Handbook, chapter 12. Earthing, lightning, safety
People skim this chapter, and that is understandable: it is dull, nothing gets calculated in it, and nothing sounds better afterwards.
It is the only chapter in the book after which nothing happens. That is precisely the point.
Where the strike will go
The only path to earth leads through your transceiver and the mains. Almost everybody does this, and almost always it passes off — until the one time it does not.
The question is not whether the mast will be struck. The question is what the strike will travel through when it happens — and you answer that question in advance, back when you are running the cable.
Lightning: three things to do
Bring the cable in through an entry panel. Not through a window, not through a vent, not «just for now». A metal panel on the outside wall, a surge arrestor inside it, and a short thick conductor from the panel to its own earth electrode. The strike must end outdoors.
Bond the earth electrodes to each other. This is what gets skipped most often, and it is why equipment burns out in houses that supposedly have an earth. If you have your own electrode for the antenna and a separate mains earth with no connection between them, then during a strike a potential difference of thousands of volts appears between the two — and the current will go to equalise it through your equipment, because your equipment is connected to both.
One system. Everything bonds to a single point.
Disconnect the antenna when you are not operating. The simplest, cheapest and most reliable measure, and one that for some reason is not taken seriously. A storm on the horizon — unplug the cable from the radio and move the connector well away. A surge arrestor is designed to save the building; it makes no promises about whether your transceiver survives.
Conductor cross-sections, the type and number of earth electrodes, the earth resistance, how to bond to the building's system — all of this is governed by building and electrical standards, and they are not the same for a private house, a block of flats and a rented space.
The drawing above is a principle, not an installation diagram. Before doing anything, check the standard currently in force and, if a block of flats is involved, check with whoever is responsible for its electrical installation. The cost of a mistake here is not «it will not work».
The mast: the thing that falls
People think of a mast as a structure, but it is first of all a lever.
It has to fall into empty space. Before you put it up, look at where it would land if it did: not on power lines, not on a neighbour's plot, not on a road. You work this out once, for the whole life of the mast.
Do not work at height alone. Not because you cannot manage, but because if something goes wrong there has to be somebody on the ground.
Look up before raising a mast. Contact with an overhead power line is the commonest fatal accident in amateur radio, and it happens not at height but on the ground: two people raise a mast and fail to notice the wires above them.
Guy wires are fixed to beams or to anchors set into the wall. Not to a downpipe, not to a cornice, not to a chimney and not to somebody else's television mast. All of those look solid right up until the wind load of a twenty-metre mast comes to rest on them.
Check the guys once a year. Ultraviolet and frost do their work silently.
A mast over eight metres is not raised by two people. This is not over-caution: the mass and the leverage are such that two people simply will not hold it once it starts to go sideways. On a sloping roof, use a safety harness with a line secured to a rafter, non-slip footwear and gloves.
Earthing: what must never be done
Three prohibitions that cost people their lives, which is why they are written into the rules.
Never earth to a gas pipe. A spark at a poor contact during a lightning discharge — and there is nothing more to explain. Heating pipes are no good either: they run through the whole building and have dozens of joints of unknown quality.
A cold-water pipe is a last resort only, and then only if it is metal along its whole length down to the ground. One plastic section somewhere in the basement turns your «earth» into an antenna.
Solder on its own does not hold. Joints in an earthing conductor are made with threaded fasteners, crimps or welding — and only then, if you like, soldered as well. Solder softens under the discharge current at exactly the moment the joint is needed most.
The earthing conductor is 16 mm² or larger and takes the shortest route, with no loops and no sharp bends: for a pulse with a microsecond rise time every turn is inductance, and inductance is the same thing to it as resistance.
Mains power and equipment
Amplifiers mean kilovolts. In a valve amplifier the anode voltage is thousands of volts, and the capacitors hold it after the power is off. Do not open the case until you have confirmed that the bleeder resistors have done their job, and do not rely on the plug being pulled.
A transceiver supply means hundreds of amps of short-circuit current. A car battery or a hefty 13.8 V supply looks harmless because the voltage is low. Its short-circuit current is such that a wire becomes a heating element within seconds. The fuse goes in both leads and next to the source, not next to the radio.
Keep one hand behind your back. The rule is as old as valve equipment, and it is about current that runs from hand to hand — that is, through the heart. Adjusting a live transmitter: second hand behind your back, tool with an insulated handle. It sounds theatrical and it works without fail.
Discharge capacitors with a grounding stick. Not with a screwdriver and not on the basis that «it should have discharged by now»: touch every point where high voltage might remain with a probe on an insulated handle that is bonded to the chassis.
Do not solder over a powered board. Dull advice that everybody ignores.
RF exposure
This is where there are the most myths and the least common sense in either direction.
What is known reliably: a radio-frequency field of high density heats tissue, and it is on that — not on «radiation» in the everyday sense — that all the standards are built. The effect is thermal, it is well measured, and the limits are set with margin.
What follows from that in practice:
- It is distance, not power, that is dangerous. A hundred watts into a dipole twenty metres from a person is nothing. The same hundred watts at the end of an antenna thirty centimetres from someone's head is another matter.
- The worst places are the ends and the feed point. That is where voltage or current is at a maximum, and that is where a chair, a bed or a children's play area should not be.
- VHF and higher frequencies are more hazardous than HF at the same power: the shorter the wave, the better the body absorbs the energy.
- Do not hold a transmitting antenna. At the end of a half-wave dipole at a hundred watts there are hundreds of volts, and an RF burn is deep and painful.
The actual permissible levels, the safe distances and the way they are computed come from the health standards currently in force, not from an article in a handbook. They differ by frequency, they differ for workers and for the general public, and they get revised.
If you are putting up an antenna where people come and go, work it out by the method in the document rather than «by eye». This is not bureaucracy: it is exactly that calculation which shows that 99 % of amateur installations have many times the required margin.
The small things that actually injure people
The statistics here are mundane, and the serious matters from the sections above barely appear in them:
- wire in the eye — while tensioning an antenna; glasses cost less;
- falling off a ladder — not off a mast, off a ladder;
- soldering-iron burns and solder splashes — again, glasses;
- cuts from a cable knife — while preparing coax;
- your back — lifting a battery or a transformer.
None of these sounds frightening, and that is exactly why they happen.
If someone gets an electric shock
The most important thirty seconds in this chapter.
Do not grab the casualty with your hands. They are live, and you will become the second casualty. First cut the power; if that is impossible, pull them away with a dry stick, a rope, clothing — anything that does not conduct.
Call an ambulance even if the person has come round and looks fine: the condition after an electric shock deteriorates with a delay, and the heart rhythm can go astray an hour later.
If there is no breathing, start resuscitation immediately and keep going until help arrives. Not ten minutes, not «it looks hopeless now»: after an electric shock people have been brought back after far longer than that.
These few lines are no substitute for a first-aid course. Take one — it is short, and it is useful outside the shack too.
The minimum worth starting from
If all you take from this chapter is the following, that is already good:
- The cable enters the building through an entry panel, not through a window.
- All earth electrodes are bonded into one system.
- The antenna is disconnected when you are off the air.
- A fuse next to the power source, in both leads.
- Masts are raised by two people, after looking up.
Next — chapter 13, your first contact: what to say into the microphone, and in what order.