his is a modem with a port for a PC, as in Figure 23.13. Figure 23.10 Cadenced ringing singal Modem Network cadenced (off part of the time, see Figure 23.10), and that a tele- phone circuit is not generally accessible to a large area of contact such as a whole hand, but only to a finger-tip. To legitimise such designs in the framework of the classical Mains Mains Barrier standards, an intermediate level was invented, called a TNV circuit (telecommunication network voltage circuit), which is allowed to have a higher voltage but less accessibility. The maximum voltage Figure 23.13 PC and modem connected to a network for a ringing signal takes its cadence into account. A TNV circuit is allowed to be accessible to the standard test finger but only on the contacts of a connector (such as a telephone jack) and only then In the present open market environment for telecom attachments, provided that it cannot be touched by a blunt version of the test a user must be able to buy a modem and a PC separately and connect them together and to the network, without having to get specific approval for that particular pair of products. But can the direct Radius 6mm attachment (the modem) be approved as being safe without exam- ining the PC to be sure that it would not harm the network indirectly, even if it had a fault in it? BS 6301 (the British Standard that has the same intent as EN 41003) requires that the port in the modem must have an electrical barrier behind it (see Figure 23.14) to protect the Handle Modem Network Figure 23.11 Blunt probe Barrier finger. This blunt probe (see Figure 23.11) has a tip radius of 6mm. Mains Mains Its use ensures that a deliberate effort (such as poking a finger into a telephone jack) is needed to touch a TNV circuit; the very limited area of contact that is possible produces a higher body impedance Figure 23.14 Barriered port and therefore a lower current through the body. 23.5.2 Indirect attachments network; alternatively the user must make sure that the PC itself complies with BS 6301 and th