Live Mains Cable Detector

[Radio Tech]

Hi Folks

I thought I would like to share this very simple but effective circuit of a live mains cable detector. Around six components, a small box to mount it in and a bit of patience. the circuit I found searching the net and I must admit that all credit goes to the author of the circuit. Its in PDF format though.

Best wishes

Ken

[Radio Wrangler]

There should also be a catching diode to the supply, else the input can get zapped by static charge.

David

[Oldcodger]

For years I've always had a telecoms pair tone trace kit around, and trusted the amplifier to indicate ( by buzzing) near a live mains cable. So any high gain amp circuit would suffice ( something like a LM386 ) on high gain.

[G4XWDJim]

It would be very useful if it was able to detect metal pipes to prevent you drilling a hole in them. You'd need it to oscillate for that I suppose.

Jim

[mole42uk]

Those are available - hand-held metal detectors. They don't detect plastic pipes though.... :-(

[Oldcodger]

Most decent pipe blokes run pipes gas/water surface. So then the problem is mains cables. It ONLY takes seconds to take a mains socket off and ascertain the direction a cable is run. OR a light switch run. A cable/pipe detector is ONLY an aid, as is a Sat nav in determining a safe route.

[lesmw0sec]

Originally Posted by Oldcodger

Most decent pipe blokes run pipes gas/water surface.

I recently installed a walk-in shower. I knew there were some pipes passing through the area where I needed to cut the concrete for the floor drain, so I used my old army mine detector to check for metalwork. To my surprise I could not locate any.
Later excavations revealed that the pipes were in fact laid very deep - bottom of the foundation level. I was most relieved, since there was no likelihood of reaching them with my work!

Les.

[Billy T]

In my opinion the description of this detector is incorrect. Voltage meters use a sensor plate and ground plane, not a coil, so this device is actually an electro-magnetic field detector.

That aside, a single-axis coil will have a deep null when the windings are parallel with the source conductor and if the source is deep, the field is weak, or the detector unit is incorrectly aligned with the source, it may fail to indicate the presence of live conductors. If the source is not loaded then the meter may read little or nothing according to the ambient EM environment.

Adding a second coil at right-angles to the first (with appropriate input circuit modifications) would help make the device omni-directional and eliminate false nulls. The ideal would be a triaxial coil system but two coils are still better than one.

The coil(s) need not be spaced, it is the magnetic field that should be searched as voltage fields are easily screened by wood or even cardboard, giving potentially misleading results. A single layer of thin cardboard can reduce an electrical field strength of 4kV per metre to less than 10 volts per metre. You won't read voltage fields through walls, floors, hand my AC Voltage field meter drops from several kV/m to zero if I put my hand over the sensor location.

There is no missing the signature of a magnetic field and reliable positive identification is assured.

My opinion is based on 50 years in the electrical-electronic industries and over 20 years as a specialist professional electrical, electronic, and electromagnetic diagnostician, serving industry, military, governnment, schools, universities, and especially hospitals and other medical facilities, working internationally as and when required.

Cheers

Billy