Part 2-Shaft Voltage And Bearing Current
Introduction
Motor bearing life has historically been from six to ten years with sinusoidal 60 Hz power to the motor.
However, with modern adjustable speed drives (ASDs), some users are seeing bearing damage in as little as
one week, damage caused by electric currents flowing through the bearings from shaft to motor frame, as
the result of a voltage potential induced between the rotor and stator. This electric potential is associated
with the use of solid-state switches, called transistors, which turn on and off thousands of times per second
to control the voltage applied to the motor windings. These new switching devices, turning on and off very
fast with extremely short rise and fall times of the applied voltages, have dramatically increased stress on
the motor windings and on the bearings. And this damage is not limited only to AC motors, but also can
stress DC motors, when driven by ASDs. Bearing damage caused by electric currents flowing through them
is called electrical discharge machining or EDM.
What causes bearing damage?
Causes of bearing damage, for the most part, can be broadly classified within three categories:
a. Lubrication
b. Mechanical
c. Electric Discharge Machining (EDM) or Bearing Currents
It is important to seek to identify the specific cause of failure in order to not repeat the failure, often within
a short period of time. Bearing current failures, for example can occur in as short a time as one week after
installation. Others, such as insufficient grease, can take several years to develop into a problem. Table 15
lists the most common causes of bearing failure and evidence to look for, during operation in some cases,
and others may require an “autopsy” to look for evidence. Bearing or shaft current damage is difficult to
prove, short of cutting the bearing apart and examining it under a microscope.
Figure 15
Bearing Failure Causes
Failure Category
Estimated % of Failures
Inadequate Lubrication
35-40
Wear
Improper Mounting
15-20
10-15
Corrosion
5-10
Fatigue
5-10
Other Causes
20
Further breaking down the leading causes of failure helps to identify the root cause and provide a guide for
corrective action. Figure 16 lists the primary causes of failure and evidence supporting the assumptions.
Shaft currents are the fastest growing cause of bearing failure today, because of the rapid deployment of
adjustable speed drives in industry, air conditioning and ventilating systems, plastic extruders, etc. Its root
causes will be presented and analyzed to provide guidance for prevention of recurrence. Service centers
play an important role in helping their customers identify and fix causes of premature motor failure. It
helps to know the history of failures or repairs, but which are sometimes just not available or the piece of
equipment transferred from one site to another.
1-26
Table 16
Bearing Failures/Malfunctions
Cause
Evidence
Overload
Spalled Races/Balls, Rollers
Foreign Matter
Noise, Embedment
Excessive Preload
Excessive Heating
Excess Grease
Immediate High Temperature
Insufficient Grease
Delayed High Temperature
Cocked Bearing
Noise, Skewed Ball Path
Shaft Currents
Spherical Craters or Fluting
Shaft Voltages and Bearing Currents
There are three primary causes of shaft voltages and bearing currents in drives today.
• Electrostatic rotor voltage caused by power supply asymmetry resulting from:
a. Unbalanced line voltages
b. Common mode voltage caused by odd number of semiconductor switches on at one
time (AC inverter driven) This is electrostatic asymmetry.
• Fast switching (high frequency ) PWM power supplies
a. Fast voltage transients (high dv/dt)
b. High frequency of PWM Carrier
• Axial rotor voltage generation caused by motor magnetic asymmetry or flux imbalance
(Figure 1) due to:
a. Rotor static or dynamic eccentricity
b. Rotor slotting
c. Axial cooling holes in rotor
d. Stator eccentricity
For AC drives the most common cause of shaft voltages and bearing currents is the generation of
electrostatic potential induced on the rotor, as a result of common mode voltage.
PWM Adjustable Speed Drive and Topology
Figure 17 shows a block diagram of a typical PWM adjustable speed drive, and Figure 18 the
simplified drive schematic, with the input diode rectifier bridge, a DC bus filter capacitor, and an
output or inverter stage.
Figure 17
Block Diagram – PWM AC Motor Control
AC
MAINS
AC Motor
2-26
Figure 18
Basic Adjustable Speed Drive Schematic
AC
MAINS
To Motor
Three-Phase Sine Waves
Referring to Figure 19, at any instant in time the sum of the three 60 Hz line voltages V A, VB, and VC add
to zero. Hence the common mode voltage for the system VCM = 0. For all balanced three-phase 60 Hz line
power, the Common Mode Voltage (CMV) is theoretically zero.
Figure 19
Three-Phase Sine-Wave Voltages
VA
VB
VC
0 Volts
Common-Mode Voltage VCM = 0
VCM
0 Volts
Inverter Technology and Common Mode
PWM ASDs generate common mode voltage because of their six-switch three-phase topology, in which
three of the six transistors are on at any given time. Referring to Figure 20, closing Switches SW-1, SW-4
and SW-5 as shown in Figure 21, gives the equivalent circuit shown in Figure 22. The resulting voltage
from the capacitor neutral to the motor neutral at time T1 then can be calculated as VCM = VB/6
Figure 20
Basic Six-Switch Inverter/Three-Phase Motor Circuit Configuration
+ VB
MOTOR
+
SW-1
Motor
Neutral
SW-5
SW-3
VB/2
Effective
System
Neutral
VB/2
+
SW-2
SW-4
CW
SW-6
Ground
Windings
0-Ref
3-26
Motor
Frame
Figure 21
Circuit at Time T1 when SW 1, 4, 5 Closed
+VB
MOTOR
SW-1
SW-3
SW-5
SW-2
SW-4
SW-6
Motor
Neutral
System
Neutral
0-Ref
Ground
Frame at Earth Ground =
Approx. System Neutral Potential
Figure 22
Equivalent Circuit at Time T1 with Switches 1. 4, 5 closed
+
RW
RW
VB/2
System
Neutral
+
Motor Neutral
+
VB/2
Voltmeter VCM
RW
Common Mode Voltage VCM = VB/6
Six-Step Inverter
By successively closing different combinations of three switches different values of CMV are generated as
shown in Figures 23 through 26. Three of the six switches are always closed (in theory) to generate an
output voltage. The control strategy used, “six-step”, sine-triangle, adjacent states, space vector modulation
determine the output voltages to the motor. As an example, by closing three switches in sequence of 1-6-32-5-4, etc., and always maintaining three closed switches at any given time generates six-step voltages as
shown in Figure 27. The name “six-step” is derived from the six voltage levels displayed in the Phase-toNeutral voltage waveform VAN. This type of inverter is NOT a PWM inverter because its RMS output
voltage is fixed by the DC bus and not by switching on and off to create a variable duty cycle.
Sine-Triangle PWM
Using three-phase sine waves as references (φA, φB, φC) shown in Figure 28 and modulating them with a
higher frequency triangle wave, pulse-width modulated “sine-weighted’ digital signals are generated. When
the reference signal for phase A, for example, is greater than the triangle signal, the upper switch SW-1 is
4-26
closed. And when the triangle signal is greater, the lower switch SW-2 is closed. The resulting phase-tophase voltage waveform VA – VB = VAB at the output of the inverter or drive is a sinusoidally-weighted
PWM voltage of peak amplitude VBUS with an effective switching frequency twice that of the triangle
carrier frequency fC and a fundamental frequency fO which is that of the sine reference signal. This PWM
algorithm has been one of the more popular PWM strategies, but is rapidly giving way to higher
performance algorithms such as Space Vector modulation, with the ready availability of higher
performance microprocessors and DSPs.
Figure 23
Circuit at Time T2 when SW 2, 4, 5 Closed
+VB
MOTOR
SW-1
SW-3
SW-5
SW-2
SW-4
SW-6
Motor
Neutral
System
Neutral
Ground
Frame at Earth Ground =
Approx. System Neutral Potential
Figure 24
Equivalent Circuit at Time T2 with Switches 2. 4, 5 closed
+
RW
VB/2
Motor Neutral
System
Neutral
+
+
Voltmeter VCM
VB/2
RW
Common Mode Voltage VCM = -VB/6
5-26
RW
Figure 25
Circuit at Time T3 when SW 1, 3, 5 Closed
Motor
Neutral
+VB
MOTOR
SW-1
SW-3
SW-5
SW-2
SW-4
SW-6
System
Neutral
Groun
d
Figure 26
Common Mode Voltage with Six-Step Inverter Operation (Switching sequence 1-6-3-2-5-4)
Phase A Switching
+VB/2
1 - ON
1 - ON
2 -ON
-VB/2
Phase B Switching
+VB/2
-VB/2
3 - ON
3 - ON
4 - ON
4 - ON
Phase C Switching
+VB/2
5 - ON
5 - ON
6 - ON
6 - ON
-VB/2
+VB
VAB Phase-to-Phase Voltage
-VB
+2VB/3
VAN Phase-to-Neutral Voltage
-2VB/3
Common Mode Voltage
+VB/2
-VB/2
T1
T2
T3
For Fundamental Motor Frequency of f0, Common mode Voltage Frequency is 3 f0
6-26
Figure 27
Sinusoidal PWM Waveform Generation and Common Mode Voltage
φA
φB
φC
Reference
Sine
Waves
Phase A
Carrier Signal @ PWM Freq. fC
Phase B
Phase C
+VB/2
Common Mode Voltage
+VB/6
-VB/6
0
-VB/2
Common Mode Frequency = Carrier Frequency fC
No. of Transitions or Steps = 6 x fC
Common Mode Voltage and Current
Motor controls have abrupt voltage transitions on their outputs that are inherent sources of radiated and
conducted electrical and magnetic noise. Most control manufacturers use IGBTs today because of their
efficient fast switching properties, with rise- and fall-times on the order of 20 to 50 nanoseconds. The
majority of control related problems are caused by conducted noise currents, the magnitude of which, are a
function of the amount of stray capacitive coupling output phases to ground, and the rate-of-change of
voltages on the control’s output. These noise currents flow through the parasitic capacitance of the motor
cables to the ground lead or conduit, and from motor windings through the parasitic capacitance of the
motor to the frame (and rotor) of the motor. These currents flowing through the system grounds and
returning ultimately to the source or cause are called common mode currents. It is these currents that cause
a voltage drop in the ground circuits, and this voltage drop is referred to as “common mode voltage.”
Because of these voltage drops, one piece of equipment’s “reference ground” may be at a substantially
different voltage level with respect to another’s “reference ground.”
7-26
By summing the three voltages at the ASD’s output terminals with respect to a reference or ground, a
voltage waveform is generated that is a function of the PWM frequency, with rapid transitions
corresponding to the switching speed of the IGBT switches. It is this common-mode voltage that is induced
in the motor circuit, creating problems as described earlier. For the sine-triangle PWM strategy of Figure
28, a common-mode voltage is generated as repeated in Figure 29. It very much resembles the six-step
waveform of Figure 27. Note the mostly uniform transitions or steps with amplitude VB/3, except for an
occasional seemingly double transition of two times VB/3 or 2VB/3. It is these larger transitions that create
the most trouble for the drive system; high dv/dt, bearing currents, EMI, etc. Referring to the waveforms of
Figure 28, it is seen that these “double” transitions correspond to “almost simultaneous” switching of
transistors in different phase “legs” of the inverter. Avoiding these “almost simultaneous” transitions by
delaying the turn-on or turn-off of the second or following transistor by no more than 5-10 microseconds, a
clear step or plateau results and has the effect of minimizing common-mode dv/dt.
Figure 28
Sine-Triangle Common Mode Voltage (from Figure 28)
+VB/2
+VB/6
0
-VB/6
-VB/2
Almost simultaneous switching of
two IGBTs in output phases.
Bearing Currents
Inverter-induced bearing currents appear to have two characteristic modes:
a. Displacement current through the bearing
This is proportional to the derivative of common mode voltage applied to the stator winding. This
current is capacitive and occurs to some degree at every switching transition of the inverter. Its
magnitude can be affected by anything that affects the rise time (dv/dt) of the applied voltage (current
loading of the switching device, dc bus voltage, reflected wave reinforcement/cancellation, etc.) or the
instantaneous capacitance of stator to rotor (rotor tooth to stator tooth alignment, air gap eccentricity,
etc.). These variations make the capacitive current a somewhat random phenomenon.
b. Discharge of electrostatic energy that is stored in the capacitance between rotor and stator.
This event also appears to be highly stochastic (non-repetitive or random) and causes peak currents
that are one to two orders of magnitude greater than displacement currents. Factors that may play a
role in the occurrence of discharge currents are rise time of the applied voltage, asperity of the rolling
elements or journals, grease thickness, grease contaminants and bearing loading (mechanical, thermal).
This is the current that causes pitting and fluting of the bearing races, and will be the main focus of this
discussion.
Bearing Currents
Often the only path available for rotor current is through the bearing races and balls to the grounded motor
end bell/frame. As static voltage builds up and discharges through the bearings, pitting and scoring of the
balls and raceways occur over time and can lead to premature failure of the bearing. If the capacitively
stored energy is great enough, this energy concentrated in a very small area raises the temperature of the
point of contact, melting the metal and creating a pit. Various solutions have been proposed and some are
in production; such as slip ring/brushes (on the shaft to ground it), conductive lubricants, ceramic ball
bearings, etc.
8-26
Bearing Current Circuit Model
Much has been written regarding the nature of bearing currents and the vast complexity of the large number
of variables, including analyses based on quantum mechanics and electron tunneling. Nothing so complex
will be attempted here; our goal is to develop a simple circuit model to explain discharge currents in
bearings. Consider as our model a steel ball suspended by a thin insulating oil film between two electrically
conducting raceways. The oil film acts as a dielectric between two conducting surfaces, forming small
capacitors between the inner and outer races, as shown in Figure 30 (b). These two capacitors COR and CIR,
effectively connected in series with the steel ball, account for the small displacement current through the
bearing. Because the capacitance is very small compared to the other motor parasitic capacitors, they are
neglected in the model. The inductance and resistance of the ball and races must be taken into account, and
are shown in Figure 30 (c).
Figure 29
Bearing Equivalent Circuit
Outer Race
SB
COR
SOR
LB
CIR
RB
SIR
Inner Race
(a)
(b)
(c)
Figure 30
Simplified Bearing Current Model
VCM
Motor Coils
VCM
VCM
LWR
CWR
VB
VB
CWS
Rotor
CWR
CWR
LB
CB
SB
SB
LWS
CB
LB
CB
CW
RB
RB
Stator
(a)
(b)
9-26
(c)
Stator-Rotor Circuit Model
Consider the lumped-capacitor stator-rotor model as shown in Figure 31. The rotor is charged to shaft
voltage VB through the winding-to-rotor capacitor CWR by the common-mode voltage, effectively on the
bearing current model. The critical path for bearing discharge current develops when the switch SB closes as
the ball makes contact with both inner and outer races. The capacitive energy ½ CB x VB2 is now dissipated
stator windings. Charge is also transferred to the stator by the winding-stator capacitor CWS by the CMV
but if the stator is well grounded (as shown) it remains essentially uncharged, with little influence on the
through LB and RB, passing a relatively large current through a very small contact surface, and in some
instances raising the temperature of the surface above melting, creating small pits in the softer raceways.
This displaced metal is trapped in the grease as sediment and further contaminating the grease.
Additionally, currents flowing through the grease causing decomposition through chemical activity.
Eventually the bearing fails.
Shaft Voltage
Shaft voltage relates to bearing current but the relationship is not direct. One cannot assume that twice the
shaft voltage will result in half the bearing life for example. However, if the shaft voltage is sufficiently
low, little or no bearing current will flow. NEMA MG 1-1993, Section IV, Part 31 states that bearing
failure due to electrical arcing on motors with frame sizes less than 500 frame series, can occur if shaft
voltages higher than 300 millivolts (peak) are present. Other sources (EPRI, etc.) suggest that voltages
greater than 5 volts will result in EDM bearing damage. However, it is assumed that the ohmic discharge of
electrostatic energy as described above, is the primary cause of bearing damage. Shaft voltages up to 30volts and bearing currents exceeding 700 milliamp have been measured in the laboratory.
Effect of dv/dt
The higher frequency and dv/dt cause voltages to build up on the rotor through charging of parasitic
capacitance from the motor windings to the rotor. Since the stator is grounded via the motor frame, voltages
appear on the rotor and shaft with respect to the frame. The higher the frequency and dv/dt, and the smaller
the air gap, the higher the voltage difference, and it can easily approach 10-15 volts or more. This can cause
problems for sensitive loads or measuring equipment connected to the motor shaft, as in high speed cutting
tools.
Shaft Voltage Measurement
By monitoring shaft voltage and its waveshape with a fast oscilloscope, we may be able to accurately
predict the presence of damaging bearing currents. The waveform of Figure 30 shows the relationship
between shaft voltage and bearing current observed in the laboratory.
An oscilloscope set to trigger on the rising or falling edge of the voltage waveform of shaft voltage will
allow observation of the ohmic discharge region. Observations confirm that the higher the voltage at the
point of discharge, the greater the bearing current pulse.
Figure 31
Shaft Voltage and Bearing Current Relationship
Shaft Voltage
≤ 30 Volts
Ohmic Discharge
Time t in
μsec.
Bearing Current
10-26
Figure 32
Illustration of Relationship of CMV, Shaft Voltage and Bearing Current
480 VAC Drive with 660 VDC Bus
+330-Volts
Common-Mode Voltage Transitions
+110-Volts
0
-110-Volts
High CMV dv/dt Induces Shaft
Voltage
-330-Volts
Shaft Voltage
Time t
Bearing Current
5 μs
Measurement of Bearing Currents
In order to obtain a measurement of the current through the bearing under actual operating
conditions, the bearing must be electrically isolated from the frame and an alternate conducting
path must be provided from bearing to the frame. The alternate conducting path, usually a lead
wire jumper from stationary bearing outer race to motor end bracket, serves as an access point for
a shunt or high bandwidth current probe. Not only does this modification require extensive
rework of the motor under test; it also alters the high frequency impedance of the bearing current
circuit.
Additionally, because of the stochastic nature of bearing currents, it is not possible to accurately
define such current with one number, such as peak current. Instead, statistical measures such as
average, median, standard deviation (assuming a normal distribution) and frequency of
occurrence may be more meaningful. Such measures require the phenomenon to be observed over
relatively long periods of time with high sample rates in order to capture the true waveshape of
each individual current pulse. Statistical post-processing is required, above and beyond what
would normally be found on a digital storage oscilloscope.
11-26
Having said all of the above, it is possible to estimate relative peak magnitudes of bearing current
based on knowledge of the shaft voltage and the stator-to-rotor capacitance. A discussion of shaft
voltage and techniques for observing and/or measuring it and stator-to-rotor capacitance follows.
Bearing Facts
An Oil film between bearing and raceway is typically 0.1 to 1 μm thick, as depicted in Figure 32.
Assume average is 0.5 μm. A typical sheet of paper is around 0.1 mm thick. It would take 200 oil
films stacked on top of each other to equal the thickness of one sheet of paper.
Figure 33
Cross Section of Bearing
Ball Bearing
0.1 to 1 μm
Raceway
Types of damage observed (Figure 33)
•
•
•
Frosting
Pitting
Fluting
Low Speed
Motors operating at low speeds, where the balls tend to maintain direct contact with the races, usually
sustain little severe damage from bearing currents. Because of this almost constant contact, the shaft
voltage cannot rise high enough to cause melting and pitting of the races. Frosting (burnishing) of the
raceways may be the best indicator of sustained low speed, low voltage operation. Consequently the only
warning may be a slight increase in bearing noise over a period of time.
There is an exception to low bearing damage at low speed, and that is if the drive is operating at high
torque, requiring high voltage to the motor. In this case severe damage can occur in the form of a random
pitting pattern, shown in Figure 33 c
Varying Speed
By varying the motor speed over even a small range, the oil film tends to be less uniform and the
discharges more random. The peak bearing currents have been observed to initially drop by more than 50%
when motor speed is stepped from a fixed speed to a new RPM. If speed is again held constant the bearing
current spikes begin to rise as the oil film once again becomes uniform. After several minutes at that new
speed the current spikes are back to their initial amplitudes.
This would imply that introducing a small dither in the speed of the motor would reduce bearing damage,
and reports from industrial installations support this hypothesis.
12-26
Higher Speed
It has been observed in the laboratory that speed does influence bearing currents and resulting failure. At
motor frequencies below 25 Hz, shaft voltages are low and Bearing currents are small. As motor
frequencies are increased above 25 Hz, discharge amplitudes grow rapidly up to 40-45 Hz, rising slightly
until around 50-55 Hz, but the incidence of discharges decreases as the bearings float on the more uniform
oil film. Above base speed, the discharge amplitudes again decrease as the PWM waveform of applied
voltage is overmodulated, and the incidence of discharges continue to decrease.
Sustained operation near base speed can result in severe pitting because of the relatively uniform oil films
created. This uniform film allows shaft voltage to build up to fairly high levels before arcing, yielding
fewer but more severe discharges.
Techniques that Protect the Bearings
Here are some of the ways one can protect the motor bearings with a relatively high degree of success,
listed in order of preference (for a variety of reasons). Some, such as the shaft grounding brush, require
periodic inspection or maintenance.
• Use Shaft Grounding Brush
• Insulate Both Bearings
• Isolated Bearings/Both (Ceramic)
• Use Bearing with Conductive Greases
• Dv/dt Filter (also called Sine Wave Filter)
• Use a Motor with Faraday Shielded Stator Winding
• Common Mode Transformer (Passive)
• Common Mode Filter (Active)
• Dual PWM Inverter (12 Switch)
• Motor with Specially Wound Stator for CMV Cancellation
Techniques that Reduce Bearing Currents, but not eliminate the potential for them.
• Reduce PWM Frequency to lowest acceptable value
• Install 3-5% Inductor between Drive and Motor
• Securely ground motor frame with low inductance/impedance cable plus conduit back to drive
• Reduce Drive Input Voltage to Lowest Acceptable Value
• Always use motor under load (minimize no-load operating time)
13-26
Figure 34
Typical Bearing Current Damage Patterns
14-26
Figure 35
Examples of Damage Due to Bearing Currents (Courtesy SKF)
15-26
Figure 36
Typical ASD Connection Diagram
Adjustable Speed Drive
Transformer
Secondary
Cable in Conduit
Motor
Frame
AC Motor
Probe to measure
CM Current ICM
Common Mode Current Path
Figure 37
Basic AC Motor Cross-Section
Stator
ODE
Bearing
Rotor
Shaft
16-26
DE
Bearing
Figure 38
Shaft Voltage and Bearing Current Caused by Axial
Asymmetry
Stator
ODE
Bearing
Rotor
DE
Bearing
Shaft
Voltage induced in shaft by magnetic asymmetry
causes bearing currents as shown by arrows
17-26
Figure39
Shaft Voltage and Bearing Current Caused by Capacitive
Coupling between Stator and Rotor
Stator
ODE
Bearing
Rotor
DE
Bearing
Shaft
Electrostatic Coupling of Stator Windings to Rotor
Voltage induced in shaft by capacitive coupling to stator windings causes
bearing currents as shown by arrows
18-26
Figure 40
AC Motor with Isolated Shaft at One End
Stator
Rotor
Insulation
Shaft
DE
Bearing
ODE
Bearing
Insulating Material for Isolating Shaft from Bearing Inner Race
Voltage induced in shaft by magnetic asymmetry but no
current can flow through bearings
Figure 41
AC Motor with Shaft Isolated at Both Ends
Stator
Rotor
Insulation
Shaft
DE
Bearing
ODE
Bearing
Insulating Material for Isolating Shaft from Bearing Inner Race
19-26
Figure 42
Outer Race Isolated Bearings From SKF
Figure 43
AC Motor with Hybrid (Ceramic) Ball Bearings
M
o
Stator
ODE
Bearing
Rotor
Shaft
Hybrid (Ceramic) Ball Bearings
20-26
DE
Bearing
Figure 44
AC Motor with Radial Shaft-Grounding Brush Assembly on ODE
Stator
Shaft
Grounding
Brush Assy
Rotor
Shaft
ODE
Bearing
DE
Bearing
Figure 45
AC Motor with Radial Shaft-Grounding Brush Assembly on Both Ends
Stator
Shaft
Grounding
Brush Assy
Rotor
Shaft
ODE
Bearing
DE
Bearing
21-26
Figure 46
Low Cost Shaft Grounding Brush Components and Installation
22-26
Figure 47
Shaft Grounding Currents Caused by Poor Motor Ground
Figure 48
Common Mode Currents and CM Filter
MOTOR
CABLE
Inverter
Isolation
Transformer
Secondary
Common
Mode
Filter
AC Motor
Optimum Path for Common-Mode Current (Green = Good)
Common-Mode Current Flowing through Earth-Ground/Building
Conduit and Loads (Red = Not Good)
23-26
Figure 49
Common Mode Mitigation Components
24-26
Figure 50
Sketch of "Triangular-Wire" Cable
Heavy Stranded
Copper
Insulation
Sheath
Phase T1
Phase T2
Ground
Phase T3
Insulated Twisted
Stranded Conductors
Figure 51
Asymmetrical Cabling
25-26
Figure 52
Sketch of "Symmetrical Ground” Three-Phase Cable
Heavy Stranded
Copper Sheath Earth Grounded
Rubber or
Neoprene
Insulation
Phase T1
Earth
Ground
Conductors
(3)
Phase T3
26-26
Phase T2