CAMBRIDGE CED 1902 - Electrical Safety and Isolation Theory

Details of operation
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To be safe for use on live human subjects, the electrodes, and
the circuitry connected to them, have to be isolated. To
understand why, first consider what actually happens when one
receives an electric shock from the mains. (This illustration
uses UK, earthed-neutral mains, which can be quite lethal. For
our purposes, AC can be regarded as slowly-changing DC.)
You usually receive a shock by touching a live point while
grounded, whereupon an electric current flows through your
body to earth. You may not be interested in what happens to it
after that, but it is relevant. At the local substation transformer,
the neutral terminal of the secondary windings is robustly
strapped to earth. As much charge will flow out of earth as has
flowed in through the victim. This happens because, were it not
to, a static voltage would rapidly build up and oppose further
current flow. So, the current has to complete a circuit.
If no circuit can be completed, no current can flow. If you stand
on a dry plastic bucket and touch one live point, you will
merely gain a static charge to mains voltage. In the case of a
person attached to mains-powered equipment, relying on
insulation is completely impractical: there is usually more than
one wire connected to the subject, not to mention intentional or
accidental contacts with other equipment and earthed
conductors, including other persons. There are too many
possible paths for current flow.
The solution is to shrink the entire power supply circuit until it
is contained within the 1902, and not connect it to earth at all.
(This is known as a floating supply.) In the 1902, a miniature
transformer provides power for the isolated circuitry; signals to
and from the rest of the 1902 pass through optoisolators. There
is an isolation gap of 6 mm between copper tracks on the two
sides. Insulation between the two sides is guaranteed to
withstand mains voltages. This greatly reduces the scope for
current-flow paths. Charge cannot flow out of the electrode
circuit so long as there is no way for it to come back; it can only
travel round the electrode wires. The isolation socket has no
connection to any mains conductors, so it cannot act as a
conduit for shock current from other equipment. To keep it this
way, 1902 electrodes should NEVER be earthed.
Theory of
electrical
isolation
Current flows in
circles
You cannot rely on
insulation
Floating power
supplies