Saturday, May 21, 2011

In this Code Tutorial about Surge-Protective Devices, I have detailed NEC 285 and placed some comments to explain the rationale of the code. This article was only about TVSS since the 2002 edition. In NEC's latest revision, it was revised to include surge arresters rated 1 kV or less. In addition, it provides SPD installation requirements for the protection of persons and electronic equipment.

I.  General

285.1 Scope
This article covers general requirements, installation requirements, and connection requirements for SPDs [surge arresters and transient voltage surge suppressors (TVSSs)] permanently installed on premises wiring systems 1 kV or less.

Informational Note 1: Surge arresters less than 1 kV are also known as Type 1 SPDs.
Informational Note 2: TVSSs are also known as Type 2 and Type 3 SPDs.  

Comment: NEC 285 covers only permanently installed SPD devices. It does not apply to portable cord-and-plug-connected types, which are intended to be used in accordance with their listings and the manufacturers’ instructions. Also, surge protective devices refer to both TVSS and surge arresters rated 1 kV or less.

285.3 Uses Not Permitted
An SPD shall not be installed in the following:

(1)   Circuits exceeding 1 kV
(2) On ungrounded systems, impedance grounded systems or corner grounded delta systems unless listed specifically for use on these systems
(3)  Where the rating of the SPD is less than the maximum continuous phase-to-ground power frequency voltage available at the point of application

Informational Note: For further information on SPDs, see NEMA LS 1-1992, Standard for Low Voltage Surge Suppression Devices. The selection of a properly rated SPD (TVSS) is based on criteria such as maximum continuous operating voltage and the magnitude and duration of overvoltages at the suppressor location as affected by phase-to-ground faults, system grounding techniques and switching surges.

285.4 Number Required
Where used at a point on a circuit, the SPD shall be connected to each ungrounded conductor.

285.5 Listing
An SPD shall be a listed device.

Comment: UL 1449, Standard for Surge Protective Devices, covers Types 1, 2, 3 and 4 devices.

285.6 Short Circuit Current Rating
The SPD shall be marked with a short-circuit current rating and shall not be installed at a point on the system where the available fault current is in excess of that rating. This marking requirement shall not apply to receptacles.

Comment: The SPD must match or exceed the system’s available fault current at its point of installation on a system. In addition, series type TVSSs are susceptible to high fault currents if located near service equipment and a risk would be present if the device rating is less than the available fault current.

II. Installation

285.11 Location 
SPDs shall be permitted to be located indoors or outdoors and shall be made inaccessible to unqualified persons, unless listed for installation in accessible locations.

285.12 Routing of Connections
The conductors used to connect the SPD to the line or bus and to ground shall not be any longer than necessary and shall be avoided unnecessary bends.

Comment: The conductor length used to connect the SPD is a vital factor to protection performance. The impedance of the conduction path is proportional to the conductor length. This results to a higher clamping voltage, which reduces the protection offered by the SPD. For maximum protection, the SPD should be as close as possible to the protected equipment. A Type 2 SPD installed as an integral component of a panelboard is a good example.

III.   Connecting SPDs

285.21 Connection
Where an SPD is installed, it shall comply with 285.23 through 285.28.

285.23 Type 1 SPDs (Surge Arresters)
Type 1 SPDs shall be installed in accordance with 285.23(A) and (B).

(A) Installation. Type 1 SPDs shall be installed as follows:

(1) Type 1 SPDs shall be permitted to be connected to the supply side of the service disconnect as permitted in 230.82(4) or 
(2) Type 1 SPDs shall be permitted to be connected as specified in 285.24.

(B) At the Service. When installed at services, Type 1 SPDs shall be connected to one of the following:

(1) Grounded service conductor
(2) Grounding electrode conductor
(3) Grounding electrode for the service
(4) Equipment grounding terminal in the service equipment

Comment: Although there are four acceptable locations for connecting the SPD grounding lead, it is necessary to follow the requirement in 285.12.

285.24 Type 2 SPDs (TVSSs)
TVSSs shall be installed in accordance with 285.24(A) through (C).

(A) Service-Supplied Building or Structure. Type 2 SPDs shall be connected anywhere on the load side of a service disconnect overcurrent device required in 230.91, unless installed in accordance with 230.82(8).

Note: The TVSS has been included to equipment allowed to be installed on the line side of the service disconnecting means. Such applications are permitted only in listed equipment, and the conductors connected to the TVSS device must have a disconnecting means and overcurrent protection. Field connection of TVSS to the line terminals of the service equipment is prohibited. 

(B) Feeder-Supplied Building or Structure. TVSSs shall be connected on the load side of the first overcurrent device at the building or structure.

Note: This is in accordance with UL 1449, connecting the TVSS on the load side of the first OCPD in a feeder supplied structure is necessary due to the exposure of external feeder conductors to lightning-induced surges.

(C) Separately Derived System. The TVSS shall be connected on the load side of the first overcurrent device in a separately derived system.

285.25 Type 3 SPDs 
Type 3 SPDs shall be permitted to be installed on the load side of branch-circuit overcurrent protection up to the equipment served. If included in the manufacturer's instructions, Type 3 SPD connection shall be a minimum 10 m (30 ft) of conductor distance from the service or separately derived system disconnect.

285.26 Conductor Size 
Line and ground connecting conductors shall not be smaller than 14 AWG copper or 12 AWG aluminium.

285.27 Connection Between Conductors 
An SPD shall be permitted to be connected between any two conductors - ungrounded conductor(s), grounded conductor, equipment grounding conductor or grounded electrode conductor. The grounded conductor and the equipment grounding conductor shall be interconnected only by the normal operation of the SPD during a surge.

285.25 Grounding Electrode Conductor Connections and Enclosures
Except as indicated in this article, SPD grounding connections shall be made as specified in Article 250, Part III. Grounding electrode conductors installed in metal enclosures shall comply with 250.64(E).

Comment: SPDs need effective grounding since the surge will be diverted directly to ground. Installing SPDs without proper grounding is considered to be more hazardous than if they had not been installed at all. It is recommended that at least two dedicated 10-ft ground rods (located at least 6 feet apart) are installed at the panel for your SPDs.  

Reference: National Electrical Code 2011

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I am a Professional Electrical Engineer with a Masters Degree in Business Administration. My interest is in Power Quality, Diagnostic Testing and Protective Relaying. I have been working in an electric distribution utility for more than a decade. I handle PQ studies, power system analysis, diagnostic testing, protective relaying and capital budgeting for company projects.