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POTS Line Isolation Transformer(s)

B

Brian Bowers

What are the best transformers to use for POTS lines, for lightning protection? Currently, two phone lines travel from one building to another, underground, for about 300 feet. We've had problems with underground lightning induced fields on these phone lines before. I would like to isolate the underground run with transformers on each end. Which ones are the best to use in this case? Thanks.
 
You call them POTS lines. Are the lines connected to telephones? It's important to know what the lines are being used for.
 
I have several runs of buried cat 5 cable running about 300 feet from one building to another. It's in PVC conduit. They aren't used for phone, but rather data and balanced audio from the studio to our STL tower. I have had a ton of of problems with lightning induced fields. The soil has a heavy iron ore content, which I suspect is a part of the problem. I have Polyphaser lightning arrestors on each end of the cables, 1:1 Altec/Peerless audio transformers on each end of the audio lines, and have bridged them with MOV's. The STL tower end has a halo ground system as well as four 10 foot ground rods, all terminated to a single 3/8" thick copper buss bar that is the common ground source for all the electronics in the building. All of this has helped immeasurably, but it has still not completely fixed the problem. From time to time, we still blow up whatever is connected to either end of the lines. To keep expensive components from frying, I've found it is fairly effective to put a sacrificial device in line at each end of the cables in question. For data, an inexpensive switch seems to protect the rest of the network. For audio, I've been using Behringer MX822 audio distribution amplifiers as a sacrificial component. http://www.behringer.com/assets/mx882_P0056_M_en.pdf Contrary to what some people will say, they sound reasonably transparent and are cheap enough to be disposable. It certainly beats blowing up the console or audio processor.

With all of this, it still isn't the ultimate answer. We still get the occasional nearby lightning strike that wreaks havoc with the equipment. I think the answer most likely installing fiber from one building to the other. I've tried wireless IP solutions using but they haven't been much more reliable. While this isn't completely the same problem as Brian is having, it certainly is similar. Any suggestions would be appreciated.
 
Frankberry - one line is connected to the in-house PBX, with a phone on the wall for it, and the other is a true POTS line, connected to the remote control, with a phone connected to the remote control. I do use TRX surge protectors on one end, but since we have had such great success with our Jensen transformers for the audio lines (basically eliminating lightning disasters in studios), I'm assuming that the best protection for POTS lines are also transformers. Didn't AT&T use transformers for Dedicated Audio Circuits?
 
Telephones require a d.c. circuit. You can't use transformers. Your best bet is to improve the grounding of the equipment on each end of the phone lines. Lightning seeks the easiest path to ground. The easiest path should not be the buried phone circuits.
 
I'm a little behind the times on current practice, but POTS lines normally depend on some flow of DC current as part of the control mechanism. (Off-hook. On-hook, etc._) Putting a good transformer into the series tends to mess up the DC current aspects of a phone circuit.

There! I've told you more than I know! :cool:
 
I will admit that since we installed the Jensen transformers (the model # escapes me for the moment) on the audio lines back in 2007, we haven't had any blow ups (I hope I didn't just jinx it), but the two phone lines still concern me. BTW, sorry, not TRX, but ITW Linx. Can I (or should I) add anymore surge protection to the phone lines, besides the ITW Linx protectors? The tower gets struck about 12 times a year, and thankfully, keeps draining it's strike to ground, without any problems. My motto is, all lightning protection is preventative.

http://itwlinx.com/product_classification/66-block-protectors/
 
To Chuck: This is what we used on our audio lines (an XLR version is available).

http://www.jensen-transformers.com/product/din-li/

We installed one (two for stereo) at the studio site, and one at the transmitter site (300 ft. in between, with underground shielded cable). These transformers instantly cured our audio console lightning problems. There is a specific method to connecting cable shields to ground (or letting them float), when using this system. The main guy at Jensen is very knowledgeable on this, and is very willing to educate one on his transformers, and how to use them.
 
Brian,
Transformers are only a band-aid. The induced surges which are caused by lightning are seeking the lowest resistance path to ground.
At the studio, you should have some long copper rods sunk into the ground (go through the building foundation, if necessary).
Likewise, you should have copper ground rods at the base of the tower and at the transmitter.
If everything is at earth ground, you should have no further problems.
 
OK, maybe it wasn't the transformers all by themselves, or maybe it was a combo of that, and at around the same time (although it took months), I read up on transmitter/studio lightning protection, and started tying all grounds together. The tower and transmitter building (one location) have very decent buried ground rods tied together. All connected to a strap which enters the transmitter building. Someone was thinking, and ran a strap underground to the office/studio building, from that tower strap. What I did was tie all grounds (building rebar, main AC electrical ground, water pipe, studios, the list goes on) in the studio/office building to that strap from the transmitter building. I also installed a busbar in the transmitter building, and ran an individual ground for each piece of equipment in the racks, to that busbar. Just around this time, everything seemed to settle down. As I understood it, when lightning strikes, you want all grounds to rise and fall at the same time, or at the same ground potential. Did I forget anything?
 
You got it pretty much correct. Everything should be at the same ground potential. That's the best that you can do.
Long lines will still get an induced voltage from a lightning strike but when buried, the induced voltage should be minimal.
 
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