WEBVTT NOTE Electrical Safety in the Station 1 00:00:00.000 --> 00:00:13.578 What actually hurts people. Not RF. The injuries in amateur radio come overwhelmingly from mains voltage, stored charge, batteries, and falling, in roughly that order. 2 00:00:13.578 --> 00:00:27.319 This lesson covers the first three; towers get their own. Current is what injures, not voltage. Voltage is what pushes; current through the body is what does the damage. 3 00:00:27.319 --> 00:00:43.986 The effects scale with how much flows and where it goes: A few milliamperes: perceptible tingle. Around 10 mA: muscles contract and may not let go. Around 100 mA across the chest: ventricular fibrillation. 4 00:00:43.986 --> 00:00:58.621 The pool asks what hazard current through the body poses and answers "all these choices", burns, muscle paralysis, cardiac arrest, because all of them are real at different levels. 5 00:00:58.621 --> 00:01:04.475 The practical consequence is that low voltage is not automatically safe. 6 00:01:04.475 --> 00:01:17.240 A 12 V battery cannot push meaningful current through dry skin, but it can push hundreds of amperes through a wrench, and that is a different kind of injury. 7 00:01:17.240 --> 00:01:29.184 Batteries. A 12 V storage battery without internal overcurrent protection is a hazard because shorting the terminals can cause burns, fire, or an explosion. 8 00:01:29.184 --> 00:01:45.874 There is no current limit worth speaking of; a lead-acid battery will deliver hundreds of amperes into a short until something melts. Charging and discharging rapidly brings a second hazard: overheating or out-gassing. 9 00:01:45.874 --> 00:01:53.989 Lead-acid cells vent hydrogen, which is explosive in air, and lithium cells fail more energetically still. 10 00:01:53.989 --> 00:02:08.000 Practical habits: fuse the battery lead within a few inches of the positive terminal, remove metal jewellery before working near terminals, and never rest a tool across a battery top. 11 00:02:08.000 --> 00:02:23.278 Fuses and breakers. A fuse exists to remove power in the event of an overload. Note what it does not do: it does not protect you, and it does not protect the equipment from most faults. 12 00:02:23.278 --> 00:02:31.867 It protects the wiring from becoming a heater. Two rules follow: Never fit a larger fuse than specified. 13 00:02:31.867 --> 00:02:49.953 Replacing a 5 A fuse with a 20 A one means the wire, sized for 5 A, now carries four times that before anything interrupts. Recall that heat goes as the square of current: sixteen times the dissipation in the same wire. 14 00:02:49.953 --> 00:03:06.057 Excessive current could cause a fire, and that is the pool's answer, in those words. Fuse the hot conductor. In a 120 V AC circuit, the fuse or breaker goes in series with the hot conductor only. 15 00:03:06.057 --> 00:03:17.041 A fuse in the neutral can open while the equipment remains connected to the hot side, still live, and now with no obvious sign of it. 16 00:03:17.041 --> 00:03:25.134 In US 120 V three-wire practice: black is hot, white is neutral, green (or bare) is safety ground. 17 00:03:25.134 --> 00:03:40.065 Stored charge. A power supply is dangerous immediately after it is switched off, because its filter capacitors hold charge. A large supply can hold a lethal charge for minutes. 18 00:03:40.065 --> 00:03:52.875 Before working inside anything with a mains-powered supply: switch off, unplug, wait, and then verify with a meter that the capacitors have discharged. 19 00:03:52.875 --> 00:03:58.814 Bleeder resistors are supposed to handle this and sometimes fail open. 20 00:03:58.814 --> 00:04:09.288 Measuring high voltages. When measuring high voltage with a voltmeter, ensure the meter and its leads are rated for the voltage being measured. 21 00:04:09.288 --> 00:04:22.326 Standard probe sets are commonly rated for a few hundred volts and will flash over above that, with your hand on the probe. Keep one hand in a pocket while probing live circuits. 22 00:04:22.326 --> 00:04:27.014 It sounds like folklore; it prevents current crossing the chest. 23 00:04:27.014 --> 00:04:42.642 Guarding against shock. The pool asks for a good way to guard against shock and answers "all these choices are correct". Those choices amount to: Use three-wire cords and plugs for all AC-powered equipment. 24 00:04:42.642 --> 00:04:58.725 Connect all AC-powered equipment to a common safety ground.. Install mechanical interlocks in high-voltage equipment, so opening the cabinet removes the power. Grounding and lightning. Two rules cover most of it. 25 00:04:58.725 --> 00:05:10.712 Bond everything. All external ground rods and earth connections must be bonded together with heavy wire or conductor, including the electrical service ground. 26 00:05:10.712 --> 00:05:26.871 Separate, unbonded grounds are more dangerous than one: a surge raises one relative to another, and the difference appears across whatever bridges them, normally your equipment, normally while you are touching it. 27 00:05:26.871 --> 00:05:42.499 Arrest at the entry point. A lightning arrester belongs on a grounded panel near where the feed lines enter the building, not at the antenna. The point is to divert the surge to earth before it gets inside. 28 00:05:42.499 --> 00:05:58.734 Check yourself. Why is fitting a 20 A fuse where a 5 A fuse belongs dangerous? Your power supply has been off for ten seconds. Is it safe to open? You add a ground rod for the antenna system. What must you also do? 29 00:06:04.734 --> 00:06:21.128 The wiring is sized for 5 A. At 20 A it dissipates sixteen times the heat before anything opens, a fire risk, which is exactly what the pool says. No. Filter capacitors hold charge after switch-off. 30 00:06:21.128 --> 00:06:31.974 Unplug, wait, and verify with a meter. Bond it to every other ground rod and to the electrical service ground with heavy conductor.