the present invention includes an insert - to - connect device body , which receives therein a grounding terminal , a transmission terminal arranged at the left side of the grounding terminal , and a conduction terminal arranged at the right side of the grounding terminal . the grounding terminal is electrically connected with a grounding bulb . the transmission terminal is electrically connected with a transmission bulb . the conduction terminal is electrically connected with a conduction bulb . the conduction terminal is electrically connected with a conduction touch - plate . with such an arrangement , a construction operator is allowed , in installing the present invention , to follow instructions for connection with polarity correctness and to identify if the polarities are set correct according to lighting of the grounding bulb , the transmission bulb and the conduction bulb and touching of the conduction touch - plate .