CHREID 2008 ELECTRICAL SAFETY Introduction and Overview of Safety Clayton H. Reid September, 2010.

CHREID 2008

ELECTRICAL SAFETY

Introduction and Overview ofSafety Clayton H. Reid

September, 2010

CHREID 2008 2

ELECTRICAL HAZARDS

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

CHREID 2008 3

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

CHREID 2008 4

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

CHREID 2008 5

What is an Arc Flash?

CHREID 2008 6

ELECTRICAL Arc-Blast

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

CHREID 2008 7

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

CHREID 2008 8

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

CHREID 2008 9

ELECTRICAL SAFETY

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

• Analyze Hazards

• Control Hazards

• Perform Work

• Feedback and improve

CHREID 2008 10

ELECTRICAL ACCIDENTS CAUSED BY ONE OF THE FOLLOWING

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

CHREID 2008 11

REQUIREMENTS

• Knowledge/Familiarity

• Mechanical Interlocks

• Electrical Interlocks

• Bus Transfer Procedures

• Lock, Tag and Try Procedures

CHREID 2008 12

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

CHREID 2008 13

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

Copyright © 2007 Rockwell Automation, Inc. All rights reserved. 14

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


1 High MV Cell


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


Two High MV Cell


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


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


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

Non-load Break Isolation Switch


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.

CHREID 2008 20

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

CHREID 2008 21

CHREID 2008 22

Voltage Detectors

CHREID 2008 23

Voltage Detectors

CHREID 2008 24

Voltage Detectors

CHREID 2008 25

Voltage Detectors

Grounding Balls

CHREID 2008 26

CHREID 2008 27

Grounding Cable

CHREID 2008 28

Infrared Sight glass

CHREID 2008 29

Infrared Scanning

CHREID 2008 31

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

CHREID 2008 33

CHREID 2008 34

CASE HISTORY #1

• MCC starter unit

CHREID 2008 35

CASE HISTORY #2

• The Reluctant Skip

CHREID 2008 36

CASE HISTORY #3

• Incorrect testing with a multimeter

CHREID 2008 37

CASE HISTORY #4

• Travelling arc and amperes gone beserk

CHREID 2008 38

CASE HISTORY #5 • Mystery of the missing fishtape

CHREID 2008 ELECTRICAL SAFETY Introduction and Overview of Safety Clayton H. Reid September, 2010.

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