We explain on a different page what a microshock is - in an electric field, different objects have different charges induced on them, and when you touch an object, the charges transfer through a tiny discharge at a single point on the skin. We give here some indications of the quantitative aspects - how big are the charges and voltages involved? See also data on what fraction of people perceive microshocks at different fields
There are several parameters involved:
- the electric field determines the overall size of any effect
- the capacitance between the two objects depends on how big they are and how close to each other, and determines how much they are electrically linked before the discharge
- the electric field induces a voltage between the two objects
- the voltage is actually calculated via something called the short circuit current, though once you've calculated the voltage you don't need to use the current any more
- the voltage and the capacitance determine the charge that transfers through the microshock
- the voltage also determines the maximum spark length and therefore how small the gap has to be before the discharge occurs
We give typical figures in the following table for three scenarios:
- a person standing upright in a vertical electric field, e.g. under a power line (we give figures for two different fields, 5 kV/m which is often regarded as a level below which microshocks are negligible, and 10 kV/m, roughly the highest field produced by UK power lines - see lots more on electric fields below power lines)
- a cyclist in the same electric field, where the microshock is between the rider and the bike (see a full explanation of how this happens)
- for comparison, the static microshocks experienced after walking across a carpet (here, the charge is produced by friction, not an electric field, so not all the parameters are relevant)
|
Person |
Cyclist |
Static schock |
|
standing in a vertical field e.g. under a power line |
cycling in an electric field e.g. under a power line, discharge between cyclist and bike |
e.g. after walking across a carpet then touching a door handle |
| electric field |
5 kV/m |
10 kV/m |
7.5 kV/m |
not relevant |
| typical capacitance |
200 pF |
330 pF |
100-200 pF |
| short-circuit current |
13 µA / kV/m |
not relevant |
| voltage (rms) |
1kV |
2 kV |
0.9 kV |
|
| voltage (peak) |
1.5 kV |
2.9 kV |
1.3 kV |
3-4 kV |
| charge (rms) |
0.2 µC |
0.4 µC |
0.3 µC |
0.3-0.8 µC |
| maximum spark length |
0.4 mm |
0.9 mm |
0.4 mm |
|
Tests on samples of people have shown that what determines the perception of a microshock is predominantly the charge - microshocks of the same charge will be perceived similarly even if the voltages and capacitances were different. So microshocks under power lines are likely to be perceived similarly (or even as less severe) than static shocks - except that it is possible to receive more than one microshock in succession under a power line before the gap is closed.
