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Am I crazy or is this problem fixable?

Discussion in 'Safety' started by squashbucket, Feb 19, 2005.

  1. squashbucket

    squashbucket Member

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    Here's the situation: My theater has 8 dimmer-packs (AS-42L http://www.lightronics.com ) each on a 20 amp circuit (but we never have a tripping issue). All of the 20 amp circuits are grouped together on a 100 amp circuit breaker.

    Our sound system is routed through three 20 amp circuit breakers and then to a master 60 amp circuit. Now both grounds are independent until they converge once again at the building main.

    For the most part there is no problem, except when we have a concert and we have to crank up the sound system. Once we bring the board up to -40 db and we turn on the dimmers for our lights, we can hear a 60 cycle hum. Now we are planning on upgrading our dimmers to dmx etc dimmers with a cam-lock hookup.

    I think if we were to have an electrician hook up a completely independent ground for our dimmers, with a ground that never meets any other ground, the problem should be solved. Now, I also know that our entire downtown area is on a gigantic block of concrete and therefore all the natural grounding rods are useless because they are dry as a bone. So my question is will re-grounding our dimmers help or not, because the cost is substantial, and I only want to move forward if it will work.

    I know that most of you have probably dealt with 60 cycle hum before so any advise you can offer would be greatly appreciated.

    Adam M. Terry
    ~Squashbucket
     
  2. stantonsound

    stantonsound Active Member

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    I am a licensed electrician in NC and have worked with numerous theatrical light and sound systems. Now I own my own sound business. Saying that, I have found several causes of 60 cycle, ranging from easy to fix to almost impossible. Try the easy fixes, then - and only then - spend the money to run new grounds.

    First and foremost: Make sure that there is no computer equipment on the same ground rod as the sound system - especially laptops. I have spent hours trying to correct a humm and realize that some moron has plugged in a laptop in the scene shop and caused the problem. There is quite often a short between the neutral (which carries the unbalanced load) and the ground. Since all of the grounds are eventually connected, this can be a big problem. A quick fix that some people will recomend is to plug the computers in to a power strip and then remove the ground post from the plug of the power strip. This leaves only a hot and a neutral. This will almost always reduce or eliminate the 60 cycle hum. (I can't recommend this, but it does work) This includes the computers that run your lights. These can be the sound engineers worst enemy - because they are always plugged in so close to the sound system (expecially the amps).

    Next, get a good ground tester. This should probably be done by an electrician. It is a meter that you clamp onto the ground rod, then you turn a hand crank (in most cases) and it sends an electrical current down the ground. Local code will determine what the rod must be able to handle. Before this is done, all of the ground cables must be disconnected or everything that is plugged in on that ground system will be damaged. In areas with a great deal of dry sand, I have had to drive in more than 20 ground rods that were 20' long each to meet code.

    A properly gounded electrical system should keep this from occuring, but if your sound board is plugged in to the same circuit as your lighting computer - you still run the risk of this occuring.

    Let me know if this helps and I would be more than happy to answer any more questions.
     
  3. squashbucket

    squashbucket Member

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    Thank you for the info. Now when you talk about the "same ground rod as the sound system" what do you mean? How does one know how many ground rods are connected to a circuit, and how does one determine which ground rod is with which ciruit? I was under the impression that a building would only have one grounding rod. Does it make a difference if two crcuit braker boxes eventually meet at a master fuse box for the whole building? Or would they be on different gounding rods?
     
  4. stantonsound

    stantonsound Active Member

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    ok...good question....i will clarify. Most buildings of meduim to large size have many ground rods. There must be enough "grounding" to handle all of the potential that is in the building. This is usually accomplished by driving several rods in the ground and connecting them all together with copper cables, then to the panel. If the building is erected with steel beams, several of the beams are usually grounded together and then to the rods. Usually all of the rods and building steel are connected together. This means that if there is a 60-cycle short in a computer on the 4th floor and your amps are plugged in in the basement, you can possible pick up this interference. The more sound that you are pushing, the more likely you will hear this. That is why you will hear this FOH more than you will in the monitors.

    Remember...it is not always computers. An improperly installed lighting ballast or a bad GFCI recepticle (Ground Fault Circuit Interuptor) will also cause this hum. Also consider where you are running your speaker cables and make sure that there are not coils of cable anywhere. You might want to take a look at your snake, too(if you are using one).

    If nothing that you try works, you might possible need to have an additional series of ground rods driven that are in no way connected to the other grounds. This can get very expensive. If it has not always been a problem, then I would consider tracking down the cause and fixing it.

    I hope this makes sense, if not....I'll try to explain it better.
     
  5. squashbucket

    squashbucket Member

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    So all of the gournds in the building are conncected? That is what I thought. If I were to trace all of the electrical compnonants to find that the dimmers were themselves the problem, would my prevous idea of a seccond powerline comming into the building possibly solve this problem? Or would more grounding rods be the only solution?

    I know that the colege I attended had four seperate hookups. One Cam loc breakout for road shows, one for our light dimmers, one for our sound with what they called and isolated ground, and then there was a nother hookup for all the offices in the building. What would the isolated ground have been for the sound do you think? Would that have been a set of ground rods only connected to the special orange outlets for sound? Thanks again for the input!
     
  6. cutlunch

    cutlunch Active Member

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    Have you checked to see where in the audio chain the hum is entering. It may be picked up in a number of places or it could be just one.

    I would start at the amplifier, disconnect the input lines and listen to see if the hum is there when the dimmers are on or off. If the hum is there, you could try an isolation transformer which would isolate the amplifier from the ground. Keep working back adding the mixer, effects, cd player. Are you running DI boxes? If yes, have you used the ground lift ?

    If you find the interference is in the mixer,cd etc then you could try an isolation transformer in the booth and connect all the sound gear in the booth to it.

    Stantonsound also mentioned cables. Do you have any long runs of unbalanced cable. The cables from the CD player, tape etc are unbalanced. Are your dimmers in the same room or fairly near to the sound gear. Or do the dimmer cables pass near the sound gear?

    Normaly when I have come across interference from Dimmers on a sound system the interference varies in level as the dimmer level varies. This is one of the ways it is easy to tell it's dimmer interference.

    If you haven't already, it is worth checking for where the hum is getting in to your system, it might save a lot of money. It may even turn out to be a be a piece of cheaper audio gear that can be replaced.
     
  7. squashbucket

    squashbucket Member

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    The company that installed the soundsystem decided to go with mackie powered drivers, so I know the hum can't be inbetween the amp and the driver. As far as I can tell the hum is entering between the mixer and the mackies. Yes we use DI boxes, and it seems to be worse in our monitors which get power from the same sourse as our mackies and our mixer. Do you mean ground lifting the DI boxes?

    Isolation Transformers are expensive, but I will talk to the sound company and see if they have one we could use to test.

    Some of the powerlines running to the dimmerpacks do run near sound gear. Unfortunatly, at the moment, there are no alternatives. However, this was a major reason for my first concept for solving this problem with a new line of camlocs to a new set of dimmers to be placed in a room adjacent to the booth. That way, I believe, by isolating the dimmers, the hum would be eliminated. But as I said before this is but speculation on my part.

    The interferience only is present when the dimmers are powered up and yes we can hear a change when the cues cause the dimmers to raise and lower the current. So, I am fairly sure it is the dimmers that are at fault. I just don't know which to isolate, the dimmers or the sound equipment. I find that sound equipment is harder to isolate because people show up and start plugging things in. So I figure if the dimmers where isolated, then nobody would be able to put any sound equipment on thier circuit.

    The more I research this problem, the more I find I don't know, and the more I feel I have a lot to learn!
     
  8. stantonsound

    stantonsound Active Member

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    An easy way to trouble shoot the issue would be to run a new set of line-level cables between the outs on the board and the powered Mackie speakers. This can be a temporary line of XLR that you just drop out of the booth window (or how ever you are set-up). Just keep them away from the lighting dimmers.

    The more that I learn about your problem, the easier it seems to fix. My guess, get some good line-level shielded cables to protect your signal from getting corrupted by the evil dimmers (darn lighting guys!!!! - just kidding) and it will probably fix the problem. This is a MUCH cheaper fix than replacing the dimmers.
     
  9. cutlunch

    cutlunch Active Member

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    Yes I meant the earth lift on the DI.

    I think you need to find where the intreference is getting into your system. Is it through the power system ground or is being inducted into signal lines.

    As I mentioned before I would start with the Mackies working back to the sound booth. Disconnect the input cables to the mackies and the monitors if powered then run the dimmers up and down. If you get inteference at this stage then it is probably comming in via the power lines. An isolation transformer may fix the problem. If you get hum while the inputs are disconnected, I would also check as to how near cables carrying lighting power are to the speakers. If they are quite near, I would start unplugging lights until there was none on that cable run, to see if that is the cause.

    Next reconnect the leads but disconnect them at the piece of equipment they connect to in the booth. Run the dimmers again. If no inteference reconnect them again but leave the equipment off , run the dimmers. If there was hum when the leads were disconnected from the booth, try Stantons suggestion and re-run the leads in another positioin. If there was hum when the leads re-connected but booth equipment off disconnect the shield ground on the cable at the end closer to the stage on all the cables driving speakers.

    Turn on the last piece of equipment , re-test , if hum try disconnecting the
    the shield ground as above. If that has no effect try the transformer.


    Keep working back in this fashion. When you get to the mixer disconnect all inputs then reconnect one by one, testing all the time. If there is hum try disconnecting the ground on a balanced line. If it is an unbalanced line try putting it through a DI with ground lift. If you get hum from the mixer when nothing is connected, but the faders are up, then try the transformer. If you get hum as you add a channel go back and remove other channels you added to ensure it is not a cummulative effect. Disconnect the shield ground at the mixer end of a balanced cable if there is hum. If that doesn't work try the transformer on that piece of gear.

    I know this is time consuming but worth the effort. Because if the fault is inducted you have saved money on changing your ground system, which I am not quite sure would fix the porblem. In places where they want to ensure that the sound and dimmers are totaly isolated they use a large transformer to provide the power to the sound system.

    Also another advantage of doing this it that you know it's not your gear. That's very helpful when someone brings in gear from outside and you get the hum back. Quite often they will try to blame your gear, and you can tell them you know for a fact it is not.

    As Stanton points out some people take off the earth pin on the power plugs. Not a good idea because of safety. Here in New Zealand it is illegal. It probably is in the States. If you did this and anyone / anything got hurt / damaged you would probably be liable also Insurance would not pay out.

    When you get the new dimmers some of this problem may go away. But it would be safer to identify it first just in case it doesn't . Hope this helps.
     
  10. ship

    ship Senior Team Emeritus Premium Member

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    Welcome to the forum stantonsound, I hope to learn a lot from you and it's always good to have more voices helping.

    I would follow the steps on testing what's easy first before jumping into a X causes Y type of un-tested theory just because it seems to be fact.

    Given the number if items, dimmers, fluorescent lights, other arc sources, motors, computers, shorts in the system all having similarities in causing noise, but different causes of it, it's not an easy subject. Yes, the dimmers seem to cause the noise but it's hard to say the exact reason that they are causing the noise in correcting it other than just citing a phenonomon. Do the simple first as test cables to the speakers would start with.

    If the case does not prove to be because of a run to speakers, than it would be by far easier and less expensive to isolate and possibly condition your sound system equipment than to correct the other problems. This after an electrician tracks down the cause and it's necessary.

    First a few things I find curious. You say you have three 20 amp individual breakers that feed a 60 amp master on what’s perceived to be some form of sub-panel for the sound equipment. I would hope that I misunderstand this description.

    Still what you describe in the ground passing by this sub-panel is typical for a Isolated ground system in it very properly terminating at the main panel as required. Path of least resistance, as long as your ground for the system is properly installed and your ground is isolated you should not get noise from the rest of the building.

    This is given you truly have a isolated ground feeding the sound system, and a good ground it terminates at. A electrician can verify this in details about doing so.
    Just because you for instance have the isolated ground wire going to the main, does not mean the receptacle itself in not being isolated from the mechanical ground of the electrical box it’s installed in qualify as a isolated ground. Unless using a specific iso ground receptacle for instance which is not connected to the also necessary separately grounded electrical box, running that separate ground wire is not doing the isolation job. In other words ensuring it's a direct end run in passing by all other outlets etc. and the outlet itself is isolated, you might have one of many potential sources for the problem.


    Given it’s not a problem in the cable coming out of the sound equipment and you really do have an isolated ground, various power conditioners, or isolation transformers can help after this along with as stantonsound mentioned ensuring the low resistance ground path at the ground rod. This than makes it less likely that noise from the building will effect your sound system. There is methods as said to check this resistance and improve upon it. Point is that you still do have to have the ground originate at the same place as the service drop. While at times it is permissible to add another ground in addition to it, and before the main system ground for a specific purpose, this is something for the electrician or electrical engineer to determine when necessary. What santonsound is talking about is more and extra grounding rods that supplement the main one for the service drop and such extra rods are very much a part of this ground so as to drop the resistance of the soil down to an acceptable level. Believe 20 amps of resistance as I remember is the threshold for what is acceptable. 20 amps can also mean there is a lot of low resistance interference factoring into electronic sound gear even with a "good ground." Another part of what stantonsound might be talking about is in adding more rods or longer rods in lowering this resistance to a more reasonable level for electronics as can be possible to do here also.


    This resistance can be a factor. All main service panels will have the neutral and various mechanical - conduit and ground wires terminating at it. Sub-panels off the main panel are directly attached to the main in isolating the neutral from ground up until that main panel. Isolated grounds also by-pass sub panel attachment to ground in a similar way to this neutral not being attached at a sub-panel or outlet to the ground.

    My next question would be about an installation of Cam Loc type plug fed dimmers. I believe what you are asking is similar to the question of it you went say twist lock instead of stage pin for the lights. One type of power outlet over another will not correct problems with system noise be it phase harmonics or noise on the neutral and ground. In the case of the dimmers, perhaps it’s a lot to do with an electrical field, could be something with un-balanced load, but the more normal answer would be in having a separately derived service drop for the dimmer packs from that of the rest of the building power. This would also include it’s own unique to it ground at the service drop and main panel. Very good option in general especially for more power but also sometimes not possible and if possible very expensive. Just because the dimmers have a portable means of disconnect, does not mean they will be powered any differently than they are now unless from another service drop. Installing them in another location on the other hand might also play a factor.

    In the end, there can be many factors in what causes noise and as theorized, it’s often not simple but there are some simple things you can check.

    As for ground lifts on equipment, given a ground lift by design is only designed to allow a grounded plug to plug into a non-grounded outlet, and is also designed to attach by way of wire or screw to the outlet box that is grounded it than becomes grounded, a ground lift to remove a piece of equipment from ground is both mis-stated and un-safe if not used as designed. Such a ground lift also is not designed to work unless the electrical box it's moving the ground from pin to box in is actually grounded.
    First at the point that the little metal ring hanging off the ground lift touches the center screw holding a receptacle onto it’s cover, you are again grounded. The intended use of the ground lift would be to install the ring under that screw in fact. Were you to remove this ring, or cut off and otherwise not use the ground wire that is available on other types, you would have something that does not ground in a similar way to that of removing the grounding pin. It's using the gear as not intended and un-safe. Were your gear designed to be not grounded it will have been.

    "But the sound system didn’t get interference" is a nice thing to explain to someone’s mom after you were responsible for putting a son or daughter in the hospital at best following a short within the equipment. No, I don’t do sound for a living as one might be able to tell, but removing the ground - really reomving the ground I know is not electrically safe on the other hand. It's a quick fix - the same as one could quick fix by just shoving bare wires into an outlet.
     
  11. cutlunch

    cutlunch Active Member

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    Ship I am wondering if there could be some confusion in the use of the term "Ground Lift" in this situation.

    I follow exactly what you are saying about removing the ground wire from an electrical appliance, as you will see from my previous post.

    But I am thinking of the term "Earth Lift / Ground Lift" in the audio context where it applies to disconnecting the ground / shield from a balanced audio cable so as not allow a current path for inteference.

    I am just mentioning this in case I have confused some newbies.
     
  12. ship

    ship Senior Team Emeritus Premium Member

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    Ah' I was seeing ground lift as a store bought electrical item that converts a three pin plug to two. Instead had not been thinking cutting the shield wire as necessary in some cases to perform as you specify which is a valid solution to loop or what ever problems it solves.

    My fear was that people out there were modifying ground lifts/adaptors - the electrical version in removing their sound equipment from having a ground. This might solve the problem but cause others thus my standpoint. I hope I'm wrong in my stance of what I understood the advice to be.
     
  13. ship

    ship Senior Team Emeritus Premium Member

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    By the way, my above posting about 20 amps worth of resistance in the ground is based upon something I remember reading. Most likely 23 amps in many situations as debated to be acceptable or not. Given this, and it being a memory thing, plus my surmising that the resistance (yes I know Amps is not resistance, it's a measure of what amperage it will take to short however by the old fuse testing method.)

    In any case I would be interested to hear stantonsound's thoughts about this seemingly large but on old installs, resistance factoring in to line noise equasion. Also if he has ever done a grounding rod specifically for the sound system ground before and in-line with the system ground.

    I know such a practice is done at times on large shows with the dimmer/distro equipment in cases of long runs to the service drop.

    As said, this is stuff I know some about but not enough given I have not gotten my licence. Thanks.
     
  14. ship

    ship Senior Team Emeritus Premium Member

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    If of any help, and I'm glad there are other methods, here is one from no doubt an older text I would certainly never try:

    Grounding Rod Efficiency:
    Ways to improve grounding rod efficiency, (1) wet or place rod near where earth will get wettest. (2) Install more rods and interconnect them at least ten feet apart. (3) Salt the ground around the rod once a year, if earth around rod contains at least 1% salt, resistance can drop by as much as 90%, or 25 ohms can become 2.1/2 ohms.
    Note: the dryer the ground around the rod, the greater the resistance to ground is.
    Resistance to Ground testing:
    Attach the hot to a 30 amp plug fused switch. Attach other end to ground to be tested. If fuse blows when switched on, resistance to ground is less than 4 ohms.
    Continue testing plug fuses by size until fuse blows. This will give the resistance to ground. 10 ohms in the city, and 25 ohms on the farm is the 1950 standard. 6 ohms resistance to ground however is best because it will blow at least 20 amps in normal conditions. Note: it is important that the ground tested is not attached with any other parts of the system ground or service panel and all down stream of it. when tested.
    Volts (120) 30 amp Fuse If Blows = less than 4 ohms, 25 amp = 5 ohms, 20 amp = 6 ohms, 15 amp = 8 ohms, 10 amp = 12 ohms,
    Amps (30) =4 Ohms.6 amp = 20 ohms, 5 amp = 24 ohms, 4 amp = 30 ohms, 3 amp = 40 ohms, 2 amp = 60 ohms, 1 amp = 120 ohms.
     
  15. darkfield

    darkfield Member

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    Isolated grounds are a complicated situation in practice. Two grounds from the same service will possibly be tormented by small but not negligable differential voltages.

    People trying to be clever spec isolated ground all the time for computer installations, but it is often not what they want.

    Focus on the two solutions above: shield the equipment and cables, and consider power conditioning/filtering equipment.

    As an outside chance, a dimmer or a rack of them may be out of whack. Can you isolate it to any one dimmer or bank more than the rest?
     

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