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Author Topic: "Safety grounds" and ground loops  (Read 3338 times)
KE4JOY
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« on: June 21, 2013, 01:23:46 PM »

Something I have always wondered.

As I understand it all equipment in the 'shack' should bond to ground at a single point, xmitter, amp, tuner, etc., all to a strap then to an 'earth ground' which is pretty much how I have my station set up.

Thing is though the transmitter and a receiver both have an electrical 'safety ground', the third pin in the AC plug that bonds to the branch circuit ground which runs probably over 100' before it is bonded to its earth ground at the serving panel. This safety ground is typically bonded to the case of the equipment so that it in essence creates a long ground loop between the station ground and the ground at the serving panel. I believe the NEC says that bonding the branch circuit ground to the earth ground out at the shack is a no no but isnt that exacly what has been done?

I think the station earth ground should be bonded to the serving panel's ground but as I said its over 100 feet away and that would be yet another ground path.

Could I get a little clearing up on this?
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AA4PB
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« Reply #1 on: June 21, 2013, 01:40:33 PM »

1) Bonding means a permanent ground connection, not one passing through a plug and cord. You are required by the NEC to bond the station ground to the electrical service ground.

2) Normally there should be no current flowing in the electrical safety gounding conductor so even though the line is long it doesn't create a ground loop. High resistance with zero current flow means zero voltage difference.

3) People often attribute all sorts of malfunctions to "ground loops" without understanding the problem. Ground loops are not as big an issue as one might think. If you have two devices connected via a shielded audio cable and the two devices are connected to two independent grounds that happen to have a voltage difference between them then you can have a ground loop that injects AC hum into the audio. This is not normally a problem in an radio installation because the equipment is located close to each other and connected to the same grounding system.
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pmraiders

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« Reply #2 on: June 21, 2013, 02:09:00 PM »

I drove a rod in the ground. Attached some heavy gauge wire to it. Ran it into my shack and connect all my equipment grounds to a ground terminal bar so I can easily expand.

With the amount of random electronics running on our house's old electrical system I wanted my own dedicated grounding system. Since you are already piping coax or ladder line into your shack from outside run a ground too!
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M0HCN
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« Reply #3 on: June 21, 2013, 03:34:35 PM »

Danger in doing that is that a fault in the suppliers network can force an awful lot of current via the earth conductor in the supply cable, the radio chassis and to earth via your earth rod, potentially starting fires.

For this reason the NEC requires that all the ground rods be bonded together.

W8JI has much on this subject on line and is worth the time http://www.w8ji.com/house_ground_layouts.htm

73 Dan.



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K6AER
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« Reply #4 on: June 21, 2013, 03:41:59 PM »

Running a separate non bonded  ground on you ham shack is a recipe for disaster.

Should the high voltage surge arrive at your home via the incoming AC mains it might not find a ground path at the circuit breaker box and go looking for an earth return ground.  Your AC panel should have a commercial surge protector at the panel. The panel ground must be bonded to all other grounds including the ham shack and the tower and antenna feed through panel.  If all the grounds are not bonded the AC surge will go through the ham equipment to the ground you just put in at the ham shack or tower.

Bottom line is the ham equipment gets fried when the grounds are not bonded together.  Most AC panels have very poor grounds and may not have any due to corrosion after a few years. It is a good idea to check the ground conductivity with a ground resistance meter.

For more information visit the article I wrote many years ago at: http://www.eham.net/articles/13461
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AA4PB
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« Reply #5 on: June 21, 2013, 03:53:44 PM »

To clarify, the NEC requires that the elecrical "grounding system" and the radio "grounding system" be boned together via a minimum #6 conductor. That implies that the ground rods or the wire connected directly to them be bonded together. Using a #14 or #12 branch circuit ground conductor feeding an outlet does NOT qualify.
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K1CJS
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« Reply #6 on: June 21, 2013, 08:26:26 PM »

The one overriding reason that all ground rods serving a building be bonded together is to keep stray currents on the ground conductors OUTSIDE the building.  In other words, to keep any current away from the equipment--AND THE PEOPLE in that building.  Their concern is to keep the people safe and protect the building.
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pmraiders

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« Reply #7 on: June 21, 2013, 11:24:38 PM »

The one overriding reason that all ground rods serving a building be bonded together is to keep stray currents on the ground conductors OUTSIDE the building.  In other words, to keep any current away from the equipment--AND THE PEOPLE in that building.  Their concern is to keep the people safe and protect the building.

Can I simply redirect my equipment grounds toward the ground rod I drove and completely bypass the problem? I will have to lay ~100 ft of #6 wire if the house ground is where I think it is.

Edit: I read that article and it seems like a relatively simple way to greatly increase the safety of my household and my equipment. A project for next weekend.
« Last Edit: June 21, 2013, 11:31:10 PM by pmraiders » Logged
AA4PB
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« Reply #8 on: June 22, 2013, 05:04:35 AM »

Years ago I had a case where the transmitter case was connected to the electrical ground and the coax shield was connected to its own ground rod outside. When disconnecting the coax from the transmitter I received a small shock. I noticed that when touching the shell of the PL259 to the connector on the transmitter I could see a small spark. If there had been a #6 bonding wire between the electrical ground and the coax ground rod then there could not have been a voltage difference between the two - no spark and no shock. That's why bonding connections are required to be permanent, not something that someone can easily disconnect.
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WX7G
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« Reply #9 on: June 22, 2013, 06:24:11 AM »

Ground loop? Yes.

Connecting your station ground to the AC service ground with a #6 wire reduces but does not eliminate the potential for lightning current through the house AC wiring.

For example, if your radio room AC power consists of with three #12 wires it offers 1.3X the resistance of the #6 wire. About 40% of the lighting current (from the shack ground to the AC service ground) will take this path through the house.
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K4SAV
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« Reply #10 on: June 22, 2013, 06:53:06 AM »

Something I have always wondered.

It's nice to hear you are thinking about the problem and not just reading something someone said and implementing that.  Living in Tallahassee, you need to know this stuff, and implement something that works.  Tallahassee isn't the worst place for lightning, but it's bad enough.
 
Thing is though the transmitter and a receiver both have an electrical 'safety ground', the third pin in the AC plug that bonds to the branch circuit ground which runs probably over 100' before it is bonded to its earth ground at the serving panel. This safety ground is typically bonded to the case of the equipment so that it in essence creates a long ground loop between the station ground and the ground at the serving panel. I believe the NEC says that bonding the branch circuit ground to the earth ground out at the shack is a no no but isnt that exacly what has been done?

Yes that's what NEC says.  You are required to tie all the grounds together, ouside the house.  Look at what how the phone line, water line, TV cable, and any other line you have entering the house, are grounded.  They all have a wire running to the AC entrance panel ground rod.

I think the station earth ground should be bonded to the serving panel's ground but as I said its over 100 feet away and that would be yet another ground path.

That 100 ft wire would satisfy the NEC requirement, and probably be an OK ground at 60 Hz, thereby providing safety for 60 Hz faults.  However that won't protect your house from lightning damage.  With that 100 ft run, as you noted, there are now two paths for the lightning current to take, one thru that 100 ft wire and one thru the wiring inside your house. Those paths are in parallel and the lightning current will split between those two paths depending on the impedance of the paths.  

The impedance of a 100 ft long #4 wire at 1 MHz in free space is about 360 ohms.  Buried in the ground it will be less and it will conduct current to ground along its length, but it still going to be a large impedance.  It's a very poor ground at 1 MHz (where lightning has a large component).  The amount of lightning current you have in this wire depends on how the rest of the system is grounded, but when you start with a 20,000 to 50,000 amp pulse distributed over a very wide frequency range, you can assume there will be a significant amount left.   Multiple that times the impedance of that long wire and you will see there are lots of volts drop.  That forces lots of current thru your house, enough to take out lots of equipment.

Since the impedance of a wire at 1 MHz is almost due entirely to its inductance (resistance is insignificantly low by comparison), and you said that the path of the wiring thru the house was also about 100 ft, for a rough assumption you can guess that the amount of current thru your house will be about the same as in that 100 ft ground wire outside.

To meet the single point ground requirement that long wire has to be short enough to drop insignificant voltage so that it doesn't force currents thru your house.

Not having a wire connecting those two grounds and having a separate ground rod for your radio is worse yet.  Probably about half the lightning current entering from your antenna or from the power lines will go thru your house.

Jerry, K4SAV
« Last Edit: June 22, 2013, 07:00:30 AM by K4SAV » Logged
K1CJS
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« Reply #11 on: June 22, 2013, 07:25:52 AM »

Can I simply redirect my equipment grounds toward the ground rod I drove and completely bypass the problem? I will have to lay ~100 ft of #6 wire if the house ground is where I think it is.

Edit: I read that article and it seems like a relatively simple way to greatly increase the safety of my household and my equipment. A project for next weekend.

You should have a ground rod located near the shack's cable (co-ax) entry point to your house.  Laying a long ground cable to a remote grounding rod doesn't give you the protection you need since that long ground cable has some resistance along it--resistance that may well cause the charge to jump to a closer grounding point.

My recommendation is to check with the local electrical inspector or a local licensed electrician.  That way you would have the opinion of the local authority--and avoid any mistakes that may cost you big money in the long run.
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WX7G
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« Reply #12 on: June 22, 2013, 07:31:12 AM »

From the standpoint of lightning current thru the house the ideal setup is to bring the shack cables and ground in at the AC power entry and ground it all together at that point only. The house/shack is now hanging off to the side of the AC/lightning ground and there will be no lightning current thru the house.

The lightning current waveform is generally modeled as having a 1.2 us rise time and voltage drop calculations/simulations are best performed in the time domain. But if you would like to use the frequency domain 300 kHz is a number to model the leading edge.
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K4SAV
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« Reply #13 on: June 22, 2013, 07:35:01 AM »

My recommendation is to check with the local electrical inspector or a local licensed electrician.  That way you would have the opinion of the local authority--and avoid any mistakes that may cost you big money in the long run.

You need to know who you are talking to.  The guy that wired my house knew all the electrical codes and everything passed inspection, but he didn't know beans about how to install lightning protection.  That may not be true for all these guys, but you can't assume that because they are licensed they know anything about lightning protection.  Typically they don't have to know that for wiring a house.
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AA4PB
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« Reply #14 on: June 22, 2013, 07:46:16 AM »

The *primary* purpose for the NEC requirement to bond all grounds in a building together is personnel safety. They want to ensure that two grounded devices in the same home cannot be at different potentials thereby presenting a shock hazard to people. Suppose you have one hand on the water faucet in the kitchen sink and touch the toaster with the other.
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