There is blue light on the hotel usb socket.

The situation is that when the plug of electrical equipment is inserted into the socket, the arc is on fire caused by excessive current.

Although there is no electricity when plugged into the socket, some inductive elements are directly connected to the power supply when plugged into the socket. At this time, it is necessary to input a large current to establish a magnetic field.

Because a large current is needed when plugging in the socket, an arc is generated at the moment before the plug and the socket contact, which is what you said. Other appliances that are not inductive loads, or appliances whose switches are not turned on, that is to say, appliances with no or little current when plugged in, will not generate sparks when plugged in.