need circuit to confirm continuity of mains transformer secondary during power cut

Discussion in 'General Electronics Chat' started by ONEBIT, Jul 12, 2016.

  1. ONEBIT

    Thread Starter New Member

    Jul 12, 2016
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    Burglars here sometimes cut incoming mains electricity supply before entering buildings. The mains supply is single phase 220V, with 110V across either side of phase to neutral. Trying to get to a circuit that would sense when the continuity of the power company's mains transformer secondary to the building goes open, so that a security alarm alert can be provided.
    Circuit would not provide an alarm when there is a standard power cut, in which instance the the mains transformer secondary winding would typically still be continuous.
    One of the issues is that devices monitoring the secondary of the power company's mains transformer would also see the inputs of all devices in the building. Any thoughts ?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    How would that happen?
    Max.
     
  3. ONEBIT

    Thread Starter New Member

    Jul 12, 2016
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    I need to differentiate between a basic power cut and a loss of power caused by the incoming lines being cut. If the lines are cut between the transformer and the building there would be no resistance across the incoming line, whilst during a standard power cut the resistance of the transformer coil would still be seen.
     
  4. BR-549

    Well-Known Member

    Sep 22, 2013
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    I would shop for a DC controlled main breaker. Monitor line voltage before the breaker. UPON proper delay of, loss of voltage, turn off main breaker.
    Use the voltage monitoring circuit to inject some current in one leg and see if it comes out the other. The voltage monitoring circuit can be used to complete the test circuit. A SHORT test. If current can be had, stop test, return to voltage monitoring mode and reset main breaker.
    Do not send alarm.
    IF no current can be detected, send alarm and play "police are coming" on building intercom. Do not rest breaker automatically.
    But do go to voltage monitor mode.
    Interesting problem.
    Check with local power company, for regulations and code. They might even have a proven solution.
    You might provide a service for others.
     
  5. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    Maybe, maybe... (But will depend on your supply conditions)
    Use the neutral line to make this determination. In normal conditions, neutral should be connected to earth by the supplier but if the cable is cut it will no longer be so connected. You could connect a DC supply via a fairly high value resistor then monitor the dc voltage on the neutral with a filter to get rid of any mains frequency.
    Will this work in your location?
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Every single non-UPS power supply of any kind on this planet (and that's a lot of supplies) will go dead if its input power is cut. So, a couple of wall warts or cell phone chargers or any other little 5 V output USB-looking critters, one plugged into each AC phase, will go from 5 V to 0 V 1 second after input power is lost. Fully isolated, safety certified, no adjustments, blah blah. Need a 12 V signal, get a different wall wart. Need an isolated dry circuit output, add a small relay. Ebay is your friend.

    ak
     
  7. AlbertHall

    Well-Known Member

    Jun 4, 2014
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    But they won't notice any difference between a 'normal' power cut and some baddy chopping through the supply cable.
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    Sorry ak, DC means battery to me. I should have stated that. A couple of car batteries and a uprocessor, one can get as sophisticated as one wants.
     
  9. ONEBIT

    Thread Starter New Member

    Jul 12, 2016
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    Thanks for the inputs. As everyone will realize the difficulty is how to measure the mains incoming transformer coil when at the same time the equipment in the building is connected to the system and providing it's own resistance.

    We plan to incorporate the circuit into our 24 hour security monitoring system so it has to be stable and not create false alarms.

    Using a contactor across the input lines will allow us to measure the secondary of the mains transformer when there is a power cut, as the contactor, when deenergized by the power cut, will disconnect the mains transformer from the building and allow us to read the secondary of the mains transformer as a separate circuit. This is a bulky and not inexpensive solution.

    What I would really like is for someone come up with a simple but elegant electronic solution. So if anyone has a good idea on how to do this let me know.
     
  10. ONEBIT

    Thread Starter New Member

    Jul 12, 2016
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    Hi Albert. Noted your comment. The neutral is also connected to the building grounding system which would read similarly. For your suggestion it would be necessary to have the neutral monitoring point made before the lines get to the building to ensure that only the incoming mains neutral is being monitored, which would be a visible connection.
    We had our lines cut recently and they merely cut the two sides of the phase and left the neutral intact so that the power cables did not fall into the street to make anyone aware of what they had done.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    While the change in DC resistance of (everything in the building) with and without the input transformer might be too small to measure reliably, my guess is that the AC impedance change will be much larger, particularly at higher frequencies. I have poked around in X-10 modules I use for some lighting control, and it is easy to see the effects of distance and device loading on the RF bursts. Something like 50 kHz or 100 kHz bridge that connects only when the AC goes away...? Sorta kinda like a water level detector. Oscillator > output impedance > connection to load > buffer/gain stage > precision rectifier > level detector > logic conditions > alarm.

    ak
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If it was me I would be looking more at the breaking and entering sensory aspect of the system not the power supply aspect being the burglars apparently know that many systems will quit working if their power is cut thus allowing them to get past the normally active window, door and other entry point plus internal movement sensor systems.

    Or ar they cutting the power lines then sawing a big hole in the walls to gain access?

    Also, how many time have you and anyone else in close proximity, <1-mile radius, to you actually been successfully burgled with this method of cutting the mains lines? o_O
     
  13. ramancini8

    Member

    Jul 18, 2012
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    This is a complicated problem. How much will you pay for a solution?
     
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  14. BobaMosfet

    Active Member

    Jul 1, 2009
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    Sounds like you need a pole monitor and an in-premises monitor that can compare what's going on at each location.
     
  15. DickCappels

    Moderator

    Aug 21, 2008
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    Apply a constant direct current through a high compliance current source and measure the DC voltage across the power line through a low pass filter that removes the AC from the signal. It might help a lot to sample the power line voltage during zero crossings so that the large peaks do not have to be filtered out.

    When the power is on, note the resistance. When the power goes off, compare the resistance with what it was when the power was on.
     
    Last edited: Jul 14, 2016
  16. EM Fields

    Member

    Jun 8, 2016
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    Nicely put, and apropos.
     
  17. EM Fields

    Member

    Jun 8, 2016
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    Slick.
     
  18. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Pulling numbers out of the air, a 50 A load on 220 Vac would have a 4.4 ohm impedance. Clearly the output impedance of the transformer would be much lower than this, but those are AC impedances at 50 Hz. The DC resistance difference between the source transformer secondary and whatever is drawing 50 A might not be all that far apart. When mucking around down in parallel milliohms, detecting the presence of a extra parallel milliohm or two sounds tough.

    My earlier thought was that the impedance difference between the source and the load (or, more accurately, the impedance of the internal power line with and without the external transformer winding connected) would be more readily apparent at 50 kHz, and I still think that is true. But depending on the load current and its subsequent low frequency impedance, a much lower exciting frequency such as 1 kHz also might work.

    ak
     
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  19. AnalogKid

    Distinguished Member

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    And another thing - the impedance of the load on the AC powerline might increase significantly when power is removed, making the effect of a milliohm transformer winding easier to detect.

    Large industrial equipment like a CNC punch press has internal contactors controlled by its control panel, and the contactors will disconnect the gazillion-amp motors when AC drops out. Old fluorescent lamp ballasts still will look like big inductors, but electronic ones will drop out, as will TVs, microwave ovens, and most other consumer or light-industrial appliances and devices. Detecting a physical connection to the milliohm resistance transformer secondary might be easier than first thought, even with 1000 A loads on it.

    Hmmm...

    ak
     
  20. BR-549

    Well-Known Member

    Sep 22, 2013
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    I don't know the power requirements, probably a little heavier than this, but something like this could isolate the building. This could be mounted on line side of main breaker.

    http://www.carlingtech.com/hm-cb-c-series-remote

    As stated before, the best solution is a security system. If you still want to detect a cut or a loss remotely, I suggest isolating the load. (building)

    Regardless of the solution, an aux power supply will be needed. Time and money will also be needed.

    Maybe some fake security cameras that light a red led when power goes out.
     
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