Control Transformer


Thread Starter


Although this may seem trivial, I am bothered by the dilemma. When grounding x2 of the secondary side of a 120 VAC control transformer, should the ground be directly bonded to the backpanel? By directly bonded, I am indicating that the ground wire is to have ring terminals crimped on both ends and the ground end tex screwed to the
backpanel as close to the transformer as possible.
The problem arose when certain individuals directed X2 to have a green ground wire ran to a grounding terminal block approximately 14 inches away, then jumpered to the neutral terminal block. I became uneasy about this practice and directed two wires to come off X2, a green ground wire and a white neutral wire, with the ground wire being mounted as describe above. Am I overreacting to this situation?

Thanks for the input
No, I don't think you are over-reacting. The method you describe is normal practice for most control system panel builders. The grounding
wire should be green and the neutral wire white, as you have indicated.

Richard Neff

Unless your panel is the incoming power distribution panel it is definately against NEC rules to connect the ground to Neutral. There is always only one place that neutral connects to ground, and that is back at the point of origen, never in the control panels.
If you connect ground to neutral in a control panel you create a hazard and you will possibly create problems for any GFIs in the area. The neutral is also called the "grounded conductor". This neutral wire is supposed to carry current back to the distribution panel and enter ground only at that one point. If you connect ground
to neutral at your panel then you will energize your ground in the panel. GFIs in the area will detect this current in the ground and possibly trip out also.
What I have always done is to have 2 wires coming out of the X2 terminal. One white (neutral of course) going to my neutral terminal blocks. The other green (ground) coming off and then I bolt it down to the nearest XFMR mounting bolt. I have never had any problems this way. I dont particularly like the idea of running one wire to the neutral bus and then using a jumper. Its all electrically the same, but it doesn't seem as secure to do it that way.
Grounding one end of a transformer is necessary to arrest leakage from the primary side voltages due to magnetic and capacitive couplings.
The correct procedure is to connect 2 wires from X2 (directly on the transformer), one green and other white. The white wire conncted to your low potential common terminals (all x2) and the green to be earthed near the transformer.
As some body think, this is not a neutral. The term neutral applies only in three phase circuits.
Do not ground X2 if the transformer is a AUTO transformer.

Bob Peterson

Perhaps where you come from neutral only applies to 3 phase circuits, but here in the USA (and a few other places) 120VAC circuits are normally connected to ground on one side. This side is definately the neutral side. I concur that auto transformers should not have one side grounded but they are not suitable for generating control volatges anyway as there is no isolation between the high and low voltage sides of the transformer.

As for your contention that "Grounding one end of a transformer is necessary to arrest leakage from the primary side voltages due to magnetic and capacitive couplings", I am not all that sure what it is you are trying to say. Grounding one of the secondary conductors is ONLY done to create a neutral. Some transformers are shielded to enhance RFI transmission, and the metal shield is then grounded, but this is the case, not a conductor.

Bob Peterson
Responding to Bob Peterson's Fri, May 17, 10:56 am, comments regarding USA practice covering "neutral" nomenclature:

By definition the "neutral conductor" is the one that usually is grounded and has equipotential (excluding phase displacement) measurements from every other conductor. Therefore your statement
regarding a single-phase two-wire system is technically correct... there is no "neutral". However, the term "neutral" is used by
convention, simply for the lack of a better term. When used in single-phase 3-wire circuits, the term "neutral conductor" is correct.

In 3-phase, 3-wire circuits, there is a "neutral point" that meets the definition above for both the conventional wye and delta derived systems. In the former, the neutral point is a "physical" one. In the latter, there is no "physical" neutral point.

For 4-wire, wye-connected circuits, the "neutral point" is usually "grounded", hence its distribution conductor becomes the "neutral
conductor". But, for 4-wire delta-connected circuits like the "grounded-corner" and "mid-point grounded" (both unusual in European
practice) the definition is not met. However, by USA convention the "grounded conductor" is labeled the "neutral conductor"!

Phil Corso, PE
(Boca Raton, FL)

Jeffrey Eggenberger

Grounding of the secondary of a control transformer is done so that a fault to ground, like a short to the equipment case will trip the fuses on the circuit. Many industrial sites here in the United States do not ground their control transformers so that "ground faults" will not stop production. We use "ground fault" indicator lights to alert the electrician on site that there has been a short to ground in the system. Using a white, or neutral is common as residential wiring uses white as a neutral. By definition the neutral wire is suppose to carry the unbalanced portion of the load, but since a control transformer is only single phase, the definition is meaningless. In this case, the white wire only shows that it is a "grounded"
conductor. A green wire is a "grounding" conductor, and not suppose to be used to carry current other than fault current.

Jeff Eggenberger
Well that's certainly a smart setup. Perhaps I can do the same on 132Kv that way I will not stop production. Yes and also get some really big wattage lights for fault indication.
Responding to Brad's (Sun, Jun 9, 11:19 am) comments on Jeffs' reply:

I'm not sure if your reply was meant in jest, or you were serious. If the latter, any power circuit, yes, even the 132 kV one you mentioned,
can be made "production safe!" If the former... fugettaboudit!

Phil Corso, PE
(Boca Raton, FL)