ECN Forum Forums General Topics of the Trade General Discussion Area wrong way Transformer

I'm thinking that since it does not have a center tap, and grounding a line conductor will not keep the voltage to ground below 150 volts per 250.20 (B) (1), the secondary MAY be grounded according to 250.21(4) but is not required to be.
I'd like opinions on the advantages and disadvantages of grounding it or not?

I see absolutely no good reason to bond one leg. It only adds a level of danger if you get between line and ground.

Exactly Greg,
Isolating transformers have worked without the secondary side of them grounded for years.
As long as both of the secondary wires are fused, I can't really see any issues with a set-up like this.
I would ground the transformer body though, just to be on the safe side.

I understand that grounding one leg will raise the voltage to ground to 230 volts, similar to the way a corner grounded 480 delta raises the voltage to ground to 480.

However, I thought that grounding a secondary conductor helps keep the voltage stable. (Scott 35?)

I have no opinion either way. Just looking to open a discussion.

Trumpy is right, you should always ground any non current carrying metal in just about everything electrical. That is why we have EGCs. I understand why you might need to ground one leg of a service to stablize voltages but in a transformer secondary feeding a single piece of equipment or a small number of local loads I am not sure where any instability would come from.

If you have issues with EMI or EMC, grounding one side or the center-tap of the secondary, will provide a place to shunt the noise to. Many EMI filters shunt noise to the chassis. If there is no chassis path to "earth" they can use the "grounded" conductor as a back up path.

Larry C

Bigger question. Why not to pursue this path in theory?

Grounding line on secondary along with grounding line on system will lead to high potentials thru out and or short circuits on EG's

Not a good practice at all.

When wired "backwards" small transformers (smaller than 3kVA singel phase) will exhibit worse than normal output voltage regulation and higher than normal inrush current. In either case these are not normally deal breakers just things you should be aware of.

The voltage regulation falls off because the "H" windings usually incorporate additional turns to compensate for the inherently poor regulation (well relatively so compared to bigger units) of small transformers. Now that the extra windings are on the output side they offer little help.

The inrush current can be up to 14x due to the location of the H and X windings in relation to the core. This is usually not a problem for transformers with less than 9A rated load on the X windings.

An isolation transformer is a separately derived system and must be grounded per article 250. It makes no difference if there is a grounded conductor on both the transformer primary and secondary, you must follow correct grounding and bonding practices.

Yes JBD, I agree , no deal breaker. But.

Not something to be taken farther than control Xfrmr. Yes?

But wouldn't having an isolating transformer with both of the windings grounded be a waste of time, considering that the ground connection would be common to both windings?.
I was always under the impression that a double wound transformer was made so that there would be no reference to the primary winding by the secondary winding, hence the term isolating transformer.