I'm a bit surprised why some people in the US were alarmed about using autotransformers as general appliance 240v-120v step down transformers.

In Asia where we use 240v. All 240v-120v transformer uses autotransformer design to produce less winding and cheaper. In the following you can obviously measure conduction between one of the leads of the primary and secondary (whereas in a real Isolation Transformer which I also have. There is no conduction between any primary and secondary leads).

enter image description here

My question is. You use Equipment Grounding Conductor (EGC) on appliances so the breaker can trip when hot wire touches the metal enclosure.

How about if an appliance has one hot wire of the secondary of autotransformer touching its metal chassis? This won't trip the main breaker because it's located on the primary panel and there is the winding inductive blocking the overcurrent, right?

So how do you properly put EGC on secondary side of autotransformer to make sure the breaker would trip when secondary hot wire touches metal chassis? Maybe a separate breaker in the unit itself? For a 500va autotransformer like the above. What must be the unit breaker or fuse so it would trip on secondary hot wires touches metal chassis?

Some of my circuits were powered by autotransformer hence I want to understand the EGC behavior of autotransformers.

  • \$\begingroup\$ Reasons for not using autotransformers include legacy premises wiring, legacy wiring devices, possible failure modes, possible wiring errors, possible misuse of devices and probably other factors. An isolation transformer can mitigate the risks of many problems elsewhere. It would require too many photos and diagrams to explain all of the risks. A lot of matters of opinion would be involved. I am voting to close the question as too broad and opinion based. \$\endgroup\$
    – user80875
    Commented Jan 23, 2019 at 14:32
  • \$\begingroup\$ I'm aware of the purpose of isolation transformers which is isolated the secondary from ground. But in autotransformers, is it not the same effect as the primary. So if you have EGC installed in any power system. It can handle the secondary side too? In Asia, almost 98% of 240v-120v step down transformers sold are autotransformers, because it's cheaper to make with less windings. So we need to know if the risk is similar to primary and grounding. \$\endgroup\$
    – Jtl
    Commented Jan 23, 2019 at 14:36
  • \$\begingroup\$ I am aware of the benefits of autotransformers. I have participated in designing one industrial product that included an autotransformer. I might use one again for a temporary experimental setup. However, this a forum for relatively narrow questions and answers, not discussions. \$\endgroup\$
    – user80875
    Commented Jan 23, 2019 at 14:42
  • \$\begingroup\$ What forums do you suggest to have discussions about it instead of narrow questions and answers? But whatever, without any EGC. The risk is the same for both primary and secondary. Isn't it. This is the question then whether the risk is same and the secondary won't make it worse. \$\endgroup\$
    – Jtl
    Commented Jan 23, 2019 at 14:45
  • \$\begingroup\$ Connect ECG to transformer case. Put ground fault circuit interrupter in mains power feed to transformer. \$\endgroup\$
    – user80875
    Commented Jan 23, 2019 at 14:50


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