Should power cords be shielded?

[Tony_Montana]

One will always get advice to not run an interconnect (or speaker wire for that matter) too close to power cords. This advise suggest that power cords could be one source of noise in a system by way of EMI )electric magnetic interference) contamination. So the question is why haven't this source of noise have been addressed in consumer products.

I think it was Skeptic suggesting that shielding after market power cord [that is not shielded] might violate UL safety codes for power cords. So given that UL approve a shielded power cord design, wouldn't this power cord outperform unshielded power cords in term of less environmental contamination, most notably the 60 Hz hums?

I believe the answer to above question would a definite yes. Just take a look at a typical HT setup with all type of wires going everywhere and twisting around each other (like my setup), and one will note that shielding power cord might not only sound like a good idea, but a necessity to reduce system noise :)

[okiemax]

One will always get advice to not run an interconnect (or speaker wire for that matter) too close to power cords. This advise suggest that power cords could be one source of noise in a system by way of EMI )electric magnetic interference) contamination. So the question is why haven't this source of noise have been addressed in consumer products.

I think it was Skeptic suggesting that shielding after market power cord [that is not shielded] might violate UL safety codes for power cords. So given that UL approve a shielded power cord design, wouldn't this power cord outperform unshielded power cords in term of less environmental contamination, most notably the 60 Hz hums?

I believe the answer to above question would a definite yes. Just take a look at a typical HT setup with all type of wires going everywhere and twisting around each other (like my setup), and one will note that shielding power cord might not only sound like a good idea, but a necessity to reduce system noise :)

My $11 Volex 17604 is a 14 awg Beldfoil shielded detachable UL power cord. I don't know how well the shielding works, since I try to keep power cords away from interconnects anyway.

[skeptic]

There are "shielded" power cables like bx. There is a big big difference between power cords and speaker cables and interconnect cables. Power cords can overheat, melt, and cause a fire. They can also expose "live parts" which have the potential to cause electrocution. Power cords are designed to safely handle current for their application when they are in free air. The circulating air removes heat. This rating is designed into it, selected for its applicaton by the engineers who specified it and verified and certified as safe by UL. This however are only for the conditions of use for which these organizations have intended and measured. That means in free air. When you apply a do it yourself shield such as wrapping the wire with foil tape, you have changed the conditions of use and all bets are off. Then the wire cannot carry as much current saftely and you have created a new arrangement which is potentially unsafe. About 10 years ago, I saw this mistake made by an inexperienced engineer on a grand scale. He tried to magnetically shield a 4000 amp bus duct in a transformer vault directly under an office area of a large building because the magnetic fields were causing problems with VDTs and there was (unjustified) alaram that it posed a health risk to employees. He had a vendor manufacture and install a magnetic shield around the full length of the bus duct, probably about 40 feet or more. It cost $100,000. When his mistake was pointed out to him and that the bus duct was no longer UL listed, he had a AW $HlT day.

You can substitute BX cable for your power cord. Frankly, it is much easier and cheaper to shield the interconnects and just as effective. There are many sound systems where neither power cords nor ICs are shielded and the owners claim that they are dead quiet. Try the prudent approach first. It can usually be made to work with complete satisfacton with just a little care and planning. The one exception in my experience is phonograph signal cables which can benefit from additional shielding which is very simple and inexpensive to install.

[kexodusc]

Hey, Skeptic

I've got to admit, the whole powercord area of my system is something I've completely ignored. My interconnects are shielded, and the powercords are moved as far away from other cables/wires as possible, but how does one detect if they have unwanted noise or interference in their system?
What are the dead-giveaways that suggest a problem with unwanted system noise?
Any tests or anything out there? Can you recommend any good reads on the subject?

Thanks.

[jneutron]

One will always get advice to not run an interconnect (or speaker wire for that matter) too close to power cords. This advise suggest that power cords could be one source of noise in a system by way of EMI )electric magnetic interference) contamination. So the question is why haven't this source of noise have been addressed in consumer products.

I think it was Skeptic suggesting that shielding after market power cord [that is not shielded] might violate UL safety codes for power cords. So given that UL approve a shielded power cord design, wouldn't this power cord outperform unshielded power cords in term of less environmental contamination, most notably the 60 Hz hums?

I believe the answer to above question would a definite yes. Just take a look at a typical HT setup with all type of wires going everywhere and twisting around each other (like my setup), and one will note that shielding power cord might not only sound like a good idea, but a necessity to reduce system noise :)

A braid, foil, or metal shield around a powercord will not eliminate 60 hz hum.

The only way 60 hz, (and about the first 10 harmonics) can be shielded from other equipment is to use a high mu material to confine the flux. Otherwise, the metal coverings are entirely useless.

For rf, it will be different, and also for e fields.

Hum invariably has to be removed by understanding the method it is being picked up...and shielding does very little to stop that method.

John

[Tony_Montana]

There are many sound systems where neither power cords nor ICs are shielded and the owners claim that they are dead quiet.

The question is would shielding on an IC would do the same job as on a Power cords, but in reverse. Purpose of one is to keep noise out, and the other is to contain it within the cord. I was thinking that we could connect the shield of power cord to ground side of a plug (which mean we have to have three prong plug for shielded power cord), but I am not sure.


The only way 60 hz, (and about the first 10 harmonics) can be shielded from other equipment is to use a high mu material to confine the flux. Otherwise, the metal coverings are entirely useless.

Yes, the only thing we want to do is to contain 60 Hz flux created by AC current. So other method of shielding such as [copper] braiding or foiling would be ineffective for containment? As I recall, mu materials are slightly expensive :)

[jneutron]

The question is would shielding on an IC would do the same job as on a Power cords, but in reverse. Purpose of one is to keep noise out, and the other is to contain it within the cord. I was thinking that we could connect the shield of power cord to ground side of a plug (which mean we have to have three prong plug for shielded power cord), but I am not sure.

The shield on an IC works because, by design, the overall loop intercept is identical between the inner and shield.... any voltage picked up as a result of b-dot intercept will be the same in both loops..the caveat is the impedance each sees, as the shield loop is lower impedance.

A power cord shield is incapable of preventing it's broadcasting 60 hz magnetic fields.


Yes, the only thing we want to do is to contain 60 Hz flux created by AC current. So other method of shielding such as [copper] braiding or foiling would be ineffective for containment? As I recall, mu materials are slightly expensive :)

A non mu metal shield will not do that.Copper and foil cannot.

I don't know how expensive mu metals are...my uncle buys what I need for work.

Cheers, John

[Walker]

What is "non mu metal"? What's the the best way to shield interconnects?
Just starting.
Thanks,
Walker

[E-Stat]

There are "shielded" power cables like bx. There is a big big difference between power cords and speaker cables and interconnect cables. Power cords can overheat, melt, and cause a fire. They can also expose "live parts" which have the potential to cause electrocution...
Yada, yada, yada. There are a number of UL listed shielded cords.



Frankly, it is much easier and cheaper to shield the interconnects and just as effective.
Which does nothing to shield the AC line from RFI generated by other components, computers, etc.

rw

[E-Stat]

So given that UL approve a shielded power cord design, wouldn't this power cord outperform unshielded power cords in term of less environmental contamination, most notably the 60 Hz hums?
Hum rejection is not the objective.

rw

[jneutron]

What is "non mu metal"? What's the the best way to shield interconnects?
Just starting.
Thanks,
Walker

All materials have some constants associated with them.

The most important for insulators used for capacitors is the DC, or relative dielectric constant..it tells us how the material works relative to a vacuum. DC of 10 means a capacitor made with it will have 10 times the capacitance of the same thing made with a vacuum, or air.

For metals, mu, or relative magnetic permeability is an important one..it tells us how magnetic the material is..with respect to free space..

a value of 1 is essentially not attracted...aluminum, copper...etc.

Steels will have values ranging from 10 to 1000, some called high mu metals, can be even higher.

If you have a magnetic field in an area, putting copper or aluminum in the area does not do anything to the field lines..ever see that magnet under the paper, iron filings on top picture? shows the field lines...

If you do that with a piece of copper on the top of the paper, the field lines will be the same..if you use iron on top, it's permeability will cause the field lines to divert..it actually sucks the field lines towards it. Think of a trampoline with an inch of water on the top..if you step onto it, you'll push the surface down, and all the water will go to your foot. That is how mu metal pulls magnetic flux.

A box made of mu metal will cause the field lines to travel along the box surface, and exclude the field from inside..real magnetic shielding..

We have to do that with the computer displays here when the magnets are powered. The magnets aren't causing the problem, the 30,000 amp DC bus runs are broadcasting the field...they make the monitors picture slide sideways, and change color..Now, we replace critical ones with LCD monitors, eliminating the need for mu shielding.

All conductors will react to a time varying field, which is what 60 hz is. But, the reaction is not strong enough to prevent the field from entering, passing through the conductor..when the frequency is high enough, then the fields will not penetrate, that is called skin effect. But, within the audio band, most shields are not thick enough to keep it out..other things are required to keep the bad stuff out of signal paths..

Cheers, John

[jneutron]

So given that UL approve a shielded power cord design, wouldn't this power cord outperform unshielded power cords in term of less environmental contamination, most notably the 60 Hz hums?

No.

John

[Walker]

Thank you John, very educational.

Quote: most shields are not thick enough to keep it out.

How do I shield an interconnect to keep it out?
Walker

[jneutron]

Thank you John, very educational.

Quote: most shields are not thick enough to keep it out.

How do I shield an interconnect to keep it out?
Walker

You cannot..for the most part.

Key things:

Keep the interconnects away from power runs.
Keep them as short as possible.
All the equip should be on the same ac run if possible.
Hooking to other stuff like tv Cable can make ground loops. (ground loops are not your friends.)

If you encounter hum, play with the cable positioning.

Lots of trial and error, for the most part..the biggest problem in eliminatiing hum is actually figuring out what is picking it up..it is rarely a shield issue, and commonly a loop issue.

Unfortunately, when a loop is causing the hum, many people break the loop by removing the safety ground....that is not wise.



Cheers, John