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Define Surge Protection

UL 1449 3rd EDITION … WHY?

Catastrophic “End of Life” failures such as spraying of ionized gas, creation of arc flashes, and complete electrical system shutdown were the primary reasons for additional safety testing of TVSS devices.


Undersizing the fuse along with using fusible links and thermal cutouts all were available options to help surge protection devices “Fail Safe.” Most manufacturers used multiple technologies to assure controlled system failures. All three options limit the ability of the suppression element to deliver the full surge current rating, thus imposing serious performance limitations on the redesigned products.

THOR SYSTEMS chose a ground-up product redesign utilizing a new Thermally Protected Metal Oxide Varistor (TpMOV) — a “Fail Safe Device” with integrated thermal and dielectric protection. The TpMOV is able to deliver its full surge current while still meeting all of the requirements of the UL 1449 3rd Edition standard.


Many surge protection specifications still reflect the now obsolete UL 1449 2nd Edition. The UL 1449 3rd Edition became mandatory September 29, 2009. The 3rd Edition had a dramatic effect on product design, construction, and performance of Surge Protective Devices (SPDs) — requiring all products to meet the new more rigorous requirements of UL 1449 3rd Edition. In evaluating and specifying surge protection it is important to consider that the product performance, specifications, and construction all fall within the applicable safety standards.

In the past, writing a specification was a “cut and paste” exercise. Taking an existing document and updating it to the new standards or using a “Guide Specification” which can lead to a proprietary specification or by using a “Master Spec” which allows a myriad of suppression technologies to be provided – all of which may or may not provide the protection needed for the application.

What makes THOR SYSTEMS’ “General UL 1449 3rd Edition Bid Spec: Surge Protective Devices” different from the “Guide Specs” and the canned “Master Specs” published by others? There are five major differences:

  1. The specification, based on UL 1449 3rd Edition Standards was created in MS Word using CSI format with Selectable Form Fields providing the means to configure the specification to the clients’ facility requirements.
  2. The specification is open and non-proprietary.
  3. The specification provides options, not mandates, for new technologies.
  4. The specification lists three approved manufacturers with defined product series for Service Entrance/Main Distribution and for Distribution/Sub-distribution/Branch Circuit Panels, who manufacture quality products providing good value to the customer.
  5. There are application-specific surge current rating charts which are relevant to various applications. There is also an Engineering Notes section addressing UL 1449 3rd Edition Standard changes, terminology, application notes, connection methods, and sizing guides for SPD applications.

In addition to providing the reference documents, we would be pleased to review and comment on any existing SPD specification, suggesting changes to bring them in compliance with the new UL 1449 3rd Edition standards. Should you have any questions, please feel free to contact us (804.355.1100).


Electrical power quality problems cost the United States industries over $100 billion annually
80% of voltage surges and transients are caused by changes in electrical demand, inside the user's facility