e drawn protection uses double or reinforced insulation (Figure 23.8), in from accessible parts through a resistance representing the human body. These values are based on knowledge of the effect of current on the human body and will ensure that even if the condition is Basic insulation noticeable, it will be safe for the great majority of situations and people. IEC 479 gives the basis of these values. So as to be unambiguous, the standard must also be clear about what is considered to be accessible. This is done by the use of a Radius 2mm Figure 23.9 Tip of a standard test finger Figure 23.7 Class I protection standard metal test finger, which is like a human finger but smaller SO as to give a margin of safety. Its main diameter is 12mm but the tip is 2mm radius (see Figure 23.9). The same test fingure is used in much the same way in all the classical safety standards. It will be noted that the criteria are binary (and therefore somewhat artificial, in the interests of unambiguous and repeatable testing): either the test finger can touch a part or it cannot; either the touch voltage is below a certain limit and safe, or it is considered to be hazardous and fails. Basic Supplementary Telecom safety considerations insulation insulation 23.5.1 TNV circuits Signals on telecom circuits, and ringing signals in particular, do not Figure 23.8 Class II protection fit into this tidy scenario. They often have a higher voltage than is accepted in the classical standards as being safe to touch, yet in 23/6 Safety and EMC telephone practice, which has a good safety record, they have not traditionally been made as difficult to touch as, for example, mains circuits. There are several reasons for this apparent difference in philosophy, of which the most important are that a ringing signal is Network Barrier Mains Figure 23.12 Directly connected equipment However, most attachments, other than simple (or clever) tele- Continuous signal Interrupted signal phones, have ports for other boxes to be connected. An example of (Cadence) t