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INSTALLING TRANSIENT VOLTAGE SURGE SUPPRESSORS
Lightning Arrester, Secondary Surge Arrester,
Transient Voltage Surge Suppressor
Where do these fit in the electrical system?
Direct bus connections between the TVSS and panel reduces; the conductor
length, unnecessary conductor bends, and impedance with bolted connections.
A Historic Perspective on Surge Protection
Surge protection was introduced into the first National
Electrical Code (NEC) published in 1897. The primary
focus at that time was lightning arresters. In 1981, NEC
Article 280 was revised and re-titled “Surge Arresters”
in order to align with industry terminology. The title
change in the NEC also recognized that surge arresters were being installed where the surge source was
other than lightning, such as utility switching, or
equipment switching within industrial and commercial facilities.
The vast introduction of electronic equipment such
as computers, answering machines,
microwaves, HVAC electronic controls, security systems, etc., during
the last 20 years has also presented
a challenge to the electrical industry
to protect this sensitive equipment
from less severe surges than lightning.
The Transient Voltage Surge Suppressor (TVSS) is the newest product becoming commonplace in residential,
commercial, and industrial facilities
in order to protect sensitive electronic
equipment. In response to the development of TVSS products, Underwriters Laboratories, Inc. (UL) published
the first TVSS safety standard, UL
1449, in August 1985.
The lightning arrester is generally installed by the
utility at the serving transformer and basically acts as a
spark plug. When lightning hits the distribution line,
the increase in voltage causes an arc to form across the
spark gap and the lightning current is diverted to
ground, protecting the utility transformer.
Surge arresters and secondary surge arresters
are generally installed at the service equipment either
on the line or the load side of the service disconnect.
They may also be found connected to non-service
panelboards where branch circuits extend outside the
building. For example, branch circuits feeding parking
lot lights are a possible source for a surge to enter the
building by backfeeding the electrical system from external sources.
Transient Voltage Surge Suppressors may be installed in any part of the electrical system starting from
the load side of the service disconnect in the service
equipment to the electronic product being protected.
The TVSS may integrate protection of AC, coaxial cable,
and telephone to establish a common ground reference for the different services. TVSS products may also
be found connected to non-service panelboards where
branch circuits extend outside the building.
TVSS products may be installed:
1. as an integral part of a listed panelboard.
2. as a field installed product within a panelboard
listed and marked for a TVSS kit.
3. external to a panelboard or switch and connected
Integral TVSS units within Busway Plug-in Units and
MCC buckets reduce the conductor length and enhance
performance of the surge protection on the system.
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INSTALLING TRANSIENT VOLTAGE SURGE SUPPRESSORS
on the load side of a circuit breaker or fuse inside
the panel.
4. as an integral component of a Listed wiring device to protect specific electronic equipment connected to that branch circuit.
Installation in Accordance With The NEC
Article 250 - Grounding and Bonding
Establishing a solid foundation for a safe TVSS installation starts with the grounding and bonding system.
Section 250-2(a) clearly states the grounding system
be connected to earth in order to limit the voltage levels imposed by surge events.
250-2. General Requirements for Grounding and
Bonding
(a) Grounding of Electrical Systems. Electrical systems that are required to be grounded shall be connected to earth in a manner that will limit the voltage
imposed by lightning, line surges, or unintentional
contact with higher voltage lines and that will stabilize
the voltage to earth during normal operation.
The connections of the grounding electrode, grounding electrode conductor, and the bonding jumper are
important to facilitate a safe, low resistance path to
ground for any surge current being diverted by a TVSS.
When adding a TVSS to an existing electrical system, it
is important to reinspect the grounding system to ensure a safe and effective path for the surge current.
Article 280 – Surge Arresters
TVSS products are not specifically recognized in the
NEC. Since TVSS products are similar in function to a
surge arrester, we must turn to Article 280 for installation requirements. The voltage rating of the TVSS must
be equal to or greater than the continuous phase-toground voltage. Section 280-4 also requires all surge
arresters less than 1000V to be listed. UL 1449 only
covers products rated 600V and less.
280-4. Surge Arrester Selection
(a) Circuits of Less than 1000 Volts. The rating of
the surge arrester shall be equal to or greater than the
maximum continuous phase-to-ground power frequency voltage available at the point of application.
Surge arresters installed on circuits of less than 1000
volts shall be listed for the purpose.
The TVSS must be connected on the load side of
the service disconnect unlike surge arresters that can
be connected to the line or load side of the service
disconnect as permitted in Section 230-82. The surge
arrester is often evaluated to higher surge current
levels than TVSS products. A TVSS evaluated to UL
1449 assumes connection on the load side of the “main
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disconnect.” The listing of a TVSS product may also
require connection to the load side of a specified
overcurrent device. UL 1449 permits this marking to be
placed either on the TVSS or in an instruction bulletin.
A service may contain six disconnects, as permitted
in Section 230-71. The main disconnect supplying the
TVSS at this service counts as one of the six disconnects. A TVSS is not power monitoring equipment.
230-71. Maximum Number of Disconnects
(a) General. The service disconnecting means for
each service permitted by Section 230-2, …, shall consist of not more than six switches or six circuit breakers mounted in a single enclosure, in a group of separate enclosures, or in or on a switchboard. There shall be
no more than six disconnects per service grouped in any
one location. For the purpose of this section, disconnecting means used solely for power monitoring equipment
or the control circuit of the ground-fault protection system, installed as part of the listed equipment, shall not
be considered a service disconnecting means.
The routing and length of conductors connecting
the TVSS is an important concern, addressed in Section 280-12. The objective of the TVSS is to get any
overvoltage conditions to ground as quickly as possible
without causing a strikeover or arc-to-ground before
the transient is dissipated to earth, ultimately protecting sensitive electronic equipment. Unnecessary bends
and conductor length increases impedance in the surge
path, driving the surge voltage higher. As the voltage
grows, the potential for a strikeover grows since the
voltage is not suppressed. The unsuppressed voltage
passes on to the electronic equipment that the TVSS was
intended to protect. Depending on the size and material
(Cu or Al) of the conductor, each foot of wire may add as
much as 165V to the suppression rating of the device.
Other Safety Issues
TVSS products function similar to surge arresters;
however, in order to provide protection for electronic
equipment, TVSS devices begin to operate (i.e., conduct electricity during a surge), much closer to the system operating voltage than a surge arrester. The NEC
does not require all electrical systems to be grounded
nor does it prohibit the installation of TVSS products
on an ungrounded system; however, there are concerns
with ungrounded systems. The voltage in an ungrounded system is not stable and the system voltageto-ground can rise as high as eight times over the normal operating voltage on the system if a ground fault
occurs on the circuit. A voltage rise of this magnitude
can be destructive to a TVSS; therefore, a TVSS should
not be permitted on ungrounded systems.
It is important to look for TVSS products that have a
INSTALLING TRANSIENT VOLTAGE SURGE SUPPRESSORS
short circuit current rating (SCCR),
even the more common single-port
or parallel devices. TVSS technology
commonly uses metal oxide varistors (MOVs) to make direct phaseto-phase and phase-to-ground connections unlike a spark (air) gap
used in lightning arresters. When an
MOV fails, a short circuit is established. The available fault current
will attempt to flow along the
shorted path. The circuit breaker or
fuse, to which the TVSS is connected, affords short circuit protection for the wire, but the TVSS may
or may not be protected unless it
has been tested and marked. An
example of markings on TVSS products that have been tested for short
circuit current ratings would be
similar to:
This TVSS is suitable for use on
circuits capable of delivering not
more than:
25,000A rms, 240V when protected by a 30A max circuit
breaker, or
10,000A rms, 240V when protected by a 30A max non-current
limiting fuse.
TVSS products can see an extremely high amount of energy before the circuit breaker or fuse limits and disconnects the short circuit
current. This energy can be destructive to the TVSS if the internal components are not electrically coordinated with the overcurrent protection. Containment of the TVSS failure within the TVSS enclosure is an
important safety concern. The rupture of the TVSS enclosure is a safety
concern whether external or internal to other electrical equipment.
The short circuit current rating for
single-port devices is a safety gap
that presently exists within industry standards and can only be evaluated by product testing. UL 1449
currently has a short circuit current
test for two-port devices but a similar test is only required at minimal
The integral TVSS is becoming commonplace in
panelboards that serve sensitive electronic
loads such as school computer facilities, office
buildings, and industrial facilities.
levels for the more common singleport or parallel device. Short circuit
current rating markings on products
is a good indication that the manufacturer has considered the available
fault current to which the TVSS is
going to be connected.
The markings on a TVSS
mounted externally to a panel
should include the SCCR directly on
the TVSS. A panel with an integrated
TVSS will have SCCR markings on
the TVSS, the panel’s wiring diagram, or panel markings that indicate acceptance of a TVSS within the
panel. The short circuit current ratings on the panel will apply for an
integrally mounted TVSS as part of
the panel’s listing. Do not confuse
the short circuit current rating with
the surge current ratings on the
TVSS, they are not the same.
The integration of TVSS products
into panelboard enclosures is becoming common. Listed equipment
is evaluated to ensure the integrity
of the panel with the TVSS installed.
The panelboard with an integral
TVSS is evaluated in accordance
with the UL 67 safety standard for
panelboards for heat rise, conductor (bus bar) spacing, wire bending
space, wire fill, short-circuit current,
etc. A TVSS will reduce the volume
of the enclosure, which impacts the
heating, wire gutter, and wire bending space. The reduction in volume
of the enclosure can also impact the
short-circuit performance of the
panelboard due to venting characteristics of different circuit breakers.
Extending the enclosure using a
panelboard extension and adding
the TVSS above or below the
panelboard will not resolve these
safety issues without proper evaluation and testing. Paragraph 30.12 in
UL 67 clearly states that components
for use in panelboards must be included in the panelboard markings.
30.12 Field-installed equipment
30.12.1 A panelboard to which a
unit, such as a circuit breaker,
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INSTALLING TRANSIENT VOLTAGE SURGE SUPPRESSORS
switch, or the like, may be added in the field shall
be marked with the name or trademark of the manufacturer and the catalog number or equivalent of
those units for which it is intended.
Look for a panel marking that indicates the panel has
been listed for use with an integral TVSS. A marking must
be provided as part of the panelboard listing that the
TVSS has been evaluated for use within the panel.
TVSS Performance Considerations
TVSS conductor length is a consideration in product
performance. If we consider the conductor lengths by
using 165V per foot to calculate new suppression ratings after connection to the electrical system, we start
looking for methods to reduce the conductor length in
order to increase our protection from surges and transient voltages. Efforts to reduce conductor length to
maximize protection from surges prompted the introduction of TVSS products being manufactured integral
to panelboards in order to minimize the impact of conductor impedance.
The entrance of utilities such as electric, cable, and
telephone at the same point on the structure is important for effective protection using a TVSS. Electronic
equipment (computers, telephones,…) has multiple
paths for surges to enter. A surge will destroy electronic
The “Whole House” surge protection unit integrates surge protection of
AC, Telephone/ Data, and Coax/ Cable and Satellite TV by establishing a
common reference where the utilities enter the house.
equipment due to a voltage differential (between AC,
phone, and coax) on the electronic boards within the
equipment. These multiple paths can effectively be
protected in a residence using a TVSS that integrates
protection of AC, coaxial cable, and telephone. By referencing all utilities to a common ground at the service entrance, we have an effective low impedance
ground path and establish a common reference for all
external surge sources. The utilities will electrically rise
and fall together when a surge enters the building, assisting in the removal of any voltage differential between
the communication system and electrical system within
the computer, phone, or answering machine.
Large residences may have multiple panelboards,
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such as two 200A panelboards. You enhance your protection of electronic equipment that is connected to
multiple utilities, such as telephones and computers,
by connecting the TVSS to the panelboard that supplies the branch circuits for those loads. “Point-of-use”
protection may also be recommended by an appliance
manufacturer or the panelboard TVSS manufacturer to
ensure proper protection of sensitive electronic equipment for warranty purposes. Point-of-use TVSS products are also listed to UL 1449 and may be found in the
form of a receptacle, direct plug-in connected, or cordconnected devices.
Summary
The NEC does not currently address the unique safety
concerns of TVSS installation as compared to surge
arresters. However, fundamental elements have been
discussed to assist in a safe and NEC compliant installation of transient voltage surge suppressors. Please
note those items covered in this article that are safety
related but not specifically addressed in the NEC at the
present. The following list provides a summary of the
items discussed and should be reviewed during the
installation of a TVSS:
1. Review the grounding system and connections to
ensure an effective ground path for surge current.
NEC 250–2(a). The TVSS should not be installed on
an ungrounded electrical system.
2. The TVSS must be listed by a recognized certification
body. You may find surge arresters installed that may
be listed as secondary surge arresters. NEC 280-4(a).
3. The TVSS must be installed on the load side of
the service disconnect. UL 1449 (Not in the NEC).
Review the TVSS markings for the required
overcurrent protection.
4. When the TVSS is installed at the service with six
disconnects, it must be connected to one of the six
disconnects. Seven is a violation of NEC 230-71.
5. Review the panelboard marking that indicates the
panel is listed for use with an internal or integral
TVSS. NEC 110-3(b) and UL 67.
6. Look for a short-circuit current rating on permanently connected TVSS products. NEC 110-10.
Alan Manche, a registered professional engineer in the state of Kentucky, is a Sr. Standards Engineer for Square D Company. He
is an active IAEI member and participates
in IAEI meetings across the country. Alan
also represents NEMA on CMP 8 of the NEC,
the NFPA 73 Technical Committee, and the
NFPA 70B Technical Committee. Alan has
held positions within Square D in product design and certification for panelboards, switches, metering, busway and
wire management equipment.