Grounding in Instrumentation Systems

© 2004, ISATI21W1 (1.1) 1

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Grounding in Instrumentation Systems

TI21W1 Version 1.1© 2004

© 2004, ISATI21W1 (1.1) 2

Seminar Logistics

• Seminar materials– Downloadable presentation– Question and Answer session (audio and email)– Survey– Earn 1 Professional Development Hour (PDH)

• Seminar length– 60 minute presentation– Three 10-minute question and answer sessions

Audio Instructions

• As a participant, you are in a “listen-only” mode.• You may ask questions via the internet, using your

keyboard, at any time during the presentation. However, the presenter may decide to wait to answer your question until the next Q&A Session.

• If you have audio difficulties, press *0.

© 2004, ISATI21W1 (1.1) 3

Audio Instructions for Q&A Sessions

• Questions may be asked via your telephone line. • Press the *1 key on your telephone key-pad. • If there are no other callers on the line, the operator will

announce your name and affiliation to the audience and then ask for your question.

• If other participants are asking questions, you will be placed into a queue until you are first in line.

• While in the queue, you will be in a listen-only mode until the operator indicates that your phone has been activated. The operator will announce your name and affiliation and then ask for your question.

Introduction of ISA Presenter

• ISA Presenter– Larry Thompson, CAP– ISA Adjunct instructor since 1984. B.A.A.S Tarleton State University.

Instructor for Texas State Technical College in Instrumentation (1977-1979), Computer Networking and System Administration (1996-1999), E-Commerce Technology (1999-2003). CCST Certificate, FCC Radiotelephone License (General, formerly First Class). Served 20 years USAF primarily in Electronic Encryption Systems. Owned andmanaged own services business since 1979.

– Designed, developed, taught and maintained industrial controls in many varied applications.

– Author of ISA books: Industrial Data Communications and Basic Electrics/Electronics for Control

© 2004, ISATI21W1 (1.1) 4

Introduction of Presenter

• Wayne Oswald recently retired after 35 years with The Dow Chemical Company. He served as Maintenance Process Leader for Electrical/Instrumentation/Refrigeration Services functions for Dow's Texas Operations. Since retiring from Dow, Wayne has taught Electrical Continuing Education courses for Associated Builders and Contractors and is currently serving as Project Director for the Greater Houston Construction Careers Initiative, involved in recruiting and training entry level construction workers along the Texas Gulf Coast.

• Wayne received his BS degree in Electrical Engineering Technology from The University of Houston. He serves as Chairman of the Texas Skill Standards Board and is active in The Associated Builders and Contractors and Houston Business Roundtable organizations.

Key Benefits of Seminar

• Know the terminologies surrounding grounds and grounding in Measurement and Control applications.

• Be familiar with the grounding philosophies employed in Measurement and Control applications.

• Understand the concepts of shielding and noise reduction as related to grounding.

• Be able to determine what is the best grounding and shielding approach for different applications.

© 2004, ISATI21W1 (1.1) 5

Section 1: Electrical Grounds

• What is a ground• What must be grounded• What may not be grounded• Intentional grounding systems• Separately grounded systems

Grounds

Connection to earth or other large conducting mass

��

© 2004, ISATI21W1 (1.1) 6

How do we connect to the “ground” ?

Instrument Power/Signals Requiring Grounding

When supplying <50 V

230 VAC

Ungrounded

When supplying 50 – 1KV

50 VAC

50 VAC

© 2004, ISATI21W1 (1.1) 7

Intentionally Grounded Systems

N

L2

L1

120/208 VAC277/480 VAC

120/240 VAC

A

B

C

120/240 VAC 480 VAC

Intentionally Grounded Systems

L1

N

120 VAC

N

C

BA

A

B

N

C

Types of Grounding

N

C

BA

Solidly Grounded

A

N

C

B

Zig-Zag Grounded

N

C

BA

Resistance GroundedN

C

BA

Impedance Grounded

© 2004, ISATI21W1 (1.1) 8

Instrument Power/Signals Requiring Grounding (cont’d)

• Separately Derived AC Systems (NEC Article 250-20(d– Commonly used in measurement and control circuits

• Direct-Current Circuits: (NEC Article 250-162)– Commonly used in measurement and control circuits

Separately Derived System - Generator

GeneratorService Panel

Transfer Switch

Feeder Neutral EquipmentGround

Neutral��

��

© 2004, ISATI21W1 (1.1) 9

Separately Derived System – Transformer #1

Ground at Transformer

DRYTRANSFORMER

EQUIPMENTGROUND

EQUIPMENTGROUND BUS

INSULATEDNEUTRALBUS

FIRSTDISCONNECT GROUNDING ELECTRODE

NEUTRALBUS

Separately Derived Transformer System Grounded at the Transformer

Separately Derived Ground – Transformer #2

Ground at First Disconnect

DRYTRANSFORMER

EQUIPMENTGROUND

EQUIPMENTGROUND BUS

NEUTRALBUS

FIRSTDISCONNECT

GROUNDING ELECTRODE

Separately Derived Transformer System Grounded at First Disconnect

© 2004, ISATI21W1 (1.1) 10

Instrument Power/Signals NOT Requiring Grounding

��

NEUTRALBUS

Review of Key Points

• What is a ground?• What must be grounded?• What may not be grounded?• Intentional grounding systems• Separately grounded systems

© 2004, ISATI21W1 (1.1) 11

Live Question and Answer Session

• During Q&A, questions may be asked via your telephone line. • Press the *1 key on your telephone key-pad. • If there are no other callers on the line, the operator will




Section 2: Types of Grounding Systems

• Ground Considerations• Single Point Ground (DC to Low Frequency Signals)• Low Frequency Multi-Point• High Frequency Single Point Ground• High Frequency/Low Frequency Model• Series Ground

© 2004, ISATI21W1 (1.1) 12

Ground Potential Difference

Ig

E = IR = 10,000 x 1 =10,000 V

1 Ohm

Power &Safety Ground

InstrumentGround

Different Earth Ground Connections

(typ)

10,000 Amps

Assume zero reference point

Mirror Earth Current

LF Single Point Ground Model

POWER &SAFETYGROUND

INSTRUMENTGROUND

INTRINSICSAFETYGROUND

COMPUTERGROUND

Equivalent Line Resistance

© 2004, ISATI21W1 (1.1) 13

Low Frequency Multi-Point Ground

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

HF Single Single Point Ground Model

COMPUTER GROUND


INSTRUMENT GROUND

POWER &SAFETYGROUND

Equivalent Line Inductances

© 2004, ISATI21W1 (1.1) 14

HF/LF Ground Model

POWER &SAFETYGROUND


INSTRUMENT GROUND

SRG SRG

COMPUTER GROUND

Short as Possible

(TYP)ConnectionLow Inductive

Isolated Ground System(IG)

Load

NN

GG

N

Safety Ground

Neutral

Conduit

Phase

IsolatedGround

CB PanelSource

IGIG

Equipment

© 2004, ISATI21W1 (1.1) 15

Simple Illustration of Instrument Grounding System

Intrinsic SafetyGround System

Instrument Ground

SG90 V 150 kASpark Gap

System

Single Point of Connection

Power & Safety GroundsPower & Safety Grounds

Service Entrance Ground

Instrument Ground

Example DCS Type Grounding

INTRINSIC SAFETYGROUND

SAFETY GROUNDGRID

COMPUTERDCS

G

N

H

SIGNAL REFERENCEGRID(SRG)

G

N

H

IS Barrier

ABC

EGN

Earth Ground

ABCNEG

Master ReferenceGround

Computer Power Panel DCS Power Panel

ELECTRONIC REFERENCEGROUND(ERG)

High FrequencyGround

�

NEC required Connection

G

�

��

��

��

© 2004, ISATI21W1 (1.1) 16

Example Plant Grid

ControlRoom

Building SteelBond(Typ) MCC Grounds(Typ)

InstrumentGrounds

EquipmentRoom

Power Triad(Typ)

To TankFarmand Flare

Instrument Triad

MCC

StructuralSteel Bond(Typ)

Example Plant Ground Grid


• Ground Considerations• Single Point Ground (DC to Low Frequency Signals)• Low Frequency Multi-Point• High Frequency Single Point Ground• High Frequency/Low Frequency Model• Series Ground

© 2004, ISATI21W1 (1.1) 17






Section 3: Shielding/Noise Reduction

• Field definitions• Electrostatic Shielding• Electromagnetic Shielding• Shielding Applications

© 2004, ISATI21W1 (1.1) 18

Field Definitions

�

�

Electrostatic Shield Model

V1

Shield

S-G

C1-2

C2-G

CIRCUIT #2 -RECEPTOR

Vnoise

C2-S

C1-G

Electrostatic Shield Model (Capacitive)

SOURCECIRCUIT #1 -

C2S-G

Path

C1-S2

© 2004, ISATI21W1 (1.1) 19

Noise in a 4-20 mA Loop

Noise in a 4-20 mA Loop

ReceiverInstrument

DC Signal + NM + CM

++

Twisted Pair+ +

DC

CM Reduced by Twisted Pair

P/S

Transmitter

4-20 mA

DC

DC

+

+24 VDC P/S

Both Normal and Common Mode NoiseReduced by Common Mode Rejectionof Input Differential Amplifier

4-20 mA Instrument Cable Shield

-

SS

DCS

-

-

Shield

Tape Back

4-20 mA Instrument Loop Shield

--

FMTBFTBTX

24 VDC P/S

Twisted Pair

© 2004, ISATI21W1 (1.1) 20

Magnetic Coupling Model

V1

CIRCUIT #1

SOURCE

CIRCUIT #2

RECEPTOR

Signal Circuits Grounded at Both Ends

System #1 System #2

System #1 System #2

Shield grounded at one endopens exposure to magnetic fields when the signal circuit is grounded at both ends.

Shield grounded at both endsreduces exposure to magneticfields when the circuit is groundedat both ends.

© 2004, ISATI21W1 (1.1) 21

Common Mode Chokes

Vg

System #1 System #2

VgSystem #1 System #2

Balun

Magnetic Core

Ferrite Beads

System #1 System #2

Vnm

Vcm Vcm

Good for >1 MHz(typ)

Symbol

© 2004, ISATI21W1 (1.1) 22

Differential Amplifier

System #1 System #2

Vnm

Vcm VcmDifferential Amp

CMRR

A differential amplifier will pass normalmode noise but will reject commonmode noise based on its common mode reduction ratio (CMRR).

Balanced Signal Lines

Vg

Vg

R L

Rs

Rs

Balanced and Unbalanced Lines

System #1 System #2

System #2

R L

R L

Vn

Noise cancels due it appearingon both circuit legs but thebalanced receiver only respondingto the difference between the legs.

VnVn

Balanced Circuit

Unbalanced Circuit

System #1

VsRS-232

RS-422/485

Rs

© 2004, ISATI21W1 (1.1) 23


• Field definitions• Electrostatic Shielding• Electromagnetic Shielding• Shielding Applications

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© 2004, ISATI21W1 (1.1) 24






Related Courses from ISA

• Grounding and Noise Considerations for Control Equipment and Computers (TI21)

• Understanding Electrical Systems (TI15)• Industrial Electronics (TI20)• Electrical Noise Reduction in Instrumentation Systems

(TI21W2)

• All ISA courses are available any time as on-site training • For more information: www.isa.org/training or (919) 549-8411

© 2004, ISATI21W1 (1.1) 25

Other Related Resources from ISA

• Electrical Instruments in Hazardous Locations, 4th Editionby E.C. Magison from ISA Press

• The ANSI/ISA12 series of standards and technical reports on electrical instruments & apparatus

• ISA Membership connects you to people and ideas in automation and control– Just $85 per year, plus $9 for each division

• For more information: www.isa.org/shopISA or (919) 549-8411

ISA Certifications

• Certified Automation Professionals ® (CAP ® )– www.isa.org/CAP

• Certified Control Systems Technician® (CCST®)– www.isa.org/CCST

• Please visit us online for more information on any of these programs, or call (919) 549-8411.

© 2004, ISATI21W1 (1.1) 26

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© 2004, ISATI21W1 (1.1) 27


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Grounding in Instrumentation Systems