances of over 10 feet away from the arcing equipment.

The intense heat of an arc flash vaporizes nearly all material within the arc. The air around the arc is superheated and expands rapidly, creating a strong outward pressure wave. As the copper conductor vaporizes, the volume of the copper expands to 67,000 times the original volume, contributing greatly to the pressure wave. The resulting blast of pressure can throw a worker across the room. For high incident energy levels, the pressure wave is often large enough to burst the eardrums of a nearby worker, cause lung collapse, or even stop the heart when the pressure burst is concentrated in the chest area. Molten metal and other debris (arc blast) may be carried with the pressure blast, creating additional hazards related to shrapnel and molten spray.

In addition to the burns, shrapnel, and hearing damage, the intensity of the light from an arc flash can cause immediate vision damage, and toxic fumes can be emitted from the burning and vaporization of products.

Anyone in the path of an arc flash or arc blast that is not properly protected with PPE and distance is likely to be severely injured or killed.

3.2 Limitations of the Human Body

In order to provide protection against the hazards associated with arc flash, it is important to understand the limitations of the human body when exposed to heat and pressure.

The pressure from an arc blast is typically measured in lbs/ft . The human eardrum will rupture at approximately 720 lbs/ft . Lung collapse can occur at pressures of approximately 1,800 lbs/ft . With these numbers in mind, recognize that on many large systems with 480 volts AC or higher voltages, pressures in excess of 2,000 lbs/ft are not unusual when measured two feet from the arcing fault. Audible sound levels from the blast can exceed 140 dBA, well above the 120 dBA limit defined for immediate hearing damage.

NFPA 70E has defined the limits of heat exposure as the maximum amount of heat which can be absorbed by the skin over a given time period without suffering a second degree burn. This transfer of heat into the skin is measured in calories or in joules.

Historical testing has shown that the human skin can absorb as much as

1.2 calories/cm (approximately 5 joules/cm

) without developing a second degree burn. Therefore, this value is used as the threshold value for distinguishing between a first and second degree burn. Note that a first degree burn (redness and pain) is still possible when standing outside of the established zone of protection.

One calorie/cm is the approximate equivalent to holding a finger over the open flame of a candle for one second. The exact unit of measure relates to the amount of heat required to raise the temperature of one cubic centimeter (one gram) of water by

1 degree Celsius.

3.3 Definitions for Thermal Measurements and Limits

The total amount of heat available at a measured distance from an arcing fault is defined as the incident energy. The amount of incident energy falls off sharply as the measurement point moves away from the arcing fault. Therefore, a listed value of incident energy must always be accompanied with a corresponding distance from the live parts.

Given this, arc flash boundary information can be conveyed in one of two manners:

• Fixed Energy Level: Calculating an unknown distance from live parts, which defines the point where the incident energy is equal to 1.2 cal/cm

• Fixed Distance: Calculating an unknown incident energy level at a particular working distance from live parts, which defines the expected thermal energy that a worker will be exposed to when working on the equipment.

Arc flash labels provided with this report provide both sets of data values.

3.4 Working Distance

The selection of PPE is based on the incident energy levels at a designated working distance from the exposed live parts. Since the person must be within reach of the equipment, but must also be far enough from the equipment to comfortably view the equipment, a value of 18 inches is typically used as a working distance.

For 480 VAC draw-out switchgear, the electrical bussing systems are located a considerable distance behind the front of the breakers. Therefore, a value of 24 inches is used as the distance between the worker and live parts. The working distance for

4.8 kV switchgear is 36 inches.

These distances reflect those recommended in Section 4.8 of IEEE 1584.

3.5 Arc Flash Boundary

The distance at which the incident energy is equal to 1.2 cal/cm defines the boundary in which the risk of second and third degree burns (as a result of arc flash) is minimal.

However, the risks of arc blast (flying shrapnel, molten spray, hearing damage, and vision damage) are still hazardous outside of this thermal boundary.

The distance at which the incident energy is equal to 1.2 cal/cm is referred to as the arc flash boundary.

3.6 Risk

The frequency of arc flash occurrences is not well documented. Many arc flash injuries are documented as general electrically-related injuries and are mixed in with injuries related to electrical shock, fire, and/or explosions. A study by Capelli-Schellpfeffer, Inc.

of Chicago reports that there are 5 to 10 arc flash accidents each day which result in hospitalization.

The amount of exposure that a worker has to electrical systems and equipment directly increases the employee’s risks associated with this type of hazard.

3.7 Classification of Arc Flash Hazard/Risk

NFPA 70E-2009 defines 5 levels of Hazard/Risk Categories (HRCs) for arc flash hazards based on the calculated incident energy at the established working distance.

The categories are assigned a number between 0 and 4, with the associated risk increasing as the assigned hazard/risk category increases. Incident energy levels associated with each HRC is provided below:

Table 1 - Hazard/Risk Categories

Category Incident Energy Level

0 0 to <1.2 cal/cm

1 1.2 to <4.0 cal/cm

2 4.0 to <8.0 cal/cm

3 8.0 to <25.0 cal/cm

4 25.0 to <40.0 cal/cm

Note that the hazard/risk category stops when values exceed 40 calories/cm

Although PPE is available with ratings higher than 40 calories/cm , the arc blast pressure at this energy level is extremely high and presents a significant risk of injury or death. For arc flash events with energy levels higher than 40 calories/cm , the arc flash PPE may protect the worker from burns but will not protect the worker from injuries related to the violent blast. Arc flash PPE does not provide suitable protection in these areas of high energy.

4. Electrical Approach Boundaries

Four approach boundaries have been established in NFPA 70E Article 130. These boundaries define specific distances from energized equipment within which workers may be exposed to an electrical shock or arc flash hazard. Precautions for worker protection are required within these boundaries.

The first boundary, and the primary subject of this report, is the arc flash boundary.

This boundary is described in Section 3.5.

In addition to the arc flash boundary, three shock boundaries should be included on all arc flash labels applied to electrical equipment. The distance of each boundary will vary depending on the voltage potentials available at the electrical equipment. These values are provided in NFPA 70E-2009, Table 130.2(C) and are described in NFPA 70E-

2009, Annex C. The definition for each of these shock boundaries is provided below:

4.1 Limited Approach Boundary

This is an outer boundary which defines the electrical work area. Only qualified persons (responsible, capable, and trained to perform the task at hand) are permitted to cross this boundary. Unqualified persons are not permitted to cross this boundary unless they are escorted by a qualified person. For 120/208 VAC and 480 VAC systems, this boundary is established at 3 feet, 6 inches from the exposed, live circuit parts. For the 4,800 volt systems, the limited approach boundary is 5 feet. At 24 kV, the limited approach boundary is 6 feet.

4.2 Restricted Approach Boundary

Only qualified persons are permitted to cross this boundary. Under no circumstances shall an “unqualified person” be permitted to cross this boundary. The qualified person must satisfy a number of safety-related conditions prior to crossing this boundary.

These conditions include the following:

• Possessing a work plan that is documented and approved by authorized management.

• Using appropriate PPE for working near exposed, energized conductors or circuit parts, and rated for the voltage and energy levels involved.

• Be certain that no part of the body enters the prohibited space.

• Minimize the risk due to inadvertent movement by keeping as much of the body out of the restricted space as possible. The employee should use only protected body parts in the space as necessary to accomplish the work.

For 120/208 VAC and 480 VAC systems, this boundary is established at 12 inches from the exposed, live circuit parts. For 4.8 kV systems, the restricted approach boundary is 26 inches. For 24 kV systems, the restricted approach boundary is 31 inches.

4.3 Prohibited Approach Boundary

This is the minimum approach distance to an exposed and potentially energized circuit part, component, or conductor, and is the closest point to prevent flashover. To cross this boundary and enter the prohibited space shall be considered the same as making direct contact with the exposed, energized parts. The qualified person must satisfy a number of safety-related conditions to cross this boundary. These conditions include the following:

• The worker must have specific training to work on energized conductors or circuit parts.

• The worker must have a documented plan, justifying the need to work that close to energized equipment.

• A risk analysis must be performed by the worker.

• The worker must have the documented work plan and risk analysis approved by authorized management.

• The worker must use PPE appropriate for working on exposed, energized conductors or circuit parts, and rated for the voltage and energy levels involved.

For 120/208 VAC and 480 VAC systems, this boundary is established at 1-inch from the exposed, live circuit parts. For 4.8 kV systems, the prohibited approach boundary is 7 inches. For 24 kV systems, the prohibited approach boundary is 10 inches.

5. System Modeling

The arc flash calculations furnished in this study have been compiled based on circuit breaker and fuse information gathered in the field. Cable distances and sizes are based on the Wayne County Job No. W-143 record drawing set (1960 original construction plan set). Where wire sizes were not available from existing record documents, wire sizes have been assigned as required to comply with the National

Electrical Code, based on upstream overcurrent protective devices and standard engineering practices.

The data used in this study are deemed to be reliable for the purpose of defining the probable incident energy levels throughout the electrical system. Calculations were performed using the arc flash evaluation tools of EDSA Paladin® DesignBase™ 3.0 software.

Calculations are based on IEEE 1584-2004 “Guide for Performing Arc Flash

Calculations”, using an arc flash boundary which is equivalent to 1.2 cal/cm of incident energy. The values produced by this software are based on laboratory values

(empirical data) in which arc current incident energy was measured and recorded under a specific set of test conditions. Therefore, the incident energy values calculated under this study can be considered statistical in nature. The actual incident energy from an arc flash may be more or less severe than indicated by these models.

Furthermore, the potentially hazardous effects of molten metal splatter, projectiles, shrapnel, pressure impulses, and toxic arc by-product are not considered in the establishment of an arc flash boundary.

Although it is possible to utilize calculations based on a “worst-case” situation, the use of excessive PPE gear introduces difficult working conditions and increases the probability of accidents. The incident energy levels provided through IEEE 1584 methods provide statistical values which allow for a balance between adequate PPE and the mobility to perform the work properly. Per Section 9.11.3 of IEEE 1584, the values obtained through the IEEE 1584 calculation methods provide a 95% confidence level that the calculated PPE will be adequate or more than adequate for an arc flash event.

Appendix A provides more information on the modeling techniques used to develop the arc flash boundaries and incident energy values for this report.

6. Electrical System Overview

The electrical system at the water treatment plant consists of utility and generator contributions, protective components (circuit breakers and fuses), and power consumption devices (system impedances and electrical loads). The primary components of the electrical system are identified below:

6.1 Contributions

6.1.1 24 kV Utility Service

The electrical system at the water treatment plant consists of two 24 kV electrical services (Navarre trunk lines 346 and 372). Fault contribution data for the electrical utility services is provided in Appendix B.

6.1.2 4.8 kV Generators

The water plant has a total of four diesel-driven generator units. Each generator has a standby load rating of 1,825 kW (2,281 kVA). The available fault current from each generator unit is inversely proportional to the internal impedance of the alternator.

Information from the generator operation and maintenance manual shows that these four generators have an internal impedance (subtransient reactance, Xd”) of 4.2% at the base rating of 1,825 kW/2,281 kVA.

The generator units at this location will produce a fault current equivalent to approximately 1/Xd” times the rated full load current of the alternator during the first 2.5 cycles of the fault (6,533 amperes per generator). This fault current will decay to approximately 3,100 amperes per generator after 0.5 seconds, and will eventually settle out near 300% of the alternator full load rating.

6.1.3 Motor Contribution

Spinning motor loads contribute power during fault events due to residual voltage and flux in the motor windings. Synchronous motors behave as synchronous generators, providing continuous fault current while the motor rotor continues to spin. Fault current from induction motors is only produced for several cycles and the current production decays rapidly as the internal flux in the motor rotor decays. The current produced from a motor during a fault is inversely proportional to the motor’s internal impedance, initially producing approximately 5 times the rated full load ampere rating of the motor.

Motors which operate on variable frequency drives (VFD’s) do not provide a motor contribution since the front-end rectification of the VFD prevents motor current from travelling back up into the electrical distribution system.

This arc flash study evaluates protective device tripping times under both, minimum and maximum fault conditions. The majority of motor contribution at this plant is sourced from the large synchronous motors.

For this study, minimum motor contribution has been defined as treating 40 MGD, with the following synchronous motors in service:

Low Lift Pump No. 1 (300 HP)

High Lift Pump No. 4 (1,250 HP)

Maximum motor contribution occurs during peak loading conditions, which is defined as treating 180 MGD. Under this condition, the following large synchronous motors have been assigned as operating simultaneously:

Three 600 HP Low Lift Pumps (LL-3, LL-4, and LL-5).

Three 2,500 HP High Lift Pumps (HL-1, HL-6, and HL-7)

6.1.4 Contribution from Passive Devices

Transformers and capacitors are passive devices. According to IEEE Std. 399 (IEEE

Brown Book), stored energy contribution from passive devices should be considered when conducting transient analysis, but this energy is negligible with respect to fault currents. Therefore, energy from passive devices is excluded when performing fault analysis and arc flash studies.

6.2 Protective Devices

Protective devices are considered passive devices in that they do not contribute or consume power of significant value. The only role of the protective devices is to isolate a portion of the electrical distribution system in the event of a fault or overcurrent condition.

This facility uses a combination of fuses and circuit breakers for fault and overcurrent protection on the medium voltage and low voltage distribution systems.

6.3 System Impedances

6.3.1 Transformers

Transformers are the largest impedances in most electrical distribution systems. The internal impedance of a transformer typically limits the downstream fault current to a value between 15 and 25 times the full load ampacity rating of the transformer.

Arcing faults at the medium voltage levels (4.8 kV and 24 kV) will have magnitudes near 95% of the calculated three-phase bolted vault value. Therefore, transformer primary protective devices are typically effective at protecting medium voltage systems for both, three-phase bolted faults and for arcing faults. However, for low voltage systems (600 VAC and below), arcing faults at 480 VAC can be as low as 38% of the three-phase bolted fault value, and as low as 25% of the three-phase bolted fault value for 208 VAC systems. As a result of the reduced fault current, arcing faults which occur immediately downstream of the 480/277 VAC unit substation transformers often have extended clearing times, since the primary protective device is incapable of quickly clearing the fault.

As an example, consider the 1,000 kVA unit substation transformers at this facility, which can produce a three-phase bolted secondary fault current that is 17.6 times the full load ampacity rating of the transformer. An arcing fault on the secondary side of this transformer will be approximately 64% of the available 3-phase bolted fault value, or 17.6 * 0.64 = 11.3 times the full load secondary current rating of the transformer. As an extra measure of safety, the IEEE 1584 arc flash calculations use 85% of the arcing fault value, which lowers the arcing current to 9.6 times the full load secondary current rating of the transformer.

Transformer primary protection must allow up to 12 times the full load primary current rating of the transformer for 0.1 seconds so that inrush current does not accidentally open the primary protective device. Therefore, by properly coordinating the transformer’s primary protective device, secondary fault currents could take several seconds or more to clear. For this reason, the first overcurrent protective device located downstream of a large transformer inherently carries a high risk of arc flash injury due to longer clearing times on the transformer primary overcurrent protection device.

6.3.2 Conductors

Conductors place an impedance on the system, limiting the fault current between the contributing sources and the fault location. Long wire lengths and smaller conductor sizes will produce the highest impedances, resulting in the smallest available fault currents.

6.3.3 Electrical Loads

Electrical loads are the only components in the electrical system which intentionally consume electrical power, converting the power into work energy. These loads consist mostly of motor loads, and are located at the end of feeders and branch circuits. Due to their location, these loads represent the points within the system which produce the lowest available fault current within any given feeder or branch circuit.

6.4 The Electrical Distribution System

6.4.1 24 kV System

The electrical distribution system consists of two 24 kV utility service entrances. These services are installed in underground ductbank from the south end of the property to the two 10 MVA outdoor transformers located on the west side of the treatment building. There are no DWSD-owned primary protective devices for these two utility transformers, so primary overcurrent protection relies on protective devices located within the utility’s electrical distribution system.

The two service transformers are connected with a delta secondary, and therefore, there is no ground return path to each service transformer. Grounding transformers have not been provided at this facility to stabilize the 4.8 kV system to ground, so the

4.8 kV system floats when operating under utility power. However, when the generator system switchgear bus is connected to the electrical system (Gen-Utility Tie circuit breaker is closed), the 4.8 kV system becomes impedance grounded via a grounding transformer located within the generator switchgear.

6.4.2 4.8 kV Medium Voltage Switchgear

A double-ended, 4.8 kV medium voltage switchgear lineup is located in the main switchgear room. This switchgear room is located on the west side of the treatment facility, with the outdoor utility service transformers located immediately outside of this room. Under normal operating conditions, the switchgear tie circuit breaker remains open, with the two 4.8 kV busses energized from the two utility services. The switchgear consists of GE Magne-Blast air circuit breakers with 8-cycle opening times.

This switchgear has no automated transfer operations, so transfer to emergency generator power must be performed manually. Thirteen of the circuit breakers in this switchgear lineup are used as motor starters for the six low lift pumps and seven high lift pumps. The switchgear has only two feeder circuit breakers, which are identified as

Feeders A and B. Feeder A feeds Bus A of Unit Substation #1 and #2, and Feeder B feeds Bus B of Unit Substation #1 and #2.

6.4.3 Generator Switchgear

The 4.8 kV generator switchgear is located outdoors in a weatherproof, walk-in housing. This switchgear was installed under the DWS-828 project in 1999, and allows four generator units to be paralleled with each other and also paralleled with the utility system. The circuit breakers are Cutler-Hammer vacuum type, with 3-cycle opening times.

6.4.4 Low Voltage Unit Substations

Two 480 VAC double-ended unit substations are located in the switchgear room.

These two unit substations convert 4.8 kV to 480/277 VAC. Unit Substation No. 1

(USS1) is fed with two 1,000 kVA liquid-filled transformers which have been integrated into the substation line-up. Unit Substation No. 2 (USS2) is fed with two 500 kVA liquid-filled transformers which have also been integrated into the substation line-up.

Circuit breakers are original 1960 vintage, but the four transformers have been replaced within the last 10 years. Power to all the motor control centers at this facility are sourced from one of the two unit substations

6.4.5 Motor Control Centers

This facility has 21 motor control centers placed throughout the treatment plant. Some motor control centers are equipped with main circuit breakers, and others are equipped with only main lugs.

An arc flash event on a circuit breaker located within a motor control center can occur on the line side or load side of the circuit breaker. The worst case clearing time is a line-side fault, since the fault must be cleared by the overcurrent protective device

(OPD) located upstream from the circuit breaker which is being worked on. Therefore, the arc flash boundary values provided for OPDs in this report represent the line side fault values (and upstream clearing times). Motor control centers with main lugs have only outgoing circuit breakers, and therefore, they all share a common line side arc flash value. Therefore, a single arc flash label will be provided for these panels, which reflects the available incident energy for each circuit breaker within the motor control center lineup. For those motor control centers with main circuit breakers, one label will be provided for the bus system and a second label will be provided for the main circuit breaker.

6.4.6 Low Voltage Systems Excluded from the Arc Flash Labeling Process

MIOSHA requires employers to assess the workplace to determine if hazards are present which necessitate the use of personal protective equipment. If hazards are present, then the employer must take steps to ensure the appropriate use of PPE in these areas. MIOSHA does not govern the steps or methods for ensuring the appropriate use of PPE. However, arc flash labeling is the most common method used to identify arc flash hazards and associated PPE criteria.

Section 9.3.2 of IEEE Std. 1584 notes that an arc flash hazard analysis does not need to be conducted on 208 VAC systems when the upstream transformer size is smaller than 125 kVA. Faults on these systems are not able to sustain themselves for longer than one or two cycles, so incident energy values are minimal. All 208 VAC systems at this facility meet this criteria, and therefore, are not considered an arc flash hazard.

Therefore, these systems are excluded from the arc flash study and are not labeled as an arc flash hazard.

The arc flash equations from Section 9.11 of the IEEE 1584 standard and the discussion in Appendix I validate the exclusion of 208 VAC and 240 VAC systems which are fed from transformers smaller than 125 kVA. The equations from Section

9.11 of the IEEE 1584 standard are used for calculating incident energy when three-phase bolted fault current values and breaker clearing times are known. The IEEE

1584 equation shows that for a 2 cycle clearing time, the three-phase bolted fault current value can reach 60,000 amperes on a 208 VAC system before the incident energy level exceeds 1.2 cal/cm

. For 208 VAC systems with 10,000 amperes available three-phase bolted fault current, the calculation shows an incident energy of only 0.3 cal/cm when the arcing fault is cleared within 2 cycles.

Similar assumptions are used to identify many of the 480 VAC branch devices located downstream of 480 VAC panelboards and motor control centers as free of arc flash hazards. For 480 VAC branch devices protected with molded case circuit breakers, the circuit breaker will clear an arcing fault within 2 cycles, provided that the magnitude of the arcing fault is at least 30 times larger than the trip rating of the circuit breaker.

Therefore, a 20A circuit breaker will clear an arcing fault within two 2 cycles if the available three-phase bolted fault current exceeds 600 amperes. Calculations based on Section 9.11 of IEEE 1584 show that for a 2 cycle clearing time, the three-phase symmetrical bolted fault current value on a 480 VAC system can reach 25,000 amperes before the incident energy level exceeds 1.2 cal/cm . For systems with

10,000 amperes available three-phase bolted fault current, the IEEE calculation shows an incident energy of 0.4 cal/cm when a 480 VAC arcing fault is cleared within 2 cycles.

Appendix I outlines the conditions under which a field device can be considered to carry an HRC 0 rating. These conditions are as follows:

Condition 1: The overcurrent protection device is a molded case circuit breaker rated 200A or smaller, and . . .

Condition 2: The three-phase bolted fault current at the circuit breaker is between 4,000 amperes and 40,000 amperes, and . . .

Condition 3A: The three-phase bolted fault current at the circuit breaker is at least 30 times higher than the trip rating of the circuit breaker, and . . .

Condition 3B: The field device is located within 400 feet of the circuit breaker location.

Table 2 provides a listing of 480 VAC motor control centers and 480/277 VAC panelboards at the Southwest Water Treatment Plant. The table uses the conditions described in this section and also the analysis described in Appendix I in order to identify any field devices located downstream of each motor control center or panelboard which may present an arc flash risk at an 18 inch working distance. In addition to this evaluation table, loads which are fed with circuit breakers rated 30A and larger have also been included in the arc flash model, regardless of the analysis results described above. The results of this modeling provides cross-checking, and demonstrates that the conditional analysis of Appendix I is accurate.

Table 2 – Incident Energy Levels for Branch Devices Fed from MCCs and Panelboards.

This table lists MCCs and panelboards which require field labels. The table applies criteria found in this section and in Appendix I in order to determine if branch devices downstream of an MCC or panelboard present an arc flash risk.

PANEL NAME

MAIN

BREAKER

OR LUGS VOLTAGE

MIN

AVAILABLE

FAULT

CURRENT

(IBF)

LARGEST

BRANCH

BREAKER HRC 0 EVALUATION

MCC-1A-1

480 12.2 kA 200A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 1A-2 MAIN LUGS 480 19.5 kA 600A

TWO 600A CBS FEED MOTORS WITH NO

LOCAL DISCONNECTS (NO DOWNSTREAM

LABELS NEEDED)

MCC 1A-3 MAIN LUGS 480 19.7 kA 600A

TWO 600A CBS FEED MOTORS WITH NO

LOCAL DISCONNECTS (NO DOWNSTREAM

LABELS NEEDED)

MCC 1A-4 MAIN LUGS 480 19.3 kA 600A

600A CB FEEDS MOTOR WITH NO LOCAL

DISCONNECT (NO LABEL NEEDED); ALL

CBS OVER 20A ARE INCLUDED IN MODEL

EVALUATION.

MCC-1B-1 MAIN LUGS 480 18.3 kA 600A

TWO 600A CBS FEED MOTORS WITH NO

LOCAL DISCONNECTS (NO DOWNSTREAM

LABELS NEEDED)

MCC 1B-2 MAIN LUGS 480 17.7 kA 600A

TWO 600A CBS FEED MOTORS WITH NO

LOCAL DISCONNECTS (NO DOWNSTREAM

LABELS NEEDED)

MCC 1B-3 MAIN LUGS 480 13.6 kA 70A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 1B-4 MAIN LUGS 480 6.0 kA 100A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2A-1A 200A 480 8.3 kA 70A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2A-1B 200A 480 8.3 kA 70A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2A-2 MAIN LUGS 480 11.1 kA 150A

400 FOOT DISTANCE IS EXCEEDED ON

SEVERAL LOADS; REFER TO MODEL

RESULTS FOR RATINGS.

MCC 2A-3 MAIN LUGS 480 7.9 kA 100A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2B-1 600A 480 9.1 kA 60A FUSE ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2B-2 MAIN LUGS 480 10.1 kA 200A

FUSE

ALL FIELD DEVICES COMPLY WITH HRC 0

CRITERIA EXCEPT THE 200A FUSED

SWITCH TO 112.5 KVA XFMR; NO PRIMARY

SWITCH ON TRANFORMER, SO NO LABEL

IS REQUIRED.

MCC-2B-3A 400A 480 7.1 kA 100A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2B-3A-1 MAIN LUGS 480 5.2 kA 7A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2B-3B 400A 480 6.6 kA 100A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC 2B-3B-1 MAIN LUGS 480 5.0 kA 7A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC HL DEHUM 125A 480 7.5 kA 25A ALL LOADS COMPLY WITH HRC 0 CRITERIA

MCC N FILTER 125A 480 3.8 kA 25A

ALL LOADS COMPLY WITH HRC 0 CRITERIA,

WHEN EVALUATED AT 3.8 KA

MCC S FILTER 125A 480 3.9 kA 25A

ALL LOADS COMPLY WITH HRC 0 CRITERIA,

WHEN EVALUATED AT 3.9 KA

PDP-M MAIN LUGS 480 6.2 kA 70A ALL LOADS COMPLY WITH HRC 0 CRITERIA

LP FA MAIN LUGS 480/277 6.0 kA 20A ALL LOADS COMPLY WITH HRC 0 CRITERIA

LP FB MAIN LUGS 480/277 5.1 kA 20A ALL LOADS COMPLY WITH HRC 0 CRITERIA

Modeling results confirm that only two 480 VAC field devices pose a potential arc flash risk; the overhead crane in the pump room, and the 277 VAC exterior roadway lighting which resides more than 600 feet away from motor control center MCC 2A-2.

Therefore, with the exclusion of these two loads, labeling of devices located downstream of 480 VAC panelboards is not required at this facility, since none of these downstream devices present an arc flash risk at the 18-inch working distance.

Note that NFPA 70E recommends that if live work presents incident energy levels of under 1.2 cal/cm , then HRC 0 PPE must still be worn. HRC 0 protection consists of non-meltable, non-flammable clothing and safety glasses (and leather gloves if the hands will be inside the working distance). This minimum PPE is always applicable when performing live work, regardless of whether the field device carries an arc flash label or not. Employees should be trained to abide by the HRC 0 PPE criteria as a minimum protection level when performing live work.

6.5 Modeling Scenarios

6.5.1 Scenario No. 1 – Minimum Fault Contribution

Scenario No. 1 represents conditions under which the minimum 3-phase fault current would be present in the facility’s electrical distribution system when operating exclusively on utility power. Under this scenario, the only source of power contribution is from the utility system and all generator units are off. Circuit breakers related to the standby power system are open. All power at the treatment facility is distributed through the two main 4.8 kV busses.

The values represented under Scenario No. 1 are based on the utility tie remaining open and with both DTE utilities in service. Motor contribution represents minimum flow conditions, and motors are distributed across the three busses in order to minimize contribution to each 4.8 kV bus.

The status of the 4,800 VAC breakers on the 4.8 kV busses and the 4.8 kV breakers on the standby power system busses are as follows for this scenario:

MAIN BKR #1: CLOSED GEN 1 MAIN (G-1): OPEN

MAIN BKR #2: CLOSED GEN 2 MAIN (G-2): OPEN

UTILITY CB 52-UM: CLOSED GEN 3 MAIN (G-3): OPEN

TIE (BUS 1 & 2): OPEN GEN 4 MAIN (G-4): OPEN

UTIL-GEN TIE (52-GT): OPEN

6.5.2 Scenario No. 2 – Maximum Fault Contribution

Scenario 2 represents conditions under which the maximum 3-phase fault current would be present in the facility’s electrical distribution system when operating exclusively on utility power. Under this scenario, the only source of power contribution is from the utility system which provides the highest amount of fault current

(Navarre Trunk 372, which feeds transformer #2). The 4.8 kV tie circuit breaker is closed, so that the sum of the utility fault current and motor contribution is applied to both 4.8 V busses simultaneously. All generator units are off, and circuit breakers related to the standby power system are open. All power at the treatment facility is distributed through the two main 4.8 kV busses. Motor contribution represents maximum flow conditions of 180 MGD, as defined in Section 6.1.3.

The status of the 4,800 VAC breakers on the 4.8 kV busses and the 4.8 kV breakers on the standby power system busses under Scenario 2 are as follows:

MAIN BKR #1: OPEN GEN 1 MAIN (G-1): OPEN

MAIN BKR #2: CLOSED GEN 2 MAIN (G-2): OPEN

UTILITY CB 52-UM: CLOSED GEN 3 MAIN (G-3): OPEN

TIE (BUS 1 & 2): CLOSED GEN 4 MAIN (G-4): OPEN

UTIL-GEN TIE (52-GT): OPEN

6.5.3 Scenario No. 3 – Generator Fault Contribution

Scenario 3 represents conditions under which the only source of power is from the generator system. The values represented under this scenario are based on the two

DTE utilities being disconnected or out of service. Under this scenario, all generator units are running and all circuit breakers related to the standby power system are closed. All power at the treatment facility is distributed through the two main 4.8 kV busses, with the tie circuit breaker closed (similar to Scenario 1, except that the source of power to these two busses is through the generator system rather than the utility system).

The status of the 4,800 VAC breakers on the 4.8 kV busses and the 4.8 kV breakers on the standby power system busses under Scenario 3 are as follows:

MAIN BKR #1: OPEN GEN 1 MAIN (G-1): CLOSED

MAIN BKR #2: CLOSED GEN 2 MAIN (G-2): CLOSED

UTILITY CB 52-UM: OPEN GEN 3 MAIN (G-3): CLOSED

TIE (BUS 1 & 2): CLOSED GEN 4 MAIN (G-4): CLOSED

UTIL-GEN TIE (52-GT): CLOSED

6.5.4 Scenario No. 4 – Generators and Utility in Parallel

Scenario 4 represents conditions under which the plant is performing peak shaving operations. The model represents all four generator units operating in parallel with a single utility source (Navarre Trunk 372).

Peak shaving operations are seldom used, but are tested for up to 4 hours on a monthly basis. This temporarily increases the available fault current on Switchgear

Bus #2, and on all loads fed from this switchgear. With this testing procedure, Utility

No. 1 (Navarre Trunk 346) remains intact, and therefore, Switchgear Bus #1 and the electrical distribution system which is fed downstream of this bus remains unaffected by this operation.

It should be noted that in the event of a full blackout, a paralleling operation will occur as the generator system performs a soft transfer of load back to the first utility bus that becomes available. This situation represents a slightly higher fault current than the peak shaving scenario, since the total motor contribution can be higher (Tie 1-2 is closed). However, the duration of this event is less than one minute, and the occurrence of this event would be rare. Therefore, this particular operational condition is not considered as a potential live working condition.

The status of the 4,800 VAC breakers on the three main 4.8 kV busses and the 4.8 kV breakers on the standby power system busses for modeling Scenario 4 are as follows:

MAIN BKR #1: CLOSED GEN 1 MAIN (G-1): CLOSED

MAIN BKR #2: CLOSED GEN 2 MAIN (G-2): CLOSED

UTILITY CB 52-UM: CLOSED GEN 3 MAIN (G-3): CLOSED

TIE (BUS 1 & 2): OPEN GEN 4 MAIN (G-4): CLOSED

UTIL-GEN TIE (52-GT): CLOSED

7. Arc Flash Calculations

Data produced from the EDSA Paladin® DesignBase™ 3.0 software is provided in the

Appendices of this report.

• Appendix A provides an overview of the assumptions used in the modeling and calculations.

• Appendix B provides information obtained from Detroit Edison related to the symmetrical, three-phase, bolted fault current available from the 24 kV services.

• Appendix C provides electrical one-line diagrams created within the EDSA software.

• Appendices D, E, F, and G provide arc flash information on the electrical distribution system in a tabulated format for the four modeling scenarios.

- Scenario 1: Minimum Fault conditions.

- Scenario 2: Maximum Fault conditions.

- Scenario 3: Operation in emergency mode; plant supplied with power from four standby generator units.