How Do GFI transformers Really Work

How Does a GFI Transformer Really Work?

There are major differences between a NEON TRANSFORMER (Electromagnetic) and a NEON POWER SUPPLY (Electronic) and both offer different kinds of protection circuitry. In addition, not all TRANSFORMERS or NEON POWER SUPPLIES have GFI circuits, even though all MUST BE UL 2161 Listed or UL 2161 Recognized.

GFI is intended to reduce or eliminate the possibility of a fire if the secondary of a neon transformer or power supply arcs to a ground due to installation problems. It is not intended to protect installers or people, although this is a nice side result. GFI has nothing to do with Overload, Underload or Broken Tube (Open Circuit).

First, let’s define some terms & applications.

Neon Transformers: Provided they are high enough in voltage to require GFI, most TRANSFORMERS will offer only GFI. They normally do not offer OPEN Circuit, SHORT Circuit or OVERLOAD protection. In addition, most (if not all) Housing Type Transformers do not even offer GFI and thus are UL 2161 Recognized only.
NEON POWER SUPPLIES: Most (but not all, dependent on voltage & manufacturer’s dedication to the sign industry) offer GFI, Short Circuit & OPEN Circuit (broken tube) protection. Some also offer OVERLOAD Protection and other features.
General: Normally, electronic neon power supplies offer more protection features than the neon Transformers and in our opinion, are safer, although all must meet UL 2161 requirements.

Some Recently asked question.

1). Do GFI circuits monitor secondary current to ground?


All GFI units must trip when the secondary current exceeds 15 mA flowing to ground. But this can get somewhat confusing concerning high frequency electronic neon power supplies since the effect of Capacitance Current (thru the air) to a ground plane can come into play and possibly result in false tripping.

2). Is the GFI mechanism really measuring current or resistance between secondary terminals?


The GFI circuit is actually measuring the difference in output current between the output leads. The difference in current between the two leads is the FAULT current, which flows to ground. If this exceeds 15 mA, the unit must trip.

3). If you have too much neon for the transformer’s rating, will this trigger a GFI?


Too much neon has nothing to do with GFI; this is an overload situation. Most transformers that we know do not offer overload protection and even many neon power supplies don’t offer this feature. If the overload finally gets big enough, transformers will “flicker” and other neon power supplies will eventually trip as the overload finally appears as an open circuit, but by that time, overheating has occurred.
As a side note, as you add footage, the mA (brightness) through the lamp decreases as the output current from a neon transformer (& most electronic neon power supplies) decreases.
Sometimes excessive Capacitance Current of the load to ground causes the brightness at the center of the sign to dim somewhat if GTO leads or glass is too close to the metal ground plane.

4). If you have too little neon for the transformer’s rating, will this trigger a GFI?


Too little glass has nothing to do with GFI. It is similar to item 3 above. Too little footage on a neon Transformer will result is excessive mA output, damaging the transformer (too hot) and possibly the electrodes. The same may apply to most electronic neon power supplies
If a unit provides Short Circuit Protection, too little glass can look to the power supply like a Short Circuit and trip (not a GFI trip). This is the reason some units have a minimum footage requirement of maybe 6 to 8 feet or so.
Ventex units do not have Short Circuit shutdown protection as they are designed so a short does no damage to the unit, and if either output lead goes to ground, the GFI will trip. This is the reason most of our units can drive footages down to about one foot without tripping.

5). If I connect primary current to a GFI transformer, but have no secondary wire attached, will the GFI trigger?


An Electronic Neon Power Supply may trip due to Open Circuit Protection, but this is not a GFI trip. A transformer will not trip at all unless a true ground fault occurs.

6). If a GFI transformer’s case is “isolated” from ground, will a GFI not be able to trigger even though the neutral wire is grounded normally where the electric company’s connection meets the building?


It is a UL 2161 requirement that the GFI circuit shall trip (or not be able to turn on), even if ground is not connected directly to the unit. The neutral line is used to determine if a ground fault exists.
However, it should be noted that units which trip due to excessive capacitance to nearby ground objects such as the metal in a channel letter enclosures, might not trip if these channel letters are not properly grounded. UL 48 however, requires these to be grounded to meet code.

How do GFI transformers work.

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