Besides short circuit surge currents flowing on safety ground wire there are also other situations where you can see 60-115 V AC between the equipment cases. In the age of switch mode power supplies the safety ground has a new job, ie EMC line filter ground. When the power supply EMC filter is not connected to electrical ground, the the line filter y-capacitor often ends up connected to the output ground putting the output at half the line voltage that is 60V or 115v depending the country you live in. The low capaciance of y-capacitor limits the available current to some low value (fraction of milliampere normally but can be up to few milliamperes), so the system is kind of hi-Z output. If the unit powered by this ps has no other ground, balanced IO requires common mode mode rejection that that can handle 60V or 115v rms voltage. The techniques that can do this are rare, this is far more than normal op-amps or RS-422/485 drivers can handle. Beware “universal” switch mode adapters!
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