Laboratories that use physiological recorders should take proper precautions to protect human subjects or patients from possible electric shocks or burns. If an accidentally high electric current passes through the skin, it can produce pain or tissue damage. If it is very high and flows across the body, it could interfere with heart activity. Ventricular fibrillation of the heart is the most frequent cause of death in fatal electrical accidents.
Modern physiological recorders use various techniques to protect subjects from shock. For example, many of them use transistor circuits (in the first stage amplifiers connected to the electrodes) to isolate subjects from high-voltage circuits. However, even though the probability of shock is very low, additional precautions must be taken. One approach is to fuse each electrode lead to prevent shocks through inadvertent shorts between the electrodes and associated circuitry. A 5-mA (milliAmp) fuse would limit electric current flow to a safe level in the event of a malfunction. This would be especially important when making recordings that require the placement of electrodes on either side of the body, as is commonly done in measuring heart activity. For example, electrodes might be placed on the right and left arms for heart rate recordings. Another approach would be to connect each electrode to ground through voltage-limiting diodes. (A diode is a device that limits current flow to one direction.)
Because electric current passed across the body is more dangerous than current limited to one side, the use of one-sided measurement is preferred where feasible. For example, skin conductance measures should be made from the same hand, rather than using two hands. Similarly, ground electrodes should be placed on the same side of the body as active electrodes whenever possible.
A number of techniques to prevent or minimize the probability of occurrence of an electrical shock have been outlined by Cromwell et al. ( 1976).
Grounding may be achieved by connecting the metal equipment case to ground by a wire or by using three-pronged plugs in which the ground connection is established by the round con