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Signal Isolation

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Hey guys,
sorry if this has been asked before but could someone explain to me what signal isolation is, and why it's needed?

I've been google-ing it but i'm not really getting a straight answer?

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I work with a 'watt meter-data logger' that measures power line wattage. It measures current in all three legs of the power line and the voltage on all there legs. Depending on how the meter is connected to the 440 power line the meter may be at 0 volts or at 440 volts. If a service person pluged his meter into the data logger he could be killed. So the data from the data logger is 'isolated'. The signal from the data logger is 0/5 volts and may be on top of the 440 power line voltage. After the isolation it is 0/5 volts at the level of the service person.
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