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Minimum distance for ground loop issues

S

SamBuca

Guest
Finally got authorization to lightning-proof the station after ANOTHER hit last night. We have 3 separate areas which need polyphasers...rack room, place by the bell demarc and main studio. It's a triangle...rack room at the point and demarc/studio at the base. It's about 50 feet from top to bottom and the demarc/studio are about 25 feet apart. Would putting the ground straps in those locations cause any ground loop issues? We're about 2-4 ground conductivity and the dirt is always dry as the building is raised with a crawl space.<P ID="signature">______________
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Hi, Sam....

Take a look at Nautel's website. They offer (I think on line but maybe
mailed on request) an excellent booklet on grounding practices for
transmitter sites and, by implication, studio sites as well.

In general, I'd venture you'd want a central cluster of several deeply-
driven copper rods, very close together, well bonded together just
above ground. Then, minimum 4" copper strap to each of the several
points where you need grounding. My experience suggests that you
should connect the straps only to the common grounding point; no
cross-strapping one to another. Take a look at the current National
Electrical Code about what you should do with the electrical system
ground; whether to connect to your central "station ground" or not.
It's been about 10 years since I dealt with this kind of situation
and have forgotten a lot.

Nautel has some strong feelings about the use of torroids on any
cable (especially RF) that enter the building and grounding of the
outer jackets on all such coax.

I followed the instructions religiously and, over a ten year period,
only had one piece of equipment slightly damaged by a nearby strike
and still haven't figured out how it sneaked by.


Good luck....


And.... as you start grounding various stuff, do a lot of audio
monitoring so that if you DO create a ground loop you'll heard it
immediately. Then deal with it before going any further so you
don't find yourself chasing multiple causes of what seems like
a single problem.
<P ID="signature">______________
Misanthropy:

Not just a hobby...a WAY OF LIFE!</P>
 
Don't forget that many lightning problems are caused by stuff coming in on the power lines. I assume you have some kind of protection at the box.

I would tie the power line ground, and any telephone or cable ground to the common ground as well. Often the power line ground is just a short grounding rod that isn't driven very deep. Telephone and cable grounds are usually worse than that. I've seen two stations (250 miles apart) with downtown studios in old office buildings where there really wasn't any ground for the power system. In one case we ran a 4" strap from a third floor loft where the studio was, across an attic crawlspace, then down the side of the building to where we could drive a couple of copper stakes into the ground. A couple of runs of #10 bare copper was run from this strap to the breaker box.

But then, I am in WV and don't have to worry about inspectors who know everything and nothing.
 
> Don't forget that many lightning problems are caused by
> stuff coming in on the power lines. I assume you have some
> kind of protection at the box.

None. There's a polyphaser on the dry copper pair from the telco (audio+control) and a polyphaser in line before the rack UPS unit on a large copper sheet ....I'm assuming that's supposed to be the grounding point. The cabinets aren't grounded, the tx lines come off the towers and go right into the phasor, current monitoring coax goes right into the potomac, the former stl antenna coax comes right off the pole and is dangling on the cabinet making contact, the old fm tx line comes off the tower and is dangling on the cabinet making contact, there's no surge suppression on the ac lines, the spark gap for tower 1 is missing, the spark gap for tower 2 is sitting on my desk, and (as far as I can see) there's no static drain on the guy wires.

As far as the studio goes, there is none at all. Standard ground wires coming off the power box/telco demarc and UPS units on all the equipment...but it keeps getting zapped through the phone line.<P ID="signature">______________
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> Finally got authorization to lightning-proof the station
> after ANOTHER hit last night. We have 3 separate areas
> which need polyphasers...rack room, place by the bell demarc
> and main studio. It's a triangle...rack room at the point
> and demarc/studio at the base. It's about 50 feet from top
> to bottom and the demarc/studio are about 25 feet apart.
> Would putting the ground straps in those locations cause any
> ground loop issues? We're about 2-4 ground conductivity and
> the dirt is always dry as the building is raised with a
> crawl space.
>
Ahh go with SINGLE point ground....it sounds like you have a ground loop issue now!!! All grounds should Y from their points in the room to a common connection that then goes back to a low impedance EARTH ground...
In your triangle, run your common ground out of the rack room to earth...
then run SINGLE large conductors (Doesnt have to be flat copper) from that point to your other rooms...tie ALL grounds to these single runs..now all your rooms have a single common ground point and zero reference spot...otherwise, putting Polyphasers in wont help (The company that OWNS Polyphasher just put a Lightning protection class on 2 weeks ago that I attended...but then I already have been doing single point grounding for YEARS)...
DONT ground each room separately...that CREATES ground loops...you want everything to rise and fall at the same potential...thus no delta V and no current flow= no damage
Around the outside of the bldg, you want to put MULTIPLE ground rods...all CADWELDED together in a ring and your SINGLE ground cable coming from inside out to this ring...(run the ground cable out through PVC conduit; not metal! Acts as an choke if you do and causes lightning to go otherpaths...NOT as you intended)
 
We used to have an AM--daytimer on 630/ 230' tower. Everytime we took a zap from lightning it came down the power line--despite having the gas-type supressors (which you can't get no more--they had a small amount of radioactive material in them). Though each time the damage was very minor--tripped breakers, blown power supply in the Moseley 1600 remote control.

At our one FM we ran a 2-ought (if memory serves me) from the power pole 300 feet to the tower. Separately from the 3 wire 220 power feed. Baffled an electrician installing our generator a month ago, but the reason was that the tower was a better ground than the electrical ground at the pole. We didn't want any EMP induced into the 220. This was tied to the tower, then strap into the building. Worked fine until this summer. This summer we started loosing suppressors, had breakers trip, etc. Finally discovered that a tower crew had cut the cable off at the tower, and the stub worked it's way underground so we didn't spot it immediately.

Looks like you have the makings of a nice winter project. At the transmitter, I would ground the coaxes as close to the entry point as possible (though you can't do that with the sample lines without throwing off the monitoring system), tie the racks all together with copper strap (scrape the paint off the rack, drill a hole or two through the strap & use the screws holding the rack together to tie down the strap); as well as tie strap to the ground point in the transmitter and phasor, then tie all the straps together at whatever passes for a ground system or screen if the tower base is close.

Surge supressors in the panel box (they install right in the knock-out holes--available from your local electrical supllier) as well as the plug-in ones you can get at the local hardware store for anything with IC's in it (remote control, processor, STL receiver). If you have any of the modern digital audio processors I would put it on the ups if it isn't already.

At the studio, I would install surge supressors in the entry box. The polyphaser or similar pass-through for any coaxial cables coming in, or at least the grounding blocks for "F" connectors for things like satellite or TV cable.

Strap or some kind of grounding to tie racks together. Old RG 8 with copper braid is great source of a rugged copper braid for tying racks, other stuff to ground points using hose clamps, racks screws, etc. Just did that today in fact for a new rack I am putting into our engineering/junk room (more of the latter than the former recently). Used the braid to tie the rack to electrical conduit & to incoming RG-6 from the satellite, as well as RPU/ EAS receivers coax feeds.

Also buy one of those electrical testors to see if grounded outlets are really grounded. As well as making sure that equipment with 3 wire ground ac cords still have the ground connected. At one station I worked at, we bought a competitor and inherited cart machines (audocords) with the ground pin removed as an "improvement," (they ran a separate #18 wire to a ground bus). Unfortunately, the power supply used a center tap ground on the power transformer which was left floating if the power cord didn't have this 3rd wire ground. Those machines liked to bite if you were not careful.

A lot of mischief was created by the folks at the old Auditronics corp. The Grandson manual had some strange ideas for grounding that could be darn near fatal. Having everything tied to a good ground is a starting point before trying to run down hums and buzzes. (Copiers!- at one station we had an intermittant buzz that was tracked down to a copier with a heater that switched on and off in idle mode. A simple fix was to solder a pair of 250 volt MOV's across the lugs of a replacement power plug, tape it up real good, and stick it into the outlet just above where the copier was plugged in. A wiser fix, in retrospect, would be one of those plug-in MOV's from The Shack.)

Have fun.
 
Had the phone company in for a consult and line testing today. Ground panel at the xmtr was balanced but the polyphaser was shorting...obviously toasted. Now I've traced more hums back to the studio.

Whoever wired the phones used untwisted wire in a spaghetti configuration and ran it right up against the 200 amp power box. As soon as I punched down twisted pairs and rerouted some of it, the hums were gone. I fully expect to find more fun and excitement like that, along with ground loops, in audio paths...it all ties into the grounding issue.<P ID="signature">______________
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My studio and FM transmitter site are combined--17 kw only 150 feet above the studio.

Resisted for years removing the old ITT phones and relay based pbx (1A2 key system). Even with this system we've had occasional RF problems since we do a lot of remotes over phones lines. But there comes a time when you can't get parts...

Went with simple AT&T (no, not The AT&T--they bought the rights to use the name) 4 line electronic phones, which have no pbx--you run tip & ring to each set for each line. To run the lines to each room I used 8 wire (4 pair twisted) CAT-5 cable. Only about $50 for 500 feet of wire. Have had no RF problems at all, phones work fine, remotes quiet.
 
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