Chapter 32: Safety Considerations
Free practice questions on Safety Considerations, chapter 32 of the SARL's Class A manual, for South Africa's Radio Amateur Examination (RAE). Every question has the right answer and an explanation. Sections covered:
- 32.1 The human body
- 32.2 Mains power supply
- 32.3 High voltages
- 32.4 Lightning
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About 20 mA
As little as 20 mA through the torso can upset the heart’s rhythm and cause a fatal cardiac arrest. 200 mA is ten times that: deadly too, but not the smallest current the manual warns about.
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30 V
V = I × R = 0,02 A × 1 500 Ω = 30 V, so a voltage far below the mains can be dangerous, especially through wet skin. Dividing 1 500 by 20 (giving 75) turns Ohm’s law upside down.
Try it: Ohm's Law & Power -
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10 times
I = V ÷ R = 230 V ÷ 1 200 Ω = 0,19 A (192 mA), and 192 ÷ 20 = 9,6, so 10 times is nearest. 100 times would take a current of 2 A.
Try it: Ohm's Law & Power -
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Its path crosses the torso, where it can upset the heart
A current that crosses the torso can reach the heart, which small currents can stop. A longer path has more resistance, not less, so it does not draw a larger current.
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They run partly on electrical signals of their own
Many of the body’s control systems use a mix of chemical and electrical mechanisms, so an outside current interferes with them. Heat can harm them too, but a small current does its damage electrically.
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Slower healing of wounds and other damage
The manual says RF fields delay the healing of wounds and other damage, on more than anecdotal evidence, and links RF to some cancers; it makes no claim about memory loss. RF heating is the accepted hazard.
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Expose yourself and others to no more RF than you need
The manual notes that power and cellphone companies have paid to disprove the links, yet concerns remain, so keep the RF reaching anyone, yourself included, as low as you reasonably can.
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Keeping RF sources screened off from the face and body
RF can heat and burn tissue, so RF sources should be screened off from your face and body. A screen must be metal, as plastic lets RF straight through.
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About 230 V AC at 50 Hz
South African mains is AC at 50 Hz; its nominal voltage is now 230 V, though the manual still gives 240 V. 60 Hz is used mainly in the Americas.
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When another part of you is in contact with ground
With a second contact to ground, such as a damp floor, current flows from the conductor through you to ground. Dry skin has more resistance than wet skin, so it is the less dangerous case.
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Concrete conducts AC fairly well, giving a path to ground
Concrete conducts AC fairly well, so bare feet on it complete the path to ground and a touched live wire drives current through the body. Concrete is not an insulator.
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Rubber-soled shoes, and tools with insulated handles
Rubber soles break the path to ground through the feet, and insulated handles stop a slip onto a live part from passing current into the hand. Bare metal tools give no such protection.
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Only once it is switched off and unplugged
Covers come off only once the set is off and unplugged: a power switch may break only one line, leaving parts live. Even unplugged, filter capacitors can hold a charge, so let them discharge first.
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Insulated wire rated for the voltage and current it will carry
The insulation must suit the voltage and the conductor must carry the current without overheating. Thin insulation can break down, and thin wire can overheat however well it is insulated.
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It is brown
The code is brown for live, blue for neutral and green/yellow for earth. Mixing up brown and blue is the classic slip; wire size and the installer’s taste mark nothing.
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Its wires may follow a different colour code
Imported equipment may use another colour code, so trace each wire before trusting its colour. Colour codes are not the same the world over.
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A good earth
With the metal earthed, a fault sends a large current to earth and the fuse blows or the earth-leakage unit trips. Neutral carries the working current, and a broken or swapped neutral could leave the metal live.
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To the metal chassis
Green/yellow is the earth wire: it bonds the metal chassis to earth so that a fault cannot leave the case live. Joining it to neutral would tie the chassis to a current-carrying wire.
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A large current flows to earth and the fuse blows
The earth gives the fault current an easy path, so the fuse blows or the earth-leakage unit trips, and the case does not stay live. Without the earth, it would stay live until someone touched it.
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As a two-pole switch breaking both live and neutral
The manual requires a two-pole switch that breaks both live and neutral. A switch in one line only leaves the set connected to live if that line turns out to be the neutral, as with reversed wiring.
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8,7 A
I = P ÷ V = 2 000 W ÷ 230 V = 8,7 A, and the plug, cable and outlet must be rated for it. Dividing the voltage by the power (0,115 A) turns the formula upside down.
Try it: Ohm's Law & Power -
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A three-pin plug rated 16 A
I = P ÷ V = 2 800 W ÷ 230 V = 12,2 A, too much for a 10 A plug, and the metal case needs an earth pin. The manual’s 15 A three-pin plug is now marked 16 A.
Try it: Ohm's Law & Power -
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The socket is overloaded: together they draw about 18 A
The currents add: I = P ÷ V = 4 200 W ÷ 230 V = 18,3 A, more than the socket’s 16 A, so it will overheat. Checking each load on its own misses the total that the socket carries.
Try it: Ohm's Law & Power -
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A master switch for the whole station that everyone at home knows
A single master switch that everyone in the household knows lets them cut all station power safely if you are hurt by a shock. A switch only you know about is no help when you are the casualty.
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Cut the power at the station’s master switch
Touching someone held by a live circuit puts you in the circuit too, so cut the supply first; that is why the manual wants a master switch the whole household knows. Never use water near live equipment.
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The mains supply to all the station’s equipment
The master switch cuts the mains to all your equipment at once, so nobody has to find each plug in an emergency. Switching off the transmitter alone leaves the power supply and the rest live.
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Antenna elements and the power supplies of valve equipment
Antenna elements can carry high RF voltages, and valve equipment can use supplies of up to 5 kV. The mains is a hazard of its own, dealt with separately, and is not one of these two.
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The tips at the far ends of the elements
Current peaks at the centre feed point and voltage at the open ends, so the tips of dipoles and verticals are most hazardous; linear loading wires can be too. The feed point is low-voltage, high-current.
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More power can drive the voltages higher
More power drives larger currents and larger voltages along the elements, so the voltages at the tips climb with the power. Voltage and current rise together; neither stays fixed.
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Only when there is no risk of the station transmitting
Never touch antenna elements while the station might transmit: their RF voltages can burn you. Low power lowers the voltages but does not remove them, so it does not make touching safe.
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To warn that RF on an antenna can burn deep into you
A surgeon’s RF knife makes the point that RF on an antenna element can burn deep into flesh: leave the surgery to professionals. Interference to medical equipment is a different topic.
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5 kV
Valve (tube) amplifiers can run on supplies of up to 5 kV, over twenty times the mains voltage. 500 V is only a tenth of that figure.
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Far more lethal, as heating and charring let even higher currents flow
A high-voltage supply is far deadlier than the mains: the heating and charring it causes can let even more current flow. A higher voltage drives more current through the body, not less.
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4 A
2 000 V ÷ 500 Ω = 4 A, 200 times the manual’s 20 mA, as high voltage breaks down skin resistance. R ÷ V (0,25 A) is Ohm’s law upside down; reading 2 kV as 2 V gives 4 mA.
Try it: Ohm's Law & Power -
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It drains the charge from the filter capacitors after switch-off
A bleeder resistor across the filter capacitors discharges them once the supply is off, so they do not keep a lethal charge. Inrush current at switch-on is limited by other parts, such as a series inductor.
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Discharge them safely, then store them away
A large capacitor can hold a dangerous charge long after it was disconnected, so discharge it through a suitable resistor, short its leads and put it away. Storing it charged leaves a trap for the next person.
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Its interlocks are in place and its capacitors have had time to discharge
Besides unplugging, the manual wants the high-voltage interlocks in place and time for the filter capacitors to discharge. Bleeder resistors are meant to drain them, but they may be faulty, so check with a meter first.
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They often live on high ground and put up tall structures
Amateurs like high terrain and erect masts and towers, so strikes near them are more likely than for most people. Radio signals do not attract lightning; height and exposure do.
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Little can be done about it; protection is aimed at strikes close by
A direct strike causes havoc whatever you do, so the manual’s measures target strikes close by. Good earthing helps against those, but it cannot make a direct hit harmless.
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Leave its mains, antenna and control cables unplugged
A nearby strike can send surges along the antenna feedline and control cables as well as the mains, so unplug them all. Unplugging the mains alone leaves the antenna path open.
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Unplug all the equipment before the lightning comes close
Unplug everything while the storm is still some way off: once thunder follows the flash within a few seconds it is past time, and handling cables then is risky. Lower power gives no protection.
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At the foot of the tower and where they enter the building
Grounding each feedline at the tower base and again where it enters the building diverts surges before they reach the shack. A ground only at the radio invites the surge indoors first; grounding does not attract lightning.
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Surge arresters
Surge arresters divert the sudden high voltage of a nearby strike to ground. A fuse reacts far too slowly, and a surge can arc straight across it.
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With earth spikes and radials, following SABS guidelines
The manual calls for grounding to SABS guidelines, using earth spikes and radials. A buried metal water pipe may help, but very few modern installations still use metal pipe.
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They may help, but very few modern pipes are made of metal
Buried metal pipes may help as an earth, but most modern buried pipe is plastic, which does not conduct. The manual requires earth spikes and radials.
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About 330 m/s
Sound covers about 330 m each second, roughly 1 km in 3 s. 300 000 km/s is the speed of light, which brings the flash to you almost at once.
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1,98 km
Distance = speed of sound × delay = 330 m/s × 6 s = 1 980 m, about 2 km. Dividing 330 by 6 gives the 55 m slip, and the speed of light gives an absurd 1 800 000 km.
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2,5 km
330 m/s × 8 s = 2 640 m, so 2,5 km is nearest; the manual likewise rounds 5 s to about 1,5 km (exactly 1,65 km). Dividing 330 by 8 gives the 40 m slip.
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The light arrives almost at once, but sound takes far longer
Light reaches you practically at once, but sound takes about 3 s per kilometre, so the delay grows with distance. Multiply the seconds by 330 m/s to find the distance.
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