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Ground-Fault Relay Advancement and
the EL731 Earth Leakage Relay
Presented by
Adam Detillieux and
Jeff Glenney, P.Eng.
WMEA San Antonio 2012
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First a Few Safety Tips
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Be EXTRA Careful when you are working and
Fatigued. Recognize the Signs!
I asked a customer his opinion on some of the signs:
What do you think Monte?
Slumped shoulders
Shut eyes
Nodding Forehead
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Age isn’t always the culprit
Wearing Appropriate PPE
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For Safety use the RM Buddy System
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Littelfuse Business Units
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Electrical Business Unit
Littelfuse offers over 50,000 SKU’s covering every
circuit protection need for multiple markets:
• Automotive
• Construction
• Electronics
• Energy/Utilities
• Engine/Generator Control
• Industrial / MRO
• Marine
• Mining
• Telecom
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Ground-Fault Products
Ungrounded DC
Systems SE-601
Solidly or Resistance
Grounded Systems
(PGR-4300, SE-701, SE-704, etc)
Ungrounded Systems
(PGR-3100 & PGR-3200)
Grounded DC System or
Resistance Grounded Systems EL731
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Ground-Fault in a Single-Phase
System
IFault 120 V
15 A
BLACK
WHITE
GREEN
RECEPTACLE
TOASTER
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Historical GF protection.
Iron Core CT’s and Spinning Discs
Older electromechanical relays as shown
required drive to spin the discs to reach the
targets and cause the relay to operate. Modern
circuits don’t require the same energy.
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Courtesy of Jack Woodham – Jedson Engineering
Power System Grounding Course
System Failures – Short Circuits (Faults)
Industrial Power Systems
1. LINE TO GROUND
2. PHASE - PHASE
3. THREE PHASE
FAILURE MODE
PERCENTAGE
OF FAILURES
MOST THREE PHASE FAULTS ARE MAN-MADE:
I.E. ACCIDENTS CAUSED BY IMPROPER OPERATING
PROCEDURE.
98 %
<1.5 %
<.5 %
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Challenges in Mining Applications
Using Ground Fault CT’s on the high resistance
grounded systems where there is much less
current available
Detection levels need to be able to detect faults
even on extremely low levels. (0.5 A grounding
resistors are common in many mining
applications)
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THE BEST WAY TO DETECT AN AC GROUND-FAULT:
• CURRENT SENSING
– Usually with a zero-sequence core-balance
current transformer (CT or ZSCT)
– Permits selective coordination and
ease of finding a fault
Sensing Ground-Faults
in a Grounded Electrical System
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Locating Phase Conductors in a
Zero-Sequence-CT Window
Symmetrically locate the phase conductors in the CT window
The gap between the conductors and the CT should be at least
12.7 mm (0.5 in)
It is preferable to have a gap of 25.4 mm (1 in) or more
GAP A
B C
SPACER
C C
A
A
B
B
FLUX
CONDITIONER
C
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Keep the conductors at a 90 degree angle
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Introducing the EL731
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EL731 Main Features
AC/DC ground fault protection and metering for solidly
grounded or resistance-grounded systems
Temperature monitoring and metering
Wide frequency response (0- 15 kHz)
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EL731 Faceplate
Trip: LED Indication
Alarm: LED Indication
CT: CT Verification LED
32 Character OLED Screen
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TYPICAL APPLICATIONS
EL731 can monitor for ground-faults on AC or DC systems
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Variable Frequency Drives
VFD/ASD’s are used extensively in
heavy industry
Benefits of VFD’s
1. Speed Control
2. Reduced power consumption
Disadvantages of VFD’s
1. Harmonics
2. Ground fault’s are also variable frequency
These drives require additional protection
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Misconceptions about VFD ground faults
VFD’s DO NOT isolate the load
– a ground-fault at the load is seen at the line side
Often, built in ground-fault protection is not
designed for HRG
– Designed for solidly grounded system
– Trip level is not adjustable (usually 50% of IL )
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SE-70x series with VFD
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Earth was obviously involved
But the ground fault was not detected!
What Happened Here?
The System:
• Low-voltage
• Variable-speed drive
• 5-A neutral-grounding resistor (NGR) Some type of fault occurred with
The Result:
• Melted copper and steel
• Luckily, no fire
Is Inability to Detect a GF in an ASD System a Problem?
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What Happened Here?
The System:
• Low-voltage
• Variable-speed drive
• 5-A neutral-earthing resistor (NER)
Could the damage be the direct result of the earth fault?
No.
P = I2R and IGF ≤ 5 A. Furthermore, for 5 A to flow, the fault
impedance R must = 0 Ω (so P=0). As fault impedance rises,
current decreases and power dissipation at the fault is small.
0
200
400
600
0
10
20
30
40
50
60
70
80
90
10
0
11
0
12
0
13
0
14
0
15
0
16
0
17
0
18
0
19
0
20
0
Watt
s
Fault Impedance (Ohms)
Fault Power Dissipation 600-Vll, 5-A NGR
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What Happened Here? A Theory
A phase-to-winding connection became
disconnected in the terminal box, but electrical
continuity was maintained by (hi-impedance)
contact with the grounded terminal-box cover.
VFD
Motor 51G > 5 A
Iθ = Imotor + 5 A, max
IGF = 5 A max
This fault wouldn’t cause an overcurrent trip
And the ground fault was not detected
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SYSTEM CAPACITANCE
All electrical systems have phase-to-earth capacitance
• distributed throughout the system
• modeled here as “lumped” values
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CHARGING CURRENT
Definition of charging current:
The current that flows when one phase of an
unearthed system is shorted to earth
Note: A core-balance CT at “1” will measure charging current
A core-balance CT at “2” will measure zero
2 1
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CHARGING CURRENT & SYMPATHETIC TRIPPING
A core-balance CT on an unfaulted feeder will detect its feeder’s
charging current when an earth fault occurs on another feeder.
• To avoid sympathetic tripping, protection must be set above the charging current level
• Protection level may be set below charging current if sympathetic tripping is acceptable
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CAPACITANCE UNBALANCE
Balanced Phase-to-Earth Capacitance: Xa=Xb=Xc
• Phase capacitive currents are equal
• Core-balance CT reads zero.
Unbalanced Phase-to-Earth Capacitance: XaXbXc
• Phase capacitive currents are not equal
• Core-balance CT reads a finite value
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SE-70x Frequency Response
Frequency (Hz)
Filte
r O
utp
ut
SE-70X Frequency ResponseVariable Frequency Peak Detection
10 20 30 40 50 6070 100 200 300 400500 700 10000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
Peak-Detection Filter
(labeled as Variable Frequency)
400 Hz 1000 Hz
Conventional ground fault
relays have a limited
frequency response.
VFD’s and DC systems need
accurate DC-60Hz
measurement.
VFD’s often have significant
high frequency harmonic
components that should be
monitored.
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Broad Spectrum Ground Fault Monitoring
Use either CT1, CT2, or both CT1: 0-90 Hz CT2
20-15000 Hz
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EFCT- Earth Fault Current Transformer
EL731 requires EFCT series current transformer
Continuously monitors CT health
CT1 & CT2 have independent trip settings
CT1: 0-90 Hz
• 30-5000 mA trip level
• 0-2 s trip time
CT2: 20-15000 Hz
• 30-5000 mA trip level
• 0-2 s trip time
• Low-pass filter 20-90 Hz
• High-pass filter 190-15000 Hz
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THIRD HARMONICS: A Special Case
In 3-phase systems third harmonics are in-phase
• their values add; they do not cancel
+ +
=
Can cause nuisance earth-fault tripping Example: Fundamental & 3rd harmonic
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Multiple primary turns supported (up to 6)
1. Wrap the primary n times around the CT
2. Program n using EL731 controls
3. View ground fault current without conversion
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Temperature Monitoring
VFD’s can cause excessive motor heat due to:
– Harmonics
– Slower internal fan speed at lower frequencies
EL731 is the only ground-fault relay on the
market with temperature protection.
Trip or alarm on over-temp
View real time temperatures
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DC Ground Fault Monitoring
Feature: Ability to detect DC faults with a CT
Unit will trip if fault is downstream of CT
Can use multiple EL731’s on
various legs of system
Alternate technology (SE-601)
allowed 1 per system.
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Outputs
3 Programmable Form C Contacts – Trip or Alarm (other functions!!)
– Fail-safe/ non-fail-safe
– Trip memory (fail-safe system)
4-20 mA analog out
Remote reset
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Communications Capable
Communications available in a variety of protocols
– DeviceNet -Ethernet
– Profibus -Modbus
Flash upgradeable via optional communication adapter
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Use Web Browser to View/Change Parameters
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Password Protected option
1st of our ground-fault relays with this feature.
Important if you know of any customers that
were removing knobs to make it more difficult to
change settings on SE-704’s or SE-701’s
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Mounting Options
Metering on the door
Panel-mount adapter not required for door
mounting.
Optionally available surface-mount adapter
(AC700-SMK)
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Rigorous Testing
100% of the relays are tested on computerized test jigs. Shown here mounted on racks for burn-in testing. The relays are powered up and kept in the burn-in room at 60 degrees C for 3 days.
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PWB – Conformal Coating… MIL 1-46058 qualified UL QMJU2 recognized
Adds resistance to moisture, corrosion and mold, lessens the impact of mechanical stress, vibrations and its improves its ability to cope in extreme temperatures.
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EL731 Overview
•Designed for: •AC Systems
•Variable-Speed Drives
•Grounded DC Systems
•Main features •Full spectrum ground-fault monitor
•Temperature monitoring
•Metering
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Conclusions
There are a wide variety of Ground-Fault Relays
available
There are some ideally suited for the hi-tech
demanding requirements of mining customers
Next WMEA will discuss our 480V and 600V
GFCI’s
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Thank you
Questions?