Grounding Antenna Question

Ref: NEC 810.15, 810.20, 810.21

Both the antenna mast and the signal lead-in (via a listed antenna discharge unit) should be grounded IAW the above sections.

Nothing prevents lightning strikes but the effects can be mitigated.

"Humbled Survivor"

** Yep.

The pole needs to be linked to metal plumbing with a heavy gauge copper wire.

** If it is isolated by a "balun", then yep - it a good idea too. ** Can I suggest you make your peace with god right now ??

... Phil

actually, grounding it will attract near by strikes, but what ever..

THe emf pulse of a strike directly on a grounded mast will more than likely cook the front end of what ever is connected to it and maybe even jump around a bit.

Jamie

But the provisions of the NEC are not to protect from direct lightning strikes to an antenna - much more elaborate protection is required. They can provide protection from other surge sources, like near strikes.

For best protection the ground from antenna line entry protectors must connect with a short wire to the power earthing system. You want to minimize the voltage between power and antenna wires.

Not according to what I read.

Hams with high antennas routinely protect from direct strikes to their antennas. The rest of us are not likely to install the protection hams install. But most of us do not have antennas as exposed as some ham antennas.

I am a HAM, so if you're getting your information from there, then you better read between the lines..

Ham radio towers get hit many times in a season, they are not immune from it.

NEC guide lines is there to protect people and that means the structures people live or work in. THey don't give a rats ass about antennas and devices connected to them, getting destroyed, they encourage it. That way the lightning will be drawn away from the structure in hopes that any thing conducting to ground, your antenna and mast, will steer it away.

In most cases as for HAM radio, the radials are horizontal because most of them put their beams at the vary top. normally you extend the mast above the mounting axes. I add a

The tower our 2 meter equipment is at has lightning rods at the very top with a beacon light, however, it does not always save the day. antennas mounted on the side of the tower still get a nice strong pulse.

We use polyphaser protection devices, they seem to work the best for what we need.

Jamie

It's illegal to use a pipe for ground in the US. The joints corrode, and are high resistance to ground after years of use. You had to install a bonding jumper around a water meter, years ago when it was legal.

Some fools have used the wrong pipe for natural gas, then another fool assumed they were water lines and used them for a ground. The results were explosive and sometimes fatal.

Water was run in galvanized pipe, and natural gas in black iron. Today, both are installed in various types of plastic. Some very early water lines were hollowed out oak, and coated with pitch.

Roger all of the above.

Re-reading the OP's question, he may have been asking if he could ground the mast to the premise ground and bond the signal downlead discharge gizmo to the mast's ground wire.

\ / \ /---. || | m || | R a || | F s || | t || | || | ,-++---[ ] discharge unit | || | | `---------RF--> | | grounding wire __|+___ || premise ground rod ||

If you get a 100,000 ampere lightning strike directly to anything other than a metal building, the V=IR (voltage = current x resistance) drop is going to raise the voltage of everything tied together many many volts. All you can do is try to keep everything at the same (elevated) voltage to avoid killing people/animals. This is done by tying everything to the same local internal "ground". By having a low impedance to earth ground using grounding rods and water pipes, you try to minimize the difference between the local ground and the earth ground. The current from a lightning strike will induce many thousands of volts in any conductor a few feet long within 1000 feet of the actual lightning strike. I have seen an arc form from a tv lead-in wire to a ground when lightning hit 2000 feet away. The arc jumped a 1" air gap, so the arc voltage was well over 1000 volts. A protection device on the lead-in wire would probably have kept that voltage under 100 volts. So grounding the antenna mast helps, but a protection device on the lead-in is adding suspenders to a very weak belt.

And hams with high antennas are very likely to have good protection from a direct lightning strike.

The OP is very unlikely to have antenna that is significantly exposed.

The provisions in the NEC, which Rich referred to, are not intended to protect from a direct lightning strike and won't protect from a direct strike. NEC compliant wiring will not 'draw' the lightning away from the structure.

The NEC *requires* a metal water service pipe (minimum 10 feet long in the earth) be used as an earthing electrode, just like it has since time began. Connection now has to be withing 5 feet of the entrance to the house, and meters still have to have a bond around. Because plastic water service pipe is becoming more common a "supplemental" electrode is now also required. A metal municipal water system is the best earthing electrode that is available at a house.

Gas service pipe is not allowed to be used as an earthing electrode by the NEC. But gas pipe is grounded by branch circuits, like at gas furnace.

CSST (corrugated stainless steel tubing) is becoming real common for interior gas pipe because it is so easy to use. It is easy to use because the wall is so thin. There have been many fires from arcing between the thin pipe and nearby grounded surfaces. I believe all the manufacturers now require the pipe be bonded to the house earthing system in a way that results in the gas supply pipe being an earthing electrode. 'Properly' bonded CSST has also caused fires. A recommendation by an electrical inspector is for electricians to not do the bonding, then they will not be named in the lawsuit.

This is a fantasy, unless you have 1/4 " x 3" bus bars running the length of your house, and from the main service panel to the ground field, which would be maybe half an acre of bare welding cable buried in the earth.

Otherwise, you simply can't get a low enough impedance to keep the IR drop anywhere within reason. And, the induced voltages across various points such as plumbing and plugged-in appliances can be lethal. (later part of your post seems to agree with this.)

So, mostly, you want to try to conduct lightning strike currents away from the building, not THROUGH the building. So, what I have is a ground rod driven directly under the antenna, and connected to the mast via 4 gauge cable. If the lightning current goes through the house, it can easily set it on fire.

Jon

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Jon, Please note that I said you can "try", I didn't say you "could" keep the IR drop low. I agree with all your other comments.

Not everyone is on municipal water, or even have a water meter. My well is physically over 100 feet from my electrical service, in another building. In fact, the actual pump is 20 feet past the original well house so a bonding wire between the pump and the pole mounted drop would be worthless. The wire is over 150 feet, and goes through three breaker boxes before it reaches the wellhead, and another 85' downhole to the pump itself. The casing is well grounded, and so is the old casing from a 50' well that went dry.

They used orange plastic pipe for gas service at the last place I could get natural gas. There was a 14 AWG tracer wire run along the side of the plastic, in case it needed to be located in the future. At that time, only black iron was allowed. I should know, since I had to replace the 40+ year old pipe. It was real fun getting a 21' piece of black iron down a 10' stairwell. You should have seen the inspector scratching his head, trying to figure that out. ;-)

I wouldn't use that, unless the local code demanded it. Even then, I would consider having no gas appliances, or furnace. I haven't used the furnace here, in the 14 years i've lived here. I am going to take it out, since the firebox is now so rusted that it wouldn't be safe to use.

"bud--"

** AFAIK - the main reason TV antennas are required to be grounded is NOT because of possible acts by a mighty and vengeful god.

The simple reason is that the antenna may become live AC supply voltage and so make the TV set (and anything attached to it) inside the premises into a lethal shock hazard.

This could be because an overhead power cable has detached in a strong wind or because of an automobile accident etc.

ALTERNATIVELY - the antenna may become live due to a faulty equipment or wiring inside the premises and so present a lethal hazard to any poor damn fool who is game enough to get up on the roof for any reason.

Grounding the damn antenna any which way you can is essential.

.... Phil

The way it is done is all wires - power, cable, phone, antenna, dish - enter at about same location. Entry protectors for the signal wires connect with a short wire to the power earthing system. A surge protector at the service panel limits the voltage of power wires to the earthing system. In an 'event' the building "ground" can rise thousands of volts, but all the wires rise together.

It is not perfect protection. During an 'event' a pad mounted A/C compressor may have its ground potential at the earth where it is located. The power system, and power wires to the compressor, can be referenced to the earthing system, which can thousands of volts different.

A ham with a high antenna would have a better earthing system and the common reference point may not be at the power service.

Suppose you have a power system earthed by a single ground rod. The antenna entrance to the house has a ground block, as required by the NEC and an 18 ft ground wire to the power earthing system, as required by the NEC. You add a ground rod at the entry protector. The NEC now wants a #6 or larger bond wire to the power earthing system. Also suppose the ground rods have a near miraculous 10 ohms resistance to earth.

Now suppose the antenna is hit by an "average" 20,000A strike. The potential at the rod will be 200,000V from 'absolute' earth potential. In general 80% of the voltage drop will be in the first 3 feet (which probably results in arcing across earth at the rod). The potential of the antenna lead will many tens of thousands of volts from the potential of other wiring, referenced to the power system ground rod. There will be major damage. The #6 (and your #4) bond wire help, but not significantly. A lightning strike is a very short event and produces relatively high frequency current components. The inductance of the wire is much more important than the resistance. Inductance doesn't change real fast with larger wire. There will be arc-over from the antenna and down leads into the house anyway.

Now suppose there is a 5,000A surge to earth on the power earthing system. The power system 'ground' will be 50,000V above 'absolute' earth potential. It will be tens of thousands of volts from the ground rod for the antenna. Power wires and antenna wires will be many thousands of volts different. That will cause damage to anything connected to both. Without the antenna ground rod the antenna lead would be lifted with other house wiring. The same thing happens with a strike to a tree in the yard with one rod nearer than the other.

I am not a fan of isolated ground rods. If you really want to protect from a direct strike you need lightning rods.

And suppose the antenna lead goes close to a CSST gas pipe, which is bonded as the manufacturer requires. There is an arc from the antenna lead to the CSST which melts a hole in the 0.008" wall. If you are lucky that just ignites the escaping gas and causes a fire. (My intent is to comment on CSST, not so much the antenna ground rod.)

Other reasons are to earth static charges from the wind and control surges induced by near strikes.

Well, if you do this, you probably end up destroying everything in the house, including the wall outlets and Romex in the walls. I have heard of people having this happen, where all the Romex was just empty plastic tubes with burn holes every couple inches where the copper vapor escaped. All the wall outlets were blown out of the wall boxes and reduces to fragments. Having to run new Romex inside all the interior walls runs something like $50K now.

My idea, which doesn't meet code, is to try to draw the main currents AWAY from the rest of the house and possibly preserve some of the appliances. Whatever is connected to the antenna will be complete toast either way, but isolation may help some of the appliances survive.

Jon

It is, essentially, how hams protect high antennas that are likely to get hit. The intersystem bonding is heavier and the earthing hams have is much more extensive.

An electrical inspector in another forum has a weather station that is on a metal post with the post tied to the power earthing system It has been hit twice. Minimal damage on the first hit and with fairly minor added protection he had no damage on the second hit. The weather system has a data lead that comes down the pole.

If you want to protect from a direct lightning strike install lightning rods. Lightning rod protection is more complicated than you antenna earthing scheme and your antenna will not substitute for a lightning rod. Side flashes into the house are likely. And damage is much more likely from power service surges and near strikes. Lightning rod systems are required to be bonded to the power earthing system.