Leakage and Ground
Currents:
Measurement Techniques
Judith M. Russell
Oct 27, 2005
Ground Currents
 Hot Button Issue
 High Visibility
 Quantifiable / Measurable
 Direct Cause of Problems
 Magnetic Fields
 Safety / Leakage Currents
 Symptom / Indicator
 Ground Loops
 Wiring Errors
 Bonding
Scope
I’m not talking about Grounding . . . .
I am talking about Measurement . . . .
(We might sneak some Grounding in, however)
Low Level Currents
 Ground Currents
 Safety grounds, reference grounds, shields
 Raceways, chassis, conduits, structure
 Wiring errors, leakage currents, signal currents
 Leakage Currents
 3 Phase + Neutral  0 Amps
 Normal:
Filters / Capacitance
 Abnormal:
Wiring Errors
 Net Currents
 Non-Zero Summary Current
 3 Phase + Neutral + Ground  0 Amps
Ground Currents
Ground Currents
Ground Loops
 Deliberate Parallel Ground Paths
 Ground rods
 Redundant grounds
 Raceway or Conduit
 Mechanical Mounting of Equipment
 System Interconnect Wiring
 Shields
 Signal Commons
 Distributed Power
Ground loops complicate
ground current measurements
Grounding for Power Quality
 Insulated fittings (raceway or conduit)
 Full-sized / Over-sized Ground Conductors
 Isolated Ground Receptacles
 Separate, Dedicated Ground Conductor
 Bonding Jumpers to Signal Reference Grid
 Interrupting Ground Loops
 Supplemental Ground Connections to Structure
or Electrodes
Show me you did something !
Prove to me that things have been improved !
Wiring Errors
 Improper connections
of Neutrals & Grounds
 Circuit Breakers
 Surge Suppressors
 UPS Systems
 Use of Ground as
Return
 Control Systems
 277 VAC from 480 VAC
source
 240 VAC Delta
Ground current can
be a diagnostic tool
Leakage Currents
Leakage Currents
L1
+ L2
+ L3
+N
=0
Ideal
A – Ground Conductor
B – Conduit / Raceway
C – Structure / Mounting
Leakage Currents
 GFCI
 5 mA
 Ground Fault Protection
 Large Panels
 Arcing Fault Protection
 10’s or 100’s of Amps
 Patient Safety (Health Care)
 5 mA (fixed equipment)
 300 uA (cord connected)
Measuring Leakage Currents:
NFPA-99
Measuring Leakage Currents:
NFPA-99
Measuring Leakage Currents:
NFPA-99
Net Currents
Net Currents
ELF Magnetic Fields
Caused by Net Currents
ELF – Extremely Low Frequency (50 / 60 Hz)
ELF Magnetic Fields
Caused by Net Currents
Z
Orthopedic Surgery
Associates Building
Y
C
X
B
A
90 Feet
Net Current Fields Do Not Fall Off Quickly
Measurements
Techniques
and
Instrumentation
Troubleshooting Techniques:
Magnetic Fields / Net Currents
Fast and Easy to Do
Troubleshooting Techniques:
Magnetic Fields / Net Currents
0.3A
2.1A
4.3A
Main
1 of 5
Main
2 of 5
1.3A
Main
3 of 5
Main
4 of 5
Meter
4.3A
Orthopedic
Surgery
Associates
Service
Main
5 of 5
1.3A
0.7A
Sprinkler
System
Controls
0.5A
0.1A
Service
0.4A
3.8A
0.2A
4.4A
1.3A
0.1A
Water Pipe
Grounds
Troubleshooting Techniques:
Magnetic Fields / Net Currents
Need to Open Panels
Troubleshooting Techniques:
Net Currents / Leakage Currents
Are Currents Leakage Related?
Troubleshooting Techniques:
Ground Currents
Tried-and-True, Use as Baseline
Troubleshooting Techniques:
Ground Loops
Ground Loops? Power Quality Run Amok?
Troubleshooting Techniques:
Ground Loops
Look for Unusual or Unexpected Sneak Paths
Neutral
Transformer
Phase B
Neutral Bus
Phase C
No
Connection
Phase A
Phase(s)
Bus
Conduit to
CPC Voltage
Regulator
Circuit
Breaker
LCG3300F
Ground
Ground Bus
and
N-G Bond
Instrumentation:
Clamp-on Current Probes / Meters
 Conventional Current Probes ?
 Electrical Issues
 Resolution
 Accuracy
 Offset
 Mechanical Issues
 Measurement Window
 Structural Measurement
Instrumentation:
Accuracy and Offset
40 A Probe
1000 A Probe
DC Offset
Instrumentation:
Measurement Window
 Conventional Current
Probes
 Low Amps
= Small Window
 Large Window
= Loss of Resolution
and Accuracy
 No way to measure
conduits, raceway,
structure
Instrumentation:
Leakage Current Meters
Instrumentation:
Flexible Current Probes
Can We Trust Flexible Current Probes?
Instrumentation:
Flexible Current Probes
Scope
120
VAC
12 Volt
DC Supply
100 Watt
Audio
Amplifier
Fluke 87
Fluke 87
Lem Flex II
Resistance
(Warm) = 4.3 ohms
Lem Flex 3000
Fluke 400i
Current Clamp
Waveform
Generator
Fluke 87
(2) x 8 ohms
Non-inductive
20 Watts
Instrumentation:
Flexible Current Probes
Error vs. Frequency
20%
0%
Error (Percent)
-20%
-40%
-60%
Fluke (ma)
LEM Flex II
-80%
Lem Flex 3000
-100%
1
10
100
1000
Frequency (Hz)
10000
100000
Instrumentation:
Flexible Current Probes
Error vs. Amperage (64.27 Hz)
50%
40%
Fluke (ma)
Fluke (uA)
30%
LEM Flex II
Lem Flex 3000
Error (Percent)
20%
10%
0%
-10%
-20%
-30%
-40%
-50%
0
0.2
0.4
0.6
0.8
1
Amps
1.2
1.4
1.6
1.8
2
Instrumentation:
Flexible Current Probes
Centered
Offset
(A)
(B)
(D)
(E)
Skewed
Deformed
Offset
(Clasp)
(C)
(F)
Deformed
(Clasp)
Case Studies
Case Study #1:
High Ground Currents
 Medical Imaging Systems
 10 Amps of ground current at some sites
 First Guess: Not real (instrumentation error)
 On site Investigations: Currents were real
Case Study #1:
High Ground Currents
 Filters for Room Shield
 120 VAC Circuits
 Lighting
 Outlets
 Surge Suppressor
 Delta vs. Wye
 Power Conditioners
 Improper NG Bonds
 Isolated Grounding
 Making things worse
Case Study #2:
Leakage Currents
 Medical Imaging - IEC / UL Testing
 High Leakage Currents in Some Test-Bays
 Not System Dependent
Equipment
Chassis
Phase A
Phase B
Phase C
Load
A
Net Current
A+B+C+N
Neutral
Ground
Leakage
Current
B
Leakage
Current
Through
Mounting
and
Ground
Loops
C
Case Study #2:
Leakage Currents
 Using IEC / NFPA
Measurement Techniques
Leakage Current
 1K in Ground
 Not-insulated
 Used Clamp-on Leakage
Meters
 Tracked IEC measurement
 Leakage Current was
flowing in mounting
R/C
Network
Ground
Volt
Meter
Measure
Leakage Current
A
All Phases and Neutral
 Real Situation:
High Leakage Currents!
Ground
A
Measure
Leakage Current
Case Study #3:
Magnetic Fields caused by Net Currents
Extremity MRI
 Ankle, Wrist,
Knee Scans
 2 mGauss
B-Field Limits
 Small Office
 Orthopedics
5'
9'
Artoscan
Room
12'
Break and
Conference Room
Field Due
to Utility Feeder
14'
Proposed
Isocenter
20'
N
22'
Field Due
to 100 Amp Panel
Bath
Field Sources
1 – 100 Amp Panel
2 – Buried Feeder
Storage
100 Amp
Panel
Case Study #3:
Magnetic Fields caused by Net Currents
 Quick Check of Conduits: Net Currents
 Traced to co-mingled neutrals (fluorescent lights)
Red = No Connection
Black = 0.2 Amps
Blue = 0.2 Amps
White = 12.2 Amps
1.5A
2.3A
11.7A
1.3A
0.3A
0.8A
0.1A
11.1A
Red = 12.24 A
Black = 0.1 Amps
Blue = 0.1 Amps
White = 0.2 Amps
Case Study #3:
Magnetic Fields caused by Net Currents
Distribution Panel
Hot
Neutral
Hot
Neutral
Broken Neutral
Line
Lighting
Circuit #1
Quick Fix !
Lighting
Circuit #2
~ 12 Amps
Improper Neutral
Connection
Water Pipe Ground
1.0 Amps
2.7 Amps
 2.6 Amps on Service
 Loop with Water Pipe
 Solution 1:
Reduce Ground Current
3.6 Amps
Distributon
Panel
Ground
Bar
 Improved Grounding
!00 Amp
Fused
Disconnect
 Isolation
2.6 Amps
Neutral Bar
Service
Ground
 Utility Involvement
 Solution 2:
Mitigation
0.06 Amps
 Shield the Feeder
Driven
Ground
Rod
 Move the Feeder
Case Study #3:
Magnetic Fields caused by Net Currents
Artoscan
Isocenter
Finished Floor
11.5'
19'
12.5'
8.5'
Magnetic
Field
Possible location if
Utility Feeder is moved
to outside of building
15'
5'
Utility
Feeder
Cost-Effective
Power Quality
Analysis
Judith M. Russell
Nov 18, 2004