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Protection Question No.21 - Secondary grounding protection
Author : EHSAN - From: IRAN
 
#1
Wed, December 3, 2008 - 20:41
UL-2161 establishes the criteria that "secondary fault protection" products must meet. UL-2161 is a new UL Standard that covers both Neon Transformers and Electronic Neon Power Supplies.

The 1996 National Electrical Code contains revisions to Article 600 (Electric Signs and Outline Lighting) that affect the design of some neon transformers. The changes require that some neon transformers must possess "secondary fault protection." These changes were implemented to minimize the possibility of fire as the result of improperly installed luminous tube signage.

Traditionally, neon transformers were constructed with the secondary winding either "End Grounded" or "Mid Point Grounded." This refers to an electrically conductive connection of the secondary coil to "ground" potential. In an "Isolated" design, there is no connection between the secondary coil and ground. The secondary is essentially floating with respect to the primary. The advantage to this design is that the secondary will not arc to ground under any circumstances, even in a broken tube situation.

A transformer assembly and method for powering a load with a secondary fault protected isolated secondary. The fault path is isolated from ground allowing voltage detection of faults and the return terminal is isolated from the midpoint for multiple load connection schemes using the midpoint as a ground connection. A power control system is connected between the primary winding and the input terminal with a ground fault detection circuit connected between the fault path and the ground terminal, where the ground fault detection circuit is operable to detect a fault and activate the power control system to disconnect the source of power from the primary winding in response to detecting the fault. Also disclosed is a high frequency filter adapted to reduce the effects of high frequency transients and a capacitive reactance connected between the input terminal means and the ground terminal. The capacitive reactance is adapted to provide a ground fault path for fault signals. Another improvement teaches the improved performance of an optocoupler using a break over device for improved bias control.
The present invention relates generally to transformers for powering luminous loads and more particularly, this invention pertains to secondary ground fault protection for neon tube transformers.
For luminous tube transformers presently utilized in industry, the output voltage from one output terminal to ground cannot exceed 7500V. To provide a design capable of producing output voltages in excess of 7500V, a “mid-point grounded” secondary design is employed in which two secondary coils are used. These coils produce voltages that are 180° out of phase with each other in order to develop a terminal-to-terminal voltage that is twice that measured from any one terminal to ground.
New industry regulations have been developed that require the addition of secondary ground fault protection to such designs. As noted by UL 2161 subsection 20.4 “An isolated output neon supply shall have a current to ground that is 2 milliamps or less when measured in accordance, with the Isolated Output Determination Test, Section 24A.” (revised Mar. 16, 1999). Subsection 24A.1 then notes: “To determine compliance with 20.4, a neon supply is to have any protective circuitry that prevents the supply from operating without an output load connected to it disabled. The neon supply is to be connected to a source of supply adjusted to rated input with no load connected to the output. While energized, the current from each output lead or terminal to ground is to he measured. The maximum current shall not exceed 2 mA rms.” (added Mar. 16, 1999). The intent is to provide a level of protection and to detect i secondary side fault to ground as a measure to reduce any potential fire hazards that may exist as a result of arcing.
 
Author : Hamid - From: IRAN
 
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