Electromagnetic field (AC current) detection

Hugh Riddle

Joined Jun 12, 2020
78
You can use a cheap AM radio with a Ferrite rod to detect electric cables that are live.
That's fascinating, Dodgydave, and makes sense. Mains cables often carry a lot of common-mode (i.e. acting as a single conductor) HF voltage and current noise, respectively creating HF electric and magnetic fields (a friend had a problem with it interfering with a magnetic deaf-aid loop system) and the AM radio's ferrite antenna must be picking up that HF magnetic noise field. I imagine you need to tune off any station and wonder from how far away it can detect a mains cable. I don't think it could be used for a consumer cable detector but, amazingly, it relies on the same basic principle as the expensive (last resort) cable detector I described. Anyone else tried it? Not sure I've got an AM radio to try it out on.
 

Hugh Riddle

Joined Jun 12, 2020
78
If you bring up the net magnetic field strength (cancels) by destructive interference you must also add the constructive interference. That is why it is called net by definition. A wiring assessment should includes Phase and wire code taught is sufficient in this area.

It is a flawed hypothetical narrative. Can a simple detector for house wiring be made from a BC547 transistor capable of 10 cm depth. The narrative goes on to complicated cable measurement systems making assertions that AC house wiring is not understood by electrical engineers. That HF measurement is somehow related, that hall effect sensor do not detect because the magnetic field is too weak. That AC magnetic field is not called B field. It is not factual narrative on house wire detection regardless of how careful it is read. The theories presented are confused with other disciplines. The assumption that house wire detection and completely different study must meet a unification is beyond the scope of principals being applied. It is easy to redirect
resentful episodes look at the riots. Keeping our head straight is top priority.

The AM radio does have merit on the appropriate subject. It can also be made accurate in both distance : volume and location : position of the loop stick.
Thanks Sparky 1, but what, in this context, is constructive interference (wave interference is irrelevant from 50/60Hz up to at least say 1 Mhz)?

This thread started with kadriguler asking 'Can we make a simple but precise circuit that locates the hidden current (cable) on the wall?' and pointed out that 'Circuits made with BC547 (3 pcs) transistors and CD4017 integrated circuit cannot detect the current inside the wall' [the youtube demos its very short range].

kadriguler later asked 'For example, a circuit to find current at a depth of 10cm ...'. I see only you quoting 'Can a simple detector for house wiring be made from a BC547 transistor capable of 10 cm depth.' Have I got that wrong?

I responded to kadriguler's 5cm and 10cm questions in the cable detection context he had set and did my best to supply a sound basis for untangling the various misunderstandings about fields surrounding mains cables that I so frequently encounter. I didn't say house wiring wasn't understood by electrical engineers but that they often had a poor grasp of fields and magnetics.

House cabling matches the thread's context perfectly and provides a good basis for explaining fields around cables, which most people can relate to much more comfortably than to physics models (e.g. 'an infinitely long straight parallel pair of conductors each of zero diameter'). The H signifies a magnetic field's 'magnetomotive force'.

It's OK Sparky 1, I'm the ignorant one on my thread in the Microcontrollers forum, get picked up over my misconceptions and have difficulty grasping explanations intended to help me.
'Anyone here using C to compose 'object-based' Microcontroller programmes?'
 
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Hugh Riddle

Joined Jun 12, 2020
78
You're mean and you hurt my feelings.
Oh dear. I'm trying to work out what or how I got so wrong. It's been a long day writing, writing but I remember you seeming to imply I hadn't read the original question and Sparky 1 Liking that. Hope I didn't misread or fail to acknowledge you. 'View Thread' doesn't currently seem to be working properly for me.
 

Wendy

Joined Mar 24, 2008
23,415
For what it is worth, you now have several moderators monitoring this thread. Unfortunately I don't have much to contribute.
 

Hugh Riddle

Joined Jun 12, 2020
78
Can we make a simple but precise circuit that locates the hidden current (cable) on the wall ?

Circuits made with BC547 (3 pcs) transistors and CD4017 integrated circuit cannot detect the current inside the wall.
Hi kadriguler. A lot has gone on in your thread. But do you feel your question has been answered at all?

If you wanted, I can sketch a circuit (incl component types and values) that you could construct and try out, which should detect a mains cable (not in a metal conduit) 3cm inside a wall and with a sensitivity control to go further, maybe 6cm. It would be pretty simple but use a fair bit more than 2 BC547s. I'd be around to ask if construction/testing problems came up.
 

Hugh Riddle

Joined Jun 12, 2020
78
Can we make a simple but precise circuit that locates the hidden current (cable) on the wall ?

Circuits made with BC547 (3 pcs) transistors and CD4017 integrated circuit cannot detect the current inside the wall.
After buying some bits on eBay, I today carried out the simple tests below:

1. DETECTING A CABLE'S ELECTRIC FIELD. With a 4.5 sq cm metal electric field pick-up plate feeding my (1M0 resistance) oscilloscope input, I detected about 15mVrms at 50 Hz at a distance of about 2cm from a portable heater's live cable. As I increased the plate's distance to around 10cm, the signal fell to around 3mVrms at about 12cm. Mediums such as plaster, especially when damp, could lessen this.

2. DETECTING A CABLE'S MAGNETIC FIELD. With the portable heater drawing 8Arms, I could detect only 0.5mVrms at 50Hz even with the ferrite slab laid hard against and at right angles to the cable and after twisting the cable (to maximise the difference between its live and neutral wires' distance from the pick-up coil).

3. DETECTING THE MAGNETIC FIELD OF A SINGLE CURRENT-CARRYING WIRE. With the heater experimentally connected to pass its neutral current through only one wire of a length of mains cable, a 100 turn winding on a 5cm long ferrite slab aligned at right angles to the cable picked up about 1mVrms at 50 Hz fat a distance of 10 cm.

CONCLUSIONS. These tests appear to confirm that the 50Hz electric field of a hidden live mains cable can easily be detected, even when carrying no current, and, with a good sensitivity adjuster, reasonably accurately located, provided the field isn't too distorted by nearby metal (big nails, conduits, rebar etc).

50/60Hz magnetic fields didn't allow detection of even a current-carrying mains cable at any useful distance. If, however, a hidden cable is temporarily connected to carry unequal 50/60Hz currents through its live and neutral wires, magnetic field detection provides a sound basis for cable detection. Unfortunately, the necessary temporary re-routing of a mains cable's current is a task best left to professionals. If you're set on trying it, you will find 'veys and meanz'.
 
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