RF Shielding NM-B

[Russs57]

I guess what I'm asking Rex....is why not do it at the end of the run. Like throw a P20 in the room with the gear.

You have way more experience.

 

[suemarkp]

This sounds unique enough I'm not sure anyone is going to have a good answer if wrapping something conductive around a cable is going to get you in trouble. May depend on the inspector.

Have you tried ferrites on the branch circuit power cable?

To me, MC cable seems like the best solution, especially in dwellings where the structure is typically wood and drywall so you're not picking up currents from anything else using structure and other circuits for a return path. You could also use the hospital version with a separate isolated ground conductor if you want and a pvc coating over the shield if you want it (Southwire HCF MCAP). Not sure if you're permitted to terminate MC in a plastic box with knockouts, but this could keep one end of the shield isolated.

 

[winnie]

I'm going to say up front that this sound like 'snake oil' to me. IMHO if the audio equipment is so sensitive to external interference that shielding the power supply cables makes a difference in the sound output, then the audio equipment is _broken_.

Ok, having gotten this off my chest, I am going to approach this as follows: 'for aesthetic reasons' the customer wants an installation where the conductors from the breaker panel to the receptacle are shielded in a fashion where the shield is grounded at the service panel end only. I don't care what the rational is nor if doing this will actually make a true electrical difference to the system, all I care about is that the customer wants it and I want it installed in a fashion that will pass inspection.

My thoughts:
1) Code generally permits cable assemblies to be sleeved in conduit without requiring the conduit to be continuous.
2) It is generally accepted to place cable assemblies in non-electrical chases, and those chases can be conductive or not.
3) If conductive metal is 'likely to become energized' it is required to connect an EGC to that metal. The EGC _may_ be the EGC of the circuit likely to energize the metal.
4) As a good design practice, I would not want to put a cable in contact with a conductor that did not have low enough impedance to trip a circuit breaker in the event of a short. However I do not believe that this is an actual code requirement for a code acceptable cable assembly.

Given the above I'd be perfectly comfortable sleeving NM in any conductive material with an equivalent conductivity to the EGC required for that application. I would be perfectly comfortable bonding that sleeve at one end of the circuit only.

I would consider materials such as tinned braided copper sleeve, corrugated copper tube, or any similar heavy gauge conductive sleeve. Something like the sleeve used for MI cable might be ideal because there are fitting available to bond that sleeve to your panel enclosure.

-Jon

 

[Rex Hungerford]

I am thinking table 250.122 might apply. If this is something that might become energized, it should be able to trip the overcurrent device in front of it, so a #12 drain.

And I probably have to have a manufacturer specified connection device from the shield material to drain wire. If I do that, I think i have a good case to say I am meeting the intent of the NEC. Probably best to have a pre inspection and get the ok before install.

 

[Rex Hungerford]

I'm going to say up front that this sound like 'snake oil' to me. IMHO if the audio equipment is so sensitive to external interference that shielding the power supply cables makes a difference in the sound output, then the audio equipment is _broken_.

Ok, having gotten this off my chest, I am going to approach this as follows: 'for aesthetic reasons' the customer wants an installation where the conductors from the breaker panel to the receptacle are shielded in a fashion where the shield is grounded at the service panel end only. I don't care what the rational is nor if doing this will actually make a true electrical difference to the system, all I care about is that the customer wants it and I want it installed in a fashion that will pass inspection.

My thoughts:
1) Code generally permits cable assemblies to be sleeved in conduit without requiring the conduit to be continuous.
2) It is generally accepted to place cable assemblies in non-electrical chases, and those chases can be conductive or not.
3) If conductive metal is 'likely to become energized' it is required to connect an EGC to that metal. The EGC _may_ be the EGC of the circuit likely to energize the metal.
4) As a good design practice, I would not want to put a cable in contact with a conductor that did not have low enough impedance to trip a circuit breaker in the event of a short. However I do not believe that this is an actual code requirement for a code acceptable cable assembly.

Given the above I'd be perfectly comfortable sleeving NM in any conductive material with an equivalent conductivity to the EGC required for that application. I would be perfectly comfortable bonding that sleeve at one end of the circuit only.

I would consider materials such as tinned braided copper sleeve, corrugated copper tube, or any similar heavy gauge conductive sleeve. Something like the sleeve used for MI cable might be ideal because there are fitting available to bond that sleeve to your panel enclosure.

-Jon

Interesting on the though for gauge of the sleeve material. I agree with the drain needs to be sized as an EGC.

I have never seen an audio amplifier or preamp that is not sensitive to the power being fed to it. They all perform much better with a clean sine wave and low impedance power source. Same for digital sources. Maybe even more so.

 

[Rex Hungerford]

If you use a high quality amplifier and feed it with a perfect 60 hertz sine wave....you will get high quality power.

You might want to consider the distortions that the audio equipment causes.

I like "balanced power" myself. Anything remotely digital has an isolation transformer and a filter in front of it.

I like vinyl and tubes.

A good rack mount balanced power transformer such as Torus is $12K.

I always show my customer how polluting their audio equipment is. They are the dirty neighbor.
Digital equipment creates massive sine wave distortion as the power supply caps fill and drain. Very sharp peaks that rag the font edge of the crests of the wave. Hence, dedicated circuits of large gauge.

 

[gar]

211113-1607 EST

Rex:

I have a feeling that the approach being taken is wrong.

Are there magnetically induced signals that are a problem?

If not, then your problem is simply noise voltages on your common grounding wires. Not hard to get a few millivolts, and extremely difficult to eliminate. So don't depend upon trying to eliminate these, you won't. Use balanced low impedance lines, and transformer isolation between separated circuits. Low impedance and twisted pairs minimizes magnetic coupling into the wiring. Balanced lines and shielding reduces capacitive coupling. Also consider optical coupling between different equipment.

For speakers use twisted pair wiring. Do not ground either wire other than possibly at the power amplifier which may be inherent in the amplifier.

Use shielded shorted wiring at all amplifier inputs. Is your noise level low enough? If not look for the cause in the amplifier..

If the amplifier is good take the longest twisted pair of Beldfoil shielded cable that might be used and connect to the most sensitive input, a microphone input. Short the signal wires at the input, but not to the shield. Is there objections noise?

Use the above information to search for a problem.

If you connect to some other equipment that is connected to an AC circuit some other place, then you need isolation in the connecting circuit.

If RF is getting into the building, then you may need to create a shielded room.

.

int

 

[tom baker]

Take a look at art 647 and read u on technical power systems. It may be more cost effective than what you are doing

 

[synchro]

• I am powering high end audio gear that is very sensitive to RF.
• Most instances there is a signal voltage of about 2mV that is being boosted either 40 to 60 times its original voltage. It is very sensitive to noise.

Of course RF itself is not audible, but nonlinearities in the active devices within electronics (e.g., transistors and diodes) can demodulate any amplitude modulation component of the RF signal and therefore produce spurious audible signals. Perhaps the simplest example of this is a crystal radio set. Amplitude modulation is present in not only AM broadcast signals but also in most digital RF modulations including those transmitted from cellular phones and base sites. I think this first became most evident in TDMA modulations like 2G GSM, where the pulsing of the TDMA caused audible clicks in audio equipment even though the modulation was constant envelope (i.e., no AM) during each slot of the TDMA.

There are some filters that block RF. On some systems they work well on preamps and digital equipment. That is one place were I have heard a back to back, filtered and not filtered. Its very apparent.

I agree with that. Obviously the electronics with the lowest audio signal levels will tend to be most susceptible to a given level of RF interference, because any interference demodulated to audio frequencies will be subsequently amplified. Also, sources of conducted interference like digital equipment can be filtered to prevent them from impacting the rest of the equipment.

I think the largest opportunity to reduce EMI susceptibility to radiated RF is in the audio signal paths using appropriate shielding, grounding, and preferably using differential signal paths through XLR connectors, etc. Common-mode chokes and unbalanced-to-balanced audio transformers can be used where appropriate. Once the RF is demodulated to audio, the damage is done and not much can be done in subsequent audio processing and amplification to improve it..
It would not hurt to put common-mode chokes on the speaker wires at the power amp outputs since they may act as antennas to pick up RF signals, particularly if the wires are long. There could be relatively low impedance paths for RF to propagate from the power amplifier back to low level audio stages.

I have never heard a filter improve an amplifier. The amp always sound better direct to the wall. So I want to shield the power to those outlets for sure.

I agree that filtering the power line supplying an audio power amplifier could be undesirable if its low frequency impedance allows the AC voltage to be noticeably affected by the current draw, such as having voltage droops at peaks of the audio output.

Hopefully this can not get too sidetracked. I understand a lot of people get consternation over rich people going to crazy extremes with their hobbies. I am trying to assist them in getting the most from what they are doing.

I can understand that. I used to attend CES when it was in Chicago and enjoyed visiting the high end audio salons.

I agree with Jon's comments that any shielding on AC supply lines should be able to carry possible fault currents.

 

[hbiss]

Hopefully this can not get too sidetracked. I understand a lot of people get consternation over rich people going to crazy extremes with their hobbies. I am trying to assist them in getting the most from what they are doing.

They don't have enough money for me to put up with their BS.

-Hal

 

[synchro]

By the way, is the customer in the vicinity of any RF broadcast transmitters or ham stations? Or has the customer experienced interference from any personal communications including cellular, digital controls, etc. If so there might be more specific areas that could be targeted to make an improvement. Or is this just because it's something that might happen?

 

[synchro]

... Given the above I'd be perfectly comfortable sleeving NM in any conductive material with an equivalent conductivity to the EGC required for that application. I would be perfectly comfortable bonding that sleeve at one end of the circuit only.

I would consider materials such as tinned braided copper sleeve, corrugated copper tube, or any similar heavy gauge conductive sleeve. Something like the sleeve used for MI cable might be ideal because there are fitting available to bond that sleeve to your panel enclosure.

Interesting on the though for gauge of the sleeve material. I agree with the drain needs to be sized as an EGC.

I agree with winnie (Jon) on his suggestions for a metallic sleeve. I would have a concern that the RF shielding paint, mylar, etc. mentioned at the beginning of this thread could add thermal resistance that would increase the operating temperature of the NM at a given current level. Or at least that's something that an AHJ could use as an argument against it.

 

[brantmacga]

Coming to a home center near you…..

Monster(c) Brand NM-B cable.

Sorry Rex, I couldn’t help myself. I’m all about making them dollars too. Go get it playa.


Sent from my iPhone using Tapatalk

 

[Little Bill]

Hopefully this can not get too sidetracked. I understand a lot of people get consternation over rich people going to crazy extremes with their hobbies. I am trying to assist them in getting rid of that troublesome money.

This sounds more like what I would be thinking!

 

[LarryFine]

I don't like MC cable/EMT due to the parallel path of the metal case which causes all sorts of ground issues.

Check out MC-AP cable. There are not two parallel grounding pathways.

 

[Russs57]

Rex, I have read stereophile and know a bit about who you are.

We live in an age where almost nothing is a linear load. A true audiophile should be taking a long hard look at everything in their house.

It becomes a lot more complicated when one camp is SET, horns, and vinyl and another is digital and class D amplifiers. IMHO nothing in the audio chain is more broken than loudspeakers.

Balanced power need not be expensive. A decent single phase 480 X 240 transformer will suffice. I wouldn't want a toroidal.

 

[Speedskater]

When it comes to EMI/RFI in audio or amateur radio systems, Jim Brown is the go-to expert! He is a Fellow in the Audio Engineering Society and a past co-chair of their EMI/RFI committee. He has about 50 papers and Power Points on his Audio System Group publications page. Many cover EMI/RFI.

Audio Systems Group, Inc. Publications

 

[Rex Hungerford]

Check out MC-AP cable. There are not two parallel grounding pathways.

View attachment 2558385

When they remove the isolated ground receptacle the yoke bonds with the metal box and the case of the MC becomes a parallel path.

 

[winnie]

When they remove the isolated ground receptacle the yoke bonds with the metal box and the case of the MC becomes a parallel path.

If the customer removes the isolated ground receptacle and installs their fru fru cryotreated dollar remover, thus defeating the isolated shielding provided by MC and isolated ground hardware.....

Clearly there is a market for cryotreated isolated ground receptacles.

Look, all this audio equipment operates on DC internally. Provide DC from a suitable battery bank, float charged directly from a matched DC solar array so that you don't have any contamination from DC/DC converters. You will need to limit to battery chemistries that tolerate uncontrolled float charging, and you will need a different bank for each required voltage.

Pure natural electrons for the most expensive sound possible

-Jon

 

[Rex Hungerford]

Rex, I have read stereophile and know a bit about who you are.

We live in an age where almost nothing is a linear load. A true audiophile should be taking a long hard look at everything in their house.

It becomes a lot more complicated when one camp is SET, horns, and vinyl and another is digital and class D amplifiers. IMHO nothing in the audio chain is more broken than loudspeakers.

Balanced power need not be expensive. A decent single phase 480 X 240 transformer will suffice. I wouldn't want a toroidal.

Thanks Russ,
The speaker and the room!!!

Yes they should take a hard look at their house. Asia is a completly different philosophy than the USA. They pride themselves on having a dedicated utility power transformer and OFC copper through out the entire audio power system. Europe is half way there. They have better cable they can use. Again, shielded OFC branch wire they spend $20 to $60 a foot on. In the USA, people ignore what is behind the wall and spend $7000 on a Shunyata or Audioquest this or that conditioner, then run $5000 per cable, power cords to everything. I have seen at least $50K in just power cords on projects. I am not going to say its snake oil, or its colorations. What I know is when I ground a house proper ( this is not easy to do), rework a panelboard properly, route branch circuits correct, use the right wall boxes, its an amazing enhancement in performance. Not only in audio, but video too. And internet data transmision is more stable. No drop outs.

I have tried a few transformers. I always hear them. They have a strong sonic signature. Sometimes they work, other times not. A 2.4KVA topaz worked great with my CJ Premier 140. It was god awful terrible with a Rega Osiris. A small triplite really brought a quiet and dynamics when powering all my date utilities (modem, router, switch). And my data has rock solid performance.

I have no desire to be a sales rep for product. If I sold a transformer and the customer did not like the sonic change it brought, I would then own the transformer. Than can end up in thousands in losses to get rid of it. Remember, my customers usually have about $200K to $600K in audio equipment. They are going to hear the transformer. And the only one I would trust to perform is a Torus. They start at $12K and go up from there. And the transformer needs to be close to the outlets. Transformers are noisy. If I put it in another room and run a non shielded wire from it to the rack, I have done nothing for RF. All I have done is block DC from the utility. And balanced the phases.

People may not realize it but we can hear every power cord with this equipment. I have heard the difference taking 3 x TEW 10 AWG, twisting them, putting them in techflex and adding ends as opposed to the same with mylar and a drain over the same set of wire. As the noise goes down, the detail comes out. Ergo, can I shield my branch circuits. I just don't want to use MC. As soon as one of these guys adds a non isolated ground receptacle the MC is a liability. Fremers home had MC for the branches. He had a non isolated receptacle at the amps and preamps. His ground loop via the case was loud. Probably 35 db at the chair. I agree MC is better. That is why its used in recording studios. But the technicians in studios don't remove the receptacles you put in and replace them with some fancy $500 duplex. They leave it alone.

My take from this thread is, every inspector is going to have an opinion. Just like here. If I spec a shield, I am going to have to have a sample on the job and have the AHJ authorize it before its built up and installed. I need the drain to be 12awg, and the shield needs to be bonded properly around the NM-B, and should be robust enough to provide a low impedance path to trip the circuit breaker.

I think I have a plan. Time to build up some test cables and install them in a system to test.

Thanks you all for your input.
Rex