Hello,

I am in a bit of sticky situation and would welcome all ideas in this regard. My client has this problem where people are, literally, frying the energy meters in their homes using apparatus that is similar to a microwave.

So, i have been given the task to find out what fault does the microwave induce in the energy meter. What i am doing i pretty simple, placing a energy meter (with battery) in a microwave for 5 seconds. What i have observed is it takes about 2-3 seconds for the meter to go dead. A spark is observed and the display goes dead after that.

After examination of the dead meter, i found out that the resistors connected with shunt and CT are fried.

I have repeated the experiment by removing the battery and the resistors, mentioned above, but the spark is still observed after 2-3 seconds and meter goes dead.

I can't seem to figure out why is the meter acting out the way it is. Any ideas?

 

placing a energy meter (with battery) in a microwave

Why would you do that? Isn´t it obvious that the 900W the microwave produces has to go somewhere, and something WILL go toast?

My client has this problem where people are, literally, frying the energy meters in their homes using apparatus that is similar to a microwave.

You seriously mean that people place electronic stuff in a microwave and then wonder why it goes poof?

<SNIP>

 

Hello and welcome, port500

I interpret your problem as people trying to escape paying for energy by frying their meters. You have a "client" that wants to know which parts burn out.

The microwave energy is inducing huge currents in the measuring device. No matter how many parts you remove, more microwave energy will induce current in the remaining parts. The solution is not in knowing which parts die first because it is not practical to put thousand watt protectors in each meter. More practical would be to tell the cheaters to pay for the meter before they will be allowed to have a new meter.

 

... people are, literally, frying the energy meters in their homes using apparatus that is similar to a microwave.

What does "similar" mean? Have vandals figured out how to fry an electric meter without leaving mechanical evidence, in hopes of avoiding an electric bill?

Anyway, IMHO the research request is nuts. It's like hitting it with a hammer and asking what the weak points are, or asking why a frog dies in the microwave.

If the client wants to repair them and wants to know what's ruined, well that makes some sense. And if they want to redesign the meter to harden it against such tampering, I guess that makes a little sense. But I'd put a microwave detector inside and charge anybody for a new meter if their detector has triggered. Just like the infamous red dot inside cellphones to indicate if it's had an unfortunate dunk in a toilet.

 

Thinking about this for a while, I have thought of putting a closed conductive loop inside the meter to short out the microwave energy. I don't know if a simple loop of copper wire will do this because high frequencies are outside my knowledge, and microwaves are, by definition, very high frequencies.

 

Thank you for the feedback.

To expect an energy meter to survive some microwaving is highly unpractical. All we want to understand is how are the PCB and components being affected.

The problem is exactly what wayneh points out, frying without any physical tempering with the energy meter. It would be very hard to prove that the meter has been tempered with if there is no physical evidence.

So, the only course of action is to either shield the energy meter, or have some way of detecting if it has been tempered with using microwaves. I believe a metallic mesh can do the job, as they are already used in microwave doors. That will allow the energy meter to be visible to the meter reader as well.

using microwaves detector is also as interesting idea. Thank you for your feedback. I will keep you posted in this regard.

 

Hello,

You could make a spot with metalic paint on the inside of the cover.
When the "meter" is tempered in a microwave, the paint will be discoulored by the metalic parts in it.

Bertus

 

Little ity bity wires placed above the electronic components with a 1mm gap will protect the components up to about 60 GHz.. Sort of like spark gaps... one end of the wire is tied directly to ground. Use a 12-GA sold wire and put a plastic sleeve over the spark gap. Put one where the power input to the board is, and spaced around the circuit board. Microwaves are directive, but the energy travels on the surface of the conductor.
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Your mesh is a good idea, but transparent mesh is not an effective EMP trap - it is used to electronically isolate a room from outside electronic communications signals - which is more of what you would use mesh for.
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On a 120-Foot Tropo Bill board Antenna we used the spark gap method to keep the light controllers operating under the obvious conditions of extreme ionized energy. The spark gaps also helped to keep the lamps lasting longer.
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You could also try this neat little trick... Buy a couple of 1N23WE diodes.... solder a wire lead to one end, and a 120 ohm resistor leading to an LED on the other end. Make sure you get the polarities right (for the Diode and LED).. Ground the LED. Put it in a clear capsule/Tube with a label - "Tamper Alert Sent" Labeled on the tube. When they energize the meter the lamp will light indicating that a tamper alert has been sent. Really worried that they may zap the led - Use a little neon bulb... trust me when they are zapping the meter the lamp will glow, and the more energy they put into it the brighter it will get. Wanna really get abnoxious - put a sonalert in line with the diode lamp and ground. The sonalert will start sounding and keep making noise until they stop.... Put the sonalert in a mesh or metallic box that is grounded and they won't be able to zap it....
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If your customers are using magnitron driven feed horns, then they are real risk takers. To fry an electric meter doesn't take all that much energy - but it is still hazardous (kinda like eliminating kidney and liver functions).
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I am curious to find out where you are located.
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I would also suspect that they are just inserting a 120V pigtail into the power tap port on the meter - that would zap it - I'm sure. But then I don't play with my meter - maybe I need to to understand your predicament. Or maybe I should just go down to the utility and talk to their resident EE......
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Still, I like the diode and lamp and sonalert --- nothing is more obnoxious than the sound of the neighbors meter going beeeeeeeep beeeeeep ....... like a smoke detector.........
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I also understand that a large magnet will have a similar effect. Maybe they aren't using microwaves after all (microwaves are not a really convenient tool for this)... Try using a heavy duty magnet (the kind that will pull an outboard motor out of the lake).... or an electro magnet.....
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I saw a neat trick with a car alarm once - the tech put a heavy magnet on the car hood - it fried all the relays and fried the Alarm controller. Since the Car alarm controller is never wired next to the cars power/relay box - it doesn't fry the car electronics.
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The magnet would freeze the little flywheel assembly and put the little motor in a stall situation, which would cause it to overload its windings and associated power tap measuring circuitry. That could fry the components as well..... then the meter would cease to function. Since the meter measuring circuitry isn't directly connected to the power it would stop measuring the power consumed and the meter would just stop - but the power would stay on....
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Seriously - you have me amused... I'm going to go and get a spare meter and try it on one of the houses I just bought to renovate...... You have my attention (and I have all the time in the world to figure it out - retirement is like that...)
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Dave
Phoenix, Arizona

 

@dataman: I love your posts

 

Well, that's one downside of modern technology. Perhaps your client could consider going back to old-fashioned electromechanical meters, which might stand up better to the zapping - or would his customers simply attack whoever had to read it?

Another low-tech solution might be a plain locked metal meter cabinet - not necessarily see-through.