Summation of Leakage (Touch) Current
Questions to the Panel:
5.1.8.2 Summation of leakage (touch) current from telecommunication networks states:
"It shall be assumed that each telecommunication port receives 0,25mA from the other equipment, unless the current is known to be lower."
Is that statement also acceptable for a telecommunication bus system like the ISDN S-bus with 8 terminal equipment? Or as an other common example the USB (Universal Serial Bus) interface in computers?
Opinions of the Panel:
The ISDN S-bus falls within the definition of a TELECOMMUNICATION NETWORK, so that all of subclause 5.1.8 applies. The maximum number of equipments permitted to be connected to the S-bus is nine (eight user terminals plus one), which could lead to a maximum TOUCH CURRENT of 9 x 0,25 mA. This is within the TOUCH CURRENT limit of 3,5 mA in subclause 5.1.6, table 5A.
The USB interface is not considered a TELECOMMUNICATION NETWORK interface, see the 5.1.8 Note making reference to data ports such as serial, parallel, etc. Therefore, the requirements of 5.1.8 are not applied to such bus systems.
74/575/INF简要解析：
ISDN S-BUS电信总线的一拖八（1主机8终端机）的设备适用于5.1.8.2条款规定“除非已知来自其他设备的实际电流较小，应当假定每个通信端口从其他设备接收的电流为0.25mA”，可以假定最大接触电流为0.25mA*9，满足连接到保护接地端子的3.5mA限值（前提是所有互连电路连到一个公共回路点，然后将该点接到保护地上）
USB接口不认为通信网络的端口，因为标准5.1.8注释规定了“通信端口是包括那些预定要附装通信网络的连接点。这些通信端口不包括其他数据端口，例如通常确定为串行、并行、键盘、游戏、操纵杆等的数据端口”
