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Karl B. Clark, E.E., E.I.T.
Power Quality Consultant
10221 Sunburst Court Spring Hill, FL 34608
(Office) 352-686-2163 (Home) 352-686-4278
(Email) kclark8@tampabay.rr.com kb_clark@yahoo.com
COMPANY CONFIDENTIAL
TVSS applications for solidly grounded, impedance grounded, or resistance
grounded neutral systems.
I. Wiring Anomalies and Improper Application Will Destroy a TVSS
The absence or presence of an electrical distribution system neutral conductor and the method by
which the neutral is connected to the earth grounding conductor determines the type of TVSS
(Transient Voltage Surge Suppressor) that is specified. TVSS are frequently referred to as Surge
Protective Devices (SPDs) or Surge Suppressors. The impact upon a TVSS by the manner in which
the neutral conductor is grounded is shown in the examples below.
Example 1. A split phase 120/240 volt distribution system used on a typical residence. In this
example, the neutral and ground conductors are “solidly grounded.” By solidly grounded, we mean
that the neutral is connected to ground without using any resistance or impedance (typically, an
inductor). In this case:
1. The neutral conductor usually has a continuous white covering for this circuit. Neutral is a
current carrying conductor. It is referred to as a “grounded conductor” and it is intentionally
grounded to the grounding conductor via a bonding jumper at the service entrance or at the
secondary of the serving transformer per the N.E.C. The green non-current carrying safety ground
conductor (or continuous, conductive metallic conduit) is connected to the grounding electrode or
electrodes.
2. The neutral conductor is solidly grounded. There are no resistors or inductors in the circuit
between the neutral and ground. This is a zero impedance or zero resistance connection at 60 Hz.
3. The Line 1-N load current is 20 amperes, the Line 2-N load current is 40 amperes, and the
Line 1-Line 2 load current is 50 amperes.
The terms “line”, “phase”, or “hot” tend to be used interchangeably and refer to current carrying
conductors other than the neutral conductor.
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Figure 1. below shows the circuit conditions for this solidly grounded neutral circuit operating at
60 Hertz.
Line 1
Solid N-G Bond
120 V
20 A
L1- N Load
6 ohms
Neutral
50 A
L1 – L2
Load
4.8 ohms
240 V
120 V
Ground
40 A
L2- N Load
3 ohms
Line 2
Observe that the following voltages (V) may be measured (subject to normal supply voltage
tolerances) at the power frequency (ignoring transient voltages that exist per ANSI/IEEE C62):
1. V Line 1-N = V Line 1-G = 120 V because N and G are solidly bonded.
2. V Line 2-N = V Line 2-G = 120 V because N and G are solidly bonded.
3. V Line 1 - Line 2 = 240 V is the nominal supply voltage between Line 1 and Line 2.
4. V N-G = 0 V because N and G are solidly bonded. We ignore transient N-G voltages per C62.
Example 2. Consider the same split phase 120/240 volt distribution system. In this example, the
neutral conductor is open (it is not continuous). Thus, we have an open neutral situation. The two
single-phase loads (L1-N and L2-N), are now in series across Line 1 and Line 2 (240 V). The
voltages from L1-N and L2-N will now vary from the desired 120 V value as these loads change.
Figure 2. below shows the circuit conditions for this open neutral circuit operating at 60 Hertz.
Line 1
Open Neutral
160.0 V
L1- N Load
6 ohms
Neutral
Ground
50 A
240 V
80.0 V
L2- N Load
3 ohms
26.67A
Line 2
Observe that the following voltages may be measured at the power frequency:
L1 – L2
Load
4.8 ohms
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1. V Line 1 - N = 160 V. This will destroy the TVSS protection mode that is designed for
120 V and quite likely damage L 1- N loads. This is the condition where the lights on this
circuit burn very brightly and the bulbs exhibit a short life. A fire may also result from this
wiring problem.
2. V Line 2 - N = 80 V. This is the condition where the lights on this circuit are dim and various
loads may fail to start or operate properly due to a low voltage condition.
3. V Line 1 - Line 2 = 240 V. The 240 V loads tend to operate properly.
4. The voltages that are observed with the open or intermittent neutral condition change with the
loads that are in operation.
If the ground-to-neutral bond is missing (an open circuit or infinite impedance), the voltage
difference between neutral and ground will vary depending upon circuit load conditions and faults
between the hot conductor(s) and neutral or ground. This can exceed the MCOV (maximum
continuous operating voltage) of the TVSS and destroy the TVSS.
Example 3. Consider a typical 120/208 V, 3-phase wye, grounded neutral distribution system.
Figure 3. below provides a diagram of this circuit and the voltages.
208 V
Phase A
208 V
120 V
Phase B
120 V
208 V
Neutral
Solid GroundNeutral Bond
See
Note
120 V
Phase C
0V
120 V
120 V
120 V
Ground
Note: The Neutral Conductor Usually Serves 120 V Loads.
As shown above, the normal operating voltages for the solidly grounded neutral, wye system are:
1. Phase A - Phase B = Phase B - Phase C = Phase C - Phase A = 208 V
2. Phase A - N = Phase B – N = Phase C - N = 120 V
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3. Phase A - G = Phase B - G = Phase C - G = 120 V
4. N - G = 0 V due to N-G bond. In practice, this may be several volts, transients are ignored.
If the neutral conductor is open or intermittent (a loose connection), the voltages in the circuit
can be different from the normal voltages depending upon load conditions and/or fault
conditions and destroy the TVSS. If the ground-to-neutral bond of this wye circuit is missing
the voltages can vary as a function of the load currents and/or fault currents. This too can
destroy a TVSS.
If the neutral-to-ground bond is missing, the neutral point is free to move with respect to ground.
Thus, under varying load and/or fault conditions the TVSS MCOV can be exceeded, destroying the
TVSS. If the neutral-to-ground bond of Figure 3. above is removed, and a phase is short circuited
to ground, this will produce a “floating ground” condition. The floating ground condition will allow
the phase voltages referenced to ground to float to a maximum of 208 V. Since the TVSS MCOV is
designed for 120 V volts for these protection modes, applying 208 V will destroy the TVSS.
A phase short circuited to neutral in the absence of a neutral-to-ground bond will produce a
“floating neutral” condition. If Phase A is shorted to neutral, the voltages referencing Phase B and
Phase C will be allowed to float. The Phase B and Phase C to neutral voltages can reach a
maximum of 208 V. Since the TVSS design MCOV is 120 V for these modes, the TVSS can be
destroyed. The phase relationships between the phases will also change, but that is not a major
concern here.
II. Delta and Wye Distribution Circuits
Typical delta, no neutral, distribution circuits are the 240V, 480 V and 600 V volt ungrounded delta
circuits. Occasionally, a 120 V delta, no neutral system will be encountered. These circuits have no
neutral conductor and are not intentionally grounded. Because two conductors separated by an
insulator (wiring insulation, air, etc.) form a capacitor, these circuits in actuality are capacitively
grounded at a high impedance level via the stray capacitance of the transformer windings and the
insulated conductor capacitance to ground.
Example 4. Consider a 480 V delta, no neutral system. Figure 4. below provides a representation
of a 480 V delta, no neutral system.
Phase A
Virtual
Neutral Point
277 V
480 V
N
.
480 V
277 V
Phase B
Phase C
480 V
Earth Ground
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For the above circuit, there is no physical connection between earth ground and the “virtual neutral
point.” The virtual neutral point does not represent an actual physical location on the transformer
secondary windings. The circuit is capacitively grounded at a high impedance value. Note that the
earth ground passes through the circuit virtual neutral point. Again, there is no neutral conductor
connected to the virtual neutral point of the circuit because a physical neutral point does not exist.
The normal operating voltages for this 480 V delta, no neutral circuit are:
1. Phase A - Phase B = Phase B - Phase C = Phase C - Phase A = 480 V
2. Phase A - N = Phase B – N = Phase C - N = 277 V. The virtual neutral point is not a physical
point in the circuit; thus, it not accessible for the connection of single-phase 277 V loads. There
is no physical connection point to virtual neutral point. The 277 V voltage vectors are shown by
the dashed arrows from the virtual neutral point to the phases.
3. Phase A - G = Phase B - G = Phase C - G = 277 V. The 277 V can be measured, but there is
no physical wiring to permit current flow. The phases are grounded via stray circuit capacitance
and are high impedances.
4. N - G will vary as the loading and current phase balance on the system change. Since there is
no neutral conductor, we can’t measure the N-G voltage.
Example 5. A single-phase of the 480 V delta, no neutral shorted to ground. Figure 5. provides a
representation of a 480 V delta, no neutral system with Phase C shorted to ground. If Phase C (or
any other phase) is intentionally and permanently connected to ground we have a “corner
grounded delta” circuit.
480 V
Phase A
Phase B
277 V
N
480 V
277 V
Virtual
Neutral Point
480VV
480
Earth Ground
Phase C
Observe the following:
1. The neutral point has been displaced from ground by 277 V. This is not a problem for a 480
V, delta, no neutral TVSS because all six of the protection modes (L1-L2, L2-L3, L3-L1, L1-G,
L2-G and L3-G) are designed for an MCOV of 480 V plus “headroom” to accommodate normal
supply voltage fluctuations.
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2. The Phase A and Phase B voltages to ground are now 480 V. Again, this is not a problem
for a 480 V, delta, no neutral TVSS because all six of the protection modes (L1-L2, L2-L3, L3L1, L1-G, L2-G and L3-G) are designed for an MCOV of 480 V plus “headroom” to
accommodate normal supply voltage fluctuations.
3. An attempt to apply a 3-phase wye, 4 wire plus ground, 277/480 volt TVSS to this circuit by
connecting the neutral and the green (safety ground) TVSS leads to the electrical system ground
will destroy the wye TVSS. This wye TVSS is designed for a nominal 277 V from phase-toground and not the 480 V phase-to-ground voltages produced from Phase A-to-ground and Phase
B-to-ground. Other wye TVSS voltage configurations will be destroyed if miss applied in this
manner.
4. When one phase of a 3-phase delta, no neutral is intentionally and permanently connected to
ground, we have what is called a “corner grounded delta” distribution system. The correct
TVSS application for a corner grounded delta system is the 3-phase, no neutral, delta
TVSS of the correct operating voltage.
Example 6. A resistance or impedance (inductor) grounded neutral 277 V/ 480 V, three-phase, four
wire plus ground, wye system. The figure below provides a representation of a resistance or
impedance (inductor) grounded neutral 277 V/ 480 V, three-phase, four wire plus ground, wye
system.
480 V
Phase A
480 V
277 V
Phase B
Neutral
277 V
480 V
Leakage and/or
Fault Current Flowing
Through N-G
Impedance
See
Note
277 V
Resistance or
Inductance
Phase C
V??
277 V ? ?
277 V ? ?
277 V ? ?
Ground
Note: The Neutral Conductor Should NOT Serve Loads.
Observe the following:
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1. The impedance or resistance grounded neutral distribution circuit is used in Europe and other
parts of the world. Equipment imported from Europe is frequently powered by this type of
system. This circuit may also have an indicator or alarm circuit in series with the neutral
grounding impedance that indicates excessive neutral-to-ground current flow.
2. The neutral conductor should be directly connected to the grounding impedance and then to
ground. NEC 2005, Article 250.36 states:
“250.36 High-Impedance Grounded Neutral Systems. High-impedance grounded neutral
systems in which a grounding impedance, usually a resistor, limits the ground-fault current to a
low value shall be permitted for 3-phase ac systems of 480 volts to 1000 volts where all of the
following conditions are met:
(4) Line-to-neutral loads are not served.”
In some parts of the world, line-to-neutral, 2-wire, ungrounded loads may be connected phase-toneutral. This is unsafe.
3. The neutral point and ground will be displaced from 0 V by some value indicated as “V ? ?”
in the above diagram. The value of V ? ? depends upon the magnitude of the leakage current
(typically due to EMI/RFI filters) and/or fault current caused by a phase-to-ground fault which
flows through the resistance or impedance.
4. The phase-to-ground voltages “277 V ? ?” which exist during a phase-to-ground fault are
approximately the same as for ungrounded systems and approach the full phase-to-phase voltage;
i.e., 480 V on the ungrounded phases to ground. This is will destroy a standard 277/480 V, wye
TVSS. It is not a problem for a 480 V, delta, no neutral TVSS because all six of the protection
modes (L1- L2, L2-L3, L3-L1, L1-G, L2-G and L3-G) are designed for an MCOV of 480 V plus
“headroom” to accommodate normal supply voltage fluctuations.
5. For impedance or resistance grounded neutral systems, the correct TVSS application
is the 3-phase, no neutral, delta TVSS of the correct operating phase-to-phase operating
voltage. We are protecting all six active modes (L1-L2, L2-L3, L3-L1, L1-G, L2-G and
L3- G). The L1-N, L2-N, L3-N and N-G modes are not to be utilized per applicable codes.
III. TVSS Applications for Delta and Wye Distribution Circuits – A Summary
The selection of the appropriate delta or wye TVSS depends upon the manner in which the
electrical distribution system neutral is connected to the electrical distribution ground connection
and whether or not the neutral (if present) serves loads. These cases are discussed below.
Case A. Solidly Grounded or Hard Grounded Wyes.
The neutral conductor or current carrying grounded conductor usually has a continuous white or
gray covering. Additionally, it may have three continuous white strips on a non-green wire or a
marking of white or gray at the wire terminations. This neutral conductor is solidly bonded
(permanently mechanically and electrically connected) to the non-current carrying grounding
conductor (the green safety ground or conduit grounding means).
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Caution: The gray color may have been used in the past as an ungrounded conductor. Use
caution when working with existing systems. Always properly identify all conductors.
1. The “standard wye” TVSS is to be applied to electrical systems where the electrical distribution
system neutral is solidly bonded to the electrical system ground and single-phase loads are served.
If there are no single-phase loads a delta, no neutral TVSS is specified. In this case, the green
non-current carrying safety grounding conductor (the green non-current carrying safety ground wire
or metallic electrical conduit) and the white neutral or gray (grounded conductor) are held at the
same potential for the applied power frequency voltages by a solid mechanical and electrical
connection between them per the N.E.C. The applied power frequency voltage measured between
the non-current carrying grounding conductor (the green wire or conduit) and the current carrying
grounded conductor, the neutral conductor (the white, gray or other approved color wire) should
measure essentially zero volts at all times at the applied power frequency. Thus, voltmeter readings
will show essentially zero volts and ohmmeter readings will show essentially zero ohms. It is
important to note that large transient voltages will occur between the grounding conductor (green or
conduit) and the neutral or grounded conductor (white, gray, striped or taped) per ANSI/IEEE Std.
C62.
2. Point of application. When the electrical system neutral conductor is solidly bonded to the
electrical system ground at or ahead of the point where the TVSS is applied, and single-phase loads
are served, a wye unit is selected.
Case B. Impedance or Resistance Grounded Wye Systems Require a Delta (No Neutral Unit).
1. The delta (no neutral) TVSS is to be applied to electrical systems where the electrical
distribution system neutral is connected to the electrical system ground through a resistor or
inductor. In this case, the green non-current carrying safety grounding conductor (the green noncurrent carrying safety ground wire or metallic electrical conduit) and the white, gray or other
approved neutral color conductor (grounded conductor) are not held at the same potential for the
applied power frequency voltages by a solid mechanical connection between them. They are
connected via a resistor or inductor. The applied power frequency voltage measured between the
non-current carrying grounding conductor (the green wire or conduit) and the current carrying
grounded conductor, the neutral conductor will vary at the applied power frequency. Fault currents
and leakage currents will flow through the impedance between the neutral and ground. Thus,
voltmeter readings will not show essentially zero volts and ohmmeter readings will not show
essentially zero ohms. Normal operation of impedance grounded systems shift the voltage levels
between the neutral and ground. Additionally, the voltages between the phases and ground and the
phases and neutral will shift. This will destroy a standard wye TVSS, essentially the same way that
a “floating neutral” will.
2. Point of application. When the electrical system neutral conductor is connected to ground
through an impedance (a resistor or inductor), at or ahead of the point where the TVSS is applied, a
delta, no neutral unit is selected.
3. Selection of delta (no neutral) TVSS voltage. The nominal phase-to-phase wye voltage is
selected for the delta (no neutral) TVSS. See the examples below.
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A. For an impedance grounded 277/480 V, wye specify a 480 V, delta (no neutral unit). The
nominal wye, phase-to-phase voltage is 480 V and the closest nominal delta TVSS phase-tophase voltage is 480 V.
B. For an impedance grounded 347/600 V, wye specify a 600 V, delta (no neutral unit). The
nominal wye, phase-to-phase voltage is 600 V and the closest nominal delta TVSS phase-tophase voltage is 600 V.
4. Always visually and electrically verify the type of grounding and bonding present at the
point of TVSS application and ensure the distribution complies with the appropriate codes
and standards.
IV. Review of TVSS Applications for Various Electrical Distribution Circuits.
The following steps will simplify the selection of a TVSS that is compatible with the electrical
distribution system at the point of application (where you want to connect the TVSS).
1. How many hot (phases or lines) conductors are present?
2. Is a neutral conductor present? Is the neutral conductor serving loads?
3. Is a ground present? It is normally either a green non-current carrying safety ground or
continuous conductive metallic conduit or the ground buss inside an electrical panel.
4. If a neutral conductor is present is it solidly bonded to ground? Or, is it impedance or
resistance grounded?
5. What are the system voltages that are present at the point of application? What are the
voltage measurements between each pair of conductors? For a 10 mode wye, we would
measure the voltages from L1-L2, L2-L3, L3-L1, L1-N, L2-N, L3-N, L1-G, L2-G, L3-G and
N-G.
6. Select a TVSS compatible with the electrical system whose specification sheet MCOVs
(Maximum Continuous Operating Voltages) are not exceeded.
7. Specify a sine wave tracking or standard threshold clamping TVSS as the application
requires.
8. Specify the required peak surge current rating(s) for the application.
Usage of this work is conveyed to Surge Suppression Incorporated (and affiliated companies) and
their staff and clients for their exclusive use. Any and all other uses including; but, not limited to
the reproduction and/or distribution in any and all forms are forbidden without the expressed written
permission of the author.
All data has been derived from sources that are believed to be reliable and source documents are on
file. The author assumes no liability or responsibility for specification changes, typographical
errors, or omissions.
 2004 Karl B. Clark, All Rights Reserved.
Rev. 04 11/30/2004