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Therm-O-Matic® Class P
Non-Class P Resetting

page number 2 from 'UNIVERSAL BALLASTS for fluorescent lamps Dec. 1, 1977'
archived by Gerard Arthus at 2010-01-31


Efforts to seek a means of protecting fluorescent ballasts from excessive temperatures have culminated in a revision of the National Electrical Code. Effective January 1, 1968, and fully applicable July 1, 1969, Section 410-71 (e) states that “fluorescent fixtures for indoor installations shall incorporate ballast protection”. The only exception to this ruling is a fixture employing a simple reactance type ballast.

Underwriters' Laboratories, Inc. promulgated standards in compliance with the aforementioned code. Accordingly, Underwriters' Laboratories, Inc. established a new ballast classification entitled, “Class P”. To meet Underwriters' Laboratories, Inc. Class P requirements, a ballast must employ internal thermal protection limiting its operating temperature so that the case temperature, except for certain specified short-term excursions, does not exceed 110°C in the event of a short-circuit in the windings or the power capacitor.

The Universal Therm-O-Matic Class P Ballast is equipped with an automatic resetting thermal protector built-in adjacent to the transformer coils. The resetting thermal protector functions as a thermostat which will open and temporarily deactivate the ballast when it exceeds the permissible temperature. It will reset when the ballast cools to a safe operating temperature. The ballast will continue to recycle until the cause of overheating is eliminated. If the ballast is defective, it must be replaced. If the cause is external, the Therm-O-Matic Class P ballast will resume normal operation after abnormal conditions are eliminated.

Although Class P ballasts are protected against excessive operating temperature, they are still subject to the normal parameters for ballast life. In proper application and for normal ballast life, the maximum coil temperature of the ballast should not exceed 105°C. A temperature increase of 10°C results in a 50% reduction of ballast life. This elevated temperature could occur without tripping the thermal protector.

According to Underwriters' Laboratories, Inc. requirements, the ballast case temperature should not exceed 90°C and the coil temperature should not exceed 105°C. However, the correlation between coil temperature and ballast case temperature will vary with ballast type and design. It is desirable to minimize the differential between case and coil temperatures — the lower the coil temperature, the longer the ballast life.

Class P requirements are not only the concern of the ballast manufacturer and the fixture manufacturer. It is incumbent upon the architect, engineer, designer, contractor and distributor to give careful consideration in the planning of a lighting installation regarding the ultimate installation and operating conditions to avoid misapplication which would cause ballast cycling.

Universal's Underwriters' Laboratories, Inc. listed Class P ballasts should be specified as Therm-O-Matic Class P ballasts and are identified by the suffix “-TC-P” after the regular catalog number. Example: Cat. No. 446-LR-TC-P. Those ballasts presently available as Class P are indicated under the column entitled, “Class P Suffix”, on the data pages of this catalog.

For Class P protection for ballasts not indicated in this catalog, please contact our Marketing Department.


Underwriters' Laboratories, Inc. requires that when replacements are necessary, Class P ballasts must be replaced by Class P ballasts. Non-Class P ballasts should be replaced by non-Class P ballasts even though the ballasts to be replaced may already contain thermal protection. This will preclude the possibility of nuisance trip-outs in an installation which was not originally designed to meet Class P specifications. Wherever possible, we recommend the use of Universal Low Heat rise (“-LH”), Very Low Heat rise (“-VLH”) or Super Low Heat rise (“-SLH”) ballasts for replacement purposes.


Universal Safe-T-Fuse Ballasts will continue to be manufactured for replacement purposes and for those new applications where Class P ballasts are not required but built-in non-resetting thermal protection is desired.

The Universal Safe-T-Fuse Ballast contains two internal non-resetting thermal protectors — one adjacent to the transformer coils, the other incorporated in the capacitor. When excessive temperature within the ballast case reaches the critical point at which failure will occur, one or both of these protectors will open and permanently deactivate the ballast, thereby providing protection against end-of-life hazards. Momentary surges of voltage and current will have no effect upon the operation of Universal Safe-T-Fuse Ballasts. The protective devices open only when ballast failure is already in progress and inevitable. Once the ballast is deactivated, it must be replaced.

Safe-T-Fuse protection is available for all CBM (ETL) ballasts and high power factor Trigger Start ballasts. To order, suffix the catalog number with “-STF”. Example: Cat. No. 446-LR-STF.


Universal non-Class P ballasts with built-in automatic resetting protection will continue to be manufactured for those applications where Underwriters' Laboratories, Inc. listed Class P ballasts are not available but automatic resetting protection is required. To order, suffix the catalog number with “-TC”. Example: Cat. No. 205-TC.

Contact our Sales Department for availability, price and ordering information regarding Safe-T-Fuse and non-Class P automatic resetting protected ballasts.


Non-protected ballasts will continue to be available for replacement purposes and for use in fluorescent fixtures where internal thermal protection is not required.

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Light Information

Light Information

Model Reference:-TC-P , -STF , -TC

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