Dr A McLeod
This mini-article explores electrocution as a cause of death - how it
kills and what the autopsy findings are. This info is largely
referenced from Knights
Forensic Pathology with additional information from the US CDC
How electrocution occurs
There are several components to an electric 'shock'
- Voltage -
if the voltage is different in two places that are connected then
current (see below) will flow between them. The more difference (higher
voltage) the more current will flow. If the voltage is high enough then
current can flow without direct contact via the air (arcing).
- Current - you can think
of this as the flow of electrons. Technically this is measured in
coulombs (amperes x seconds) - time plays an important part in level of
injury as even a low current can cause a lot of damge when exposure
time is long. In practice amperes (or milliamperes) are used. Currents
under 30 mA (milliamperes) rarely cause damage. Levels of 50 mA or
above, over a few seconds are usually fatal.
- Another danger of currant is that it can cause muscle contraction
- in the hands the flexors are more powerful than the extensors so the
hand clenches shut - often around the source of the current - this is
the 'hold on' effect. Currents of 9-10 mA are enough for this to occur.
- Resistance (impedence in
alternating currant). The resistance must be overcome before currant
can flow. Higher voltages have a higher ability to overcome resistance.
Resistance varies over the body from 1-2 million ohms on the thick skin
of hands and feet to a 500-1000 ohms elsewhere. The million or so on
the hands can drop to a few thousand when the hands are sweaty and less
than a thousand when wet thus explaining why electrocution is more
likely with wet hands!
|More on Voltage, current and resistance:
To calculate voltage (V) in volts: put your finger over V,this leaves
you with I R, so the equation is V = I x R
To calculate current (I) in amperes: put your finger over I, this
leaves you with V over R, so the equation is I = V/R. Note that
damaging currents are in the milliampere range rather than amperes!
To calculate resistance (R) in ohms: put your finger over R, this
leaves you with V over I, so the equation is R = V/I
It is the current that causes tissue damage.
You can see that (I=V/R) as either voltage increases, or resistance
decreases the amout of current goes up - causing more damage.
Effects of Electrical Current on the
Body (from CDC)
|1 milliamp (mA)
||Just a faint tingle.
||Slight shock felt.
Disturbing, but not painful. Most people can "let
go." However, strong involuntary movements can cause injuries.
|6-25 mA (women)
9-30 mA (men)
|Painful shock. Muscular
control is lost. This is the range where "freezing currents" start. It
may not be possible to "let go."
||Extremely painful shock,
respiratory arrest (breathing stops), severe
muscle contractions. Flexor muscles may cause holding on; extensor
muscles may cause intense pushing away. Death is possible.
(heart pumping action not rhythmic) occurs. Muscles contract; nerve
damage occurs. Death is likely. 3A is the smallest fuse in common usage.
|10 amps (A)
||Cardiac arrest and
severe burns occur. Death is probable.
||The typical current at
which the fuse blows in an item like a hairdryer or electric drill!
The flow of current through the body after bypassing the skin is fairly
easy - it is well conducted by the (low elecrical resistance) organs
and fluids of the body in a fairly straight line from point of entry to
the earthing point. Certain entry and exit points are known to be more
dangerous than others.
- Entry right hand and exit left foot is the most dangerous - the
current passes through the heart at the axis most likely to cause
- Current from right hand to left hand is next most dangerous and
then left hand to right hand. Both of these pass directly though the
Mode of death
- The commonest by far is cardiac dysrhythmia - usually ventricular
- A small amount of electrocution victims suffer paralysis of the
respiratory muscles in the chest and/or diaphragm.
- A very small amount suffer damage to the brain stem - this
presumably leads to either of the two above.
- At higher currents (approx 4 A) clots may form in blood vessels.
- In addition to direct damage electrical shocks may be responsible
for fatal falls.
Circumstances of death
- Usually accidental
- Occasionally suicidal - becoming more common - especially in
- Rarely homicidal
Indicators of electrocution as a mode
These can be very subtle - the best indicator is the circumstances
surrounding the death. These may prompt the search for specific signs.
Typically electrical burns or blisters are confined to the site at
which current enters the body - the earth point is usually lesion free
but burns here may be seen sometimes.
There are two main type of electrical lesions:
It is very important to look for these lesions - hands, feet and head
must be carefully examined. Lesions may be concealed by the clasped
fingers of rigor mortis and if necessary the tendons must be cut to
release fingers for a proper examination.
- The typical electrical burn lesion from direct contact
occurs when heat generated by the electrical current turns fluid to
steam, splitting tissue layers. The blister flattens again when cooling
occurs and may burst if current is high enough or maintained long
enough. These lesions are surrounded by a white area - this is almost
pathognomonic for an electrical burn. This white area may be surrounded
in turn by a hyperaemic area and may be in the shape of the contact
- The other type of lesion occurs with arc burns.
These occur when a spark of electricity makes contact with the body. A
spark of 1000v can travel millimetres and 100,000v can travel 35 cm. At
these massive voltages considerable heat can be generated. This melts
the keratin of the skin leaving a hard nodule of keratin.
Other important areas to check for lesions are:
- In the mouth - especially in children where electrical devices
may be put in the mouth.
- On breasts where a sado-sexual component is plausible
- Within the urethra, vagina and anus. Again especially where
a sado-sexual component is plausible.
|Electrocution in water:
This provides special problems - the entry
point may be over a wide area and at low resistance - there may simply
not be any electrical burn lesions. Here the context may be the only
clue to electrocution as cause of death.
may result from either massive currents or lower currants over a long
time. Even the current from an electrically powered vehicle may cause
severe burns over the course of hours (see pic right).