IDCI (Immersion Detection Circuit Interrupter) — A component device that interrupts the supply circuit to an immersed appliance. When a conductive liquid enters the appliance and contacts both a live part and an internal sensor, the device trips when current flow between the live part and the sensor exceeds the trip current value. The trip current may be any value below 6 mA sufficient to detect immersion of the connected appliance. The function of an IDCI is not dependent on the presence of a grounded object. Listed products that contain IDCIs or ALCIs have been investigated for installation in applications in accordance with Section 422.41 of the NEC.
Being double-insulated has nothing to do with it.
They're installed because not every bathroom has GFCI receptacles. And not all GFCI's that are installed actually work as intended.
They might be insulated against incidental contact, but they're not insulated against submersion.
And while there might be one or two celebrated cases of people less than an electrical-engineering education using them as a submersion heater to warm up their bathwater, there's an ever-present risk of one accidentally falling into the bathtub, especially if two or more people share a bathroom.
So it saves lives half the time -- especially in the older houses that are least likely to have a GFCI installed. That's better than nothing.In which a GFCI would do nothing if the person was immersed in an ungrounded bath tub. ...
So it saves lives half the time -- especially in the older houses that are least likely to have a GFCI installed. That's better than nothing.
And I suspect most "ungrounded" tubs have enough capacitance to trip a GFCI.