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)