Contradictions in conductivity

Question

I mostly know how electricity flows but cannot understand some aspects, and have three examples that I cannot fathom:

1. The pilot lighting electrode wire on my house central heating water heater /furnace generates a high voltage spark and is covered by approximately 0.5mm of insulation. You can hold it with no effect whilst it's sparking. When I stand on my foam lino floor wearing plastic soled trainers and stick an electrical screwdriver (one of those that light up when there's voltage at the tip) into a regular power socket (240V), it lights up.

2. Some people recommend against using a hair drier whilst in the bath because you might get an electric shock if it falls into the water. Super grid (400kV) pylons get rained on and totally covered in freezing ice, from the wires themselves, over the insulators, right to the metal support structure. Nothing happens.

3. Train surfing is ill advised by some as you might get a shock by arcing from getting too near to the power cables (25kV). The same cables sometimes pass within 100mm of cast iron bridge beams in low clearance situations and there is no arcing to ground.

These examples seem contradictory. I know that this is crazy, but it almost seems that anything that I can do personally is dangerous, but anything done commercially works safely. Can anyone explain them?

Answer 1

It is all about conductivity (resistance) as you mention. An electric circuit needs a complete path from the source, and then back to the source. Danger exists whenit is the human body that completes the circuit, either through direct contact or capacitance return. The human body reacts differently to voltage applied and to current through the body. Also, increasing the path length (higher resistance) or reducing the capacitance in the circuit results in less current (ohms law).

1. Your linoleum floor and plastic soled shoes make a very high resistance, resulting in low current and capacitance is likely minimal. I would nominate you for the Darwin award since I know what it is like to touch the ignition wires on automobile spark plugs.

2. A hair dryer falling into the bath lets the electricity go through water (Low resistance) directly to the human skin (short path length), resulting in higher current. High voltage grids use corrugated insulators to increase the length of the path of resistance. The electricity has to follow the path of the corrugations rather than in a straight line, thus increasing the resistance. The resistance of Air is quite high.

3. Same as answer 2.

   The resistance of Air is quite high.

Answer 2

1. It's all about the path to complete the circuit. Are you part of it? In the first case, you state that you have 0.5 mm of insulation between you and anything conductive. That insulation is meant to prevent anything arcing through it. I assume you also had a vast air gap (high resistance) between you and the metal conductor so you didn't get sparked.
   The electric detecting screwdriver is capacitively coupling to ground through you. See here for more info.
2. Water is very conductive only if it has a lot of dissolved solids in it (ions). The pylons with freezing rain on them is likely very clean water since it just got purified by the water-cycle. The water in a bathtub is likely full of chlorine, dirt, soap and who knows what else which makes the bath water highly conductive.
3. You'd need to show a picture of the power rails coming that close to a cast iron bridge, according to the diagram below, that's not possible without arcing. Air is mostly N2. At 10 cm you only need 1000V to cause an arc. 25kV can jump 30ish cm. More than likely, what you thought were power rails were instead the supporting structure for the power rails and weren't actually "hot".

Answer 3

1) Touching a sparking conductor: Here is a picture of a typical vehicle spark plug:

You can see that the plug has a special structure which facilitates sparking - a pointy head and a very small gap between other terminal. Also, whenever there is a spark, it doesn't mean there is a huge electrical current flow. When you take off your sweater in winter, you might hear a crackling sound (and see sparks if you are in a dark room) but you don't feel anything because there wasn't a lot of current. Besides this, current will always follow the most convenient path. If you touching the wires going to the spark plug, you won't get a shock because current will flow through that specialized gap and not through you.

2) Lighting of screw driver (tester) even when you are wearing plastic trainers: Remember that you are working with ac here. ac can also pass through capacitor besides resistor. When you are isolating yourself from ground by using plastic trainers or carpet, you are just increasing the resistance for the current path. However, there is still a capacitance present by which you can conduct some current (and get shocked if you touched the Live wire during your evil experiments). If you compare two cases - One where you are standing on floor with bare feet and Second with shoes or something insulating, you will find a difference in the brightness (depending on what model you are using).

3) Hair dryer myth: This won't be as bad as being shown in movies where a person dies when a hair dryer is thrown in his bath tub. Applying the same principle - Current path will be the most convenient one. Current from Live wire will prefer flowing to Neutral directly because it's the most convenient path and close by. If you are kept in between those two wires, then you will feel a shock but not very bad shock unless you come in a direct contact of the Live wire. Here is a video demonstration for this.

Regarding those ico covered pylons, nothing happens because snow/rain contains almost no salt. Pure water is insulator. So, snow covered pylons is almost as good as air. Well almost because it has a relative permittivity of around 80 which will increase the chances of sparking but the gap between High Voltage and Ground must have been designed to take this in account.

4) Train surfing: Again the same concept as above. 25 kV will discharge if some grounded conductor comes within the distance required for a discharge. Either 100 mm is too large for discharge or that beam might be isolated from ground (chances of which are highly unlikely). A person however, if he is on a moving train, he might tip or somehow get into that range and roast himself.