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A text mentions about the importance of connecting the earth wire to an instrument case/enclosure as follows:

Instruments with ungrounded cases can become lethal devices in the event of transformer insulation failure or accidental connection of one side of the powerline to the case.

I have couple of questions regarding this practice above:

1-) I can understand that how earth would save the situation for accidental conduction of power line to the case. But what is meant by "transformer insulation failure" and can you give an example scenario how can that be lethal issue?

2-) I sometimes encounter enclosures which are not metallic but plastic(sometimes DIN rails inside the plastic enclosure). In those would earth wire still be connected to the case?(It doesn't make sense to me but just curious to be sure)

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  • \$\begingroup\$ Note that if grounding and bonding are done correctly, in a situation where a casing (or anything else metal that isn't intended to carry current, ductwork, metal framing, a fence, structural steel) becomes energised, enough current should flow to immediately trip the upstream current control device, revealing which downstream circuit has a problem and removing the danger. Plastic cases do not need to be grounded, but can have a common attachment point/bus for the ground wires of conduits entering the enclosure. \$\endgroup\$ – K H Jul 29 '18 at 0:28
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  1. Several occasions can lead to this.

-Imagine the power cable passing through a small hole of the metal enclosure to reach the transformer. The sharp edges around the hole are an easy way for the plastic insulation of the wire to start wearing out. Given some time and some rough handling the hot wire can touch the metal enclosure and transform it to a deadly trap for whoever touches it.

-Also, not all devices have a transformer, who adds galvanic isolation. If, for example, the device uses a capacitor dropper, if ANY high voltage component touches the metal enclosure, the current can find a way from the outlet, through your body and to the ground.

But how will a component come in touch with the enclosure? You may drop the device, or maybe shake it, or maybe some internal damage lead to some wire insulation to burn out, or to some component desoldering from its place. For all this cases, it is mandatory to ground the enclosure.

  1. If the enclosure is 100% plastic and is ZERO chance of someone touch any metal part that's inside, then there is really nothing to put the "ground" on.
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A transformer consists of insulated copper wire wound on an iron core. If the wire insulation is damaged so the copper wire contacts the iron core, the core could then be at line voltage. If the transformer core is connected to a metal case (a common construction method), the case would also become "hot".

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  • \$\begingroup\$ This damage can be caused by heat, mechanical damage, or high voltage transients. \$\endgroup\$ – K H Jul 29 '18 at 0:15

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