CHREID 2008

ELECTRICAL SAFETY

Introduction and Overview ofSafety Clayton H. Reid

September, 2010

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ELECTRICAL HAZARDS

• Electrical shock• Electrical arc-flash• Electrical arc-blast

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ELECTRICAL SHOCK (part one of two)

• Body resistance

• Wet or dry skins are major factors of resistance

• Circuit voltage

• Amount of current flowing through the body

• Current through the body

• Area of contact

• Duration of contact

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ELECTRICAL ARC-FLASHArco electric-flash

0102030405060708090

100

20-29.9

30-39.9

40-49.9

50-59.9

Age range years Anos de edad

Burn Injury - Probability of Survival

25% Body Burn50% Body Burn75% Body Burn

• Body Burn

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What is an Arc Flash?

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ELECTRICAL Arc-Blast

• Rapid expansion of the air caused by an electrical arc, referred to as an electrical arc-blast or explosion

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CONTRIBUTING FACTORS TO ELECTRICAL ACCIDENTS

• Faulty Insulation

• Improper grounding

• Loose connections

• Defective Parts

• Ground faults in equipment

• Unguarded live parts

• Failure to de-energize electrical equipment when it is being repaired or inspected

• Intentional use of obviously defective and unsafe tools

• Use of tools or equipment too close to

energized parts

• Tools left in electrical cubicle

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PREVENTING ELECTRICAL ACCIDENTS

• Largely preventable through safe work practices

• Examples of some safe work practices

– De-energizing electrical equipment for inspection and repair

– Keeping electrical and equipment properly maintained

– Exercising caution when working near exposed energized lines or equipment

– Using appropriate personal protective equipment and insulated tools

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ELECTRICAL SAFETY

• ENVIRONMENT, SAFETY AND HEALTH PRINCIPLES (used by DOE)• Plan Work

• Analyze Hazards

• Control Hazards

• Perform Work

• Feedback and improve

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ELECTRICAL ACCIDENTS CAUSED BY ONE OF THE FOLLOWING

• Unsafe work practices • Unsafe equipment or installation • Unsafe environment

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REQUIREMENTS

• Knowledge/Familiarity

• Mechanical Interlocks

• Electrical Interlocks

• Bus Transfer Procedures

• Lock, Tag and Try Procedures

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REQUIREMENTS

• Workers: Trained and Qualified

• One Line Diagrams: Up- to- date and attached to procedures

• Test Equipment: Good working condition and calibrated

• PPE( Personal Protective Equipment) Good Working Condition and tested per standards

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SAFETY INTERLOCKS IN MEDIUM VOLTAGE STARTERS

• Protective Barriers for Safe Operation of MV Starter Isolating Switches

• Design And Function Of Safety Interlocks

• Maintenance of Safety Interlocks in MV Starters

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3 Pole, Non-Load BreakIsolation Switch3 Current Limiting Power Fuses(Clip-On or Bolted Type)

3 Phase Vacuum Contactor

3 Bar Type Current Transformers

1 High MV Cell

Control Power Transformeris located behind the contactor

Power Cell Compartment

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1 High MV Cell

Power Cell Compartment

• The power cell is designed to allow easy access without the need to remove components.

Copyright © 2007 Rockwell Automation, Inc. All rights reserved. 16

Two High MV Cell

Power Cell Compartment

• The power cell in a Two High structure includes, as standard, a swing-out Low Voltage panel, allowing for increased working area.

Copyright © 2007 Rockwell Automation, Inc. All rights reserved. 17

The standard components housed in the panel are:

Normal-Off-Test selector switch

Male test power receptacle

Rectifier Bridge CR1 and CR2 control

relays Motor protection relay(s)

Low Voltage Compartment

Copyright © 2007 Rockwell Automation, Inc. All rights reserved. 18

When in the “OFF” position the isolation switch is connected to “Ground Potential” via grounding pins.

Non-load Break Isolation Switch

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Non-load Break Isolation Switch

• It is mechanically and electrically interlocked with the contactor to ensure it can’t open or close when the contactor is closed.

• It is also mechanically interlocked with the power cell door.

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Maintenance Aids

• Voltage detectors

• Viewing windows

• Infrared sight glass • Grounding balls

Voltage Checking Devices

• Non Contact Voltage Detectors (NCVD)• Voltage Indicator (low and high voltage)• Voltmeters

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Voltage Detectors

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Voltage Detectors

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Voltage Detectors

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Voltage Detectors

Grounding Balls

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Grounding Cable

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Infrared Sight glass

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Infrared Scanning

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ArcShield Two-High: Top Cable Entry/Exit

New plenum design enables

top cable/conduit connections!

1512B1512B

1512B1512A

1591A

32

• Arc vent on the unit roof– Aluminum plate designed to open

under high pressure associated with arc flash conditions

– Offset to rear of structure (allows use of LV wireways)

– Installers must not step on this area (suitable warning label is provided)

ArcShield Overview

Unusual Electrical Events

• Unusual Electrical Events

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CASE HISTORY #1

• MCC starter unit

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CASE HISTORY #2

• The Reluctant Skip

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CASE HISTORY #3

• Incorrect testing with a multimeter

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CASE HISTORY #4

• Travelling arc and amperes gone beserk

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CASE HISTORY #5 • Mystery of the missing fishtape