grounding in instrumentation systems
TRANSCRIPT
© 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.
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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
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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������������ ��
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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
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.
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
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HF Single Single Point Ground Model
COMPUTER GROUND
INTRINSICSAFETYGROUND
INSTRUMENT GROUND
POWER &SAFETYGROUND
Equivalent Line Inductances
© 2004, ISATI21W1 (1.1) 14
HF/LF Ground Model
POWER &SAFETYGROUND
INTRINSICSAFETYGROUND
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
Review of Key Points
• 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
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
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.
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
Review of Key Points
• Field definitions• Electrostatic Shielding• Electromagnetic Shielding• Shielding Applications
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• Poll Slide• Click on the appropriate number indicating the number of
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© 2004, ISATI21W1 (1.1) 24
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
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.
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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