What causes an iPhone charging cable to shock?

if it's not just static electricity Static electricity - Wikipedia

then since the charging is only 5volt dc one should never be able to register it, so if it's not static then I would suspect that the ac charger / cable was leaking 110volt or 230volt AC in which case it could kill the device and harm you, so if that was the case I would avoid ever using cable or ac charger again

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