What is the Difference Between a Surge Arrester and a Transient Voltage Surge Suppressor?

    New National Electric Code requirements make it important to understand the difference between surge protectors, surge arresters, and transient voltage surge suppressors…and to fully realize the benefit provided by “dual rated” surge arresters and transient voltage surge suppressors offered by Stedi-Power.

    The term “surge protector” describes any device that protects against surges.  These can be either sustained (more than 16 cycles or about 3.75 seconds) or impulses (also called "spikes").  Sustained over-voltages are more commonly referred to as "swells" in the power quality industry.

    Surge Arresters and Surge Suppressors both protect against excess voltages a nd many people interchange the terms.  This practice is incorrect because they must meet different standards to reflect their different roles.

Surge Arresters

    A Surge Arrester is the first line of defense in protecting your facility from externally generated impulses.  They are usually designed specifically with lightning in mind and as a result, are sometimes referred to as "lighting arresters".  The US National Electrical Code refers to these devices as "Secondary Surge Arresters" or SSA's

    A Surge Arrester's design reflects the location it is designed to be installed at, which is BEFORE the customer's first means of disconnect.   Equipment with this rating requires no other protection (such as a fuse or a circuit breaker supplied by the customer) because it is designed as protection devices.  Their design takes into account the immense magnitude of voltages and currents they are designed to divert

    Since a surge arrester is intended to be installed in a location distant from the customer, and in understanding that no device can withstand a direct lightning strike without significant damage, it is allowed to meet more relaxed standards in respect to failure conditions.

    Specifically, while surge arresters and surge suppressors are tested at the same maximum amperage (10,000 amps), a surge arrester is only considered "qualified" if it survives undamaged and maintains performance to within 5% of its initial rating.  A surge suppressor, on the other hand, is only required to "survive" (no performance standard) and "fail safely" under the same conditions.
  

Type	Test/Wave	Requirement
Surge Arrester	10,000 A impulse (note 1)	Survive undamaged and continue to function within 5% of initial tested clamping voltages (note 3)
Surge Suppressor	10,000 A impulse	Survive or fail safely (note 2, 3)
	3,000 A impulse	Survive undamaged (note 3)
	2-times normal AC Voltage at AC Fault Interrupt Rating (AIC)	Survive or fail safely (note 3)
	2-times normal AC Voltage (0.125A to 5A, low current, long-term)	Survive or fail safely (note 3)
note 1:  Impulse = Current pulse with 8μs rise time , 10μs full-width at half maximum (also called the 8x20μs pulse or "combination wave")
note 2:  Fail Safely = Test does not breach the housing or make the protector emit sparks or flame.
IMPORTANT NOTE
note 3:  All Stedi-Power Surge Arresters/Suppressors were designed and have been tested to meet the "survive undamaged" requirement for all conditions.

Surge Suppressors

    A Surge Suppressor is intended for installation within the facility, in close proximity to the customer and his equipment.  As such, it requires protection supplied by the customer (in the form of fuses or circuit breakers) and must meet stringent standards related to its failure mode.  Specifically, it may not "explode" or emit flame or sparks during failure.  For this reason, a device rated as a surge arrester is not necessarily rated for installation within a facility because it has not been tested to "fail safely".

National Electrical Code Changes: Surge Arresters and Surge Suppressors 

    The 2002 National Electrical Code (NEC) published by the National Fire Protection Association, was changed to define the specific locations both devices are allowed to be installed.  The NEC is in use in all 50 states in the United States. Countries that have formally adopted the NEC include Mexico, Costa Rica, Venezuela and Columbia. Many countries in South America use the Code, and it has been tr anslated into Spanish, Korean and Japanese.  While it has similarities with international requirements (IEC 60364) it is a much more comprehensive set of standards.  
See:  NEC and IEC 60364: Comparison and Contrast

    These changes have produced a condition that can create confusion with installers and specifiers and has resulted in "illegal" installations.  This is not only unsafe, it can create a substantial liability for the property owner, as well as the installer.  Most often, distributors protect themselves with such phrasing as "must be installed in accordance with...."

    For the protection of our customers, distributors, and installers, Stedi-Power pursued a program of rating all of our products under both standards (Surge Arrester and Surge Suppressor).  These Dual-Listed Devices may be installed in either location.

The most important changes relate to the following:

1.  Only overvoltage suppression equipment rated as a  “surge arrester” can be installed before the service disconnect [Art. 280 and 230.82(3)].

2.  Only overvoltage suppression equipment rated as  “transient voltage surge suppressors" may be installed after the service disconnect (Art. 285).

    The National Electrical Code (US National Electrical Code 2002 Art 285.6)  now requires surge suppressors to be marked with their short-circuit  current rating (also known as "fault current rating").  Another change (NEC Art 285.3) requires that the MCOV (Maximum Continuous Operating Voltage) of surge suppressors be considered during installation and does not allow equipment with a sufficient rating.

    Previously, these requirements were limited to Surge Arresters, and in fact, Transient Voltage Surge Suppressors were not even tested to determine Maximum Continuous Operating Voltage.

    Since the harshest environment is at the service entrance, surge arresters must withstand the ten-thousand amp test pulse without incurring damage. 

    Surge suppressors are exposed to much smaller surges but are used in close proximity to people inside the building. Experience shows that significant exposure to severe AC overloads can result from power cross brought about by accidental contact between high-voltage and 120 volt circuits, from utility problems, and when electricians work on live wiring, so all surge suppressor products must survive or fail safely under severe overload.

    Finally, devices rated solely as surge arresters may not be installed downstream from the service entrance because they haven't been tested for interrupt rating or other sustained AC overvoltage events.

Conclusion: The best solution is dual-rated power protection equipment

    Obviously, “dual rated” devices are required to meet more stringent requirements than single-rated devices. You can be assured that all devices designed and tested to both standards offer the maximum in durability and safety. This is why Stedi-Power only offers dual rated products.

Stedi-Power, Inc
5044 B U Bowman Drive #102
Buford, Georgia 30518
PHONE: (678) 546-6780

Last Updated: 07 Jun 2004
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