Why should a double insulated electrical appliance not be earthed?

Answer

If we only look at the double-insulated appliance, it would make no difference: according to the definition of a double-insulated appliance, no touchable part can become live in the event of a fault. Grounding possibly touchable metal parts therefore offers no advantage.

If the device had touchable metal parts that we ground, then if the cable is damaged and contact is made between the grounding conductor and a live conductor, these parts would be live. In principle, the earth leakage circuit breaker must then interrupt the supply, but when the appliance is connected to an ungrounded socket (or to a multiple plug or reel with a damaged cable, which is certainly not inconceivable with reels that are used in construction, for example) , that doesn’t happen. Even if the appliance is used together with another appliance with an earth fault in an ungrounded or incorrectly earthed multiple plug or reel, touchable parts can still come under voltage. In practice, these are not common, but certainly not impossible situations.

In summary, there is no extra protection by grounding the device, but in the event of earthing errors (via the cable or another device), dangerous situations can arise if we ground touchable metal parts.

The same happens in the event of an earth fault with a non-double insulated appliance. However, it is estimated here that the risk of a problem with the device is greater than the risk of a ground fault.