What is “capacitance effect” in sign systems? Why is a 20 feet limitation placed on GTO in metallic conduit in a neon sign system?
The 1999 National Electrical CodeÒ limits to 20 feet the maximum run of GTO cable in metallic conduit from a neon transformer’s high voltage outputs to the first electrodes in a neon sign system. A good reason exists for this limitation. GTO in conduit adds capacitance between the high voltage output of the neon transformer and ground and the longer the conduit, the more the capacitance. This capacitance acts with the output impedance of the neon transformer in such a way as to increase the peak voltage and currents within the transformer’s high voltage secondary coils. Increases in peak voltage and currents in the transformer reduce the life of both the transformer and sign system’s other high voltage components. As the length of GTO in metallic conduit starts to approach 20 feet, the increased capacitance in the GTO/conduit begins to approach something called “resonance” with the output impedance of the some transformers. This resonance effect makes the peak voltage and currents incr
