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eHam Forums => Elmers => Topic started by: KC1BMD on September 03, 2017, 05:50:25 PM



Title: Common mode current on AC wiring?
Post by: KC1BMD on September 03, 2017, 05:50:25 PM
Situation:
- 2nd floor shack in townhouse condo.
- two separate fan dipoles in attic above (40/20/10m and 17/12m) both center fed with RG-8x and current baluns at feed point for each.
- Power 100W max.
Problem:
- Plug-in CO detectors (with battery backup) occasionally get tripped, only when plugged in, not when unplugged on battery.
- So it appears to me it is not being caused by direct radiation into the detector (e.g. wrapping in foil does not help) but rather caused by RF current coming in via AC wiring.
- Problem mitigated to some degree (at lower power levels anyway) by switching to different brand CO detector.
- Unable to isolate to specific bands or frequencies, or power levels (can happen when tuning up at 10-15W too).
- Neighbor with unit attached to our end unit (3 units in building) also affected but we switched his also to another brand.

I tried to make some measurements and plugged one of the more susceptible CO detectors into an outlet at the end of a few feet long extension cord. I measured some RF current using an MFJ-854 (of questionable accuracy) which varies but appears to be e.g. 10-80mA, or so, when sending CW key down at about 25W. Interesting, the measurement seems to vary as I slide the meter up and down to different spots on the cable but could be my imagination or the MFJ meter quality :).

Questions:
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?
- Is this common mode current, and if so is it: a) getting past the current baluns on coax outer shield, or b) being induced/radiated onto the AC cable by the antenna near field radiation?
- Aside from moving to a single family home with more land (which I'm searching for), or moving the antenna away out of the attic and further away from the operating position while still in the condo (not able to do that), are there any other comments or suggestions?


Title: RE: Common mode current on AC wiring?
Post by: K6BRN on September 03, 2017, 06:30:53 PM
Your house wiring is in the (very) near field of the antenna, where capacitive and inductive coupling is dominant and very strong.  You need to 1.  Move the antenna elements away from any wiring, particularly parallel runs, 2. Consider running any nearby wiring through toroid inductors, 3.  Make sure the antenna match is as good as possible,  4.  Reduce power output, and 5.  Perform an RF exposure (safety)  assessment as described on the ARRL site.

In summary... your antenna is probably TOO CLOSE. to your house wiring and to you to run 100 watts safely.

Brian - K6BRN


Title: RE: Common mode current on AC wiring?
Post by: AA4HA on September 03, 2017, 07:36:57 PM
With the house wiring being so close to the antenna element it is making the house wiring act as part of the antenna system. Even if you achieve a good match with your antenna for minimal SWR the wiring beneath the antenna is some fraction of a wavelength away. It would not surprise me to find that the radiated pattern for your antenna is more like a "cloud burner" and is going more upwards than to the horizon.

As sensitive electronics like a CO or smoke detector is making a decision on an alarm threshold based off of microvolts of differences in its circuity it is not surprising that you are getting false trips. GFCI's are also subject to the same sort of upset.

If your installation "had" to be the way that it is you might be able to reduce the impact by putting the detector inside of an RF-proof enclosure (like a mini-Faraday cage) with PI filters on the enclosure metal for the incoming AC power. It's a tricky design process that is more of an art if you are not totally familiar with the science behind shielding.

You may find it easier to move your antenna. As has been said, your power levels at 100 watts might be above suggested limits for human exposure to the general public. You should run the calculations to be sure.

Also, making a double walled, shielded Faraday cage with a double layered copper mesh window to allow the air flow in and around the detector might also invalidate the performance of the CO or smoke detector. I bet you would never get anyone to sign-off on you modifying a detector that is going in someone else's apartment or condo.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 03, 2017, 08:08:25 PM
Thanks for the comments.

K6BRN: I have already performed an RF safety assessment and am within safe limits (for both controlled and uncontrolled areas) for 100W and up to an including 12m. At 10m I believe it was 85W max to be within limits, but I haven't been using 10m much due to poor propagation in the sunspot cycle. I don't currently operate any VHF or UHF and if I do, it would likely have to be portable.

AA4HA: Faraday cages for the detectors would not really be feasible for reasons you cited. I doubt it would help since the RF is apparently coming in via the AC wiring (although you mentioned filters -- but implementing that would be problematic, if it would even work.

As an experiment, I added several ferrites to the AC extension cord with as many windings going through them as I could manage an that seemed to reduce the RF somewhat but not to the point that it prevented tripping. I guess I'm stuck with the situation for now until I can find a suitable place to move (a longer term solution).


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 07:50:41 AM
I used this calculator: Amateur Radio RF Safety Calculator (http://hintlink.com/power_density.htm)
It has an entry for: The distance to the area of interest: From the centre of the antenna, in feet

For my fan dipoles, would that be indicating that the area of concern is the high current portion (center), rather than the high voltage portion (ends)? The reason I ask is because I am closer to the ends of these dipoles than the center. In any case, my numbers showed not much exposure even at 100W PEP (lower avg-pwr on SSB/CW) at relatively close proximity (even at ~10ft).


Title: RE: Common mode current on AC wiring?
Post by: W9IQ on September 04, 2017, 08:06:03 AM
The radiation of the antenna is proportional to the current in that part of the dipole. In a dipole antenna the current is highest in the center and tapers off to near zero at the ends. So the majority of the radiation occurs in the center of the antenna with the ends contributing very little. As a result, the RF exposure is much less near the ends of the dipole antenna than the center.  However, as frequency increases - the human exposure limits decrease and the distance between the center and the end of the dipole become less so the current distribution in a dipole has a diminutive effect on RF exposure as frequency rises

- Glenn W9IQ


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 08:38:47 AM
Thanks Glenn. I won't be going over 100W PEP at my current location and usually operate no more than 75-80, sometimes only 25-50 (use only what you need as the guidelines say). At least using the calculator reference above I'm well within safe limits for controlled and uncontrolled areas.

I just wish I could tame the CO detectors. It seems that if I went to battery-operated (not AC plug-in) they would not trip and I believe such devices acceptable in my location (would need to double-check). However, I'm not sure I could convince my neighbor to do that. That would be a last resort if the issue rears it's ugly head again.


Title: RE: Common mode current on AC wiring?
Post by: KD0REQ on September 04, 2017, 09:53:14 AM
any way you can set up an outdoor vertical? that will get you distance from the wiring, and perhaps reduce the intersection of copper to radio waves. you're pretty tightly coupled right now.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 10:43:11 AM
I couldn't put up anything permanent outside on the common land area (i.e. back and side yards). I do occasionally put up a couple of different antennas on a push up mast on tripod on nice days but take them down when done (40-10m EFHW, 20-10m cobweb, 15m PAR rectangle). I don't believe these interfere with anything but haven't tested them enough to see and only at relatively low power so far (well under 100W). If I was able to put up a vertical, it would have to be on my deck (which I have exclusive use of) but that's also adjacent to the living areas (and so wiring). That also brings a new set of challenges like how to bring the feed line in, whether to locate the shack in the basement, instead of 2nd floor, lightning protection, RF grounding, how to manage radials, etc. But thanks for the suggestions.


Title: RE: Common mode current on AC wiring?
Post by: KC2MMI on September 04, 2017, 10:58:53 AM
If these are simple plug-in CO detectors, it could be worth asking some mfrs. and possibly the ARRL folks, if there are another brand or model that are known to suffer less from an RFI problem. And if no one knows a better brand, perhaps to buy a couple from Amazon or Costco or other sources that will let you return them if they don't work any better.

From the next door neighbor's and the association point of view, it shouldn't matter which detectors you wind up, as long as they are a typical approved commercial unit.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 11:09:50 AM
I did a lot of research. A few others I tried are no better. I contacted the manufacturer of the original's tech support. They had no solution. I did find one brand/model that is less susceptible but not completely. It was still an improvement even if only marginally.


Title: RE: Common mode current on AC wiring?
Post by: WA3SKN on September 04, 2017, 11:40:59 AM
Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 12:12:44 PM
Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.

If it was only my issue, then I could try that for the 3 required CO detectors I have (one on each of three living area levels). However, I won't modify anything in my neighbor's unit. In ours, it is only one CO detector that trips sometimes (the one on the 2nd floor where my shack is and closest to the attic antennas -- if that matters or perhaps just coincidence -- I say that because at least two of my neighbor's detectors were affected). A complicating factor is that the electrical feeds for the building's main AC panel, in a utility closet attached to my garage) comes through my attic to the neighbor's units (two more units) where it feeds the individual AC panels in each unit's basement. Definitely very sub-optimal situation! I thought about getting one of the massive clamp on ferrites (that I think Fair-Rite sells) but not sure where to place it or if it would even help. Thanks anyway for the suggestion.


Title: RE: Common mode current on AC wiring?
Post by: WB6BYU on September 04, 2017, 12:31:59 PM
One of my standard RFI test tools is an extension cord wrapped around a ferrite
rod.  Makes it easy to insert it in the power cord and see how much difference
it makes.

That isn't necessarily the most effective arrangement:  a shorter cord run
through a toroidal core would be a better filter, but I couldn't do that
with the existing plug.  (One of the split cores could also work.)

For loaning out to someone, or testing with a neighbor, using an unmodified
commercial cord may have an advantage from the perception of safety.
For my own use, I'd probably make my own cord to suite the required
current draw, frequency range, etc.

Note that, for common mode current, a ferrite choke will be more effective
than bypass capacitors.


Title: RE: Common mode current on AC wiring?
Post by: W9IQ on September 04, 2017, 12:50:14 PM
I do agree with Dale that a couple of turns of the AC wiring around a 31 toroid core would be worth testing. Fitting the core in the j box and determining if it is permitted is another matter.

- Glenn W9IQ


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 04, 2017, 12:53:15 PM
WB6BYU and W9IQ: There is no room in the AC wall outlet to really fit much of anything. I tried plugging the CO detector into an extension cord to the outlet and used ferrites (mix-31) on the cord to see what effect it had. I used snap on ferrites of varying sizes on the extension cord putting as many turns in as would fit. It did not silence the detector, although the ppm indicator on one of the CO detectors with such an LED display showed a lower reading, indicating that the RF current was reduced. These CO detectors are so sensitive, I think it would need to be reduced to near zero to have any effect and I was unable to accomplish that.


Title: RE: Common mode current on AC wiring?
Post by: NK7Z on September 04, 2017, 03:34:20 PM
Why not just make some filters?
Some .1 to .01 uf disc caps (proper voltage) and a couple of coils like the old "Brute Force" filter in many ARRL handbooks should do the trick.
You don't even need toroids to build one.

-Mike.
When doing this on AC lines, I use X and Y caps, not disk caps...  See:
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/ (https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/)


Title: RE: Common mode current on AC wiring?
Post by: G8JNJ on September 05, 2017, 03:39:56 AM
Hi Norm,

You may need to optimise the type of ferrite and number of turns for the frequencies that are problematic.

If you are just using small single clip-on ferrites, you would probably need at least 10 of them attached to have any sort of noticable effect at 7MHz.

That's why multiple turns on large cores are more cost effective.

Steve, G3TXQ's balun chart is very useful in helping to understand what is likely to work the best.

http://www.karinya.net/g3txq/chokes/

Adding X and Y capacitors or an off the shelf mains filter may also be required if the problem is really severe.

If this still doesn't work, it may be necessary to modify the detector and add additional RF decoupling capacitors to the circuit. However this may invalidate your insurance, so personally I wouldn't consider it, unless the manufacturer can provide modified units.

Regards,

Martin - G8JNJ






Title: RE: Common mode current on AC wiring?
Post by: KH6AQ on September 05, 2017, 05:01:04 AM
G3TXQ has a nice chart that will show you how many turns of power cord thru which core will give you maximum CM impedance at the frequency of interest. What makes a CO detector a challenge to CM filter is that it is small and has little capacitance (that means a high impedance) for the CM inductor to work against.

http://www.karinya.net/g3txq/chokes/


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 05, 2017, 05:42:44 AM
G3TXQ has a nice chart that will show you how many turns of power cord thru which core will give you maximum CM impedance at the frequency of interest. What makes a CO detector a challenge to CM filter is that it is small and has little capacitance (that means a high impedance) for the CM inductor to work against.

http://www.karinya.net/g3txq/chokes/

Thanks, I've also seen that nice chart. I think I convinced myself that no reasonable amount of ferrites will solve it.


Title: RE: Common mode current on AC wiring?
Post by: W6EM on September 05, 2017, 06:06:43 AM
Have you contacted the manufacturer of the detectors and explained your situation to them?  Perhaps they have a modified detector for use in RF environs.  I would think that by now, CO detectors installed in RF environments would have had a solution from the manufacturer side, just like AFCIs of recent manufacture have.

It is, by definition, a classic appliance Part 15 problem.  And, you've done almost everything imaginable to try to suppress RF externally.  Manufacturing devices highly susceptible to RF falsing would rule out their application in many buildings subject to mild RF.  And, that's probably a lot of places.

The fact that you don't have the problem when operating from batteries should steer the manufacturer in the right direction.  Perhaps some internal filtering would solve the problem, but doing it yourself would risk invalidation of warranties, liability (in the case of your neighbor's  etc., and insurance company denials if ever a bad situation).

73.

Lee

 


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 05, 2017, 06:07:53 AM
AA4HA: You commented: "It would not surprise me to find that the radiated pattern for your antenna is more like a "cloud burner" and is going more upwards than to the horizon."

I've been fairly lucky with DX running at most 85W or so (if my ARRL DXCC and WAS over the last year or two says anything about that). However, I have not been able to reach Japan and other parts of Asia. Close contacts have been difficult but not impossible (e.g. immediately surrounding states). It seems intermediate distances are easier than the close distances leading me to believe that at least on some bands the take-off angle is reasonably low as opposed to more straight up.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 05, 2017, 06:23:12 AM
Have you contacted the manufacturer of the detectors and explained your situation to them?  Perhaps they have a modified detector for use in RF environs.  I would think that by now, CO detectors installed in RF environments would have had a solution from the manufacturer side, just like AFCIs of recent manufacture have.

It is, by definition, a classic appliance Part 15 problem.  And, you've done almost everything imaginable to try to suppress RF externally.  Manufacturing devices highly susceptible to RF falsing would rule out their application in many buildings subject to mild RF.  And, that's probably a lot of places.

The fact that you don't have the problem when operating from batteries should steer the manufacturer in the right direction.  Perhaps some internal filtering would solve the problem, but doing it yourself would risk invalidation of warranties, liability (in the case of your neighbor's  etc., and insurance company denials if ever a bad situation).

73.

Lee

 

Thanks Lee. Yes, I have contacted the manufacturer (Kidde in this case). I'm not saying all their detectors have issues but the 2 or 3 that I tested did (e.g. model KN-COB-DP-LS, 900-0235). Their tech-support said they have not had any reports of the issue and there are no suggestions for fixing it. I provided information from my testing but I doubt it went anywhere. Switching to a different brand (First Alert model CO615) helped somewhat. Note that some people have had issues with some models from First Alert also. You only have to search this and other sites (e.g. "Kidde") to see some of the issues reported -- albeit, many of those posts could have come from me :)). I suppose there are two sides to many issues and someone could possibly make an argument that my installation is not typical, nor recommended, in the amateur radio community. So I have to wonder at what point the blame starts to shift to my side (causing the interference) compared to blame on their side for not being able to adequately filter and suppress it. I am not sure if they are required by the FCC to eliminate any and all interference regardless of antenna/equipment installation and proximity to their device. This is a  gray area and point of confusion for me.


Title: RE: Common mode current on AC wiring?
Post by: KH6AQ on September 05, 2017, 09:00:05 AM
You are not required to fix the interference problem and the CO detector manufacturer is not required to fix the interference problem. I believe the the FCC's position is that the user of the CO detector can have it modified to reject RF or they can stop using it. If you are renting, or even if you own the condo I can see the management making things hot for you.

The simple fix is to disconnect the AC and operate it off the battery. You might supply lithium batteries for maximum battery life. Another way to deal with the issue is to reduce RF power.

Infinite CM filtering will not solve the problem if the dominant RF signal mode is DM (Differential-mode). There can be CM-to-DM conversion on the AC wiring due to asymmetry of the line, neutral, and GND wiring. The fix could be to install an EMI filter having both DM and CM attenuation. But, even using an off-the-shelf, UL listed EMI filter requires getting into the AC wiring and that's a potential safety issue.  

  


Title: RE: Common mode current on AC wiring?
Post by: N7EKU on September 05, 2017, 10:51:19 AM
Hi,

Both battery and AC CO detectors perform the same.  I don't see why anyone would object to replacing AC ones with battery operated ones.  Plus an advantage to battery ones is that you have more freedom to optimize their placement (height above floor level) and they can leave more outlets free.  Perhaps you can "sell" you neighbor using these points.  The only disadvantage is that you eventually have to replace the batteries, but they beep at you just like smoke detectors when they need replacing so there is no worry there (just the cost).

Another advantage is you can replace CO-only detectors with combined smoke-CO units and add even more safety!

Otherwise, probably you could build up a small plug-in box that contains filter capacitors with inline choke, and an AC socket, to plug between the detector and wall outlet.  As using a choke alone on different bands doesn't seem sufficient, I think you need the capacitors to bypass the RF.

73,


Mark.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 05, 2017, 11:14:52 AM
KH6AQ: Thanks for the reply. Even if I could somehow apply a filter, I have no clue whether the intruding RF is CM or DM. I could always just unplug my offending detector when I want to operate but would never expect my neighbor to do that and would never ask. That's only a short term workaround anyway, since when unplugged and on battery, I believe it beeps every couple of minutes to remind you.

N7EKU: If it becomes more of a problem affecting my neighbor's detectors, I'll probably ask if they would consider switching to battery-only operated detectors. Not really knowing what I was dealing with when it started, I tried plugging th detector into this TrippLite surge suppressor/noise filter (https://www.amazon.com/gp/product/B00006B81D/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1). It didn't have any effect. Maybe I could use such an empty case to add caps and choke. Anyone have a design they tried already?


Title: RE: Common mode current on AC wiring?
Post by: KH6AQ on September 05, 2017, 11:26:42 AM
Having an AC powered CO detector might be required by the local (national?) electrical code. I'm getting my new house inspected soon and was told that the smoke detectors had to be AC powered.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 05, 2017, 11:45:13 AM
Last time I checked (in MA, about 3 years ago) battery operated CO detectors met the requirement. I just checked on-line and they have the requirement from 2015 which states in part:

There are several types of alarms that are allowed; they include:
• Battery powered with battery monitoring;
• Plug-in (AC powered) units with battery backup;
• AC primary power (hard-wired – usually involves hiring an electrician) with battery backup;
• Low-voltage or wireless alarms with secondary power; and
• Qualified combination smoke detectors and CO alarms.

Of course, that could differ by location.


Title: RE: Common mode current on AC wiring?
Post by: N7EKU on September 05, 2017, 05:46:10 PM
KH6AQ: Thanks for the reply. Even if I could somehow apply a filter, I have no clue whether the intruding RF is CM or DM. I could always just unplug my offending detector when I want to operate but would never expect my neighbor to do that and would never ask. That's only a short term workaround anyway, since when unplugged and on battery, I believe it beeps every couple of minutes to remind you.

N7EKU: If it becomes more of a problem affecting my neighbor's detectors, I'll probably ask if they would consider switching to battery-only operated detectors. Not really knowing what I was dealing with when it started, I tried plugging th detector into this TrippLite surge suppressor/noise filter (https://www.amazon.com/gp/product/B00006B81D/ref=oh_aui_detailpage_o08_s00?ie=UTF8&psc=1). It didn't have any effect. Maybe I could use such an empty case to add caps and choke. Anyone have a design they tried already?

Hmm,

I looked at that TrippLite unit and it seems pretty good as includes ferrites and capacitors designed for HF/VHF, so I would have expected that to take care of it.  Probably there is a lot of HF getting into the AC line just because it is of an unfortunate length.

If battery-only is OK in your area, that seems to be the way to go.  Other than that, you could try rotating the dipoles 90 degrees and see if that makes a difference.

Or QRP to a level that stops tripping things and don't go above that.

Good luck and 73,


Mark.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 06, 2017, 03:12:40 AM
When I purchased it, I don't recall seeing the inside shot with the included components. It only had a general description about surge suppression and 'noise filtering'. You would think with all those components it would have cured it! I don't think I could do any better job building one.


Title: RE: Common mode current on AC wiring?
Post by: N4MQ on September 07, 2017, 04:39:43 AM
Have you thought about installing the detector in a faraday shield  that is grounded to prove the signal is on the power line and its not detecting the rf in the wiring of the detector?   It would be easy to wrap it in foil, and ground it for a TEST to see how the signal is being picked up.

The problem is either on the power line or direct pickup due to sensitive design, knowing which would be helpful.  In the Army I was taught how to quickly kill a snake, by cutting off its tail ....all you needed to know was which end was the tail.

Enjoy, Woody


Title: RE: Common mode current on AC wiring?
Post by: G3RZP on September 08, 2017, 01:09:36 AM
the OP asked

Quote
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?

FYI and FWIW, European EMC standards - which are mostly derived from IEC (International Electrotechnical Committee) standards - require immunity to 30mA modulated 80% by 1kHz in the HF bands on leads such as power and signal.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 08, 2017, 02:01:43 PM
Have you thought about installing the detector in a faraday shield  that is grounded to prove the signal is on the power line and its not detecting the rf in the wiring of the detector?   It would be easy to wrap it in foil, and ground it for a TEST to see how the signal is being picked up.

The problem is either on the power line or direct pickup due to sensitive design, knowing which would be helpful.  In the Army I was taught how to quickly kill a snake, by cutting off its tail ....all you needed to know was which end was the tail.

Enjoy, Woody

Yes. In my starting post I wrote under "Problem:
     "...get tripped, only when plugged in, not when unplugged on battery."
and,
     "...wrapping in foil does not help."
I figured these two tests indicated it was coming in on the AC wiring.


Title: RE: Common mode current on AC wiring?
Post by: N4MQ on September 08, 2017, 08:35:41 PM
I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody


Title: RE: Common mode current on AC wiring?
Post by: WB4SPT on September 09, 2017, 06:28:43 AM
the OP asked

Quote
- Is there some level of such RF current that is significant (e.g. some level that can be ignored besides 0Ma)?

FYI and FWIW, European EMC standards - which are mostly derived from IEC (International Electrotechnical Committee) standards - require immunity to 30mA modulated 80% by 1kHz in the HF bands on leads such as power and signal.

Yep.    2 days ago, I had a design of mine tested to iec61000-4-6 to 10V conducted;  .15 to 80MHz;  For my stuff, 10V or 67mA input RF current CM.  Not a peep from a classic opamp design, but it took  a bit of on board ferrite and bypass to get there.  

To the OP;  stop the madness, go with battery CO units,  I did at my own home.  Even networked, the battery lasts much longer than a year.


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 09, 2017, 06:45:26 AM
I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody

It seems I completely missed your suggestion: "faraday shield  that is grounded". Note to self: must read more carefully. I thought about it but didn't try it. Not sure where I would ground it being on the second floor. If I grounded it to the AC safety ground (3rd prong of outlet) could that make for a bigger set of rabbit ears? Also, not that wrapping in tin foil is a perfect classic Faraday Cage but in the classic example, at least, I don't think it needs to be grounded to shield the contents, correct?


Title: RE: Common mode current on AC wiring?
Post by: KC1BMD on September 09, 2017, 06:46:35 AM
... To the OP;  stop the madness, go with battery CO units,  I did at my own home.  Even networked, the battery lasts much longer than a year.

I will certainly consider going that route if the replacement detectors start misbehaving.


Title: RE: Common mode current on AC wiring?
Post by: WB4SPT on September 09, 2017, 09:25:43 AM
I was suggesting that the shield be grounded, if it was not it would just act like rabbit ears. Woody

Also, not that wrapping in tin foil is a perfect classic Faraday Cage but in the classic example, at least, I don't think it needs to be grounded to shield the contents, correct?

Correct.   Shields do not require "grounding".  However, cable shields do need to be attached to metallic housings to keep the overall shield construct continuous.  Sometimes folks call the attachment of shields and drain wires "grounding".  But, for shield purposes, attachment to earth has no purpose.