Surge Arresters and Transient Voltage Surge Suppressors (TVSSs)

NEC Basics by Mike Holt for EC&M magazine

Transient voltage surges, sometimes called “spikes”, are short-term deviations from a desired voltage level or signal, which can cause equipment malfunction or damage. A transient voltage surge on the 60 Hz AC sine wave is very brief in duration (measured in microseconds), and the surge voltage is much higher than the AC line voltage. Equipment driven by microprocessors is especially vulnerable to transient voltage surges.

SPDs are designed to reduce potentially damaging short-duration transients present on utility power lines, data networks, telephone lines, closed circuit and cable TV feeds, and any other power or control lines connected to electronic equipment.

How Surge Protective Devices Work
In the simplest terms, SPDs prevent damaging transient voltage surge levels from reaching the devices they protect by providing a shunt path for transients away from the load before they can enter the equipment.

A surge arrester and a TVSS both operate in the same way: They shunt high voltage surges to ground [280.2 and 285.2]. A surge arrester is listed to be installed on the line side of service equipment, where overcurrent protection is not provided. A TVSS device is listed to be installed on the load side of service equipment, where short-circuit and ground-fault protection is provided.

Author’s Comments: For more information, download Leviton’s Applications Manual and Reference Guide for Surge Protection and Line Conditioning Products and EPRI’s Developing a Consumer-Oriented Guide on Surge Protection at: http://mikeholt.com/Powerquality/Powerquality.htm.

ARTICLE 280 Surge Arresters

I. General

280.1 Scope. This Article covers the installation and connection requirements for surge arresters that are permanently installed on the line side of service equipment.

Author’s Comment: According to Article 100, Service Equipment is the necessary equipment, usually consisting of a circuit breaker(s) or switch(es) and fuse(s), connected to the load end of service conductors, and intended to constitute the main control and cutoff of the supply.

280.2. Definition.
Surge Arrester. A protective device for limiting surge voltages by discharging or bypassing surge current, and it also prevents continued flow of follow current while remaining capable of repeating these functions.

280.3 Number Required.
Where used, a surge arrester shall be connected to each ungrounded conductor of the system and a single surge arrester shall be permitted to protect all of the line conductors.

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 phase-to-ground voltage at the point of connection.

Surge arresters installed on circuits of less than 1000 volts shall be listed for the purpose.

FPN No. 2: See the manufacturer’s application rules for the selection of an arrester for a particular application.

II. Installation

280.11 Location.
Surge arresters shall be permitted to be located indoors or outdoors.

280.12 Routing of Connections. The conductors for the surge arresters shall not be longer than necessary, and unnecessary bends should be avoided.

III. Connecting Surge Arresters

280.21 Installed at Service Equipment.
The grounding conductor for the arrester shall be connected to one of the following locations:
(1) Grounded (neutral) service conductor.
(2) Grounding electrode conductor.
(3) Grounding electrode for the service.
(4) Equipment grounding terminal in the service equipment.

280.22 Installed on the Load Side Service Equipment.
A surge arrester shall be permitted to be connected between any two conductors - ungrounded conductor(s), grounded conductor and grounding conductor.

280.25 Grounding.
Grounding conductors for surge arresters shall not be run in metal enclosures unless the metal raceway is bonded to both ends to the grounding conductor.

Article 285 - Transient Voltage Surge Suppressors (TVSSs)

I. General

285.1 Scope. This Article covers the installation and connection requirements for TVSSs that are permanently installed on premises wiring systems. Figure 285-1 285-01 cc285-01.cdr

Author’s Comment: The scope of Article 285 applies to devices that are listed as TVSS devices. It does not apply to devices that incorporate a TVSS device, such as a cord-and-plug connected TVSS unit, a receptacle, or an appliance that has integral TVSS protection. For more information about TVSS devices, visit www.mikeholt.com/Powerquality/Powerquality.htm

285.2 Definition
Transient Voltage Surge Suppressor (TVSS). A protective device for limiting transient voltages by diverting or limiting surge current; it also prevents continued flow of follow current while remaining capable of repeating these functions.

285.3 Uses Not Permitted.
A TVSS shall not be used for:
(1) Circuits exceeding 600 volts
(2) Ungrounded electrical systems
(3) Where the rating of the TVSS is less than the maximum phase-to-ground voltage at the point of connection.

FPN: For further information on TVSSs, see NEMA LS 1-1992, Standard for Low Voltage Surge Suppression Devices. The selection of a properly rated TVSS is based on criteria such as maximum continuous operating voltage and the magnitude and duration of overvoltages at the suppressor location.

285.4 Number Required.
Where used, a surge arrester shall be connected to each ungrounded conductor of the system.

285.5 Listing.
A TVSS shall be a listed device in accordance with UL 1449.

285.6 Short Circuit Current Rating.
TVSS devices shall be marked with their short circuit current rating, and they shall not be installed where the available fault current is in excess of that rating.

WARNING: TVSS devices of the series type are susceptible to high fault currents if located near service equipment, and a hazard would be present if the device rating is less than the available fault current.

II. Installation

285.11 Location.
TVSSs shall be permitted to be located indoors or outdoors.

285.12 Routing of Connections. The conductors for the TVSS shall not be any longer than necessary and unnecessary bends shall be avoided.

III. Connecting Transient Voltage Surge Suppressors

285.21 Connection. Where a TVSS is installed, it shall be connected as follows:
(A) Location.
(1) Service Supplied Building or Structure. A TVSS can be connected anywhere on the premises wiring system, but not on the line side of the service disconnect overcurrent device. Figure 285-2 285-21A1 cc285-02.cdr

Author’s Comment: Care must be taken to ensure that no more than one conductor terminates on a terminal, unless the terminal is identified otherwise [110.14(A)].

Exception: A TVSS device listed as a surge arrester in accordance with 280.4(A) can be connected to the line side of the service overcurrent device.

Author’s Comment: TVSSs are listed to be located only on the load side of service equipment. TVSS devices cannot be installed on the line side of the building or structure overcurrent device because of the concern that they might be exposed to lightning-induced surges.

(2) Feeder Supplied Building or Structure. A TVSS can be connected anywhere on the premises wiring system, but not on the line side of the building or structure disconnect overcurrent device.

(3) Separately Derived System. A TVSS can be connected anywhere on the premises wiring of the separately derived system, but not on the line side of the separately derived system overcurrent device.

(B) Conductor Size. Line and ground connecting conductors shall not be smaller than 14 AWG copper.

(C) Connection Between Conductors. A TVSS shall be permitted to be connected between any two conductors - ungrounded conductor(s), grounded conductor and grounding conductor.

285.25 Grounding.
Grounding conductors for surge arresters shall not be run in metal enclosures unless the metal raceway is bonded to both ends to the grounding conductor.

Mike Holt’s Comment: If you have any comments or feedback, please let me know, Mike@MikeHolt.com


Copyright © 2002 Mike Holt Enterprises,Inc.
1-888-NEC-CODE (1-888-632-2633)