Severe AC hum on service entrance

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DougSemon

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Greetings!

I'm a EE and a former engineer at Ohio Edison, and I have a problem at home that I've been unable to resolve, so I'm looking for advice and possible solutions.

Background: I live in an area of Denver with underground distribution -- I think a typical two-phase 220 system. In a downstairs room, I build a modest audio recording studio as a hobby. It's NOT a commercial facility, just my private retreat.

My primary instruments are electric guitars, which you may know are susceptible to induced hum through the coil pickups on the guitars.

The service entrance from the side of the house runs directly overhead in this studio, behind a low suspended acoustic ceiling, to the breaker subpanel in an adjacent furnace room. The wire between the main panel on the outside of the house to the subpanel appears to be 0000-triplex.

The problem: I have severe induced hum in all of the guitars which adversely affects the recorded sound. To diagnose the problem, I took a cheap guitar pickup and connected it directly to a battery-powered amplifier (to eliminate the possibility of a simple ground loop). I can easily move this pickup around to see where it gets worse and better. The bottom line is this: it's MUCH worse as it nears this 0000-triplex, and the closer to the outside main panel, the worse it gets. Of course, there is some hum as the pickup gets near flourescent lights, transformers, computer monitors, etc. I'd expect that, but this very objectionable hum seems to be coming in from Exel Energy, not from withing the house.

As an experiment, I took the "test" kit outside and shut off the main breakers, so that NOTHING in the house could be feeding back into the secondary. The hum is still there.

A couple of months ago, I called Excel and got someone in "engineering." He was very helpful and sent out a tech to check the ground, etc, but found no problem. Unfortunately, I was unable to be there to show him (actually, let him listen to) the problem. I have no complaints about Exel, they were great, but the problem persists.

Question: is it possible that his hum is actually coming down the Exel secondary? I've done extensive internet research, and frankly, haven't found any relevant citations. The one test that I haven't done is to take my "test kit" to a neighbor's house, and I should probably do that. Going a step farther, I could take this test to another area of town and see if the hum is just normal 60 Hz EMI, but having lived a few different places, I'm pretty sure that my case is the exception, not normal. I've played guitar for 40 years in six different homes, and have never had such a serious EMI problem.

Any advice would be appreciated, and thanks in advance.

-doug-
 
Please keep in mind that this is not a 'do it yourself' forum. There are lots of professionals with _different_ jobs, and as I understand how DIY is interpreted, we can't help an electrical engineer 'DIY' as an electrician. My thoughts below are to help you better analyze your situation and if necessary better communicate with a licensed electrician who would do any necessary repair work.

My _guess_ is that there is net or unbalanced current running on the feeder that you describe. If that feeder is in fact 'triplex' (three conductors only), then there must be a ground-neutral bond at the 'subpanel'. Improper bonding is a great way to have current flow along parallel paths and end up with net current on a cable.

I would suggest having an electrician evaluate where you have ground-neutral bonds, where your grounding electrode conductors terminate, and if you have any net current flowing on this feeder.

Good Luck,
Jonathan Edelson
 
Jonathan:

Thanks for the quick reply. I do realize that this isn't a DIY forum; I didn't intend to do any of the work myself.

What I was seeking was two things:

1. From the experience of those on this forum, IS it possible that this hum is coming from the power company?
2. Suggestions on who to contact/hire to remedy this.

I have a licensed residential electrician that does all of our remodeling and new construction work. He's great, but he has no experience with recording studios, thus has no idea where to start.

Still seeking advice; I tried the local yellow pages trying to find a contractor who specializes in audio/radio/TV/studio electrical, but came up empty-handed.

-doug-
 
I don't know if it is useful to answer 'does the hum come from the power company'.

As I said, my guess is that the hum comes from unbalanced or net current flowing on the feeder. Clearly, the power company is supplying this juice.

The question to answer is 'What is the cause of this net current flow and is that cause under the control of the POCO or someone else?' followed by 'If the cause of this net current is under the control of the POCO, is it considered a defect that they would fix, or part of the normal operation of the system?'

When electric power is delivered to your home, the neutral conductor in the service is _grounded_ at your service equipment. This is an intentional connection between a current carrying conductor and a system of earth electrodes. Also connected to this location should be all 'bonded' metal.

There should be one, and only one connection between the neutral and grounding electrode system. With only a single connection, there is no closed circuit, and thus no current flow on the bonded/grounded metal.

However when you have _multiple_ residences all sharing the same transformer, each with their own neutral-earth bond, you get multiple connections and closed paths. In urban areas, it is very common to have these multiple paths, especially since one important bit of bonded metal is metal water piping. If you share the same transformer and the same metal water pipes with another house, then you _will_ have a parallel neutral path. What this means is that neutral current will have multiple paths back to the transformer, some through the proper neutral wire, others via the grounding electrodes and that water pipe.

This parallel path can mean that current may flow on your neutral conductor that isn't balanced by the flow on the hot conductors. This _net_ current flow on your service would show up as a 60 Hz magnetic field surrounding the service cable. A bad neutral connection, either on the customer side or the POCO side, in your house, or a neighbor's house, could cause severely unbalanced neutral current flows, but even with everything _perfectly_ to spec, the parallel paths would still result in some unbalanced current. While the POCO or an electrician would correct any _bad_ neutral connections, they won't be obliged to fix unbalanced current if everything is up to snuff.

On top of this, you described a main disconnect outside, feeding a panel inside, but with _triplex_ between the two. This pretty well implies that you neutral to ground bond is located at the inside panel. If this is the case, it is violation of current code, and may also be causing the current flow that you are detecting. Correcting this issue might require replacing the feeder conductors between outside and inside, as well as moving any grounding and bonding connections. If you are having issues with current flow via these connections, then they would need to be carefully routed, otherwise you might spend lots of money to move the problem from one cable to another.

I repeat my suggestions above for your electrician. Look for improper ground-neutral bonds, look for improper location of the required ground-neutral bond, and try to measure net (unbalance) current on the feeder and grounding conductors. Your electrician will know how to evaluate proper grounding, and have the tools to measure current flow on the cable (it just takes a standard 'clamp on' current meter, but large enough to clamp around the outside of the entire cable set.)

-Jon
 
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