NLSI Recommendations for Surge Protection Devices
By Richard Kithil, President & CEO, NLSI
- The Problem. According to recent NEETRAC (1) and other testing,
many surge protection devices (SPD) have failed to perform according
to ANSI, IEEE, UL, IEC or other required specifications. How can the
concerned applications user be assured that this equipment will perform
satisfactorily under abnormal power quality conditions?
We provide this generalized reference guideline for
selecting appropriate power and data/signal devices.
- How SPDs work. If a transient suddenly is imposed upon an electrical
system, there are several methods of defense responsiveness: protection
by open circuit or protection by short circuit. Voltage breakdown devices
(gas discharge tubes and spark gaps), voltage limiting devices (metal
oxide varistors or avalanche diodes), bandwidth suppressors (various
filters, inductors and capacitors), and isolation devices (opto-isolators
and fibre-optics) are the principle technologies employed. Each type
of SPD has various advantages and disadvantages.
Hybrid SPDs – incorporating some combinations of
the above technologies – are more costly but more beneficial in handing
different transient waveform components. Purchasing SPDs based solely
upon cost is a (costly) mistake.
- General Requirements. Per IEEE, SPDs shall be installed at
designated Cat. C main panels, at appropriate Cat. B secondary feeder
panels, and at Cat A plug-in outlets as necessary. All power, data and
signal lines entering or leaving the structure should be considered
carefully for surge protection. (See IEEE 1100, below, for more details.)
SPDs must limit peak let-through voltages to below
equipment vulnerability levels. Three way protection of Phase to Neutral,
Phase to Ground, and Neutral to Ground must be standard. SPDs should
have diagnostic signaling – audible and visual is preferred – to indicate
status condition. If internal modules within the SPD are subject to
degradation and/or failure, they should be replaceable. Manufacturers
must provide clear, detailed installation instructions (short leads
and twisted cables are essential to reduce impedance and magnetic effects).
SPDs shall not interfere or restrict normal power quality.
They shall not corrupt incoming power; shall not interrupt the power
supply during operation; shall not have excessive current leakage to
- Conformance to Quality and Testing Standards. In order to distinguish
reputable from not-so-reputable products, we suggest selection of the
SPD be based upon:
- Conformance to UL 1449 Second Edition Testing. An optional part
of UL 1449SE is called Adjunct Endurance Testing. Performance characteristics
are rated according to three Grades - Classes - Modes.
- GRADES = Type of applied surge: Grade A 6000V and 3000A
(this is called for by IEEE 62.41); Grade B 4000V and 2000A; Grade
C 2000V and 1000A.
- CLASSES = Type of let-through-voltage: Class I = 330V;
Class II = 400V; Class III = 500V.
- MODES = Mode 1 does not contaminate ground; Mode 2 does
- Conformance to all relevant IEEE and IEC Standards.
- Conformance to the Federal Commercial Description (CID). This is
similar to the UL1449SE Adjunct Endurance Testing. Federal purchasing
agents can request the CID of an SPD and be assured of known, tested
and guaranteed performance results.
- Manufacturer compliance with ISO 9000/9001 Quality Control procedures.
References and Further Reading
- NEETRAC – National Electric Energy Testing, Research, and Applications
Center, Atlanta GA. WWW: http://www.neetrac.gatech.edu
- Standler, R.B., Protection of Electronic Circuits from Overvoltages,
John Wiley, NY, 1989.
- IEEE Std. 1100-1999, Powering and Grounding Sensitive Electronic Equipment,
IEEE, NY, 1995.