Microshocks

 In certain circumstances, a person exposed to a high electric field could experience small spark discharges on touching other objects. 

This can happen two different ways.  In both cases the common feature is the person touching an object, where one is at earth potential and the other, which is not earthed, has been raised to a higher potential by the electric field.  When the person touches the object, charge flows so as to equalise the potentials, and this charge, concentrated on the small area of skin where contact is first made, creates the microshock. 

In the diagram below, on the left is the situation where the person is grounded through their feet and then touches an ungrounded object.  On the right is the other situation where the person is isolated from ground (because they are wearing insulating footwear or are standing on an insulating surface) then touches a grounded conducting object. (for more detail on how the charge actually flows when this happens see toggle)

How microshocks occur

The sensation of a microshock is similar to that caused by the static discharges commonly experienced in dry atmospheric conditions after frictional contact with a nylon carpet or car seat. Normally, any sensation is confined to the momentary spark discharge as contact is made or broken.

How do charges flow in a microshock?

A microshock happens when a person and a conducting object acquire different potentials in an electric field.  At the instant of coming into contact, the two potentials are equalised by a transfer of charge, and that momentary transfer of charge, concentrated at a single point on the skin, constitutes a microshock.

But what actually happens to the charge - where does it come from and which way does it flow?

Consider the situation where a grounded person touches a floating object (the other situation where a floating person touches a grounded object is exactly analagous).  As shown in the left-hand diagram below, before the person touches the object, the electric field induces opposite charges on the two sides of the floating object.  There is no net charging, but the charge is separated within the object.  For the person, there is a net charging: the field induces a charge on the top of their body, but instead of an equal and opposite charge on the bottom half of the body, charge flows from the ground to cancel this, leaving them with an overall net charge.

 microshocks with charges

The right-hand diagram shows what happens when contact is made.  To a first approximation, the charge on the top of the object stays the same.  It is the charge on the bottom of the object which is neutralised by charge flowing to and from ground through the person.  So the object started out with no net charge and after the contact is made it acquires a net charge.

This is perhaps counter-intuitive. People (including us!) sometimes say the object is "charged" in the electric field and then "discharged" by the person touching it.  The correct version is actually the other way round, though we all understand what is meant by this simpler version.

Note that touching the object will in fact change slightly the charges on the top of the object as well because it will alter the shape of the ground plane.  And note that because these are alternating field, the sign of all these charges changes every half cycle as the field changes direction.

What determines the size of a microshock?

As described above 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 toggle 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 Shock

 

Standing in a vertical field e.g. under a power line

Cycling in an electric field e.e. 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

13 µA / kV/m

Not relevant

Voltage (rms)

1 kV

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.

How sensitive are people to microshocks?

We describe elsewhere how microshocks can arise when a person touches an object in an electric field.  We also give quantitative information on the sizes of voltages, charges etc involved.  Here, we give information on how big they have to be for people to perceive them or to regard them as annoying.

These data come from tests in America.  They took a sample of 136 adults, standing upright in an electric field such as you might find under a power line, and asked them to say when the microshocks were first noticeable ("perception") and when they became annoying.  They did this when the microshock arose by touching a grounded object ("finger") or by discharging the person through a contact to their ankle (you might get this when brushing through long grass wearing sandals or with bare legs).  The results are shown in this graph:

 microshocks graph experience

This shows, for example, that in a field of 5 kV/m, most people - over 80% - will perceive a microshock when they touch a grounded object, but only about a quarter will describe it as annoying.

Note: we have recreated this graph from a printed version so it may have lost some accuracy in the process.

Control of microshocks in the UK

The principles for controlling microshocks in the UK are set out in a Code of Practice published in 2013.

How do the exposure guidelines deal with microshocks?

Some of the EMF guidelines published include limits or guidance on indirect effects, such as microshocks.

Microshocks and bicycles

One particular way a microshock can be experienced is by riding a bicycle under a high-voltage power line.