Abstract: Maintenance information is collected by a sensor on an electric power cable installed in a manhole. The information is transmitted to a radio transmitter which is positioned in a vicinity of a bottom plane of an inner lid in an opening of the manhole. The information is transmitted by the radio transmitter, and received by a receiving antenna mounted on a bottom plane of the inner lid. On a top plane of the inner lid, an electrode, a connecting lead or a transmitting antenna is provided to be connected to the receiving antenna by an impedance matching circuit. The information is supplied to an outer lid in the opening of the manhole by the opening of the manhole by the electrode using a spatial coupling capacitance relative to the outer lid, or the connecting lead directly connected to the outer lid. Thus, the information is transmitted as a radio signal by the outer lid functioning as an antenna. Otherwise, the information is radiated below the outer lid by the transmitting antenna.

Abstract: A detector for detecting an extraordinary approach of objects having earth potential toward a power transmission line comprises a charging current detection circuit for measuring variations in a charging flowing from the power transmission line into a stray capacitance formed between a detector casing and the earth and a transmitter for sending the value measured by the charging current detection circuit to a receiver installed outside the detector casing by wireless means or a wire. The detector is always able to monitor the extraordinary approach of objects such as trees toward the power transmission line, thereby preventing in advance the accidental grounding of the line.