Prior to the publication of the Standard UL 1778 3rd edition (issued on March 14, 2003) and 4th edition (issued on September 15, 2005), UL has been using the UL 1778 2nd edition (issued on October 25, 1994) to conduct construction review and safety certification of Uninterruptible Power Supply (UPS) equipment and the related products.

However, in the North American standards systems for UPS equipment, there are already differences in the safety requirements between the UL 1778 2nd Edition and the CAN/C22.2 NO107.1-01 of CSA, (Canadian Standard Association). In addition to that, among the North American standards system, the International

Electrotechnical Committee (IEC) standards, and the TUV standards for Europe, there also exist differences in construction review and testing requirements. As a result, UPS equipment manufacturers need to adopt different product constructional designs based on the different requirements of the North American and European standards systems.

To address manufacturers' concern over the needs to conform with the different safety requirements, UL published the UL1778 4th edition on September 15, 2005. This was a step towards international harmonization of standards, which in turn would help manufacturers raise production efficiency and lower production cost.

International Harmonization of Standards
The requirements of UL 1778 4th edition are highly identical with those of IEC standards. In addition, both CSA and UL will conduct review on UPS equipment and the related products based on UL 1778 the 4th edition. That means the differences in the standards requirements between CSA and UL will no longer exist.

Harmonizing the international standards including the American and European standards, UL 1778 4th edition is based on UL 60950-1 1st edition and CAN/CSA-C22.2 No 60950-1-03 (These two editions of standards are mutually recognized in the U.S. and Canada). These two editions of standards are listed as "Reference Document" of UL 1778 the 4th edition. Besides, UL 1778 4th edition adopts the requirements originally defined in UL1778 the 2nd edition and CAN/C22.2 No.107.1-01, and additionally includes the EN50091-1-1:1996, which is the European standards for UPS equipment.