Electronic – Grounding and why charge leakage occurs

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A basic nugget of info is missing in my end-to-end picture of how grounding works and why it's important. When a voltage is applied in a circuit the electric current starts flowing (or the field establishes itself). Now in an AC home circuit the current flows through the circuit like in DC, but happens to reverse directions 50 or 60 times a second too (Hz).

So why do some appliances have electric current leak onto their metallic surfaces at all in the first place. Should'nt all appliances's internals be designed so that there can never (or rarely) be any leakage of current ?

The whole point of my question is that why do we blame the lack of grounding when an appliance gives an electric shock – isn't the appliance equally to blame for being designed in a way that allows for charge leakage ?

Hence in the event of an electric shock isn't it equally important to investigate the appliance (in this case it's actually a custom assembled desktop computer) to find out why its circuit is leaking charge to its metallic body parts, instead of always expecting the grounding to remove that excess charge to the earth.

Another way to paraphrase this question is – are some appliances (especially assembled computers) likely/expected to leak charge. Hence in the event of rare shocks sometimes shouldn't it be more important to investigate the appliance itself for having the proclivity to receive leaked charge instead of blindly checking the grounding

Best Answer

It is not unusual for the mains supply to be deliberately connected to ground via small high voltage capacitors, to reduce emitted radio interference. These capacitors are rated to withstand high voltages safely, and to "fail safe" (i.e. not form short circuits in the event of an accident or excessive temperature.) They are usually identified as "Class Y" or "Class X2" on their case markings, typically 0.1 uF 275V or 400V.

These will conduct a small AC current to a metal case, and if the metal case is NOT correctly earthed, it is possible to get a mild shock from this current, but it should not be dangerous.

I have also measured about 110V AC on exposed metalwork simply from the capacitance within a (230V) mains transformer (the short circuit current was only 30 microamps but the "tingle" could be felt)

I would however agree that any other source of leakage from AC mains to metalwork should be investigated - the dangerous ones would usually show up with DC resistance measurements, unlike the above.