As far as I know, the system for detecting a contactless voltage is magnetic. To be able to read a magnetic field, the magnetic field must be present. Without an alternating component or a circulating current, a magnetic field is not created.
…exact, in fact I need to detect power failure (troubleshooting) in a 50V direct current circuit, normally traversed by a current of about 0.2A. The circuit is about 100 m, the pair of wires is sectional 1,5 mm2, placed inside a corrugated. The absence of contact is useful because I have no possibility to dissect the installation. If that's not possible, I can dissect i 50 Vcc and inject a frequency signal to search for it with a paired receiver, do you think it is possible?
With an injected signal it is simple to detect the interruption. At the weekend I try to design something simple. First I have to do the miracle of adapting an exponential output sensor with a control unit that accepts linear variation. A bit’ how to plug a round hole with a square plug. It will be tough but I won't give up.
…exponential and linear variation output! you might consider using an op amp in a logarithmic configuration. for my problem, I am thinking of studying a tone generator and relative probe.
thanks and good job!