Lightning protection

[VK4WDM]

Hi Barry

I believe that the cable should be bare copper not insulated, correct?

73

Wayne VK4WDM

[VK4TI]

I was tempted to take a shot but in a word , yup

[VK2XSO]

but what say the tower was 25m away from the shack how can you ensure that the radio and the tower are bonded to the same earth then?

Use good Earth cable !
No.. not 2.5mm auto electrical cable

In the real world if my tower was 25m away, I'd use galvanised copper strap. I look for it as scrap. There is always offcuts of it being thrown out.
It's just a matter of drilling two holes in each end and joining the pieces together. You can buy it longer lengths but hams aren't well known for spending big on important things like earth cable.
http://www.earthing.com.au/ACT_EARTHING ... ducts2.jpg

Of course you can use just steel cable (old power cables or offcuts) That's easy and cheap to find as scrap. Same for insulated or uninsulated copper wire. It really doesn't matter what you use.
The important point is that it is low resistance as possible. It can be insulated, but my preference is not as it's part of the earth mat. Earth stakes at each end are a compromise.
35mm^2 earth cable is thrown out all the time and you can buy it from scrappies. Don't be afraid to join them with the uninsulated earth clamps and just burying them or wrapping them in denso tape if you're worried about them corroding.

[VK4WDM]

Dear All

Once again we have a collection of excellent advice from experts in the ham community on something that is absolutely essential to the safe operation of our stations. IMO this is the sort of material that should be published in AR mag and I will be contacting the editor with that suggestion, along with the suggestion that they run an article on the correct way to terminate and waterproof feed line connectors as per another thread under my name.

As for my own station, I have a piece of 25mm stranded copper cable that I am going to run between the station/antenna mast grounding system (three ground rods connected together with heavy power cable) and the vertical grounding system (four ground rounds connected with copper pipe to which the radials are connected). The mast and the vertical are only 9m apart so this will be easy to do. I will still disconnect all cables and power cords whenever there is a storm in the area or when we are away from town.

73

Wayne VK4WDM

(Trash, I am sorry I called you a VK4 earlier, but you would be very welcome up here ).

[VK4BXI]

As has been said in a couple of places its the resistance or lack of it that counts !
For me earthing cable does two things.
1 it drains static charge from the mast and aerial. ( very low current but very important)
2 If lightning does hit it provides a path to earth. (in this case the inductance (or rather the impedance) of the earthing wire is as important as the resistance).

When you want to carry 20~50kA of current which has a couple of micro seconds of rise time then you want bare copper flat strap with as much width as possible to get the minimum inductance. The strap should not have any kinks in it and should not have any ferrous metal near by (use brass bolts to join or connect), otherwise if there are impedance "bumps" in the earthing cable then the lightning current could very well jump off of the cable at that point and find a path to earth through the air, at which point it could go anywhere (non preferred).

Another thought would be to use aluminium tubing as the earthing path. Providing one can do good joints it should be the best way to go for most installations. so use (say) 25~30mm tubing with 2 ~3mm wall thickness insulated off of the tower and ideally weld lengths of it together to make one straight length from top to bottom or join with sleeve of larger pipe over the joint. Aluminium tubing straight into the ground may not be such a good idea due to corrosion so that bit of it could be copper. I prefer to use ordinary hard or semi-hard straight copper plumbing piping where I can. I install it with a water supply connected to the top and a couple of teeth cut into the bottom of the pipe. The water softens the soil at the bottom and pushes the debris out of the way (doesn't work for big rocks !) the hollow pipe can be used to put water down to the bottom after to keep things moist down there.

If you are going to use an aluminium pole for the mast then that is all you need to use if you pay attention to any joints in it.

Regards
Bob
..... now come out of your corner fighting !

[VK2AAH]

The function of surge diverters in antenna systems is not always fully understood.

In a proper installation the outer of the coax feeder should be earthed at the top and at the base of the mast and if a long run preferably periodically along the mast as well. The mast in turn should be connected to a low impedance earthing system at the base (especially so if the site has a metallic connection of any type (eg; cable, pipes) to a remote earth).

Why are antenna surge diverters used?

The function of a surge diverter at the base of the mast is to minimise the voltage difference between the inner and outer conductors of the coax at the location of the surge diverter. This is all it does. The lightning energy present still needs to be dissipated somewhere.

During a lightning strike a voltage potential develops between the top of the mast (where the antenna is located ) and the base of the mast due to the very high currents flowing.

A simple example is a side mount dipole. Here, the centre conductor of the coax connects through the dipole to the mast. The centre conductor of the coax at the base of the mast is thus connected to the top of the mast. It will as a result have the voltage difference between the top and bottom of the mast.

The surge diverter prevents the voltage difference between the top and bottom of the mast from at the equipment antenna input (centre pin to ground). That is all it does. The lightning energy is still there to do damage. This can be avoided or at least minimised through a good low impedance earthing system and short heavy bonding conductors.

Lightning protection is in a way like insurance. I does not guarantee it will not happen but at least if it does it will be less painful. Disconnection serves to increase the insurance.

I've seen lightning protection implemented properly save everything on the equipment side of the surge suppressors. I know my old boss lurks here regularly and he will remember a strike at Taree many years ago that hit the antennas reducing a VHF colinear to a steel stump and a few shards of fibreglass, welded the LDF4-50 to a parallel metal surface... ok all of the fuses in the gear popped but the gear powered up after they were replaced! OK... it did travel down a copper phone line and blow gear up at the hospital up the road!

If people follow the advice you have given Igor it is not rocket science or black magic. It works. I've seen good engineering practice minimise the damage well as long as basic common sense concepts are understood:

1. Try to keep the threat as far from you (or your equipment) as possible. In other words, place the barrier (earth bond, surge suppressor etc) as near to ground and as far from you as possible. Don't put the surge suppressor where the cable goes into your rack and expect it to work (seen this a few times...). Even better... leave the rack "floating"! Seen that too!

2. Deal with the peak current by a low low resistance/impedance, short, straight, conductor to ground.

3. Deal with the VOLTAGE rise by bonding everything to a common ground point. This doesn't take a heavy conductor... just bond everything you can. Keep the resistance between equipment as low as possible. If the potential difference can't rise it can't do any harm...

I recommend Polyphaser surge suppressors and will be bringing some back from the US later in the year. The price here is silly compared to what you can buy them over there, and HRO are selling the high power, HF rated versions now...

Cheers


Richard
VK2AAH

[VK2XSO]

The strap should not have any kinks in it and should not have any ferrous metal near by (use brass bolts to join or connect), otherwise if there are impedance "bumps" in the earthing cable then the lightning current could very well jump off of the cable at that point and find a path to earth through the air, at which point it could go anywhere (non preferred).

Finding a path through the air rather than through or along the metal, I can't but help think this is another ham urban myth that we don't ever question.
While I haven't seen that many strikes, none of them have hit an earthed object at one end and then leave the same earthed object at some point to find a better path to earth.

I'm not so sure about "impedance bumps" either. While there is almost always a change in impedance where ever there is a connection, this is not going to cause the majority of the current to leave the conductor. In terms of DC, the stroke is going to follow the low resistance. The minor change in impedance from joining two pieces of similar material is trivial even taking into account the short rise time of a lightning strike.

Commercial earthing systems like those in big telecommunications earths use stainless steel bolts to join earthing.
Same in electrical transmission switching yards short of cadwelding the earth mat. But this has nothing to with impedance of the join, it's just about corrosion.

Sharp bends are also not a problem in terms of impedance. Bends sharp and gradual occur everywhere in the earthing system.
The reason to avoid them if possible is metal fatigue from the physical stress of bending them several times during installation.
Somehow this has been lost in translation to ham radio and joined the "ham myth" club.

So while impedance is something to think about, keeping resistance as close to zero is more important.

[VK2AXL]

then the lightning current could very well jump off of the cable at that point and find a path to earth through the air, at which point it could go anywhere (non preferred).

Last year I was sitting reading a book in my sun room (a flat roofed extension on the back of the house) with my feet up on the arm of the chair.
Lightening hit the ground about 3m outside the room. It travelled along the ground, possibly via steel reinforcing in concrete, causing the concrete to explode and leave the area. Pieces were found five houses away. Two large lumps went straight up and came down punching holes through the house roof. The lightening then passed through the wooden wall and earthed into the wiring. This caused a 1m by 0.5m hole to be blown in the wall about 1m away from my posterior. My garden was covered in wood and insulation fragments. Inside the room a coffee table, TV and DVR were thrown across the room. Pieces of wood and steel were embedded in my arm chair. The charge then blew a smaller hole out through the other end of the room, travelled down the supply cable to my garage and fried the electric door opener. Funnily, all the tools that were hanging on nails on the garage wall, fell off.
The main 100A fuse in the fuse box, not only blew, but exploded. The fuse box door blew open, the fuse flew down the driveway, and the door closed behind it.

I have no recollection of the actual explosion. My memory is fine one second, the next I'm looking at the carnage after it had all settled. I didn't have a scratch on me.

A better man than I would have checked the neighbours were still breathing. But I thought, that's the end to all my radios and the new TV. But apart from the TV (old CRT) and DVR which were in the blast zone, and my computer mother board (inevitably) everything was fine apart from a kit built digital SWR meter. All the radios were OK even thought the lightening strike passed within 4m of all my antennas.

I put my amazing escape with no injury down to the coffee table which had a low shelf which deflected the blast downwards and away from me. Without it, the fragments would have been getting pulled out of me, not the chair. I kept the table even though my wife wanted to throw it out as one of its legs was split in two with wall insulation driven into it. I named it - the table that saved my life. Until six months later when I walked into it in the dark and broke my toe. Then the name was cancelled.

Anyway - point being - you cant predict lightening paths. Metal, air, wood, concrete, its all the same. Sometimes it can take the path of most resistance.

My advise, unplug your antenna and hide under the stairs during storms.

[VK4TIM]

I always ensure that any antenna towers/poles verticals etc are grounded with a good earth stake, for a minimum impedance path to ground as possible.

Probably 15 yrs ago we had a nearby strike hit a house across the road down a long driveway, much lower in elevation that my place at the time, absolutely no damage done to my radio gear, which was connected to an antenna at the time...

The house that was hit copped it, most of the domestic 240V wiring vaourised, all the phone wiring vapourised, part of the roof blown to splinters (steel roof), the supply meterbox destroyed. Most of the houses in the neighbourhood suffered damage to devices connected to the phone line, alarm systems, modems, phones, and phone service too about 4 days to be restored because of damage to the exchange and cabling. All services were underground, phone, power, no cable TV.

I lost a PC, completely dead, flatmate's PC suffered damage that became fatal shortly after the event, normal wired phone written off, nut my Uniden 900 MHz phone (that I still have today) was completely unaffected. Seems the spike up the phoneline may have been a commone mode event, finding a path to ground through everyone's devices that had a mains earth connectin the chassis to earth.

The path that a strike will take is very unpredictable.

A Tetra network I commissioned took a hit, the system did a reset, and came back up and carried on as if nothing had happened. I suspect that the reset event was as much a power supply event rather than related to lighning hitting the building or antennas.
The system is very well grounded, LDF5-50s earthed top and botton, equipment hut and antenna bulding well bonded and well grounded with many grounding stakes.