Ok, you've helped Philip w/ his hum, now help me w/ mine...

Discussion in 'Archived Threads 2001-2004' started by Jon G., Jan 11, 2003.

  1. Jon G.

    Jon G. Stunt Coordinator

    Jul 18, 1999
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    I live in a old ( ca. 1942) duplex whose power is supplied by a 60amp fuse box. I was poking around the fuse box the other day and noticed that it wasn't grounded, but my neighbors was.

    Being the dummy I am, I decided to ground the fuse box. I bought a few grounding clamps and attached a 12gauge copper wire from the rod the white wires are attached to a metal pipe (the neighbor's fuse box is grounded the same way, and a satelite cable is also grounded to the same metal pipe).

    The wiring in my house is all two wire copper wire. There is no ground cable in the house's electrical wiring...

    I turned the electricty back on, everythings good, but now I have a very slight hum from my speakers.

    A few weeks ago I was running more speaker wire under the house for my new 7.1 set-up and I also grounded the outlet that feeds my HT. I crawled under the house and ran a copper wire from the power outlet and attached it to a grounding clamp that I attached to a water pipe.

    What should I do to eliminate the ground loop? Disconnect the ground to the power outlet feeding the HT, or disconnect the gound at the fuse box? Does grounding the outlet but not the fuse box actually do anything?
  2. brucek

    brucek Second Unit

    Dec 29, 1998
    Likes Received:
    Your first order of business would be to remove that insufficient 12 gauge ground wire you installed and use a proper #6 gauge bare copper wire. That's the code in most areas. That wire must connect to your house metal plumbing ahead of the water shutoff valve as it leaves your house to pass outside. This ensures the lowest possible impedance path to ground.
    They use to allow you to jumper your water shut off valve and connect anywhere to your plumbing, but most areas don't accept that any more. If this can't be accomplished you must resort to installing rods outside.
    This #6 copper wire must connect in your fuse panel directly to the white neutral wire and then this point must be bonded to the fuse panel case.
    Once this is done you now have a proper safety ground capability at the fuse box to allow you to run cable with a safety ground wire for three prong receptacles. You have now ensured that the neutral in your panel is at an earth ground potential. The neutral comes from a center tap transformer on the pole. The transformer has two 120-volt outputs 180 degrees out of phase with reference to the center tap. We want to ensure at the house that this neutral lead is referenced to earth ground. This allows us to use a separate non current carrying safety wire to every receptacle in the home for protection against shock.
    Since wire isn't a short and has a finite resistance, using #12 gauge wire isn't a good idea. You need to lower the potential as close to earth as possible, so use a #6 wire.
    Once this is done, your fuse panel is grounded properly. Now, if your house utilizes only two wire cable without a safety wire, you should realize no increased hum or interference in the speakers of your system. It can happen, but it wouldn't likely be the cause.
    Generally, it's accepted that the optimum grounding grid is a star system. This means, you choose a proper single ground point for your load center at a rod or plumbing as it exits the home, and then every safety ground wire in each cable connects to the fuse panel (like a star). This ensures your best chance of no loops. Each safety returns to a common point and will hopefully be at the same potential. This means that if you use two receptacles for your stereo, the safeties will be at the same potential and no micro current will flow between them through your system causing hum.
    If you attached a safety wire from a receptacle to a plumbing fixture somewhere in the house, you've now increased the chances of created a loop. Certainly the path the neutral in that cable has to travel is completely different than the path the safety has to travel through the plumbing. I'm not saying it's unsafe, it's just that the probable 'potential' on the safety is not conducive to creating a quiet sound system. If you were using two outlets and two different safeties, the problem would be increased.
    I'll go over something you likely already know.
    The safety ground is a cold conductor designed to provide a path to ground for safety protection against internal shorts inside your equipment. This safety ground is attached directly to the metal case of your equipment (through the third prong) and if the hot 120 volt shorts to the metal case because of a fault, a breaker will trip and protects you against a shock. That's the good news.
    Even though the safety is a cold conductor, it can, and usually does, develop a small potential, through mutual inductance and various other reasons that can actually be different at different receptacles in your house.
    When I plug a power amp into one receptacle and a preamp into another receptacle, the metal cases of these two units can have a small potential difference in their safety grounds which means that their metal cases are at a slightly different potential. When I connect a single ended (RCA) cable between these two devices, a small AC current can flow because of the potential difference. This signal is in the loop circuit and can cause a hum. An interconnect circuit has a loop path (completed circuit) that flows through the centre conductor of the interconnect cable and back on the shield. If there is an AC signal on the shield flowing because of the ground difference potential, you'll hear a hum.
    Exactly the same situation can occur, except usually worse, when you introduce a new ground into the system from cable TV or a satellite. Their ground on the shield may possess a different potential than the ground in your system and current will flow in all the interconnects. By centralizing and bonding all external grounds to the common house ground you're at least giving yourself the best chance of reducing this problem....
    So, what's my point (and there is one) [​IMG].....
    You must first ensure you have a properly grounded load center. Nothing else will do.
    Then, if you cannot run a new cable to your stereo with a safety wire in it, the next best thing to afford you a safe situation is to attach the third prong to nearby plumbing. This will no doubt create a hum in your system that you may not be able to get rid of. Safe and noisy. [​IMG]
    My recommendation is to fix your panel and then run that dedicated cable with a safety wire in it.... Hum gone.
  3. Lee Bailey

    Lee Bailey Second Unit

    Jun 8, 2000
    Likes Received:
    Central California
    Real Name:
    Lee Bailey
    Get an 8 foot copper grounding rod and grounding connector from Home Depot or Lowes. Pound this into the ground as close to the fuse box area as possible. Then use 4 gauge copper ground wire to connect to the fuse box.(You really should hire an electrian to do this.) If you can afford it, replace the entire fuse panel with a new breaker panel, then lastly, rerun the house with new wire and 3 prong receptacles. This is of course, if you are the owner of the house.

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