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I am not an electrical engineer and do not have a deep understanding of all the different factors in transformer design. My question is about a plug-in 2:1 step up/down transformer used for 120/240 volt conversion in a home. If such a device is plugged into a wall outlet what is the correct approach to wiring its ground pins?

Say the power supply into such a transformer includes a correctly wired ground. It’s connected to the transformer’s housing, and neutral/ground bonding is done correctly at the appropriate place within the building it is in.

On the isolated delivery side of the transformer, how should ground be dealt with? A) None. It is not possible to provide a safe and useful grounding function, so none should be provided. B) Output ground pin should be bonded to the transformer’s output neutral, but not to the case or the building ground. (isolated neutral/ground bond) C) Output ground pin should be bonded to the transformer casing and thereby to the building ground but not to the transformer’s output neutral. (Transformer output fully isolated). D) Output ground pin should be bonded to both the case AND to the neutral side of the transformer output. (Transformer output referenced to actual ground).

I can see benefits and problems with each of these, and do not understand enough of the theory and practice of either transformers or grounding safety to know what's right.

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  • Can you give us the model number and diagram of said step transformer? Also, we will need to know what country/power system you are in: the answer is totally different EU vs NA vs Philippines.
    – Harper - Reinstate Monica
    Mar 5, 2021 at 18:33
  • @Harper-ReinstateMonica this is a theoretical question, attempting to generalize my understanding of what my expectations should be from a device like the one in another recent question. I'd enjoy hearing country-specific answers. If you'd rather answer a specific case, let's say a Square D 1H721 used in the USA with appropriate plugs and sockets attached, or alternately a Grainger 30C520, which is a complete unit but then my question is not how it has been built, but how it should be built -- let's say for step-up in the USA, and/or step-down use in the UK. I'm flexible.
    – jay613
    Mar 5, 2021 at 19:56
  • The background for this question was 1) This question diy.stackexchange.com/questions/217955/…. and 2) various things I have read about why some laptop PSUs have grounded power cables, e.g. electronics.stackexchange.com/questions/78079/…
    – jay613
    Mar 5, 2021 at 19:58
  • The consumer devices are NOT 2:1 stepdown transformers. The Square D is an isolating transformer, which works differently than stepdown transformers. The Grainger is a cheapie step-up/down auto-transformer. Those are tricky.
    – Harper - Reinstate Monica
    Mar 6, 2021 at 0:58
  • I didn't know any of these were autotransformers. Tricky how? The neutral is a straight passthrough, if you run the ground as a passthrough it seems more like any standard device, just with the live at a different voltage. No? Isolating transformer seems more tricky to me ... but they all do, hence my question!
    – jay613
    Mar 6, 2021 at 1:42

2 Answers 2

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Ground is still, always, ground.

On the other hand, nothing should be connecting anything to safety ground. That only happens during fault conditions. Safety ground is not for normal operating currents! There is no reason for a voltage converter to interact with it electrically (unless it's a true isolating transformer and it's establishing its neutral reference).

The ultimate appliance still needs a safety ground that is pegged at the potential of actual earth, i.e. so there isn't a potential between it and other grounded objects the user might encounter. So the appliance should get it; that is, the converter should pass through the ground it receives from the structure's Equipment Grounding system and Grounding Electrode system.

If the transformer does not isolate (or fails to isolate), then it's especially important not to float a ground. Even a neutral-ground fault is dangerous in the North American 240V -> into 120V scenario.

There's a corner case which is big trouble

If you're taking European 240V and making 120V.... or taking North American 120V and making 240V... all those things are fine. There is no need for isolation, and you can simply "pass through" neutral. That means a cheaper, non-isolating type of transformer called an autotransformer can be used -- or the electronic converter does not need to provide isolation.

However, there's a scenario that creates a hazardous condition. North American and Philippine 240V are actually two 120V legs, each 120V away from neutral/ground. This means neither wire is neutral. So if you try to derive 120V from that, a non-isolating device is creating a "neutral" that is hot.

This is an unworkable and unsafe situation. In theory, the synthesized "hot" wire will be in the neighborhood of neutral, but that isn't necessarily so. If it creates it opposite-phased, it could be 240V from true ground! Even if it is correctly phased, this "fake neutral" is not safe at all.

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  • Hey I know it has been a long tiime but I think the second half of this answer is thorough and useful, but the first half, even though it is correct, is a distraction. Why would anyone use a transformer to synthesize 120V in a primarily 120V country? It's not the problem that 99.999% of readers will be solving here. If you wouldn't mind removing it, or making it a footnote rather than the leading text, I'd like to Accept this answer.
    – jay613
    Jan 31 at 16:35
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    @jay613 We see people try to add 120V to 240V circuits all the time. Typically a well house, but A/C units too. The warning needs to stay but I'll put it at the bottom.
    – Harper - Reinstate Monica
    Jan 31 at 18:57
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On the isolated delivery side of the transformer, how should ground be dealt with?

As the output is isolated no ground connection is needed.

Connecting output ground to input ground would not cause any problems, but neither would it solve any.

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