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I am wondering if it makes sense to use MC cable to avoid RF interference. I have a wireless network transmitter in close approximation to my system. My neighbors also have such transmitters.
I asked a guy a Hubbel tech support about using MC cable for this purpose, and whether or not it would be advisable to use the cable with plastic outlets, since, I have heard, the metal jacket can act as an antenna and introduce its own problems, requiring special grounding treatment.
The NEC requires that metal conduit or armored cable be bonded to ground at both ends in order to (quickly) pass fault current. In a situation where you have a ground conductor within metal conduit/armor that is grounded at only one end, that conduit/armor becomes a choke to fault current that will impede its flow.
I had two dedicated audio AC lines installed when we moved in, and had the electrician install them with armored cable. Later, I had two more lines installed in plastic cable. Thus, I can run my Wadia CD player from either type of circuit. It seems to sound better on a plastic cable circuit, as long as I load the circuit with a large filament transformer with an R-C network on the secondary, where the R is chosen to appear as 120 ohms to the AC line.
Dedicated circuits are resonant objects. They are so long that they do not pick up higher-frequency RF noise easily unless the noise matches a harmonic of their fundamental resonance frequency. Loading the line with something close to a matched impedance reduces the chance of this happening.
Armored cable has increased capacitance from hot to ground (safety earth) because of the armor. Coupling noise on the hot wire to the safety earth causes degradation in audio systems with three-wire power cords. If the armor kept all the noise out, this would not be an issue. However, aluminum armor does not block magnetic fields, and noise can get in from the power cords or equipment itself.
What if you used twisted wires in the conduit to reduce magnetic pickup, and an insulated technical ground wire?
The conduit and the insulated ground wire are joined at the source and load ends of the circuit, so they make a loop. Will this loop pick up noise from magnetic induction?
Thanks a ton, Al.
You have given me some excellent advice on this subject over the past several weeks, and even a couple of years back, I think, when I added dedicated circuits to my previous home.
I am tempted to take your earlier suggestion: to install one MC line just for the heck of it, and use romex for the others.
So I take it, you think MC will not be necessary, even in proximity with my wireless network transmitter. Also, you don't seem concerned about the "single-ended" ground for the metal conduit. Go easy on me, Al. I'm kind of a "plug-and-play" audiophile who can be easily baffled by your more technical arguments.
We've also got a wireless router for our computers, but it is not next to the routing path for the dedicated audio power circuits. Thus, I can't say definitely that the armored cable would not give you better performance in your situation. IME, the biggest problem with electrical noise comes from appliances that contain computers with their associated switching power supplies: TiVo DVRs, microwave ovens, garage door openers, high-efficiency furnaces, and, of course, the UPS devices that feed computers. The kind of cable you use for your dedicated lines would not affect the noise that these things put into the AC and safety earth wiring.
I'm using the armored cable circuits for a pair of Gilmore Audio Raptor switching amps. These don't seem to be as sensitive to power conditions as the Wadia CD player.
I don't think you will find an electrician to install a conduit or armored cable with only one end grounded, so there you have an obvious problem with parallel, dissimilar ground paths (the ground wire and the conduit or armor jacket). This might be a case where an isolated ground outlet device would come in handy: connect the conduit or armor jacket to the junction box, and the ground wire to the isolated ground terminal on the outlet. I'm not an electrician, so please discuss this with your electrician when you decide the scope of the job.
This might be a case where an isolated ground outlet device would come in handy: connect the conduit or armor jacket to the junction box, and the ground wire to the isolated ground terminal on the outlet. I'm not an electrician, so please discuss this with your electrician when you decide the scope of the job.
Hospital grade MC cable:
Steel armor jacket.
2 insulated equipment grounding conductor wires. One for the metallic box ground the other for an isolated grounding type receptacle.
I would have to check NEC, but I do not believe there would be a problem with using a plasic box with MC cable. Providing the box used has a 1/2" KO hole to receive the MC approved connector.
Yes. You use an isolated ground receptacle to keep the grounds separate. The conduit acts as a shield for all the wires. Look up "technical ground" practices.
Incidentally, I don't understand your aversion to the capacitance between the hot and the conduit. How is this a bigger disadvantage than physically installing a cap from hot to ground in a power supply filter to shunt high frequency noise?
In my experience, it puts more noise on the ground and degrades the sound.
R-C filters, where R is about 120 ohms and C is an X-rated cap, installed from hot to neutral reduce normal-mode noise on the AC line. Similar filters where C is a Y-rated cap may do more harm than good when installed from hot to ground.
A capacitor that looks like a short does not shunt RF noise, it reflects it. The R is necessary to dampen the noise.
The distributed capacitance from hot to ground associated with armored cable (or conduit) does not reflect the RF noise. It is part of the impedance of the setup. However, it will appear as a lumped capacitor to lower frequency noise.
"In my experience, it puts more noise on the ground and degrades the sound."
to safety-earth. If connected to safety-earth, the noise will get back to the equipment and cause degradation. The short-circuit to the noise represented by the capacitor will still aggravate standing waves on the ground conductors, of whatever flavor.
If you wish to 'stretch' the code and get approval, you will have to schmooze the inspector. See linked paper for comments on how to do this in Canada.
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