Ground Fault Detection and
Isolation on Ungrounded DC
Systems
Jose´ A. Marrero
Southern Company
DC SME
General Comments
The interaction of you detection
system/equipment and your DC system must be
understood along with their respective limitation.
Also the limitations of your detection method
must be understood. The figure below shows us a
typical system and some of the interaction during
a ground fault.
Concerns

Must let all know what to expect, cooperation
from Maintenance and Operation Management
 Preparation and planning
– Early/quick problem detection, isolation and
resolution

If multiple grounds are present or suspected
– Caution is needed


4-8 mili-Amps and 30 - 46V is all that is needed for certain relays and
solenoids to actuate/drop out.
In other words High Visibility
– Must remain focused
Basic Problem

Identify and locate low level DC ground
fault current
– Low-to-high impedance ground faults in the
presence of larger DC load current

Identify them in a complex multi-feeder
source(s) and loads
 Perform quickly and efficiently
– Without interrupting the service of critical
control and plant systems
Quasi Typical DC System
Simpler Example
Types of DC Ground Faults

Continuous or Transient
Types of DC Ground Faults(GFs)

Steady
– comes in and stay in at the same level
 (can ⇑ or ⇓)

Intermittent
– Comes in & out, increasing or decreasing
levels, but with ‘Long’ durations(>>15min.)

Transients
– Pluses
– Spikes
Types of DC Ground Faults(GFs)

Their periods can be either of a fixed or
‘random’ nature, this can pertain to the either
on/off portion equally
 All GFs are of large enough amplitude to be
detectable
 There is no such thing as zero crossing
Caution
Multiple GFs

Multiple GFs of opposing polarities, can
cause components/system failure without
indication

Reliability of your system may have been
compromised
General Comments
Detection Scheme
THERE CAN BE ONLY ONE
Location of Detection Scheme

DC System monitoring
– Local and Remote Point(s)

Charger(s)
 Inverter(s)
 UPS
 Load(s) Control System like Mark V & VI
 Plant DAS
 Battery Monitors
Detection Scheme

Voltage Divider System
– Numerous variations to this scheme
 Resistor can be replaced with relay coil to annunciate
 Voltmeter can be replaced with amp meter to detect
magnitude of the ground fault
 Symmetrical & Asymmetrical forms

System utilizes voltage imbalance measurements
– Only indicate that a fault condition exists
– Are unable to determine the source, location or
number and magnitude of fault(s)
Some Commercial Avaliable
Detection Units
A Common Detection Scheme

Voltage Division
– There are many variations
Fault
+
-
1
2
PN
Load
What Information Is Needed?

What type of indication do you have?
– Ex. Lighting, amp meter, alarm circuit
 Always check light bulb and fuses prior to
continuing

What is the alarm trigger point?
 What are the limitations and what
information can be obtained?
– What side is it on?
– What is the magnitude?
– Is it a constant or transient ground fault?
What Information Is Needed?

Important parameters to know on your system
(especially on multi-feeder distribution system)
– DC voltages between battery polarities
– DC voltages between the polarities (pos. &
neg.) with respect to site ground
– DC ground fault from both polarities(pos. &
neg.) with respect to site ground
– AC voltage between polarities with respect to
site ground (phantom ground)
Methodologies

Voltage measurements
 Selective tripping, isolations, circuit breaker
 AC signal injection
 DC signal injection
 DC Ground Loop Interrupt
– (Resistive)/Magnetic coupling
– Use of differential current

Event analysis
Capabilities/Limitations

Voltage Measurements
– Provides detection and limited (inaccurate)
information on location
– Considerable amount of time required to locate
the fault
– Expect considerable interference from system
– With multiples grounds on the system, reading
can be very misleading
Capabilities/Limitations

Selective tripping, isolation of circuit
breaker, fuses and other interrupting devices
– Must be methodical
– Considerable amounts of time required to locate
fault
– Interruption of critical equipment and service
– Interruption of circuits
– May inadvertently trip circuits due to transients
caused by opening and closing breakers.
Capabilities/Limitations

AC signal injection
– Will locate fault, minimizes circuit and service
interruption
– High system knowledge is required
– Ineffective and should not be used on system with
large stray capacitance or High AC Ripple Content
– Ineffective in multi-feeder distribution systems
having large stray capacitance and strong
electromagnetic field interference between the
distribution line and ground
Capabilities/Limitations

AC signal injection
– Requires a large AC signal to overcome low
impedance to ground
– Sensitivity varies in proportion to signal
strength
– Use of large amplitude AC signals may trip or
damage sensitive devices/circuits
Capabilities/Limitations

DC signal injection
– Will locate fault, minimizes circuit and service
interruption
– High system knowledge is required
– Use of large amplitude DC signals may trip or
damage sensitive devices/circuits
Capabilities/Limitations

DC Ground Loop Interrupt (Resistive)
– Can use magnetic coupling & sensors
 allowing you to take reading through conduits and in wire
bundles
– Locates fault with minimal circuit/service interruptions
– Usable on energized/de-energized circuits
 (w/power supply)
– Uses differential current (fault current ) to locate
ground
– Sensitivity of around 3 mA (greater sensitivity
depending on unit and experience)
– Accommodates large range of cable and circuits
Injection Point & Length:
Does it Matter?

Power Bus
 Site Ground
 Continuous
 Pulsed
Event Analysis

This involves the use of multiple event
recorders (with voltage & current inputs)
placed/positioned around your distribution
network in a methodical manner then
waiting for events
 Time & Date stamp all units used(to the
same ref)
 Give or request equipment start/stop logs to
operators
Event Analysis

Look for events occurring at the ~same
time(slope & amplitude)
 Particular circuit in question can be isolated
by the use of an external power supply &
opening the associated breaker to the
suspected circuit
 Monitor upstream points for transient
reoccurrence(at least one period)
Caution

Injection of any large amplitude signals in a
distribution system has been known to
damage sensitive digital equipment and/or
inadvertently operate or prevent from
operating critical loads
Note

Multiple ground faults
– Largest will only be detected first
– Must be isolated to detect others
– Ground faults can be of opposing polarities

Re-evaluation must occur upon removal of each
GF

Magnitude of remaining GF can increase
significantly
Predictive Maintenance

Measuring and trending of the following
parameter should be performed periodically
– DC overall voltage
– DC polarity voltages with respect to ground
– Monitor ground faults on your different DC
buses
– AC voltages between polarities and ground

Train maintenance personnel on DC ground
fault troubleshooting & isolation
How We Help Ourselves
How We Help Ourselves
Ground Faults in Batteries

Concerns about ground faults in batteries.
 Causes of ground faults in batteries.
 Results of Ground Faults.
 Methods to Detect Possible Ground Faults
in VRLA.
Voltage to Ground –
UPS Battery Systems

Due to the elimination of the input
transformer in most large UPS systems
there is a very large and dangerous voltage
to ground.
 On a 240 Cell battery system the voltage
ranges from 200 to 500 V depending on the
point along the battery the measurement is
taken.
Voltage to Ground Waveform
Causes of Ground Faults in
Batteries

Dirt on the surface of the container.
 Hole in the container.
 Jar to cover seal failure.
 Post seal leak.
Results of Ignored Ground
Faults
FIRES!
Useful Tool
Insulation resistance tester.
2gohm @ 1000VDC