| 摘要 |
<p>PURPOSE:To enable accurate detection of a possible contact of an electrically conductive object with a buried tube, free from effect of noise current, by monitoring an input impedance of the underground buried tube as viewed from a proper point to detect a drop therein. CONSTITUTION:When an electrically conductive object 3 of an excavating machine gets in contact with a buried tube 2 at a point (a) at a time t1 to cause a leak of a current Ie through the object 3, an input impedance Z of the buried tube 2 lowers as viewed from a point 1. This impedance Z is so free from effect of a noise current that there is little leak caused by other reasons than the contact of the object 3 with the buried tube 2. Therefore, only monitoring of the impedance Z of the buried tube 2 viewed from the point 1 ensures accurate detection of the contact of the object 3 with the buried tube 2 by a change therein. To achieve the monitoring, an AC signal current supply means 4 to the buried tube 2 is provided with detecting means 5 and 6 for a supply current and voltage while the impedance Z of the buried tube 2 is derived from detection signals of the means 5 and 6 to be checked by an input impedance monitoring means 7.</p> |
