Rental Property Management Software

My cable coax caught fire!

NOTE TO THE READER: This conversation occurred in late 2003 on a message board where I was a regular. At the end of this conversation, I make the comment that this "is so obviously a bad practice..."

Since that time, I have discovered a substantial number of cable installations that ground themselves to water pipes. Be aware that this violates proper procedures and is extremely dangerous. All that is needed is for anyone anyplace in the building to have EVER made a connection between an electrical neutral or ground to a waterpipe and you have an enormous fire hazard if the cable system gets grounded to the waterpipes.

Grounding electrical systems to waterpipes is a common practice in older homes.

Most people don't understand grounding systems and ground loops - and the danger that a ground loop represents. Pay careful attention to the conversation here; it could save you from having a fire.

I never thought this could happen. I have a new cable modem, and I needed to split my signal 3 to the modem, one to the tv, and the other to my computer's tv tuner card.

I bought quad shielded cables and crimp connectors and made my own cables and set everything up...

but I was getting hum on my computer speakers that went away when I removed the cable input from the back of the tv...a sign of a ground loop between the electrical ground and the cable ground. No biggie for now, I'll just get a groud loop interruptor and see what happens. I had not done that yet when last night...

I'm sitting watching tv when I see a spark from behind the tv. By the time I walked up to it, there was a 3 inch flam coming out of a melted part of the coax cable that ran to the computer! I stomped it out and removed the cable from the wall. Had I not been in the room I could have lost my house to a fire!

Has anyone else ever heard of anything like this? Has anyone ever thought you could have a fire from your CABLE line? Home-made cables or not?

You have a ground loop in your wiring. I have seen ground loops carry many hundreds of amps. In your situation, I would take a look at the geometry of the ground loop, and I would then look around for nearby field sources such as high tension lines.

No, I have never seen a home cable coax burn. But I have seen other things burn from ground loops, and I can readily believe it.

You need to take a look at your wiring topology to make sure it is correct throughout the building.

Well, all of my outlets are wired correctly so there can't be any shocks from that. I live in a condo, so I can't check the entire building. Could there be a short from some other piece of equipment to my cable coax? I have played with this thing further, and I can see a small spark from the cable shield to the TV when I try to connect them.

Any such short will very quickly be very visible. It'll shock you. Literally.

Sparking when making the sheld connection basically says you have a substantial voltage across the gap. If this is shield to ground sparking, then you either have a serious fault in one of the devices connected, or you have a BIG ground loop, that is taking in a LOT of flux from some nearby source - probably the power lines in the building. This will be in-phase signal, so if the geometry is right you could induce very large currents in a big enough loop.

Assuming this is a ground loop problem, will a "cable circuit ground breaker" solve the problem?

Your grounding system, throughout your house, should always resemble a tree with branches. All grounds are connected to one point only, at one end only, and the ground for the building goes to earth at one point only.

If your grounding system is somehow not right, then one of those devices will work fine. You possibly can deduce your ground topology by buying one of those things, and connecting it in different places in the system and measuring the ground currents.

Given that you live in a condo that has undoubtedly been worked on by many different people over time, with little or no appreciation for the "finer points" and given that the cable system is a retrofit, then you could well have a ground loop that extends well outside your unit. Most of the time, the geometry of the ground loop will be closed enough that not a lot of flux is trapped, or the loop may geometrically resemble a figure 8, so that induced voltages in one part cancel induced voltages in another part. Consequently, the currents don't get too large.

If you indeed have a ground loop, then it must be a big, wide, open loop that is trapping a lot of magnetic flux, probably from power lines.

Alright. I have tracked this whole thing down. Made a lot of tests, and here is what I learned. First, all of the electrical outlets were wired correctly.

2nd, I measured 20 Volts AC between the cable coax shield and the electrical, 3rd prong ground plug of every outlet I tested.

I placed a ground interrupt in the incoming catv line, and could also measure 20V AC between both ends of the device, confirming a ground loop.

I have 2 televisions hooked up to two independent catv inputs (meaning the cable company did not just split one line to get my two catv inputs into the house, each one has its own connection to the junction box). There is about 20 VAC on both catv inputs.

This suggests an improperly grounded cable junction box.

I went out to the cable box on the outside of the condo complex. It seems they have grounded the box to a water pipe of some sort. From what I have read on the internet, this kinds of cable boxes must be grounded to the main electrical ground wire (the one that actually does go into the physical ground). However, the electrical meters/boxes/etc are more than 50 yards away on the other end of the complex. So maybe it was impractical to do this...

BINGO! Actually, though I hadn't said anything, I was thinking to myself "I'll bet the cable system is grounded to a water pipe." I didn't say anything because grounding a cable system to a water pipe is so obviously a bad practice that I am somewhat astonished that anyone would do it. Clearly, the cable company needs to do some competency testing/training of its installers.

Hence your giant ground loop: along the water pipe (basement?) through the entire building. Then, through the coax cable routed through the attic, to your TV and electrical equipment, then to the ground in your equipment, through the house wiring back to your meter panel, thence to the neutral in your wiring, and I will bet you a BUNCH that someplace in your building there is a ground connected from electrical service to that water pipe - thus completing (via the neutral in the House electrical feed) a giant ground loop that traps a LOT of flux.

About the Author: Jim Locker is a technical guy who has done a lot of real estate investing and landlording. The experiences he writes about and advice he gives are either first hand, or in answer to specific questions posed by others. He is commonly known as jiml8 around the internet.


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