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In Reply to: outlet strap question posted by bkst140 on December 17, 2006 at 21:21:58:
It depends on whether the outlet is mounted in a steel, nonferrous, or plastic box.For any type of box, the strap (very cheap outlets do not have one) is a steel or brass rod in the middle of the hot and neutral wires. Steel has a nonlinear magnetic response and acts like the core of a one-turn inductor. This inductor is in series with the AC voltage and causes distortion in the current from the nonlinear magnetic behavior of the steel. The magnet trick bartc mentioned helps by biasing the steel into a more linear region of its magnetic response curve, so that it produces less distortion. A better solution is to use an all-brass outlet.
For conductive boxes (steel or cast nonferrous alloy), the strap forms a conductive loop with the box that again is coupled to the hot and neutral wires because they pass on either side of the strap. This is a one-turn, shorted transformer, and a steel strap outlet in a steel box will have similar nonlinear response issues. A brass strap in a nonferrous alloy box will still have some effect. The simplest thing to do is to break the loop at one end of the outlet by using a Nylon screw and washer to prevent electrical contact and create a magnetic gap. The other end of the outlet should be fastened with a metal screw to properly ground the box, or you can wire a separate ground pigtail to the box.
The steel screws used on the outlet are also problems and should be replaced with brass or nonmagnetic stainless steel.
Follow Ups:
Thanx for providing such detailed information. I guess it won't matter whether I use ground connections or not. More dumb questions, is it necessary to use nylon screws for on-wall installation? Is the washer also made of nylon? Will an IG outlet make a difference?I also have one DIY'd Bell 5324 single gang box made of aluminum with a Marinco/Leviton outlet installed inside. How about the screw which tightens the outlet cover? Does it need to be nylon or brass?
And there's a debate with outlet covers, too. Should I replace my steel ones with brass or nylon? Some salesman told me steel covers would sound better than nylon, brass would sound better than steel, and aluminum is better than brass. Go figure. I hope I'm not alone in this outlet material madness.
The Nylon screw/washer trick is used for any installation where the box is metal, whether in the wall or in a power strip or box. The idea is to use one Nylon screw and washer and one metal screw, to preserve the ground connection to the box, but to break the loop formed by the box and outlet ground strap. The strap is electrically connected to the box through the metal screw, but there should not be electrical contact at the other end with the Nylon screw. The washer is to make sure of this and to provide a little air gap for the magnetic field. You can find #6-32 Nylon screws and washers at Home Depot or similar stores, in the massive Wall-O-Hardware bin section.Your aluminum box device should have a metal screw to ground the cover plate if it is metal. Just be sure the cover plate does not complete the circuit between the outlet ground strap and the end of the outlet that is insulated with the Nylon screw: a little tape on the underside of the cover plate at this end may be all you need.
There has been a lot of debate and experimentation on cover plates. Using no cover plate sounds better than almost any existing cover plate. IME, Nylon cover plates are almost as good. The best is a home-made carbon fiber slab with holes painstakingly machined to accommodate the outlets and cemeted to a plastic cover plate with epoxy. This is similar to the Oyaide design, and the carbon fiber material serves to damp standing waves on the power cords.
Metal cover plates reflect RF on the power cords and help sustain standing waves. Nylon plates allow RF to pass through and be reflected by the internal metal of the outlet and box. Carbon fiber is resistive, so it absorbs energy and damps standing waves.
Be very careful about putting anything inside an outlet box, as the corners on a ceramic magnet may cut the insulation on the wires. If you wish to proceed, get some rectangular ceramic magnets from Radio Shack that fit on the outlet strap. Secure them with polyurethane construction adhesive, and this will damp the strap against acoustic vibration as well as bias the steel's magnetic response.
An isolated ground outlet is only useful where you have armored cable or metal conduit with an insulated ground wire inside feeding a metal box. The armor or conduit will serve to ground the metal box, and the insulated ground wire will ground the outlet. The two grounds are connected at the circuit breaker box, but not at the junction box. This provides a method of shielding the power wires and equipment AC ground. It is still a good tweak to use one Nylon screw on such an outlet, as the metal loop comprising the outlet strap and metal box still exists.
You'll be famous if you have a website like Jon Risch's. As for the cover, I guess I'll leave it as it is. I'm a lazy tweaker and probably will dump all my steely damned outlets for those brassy babes.
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