| ??? 07/01/06 03:40 Read: times |
#119499 - PEC Responding to: ???'s previous message |
Andy said:
In a balanced system, the shield is only the shield, and not a signal conductor. This means that the shield is simply an extension of the enclosure. So, connecting the cable shield at both ends to the chassis ground means nothing less than to expand the Faraday cage over the whole arrangement! A closed Faraday cage expanded over the whole set up, means expanded over all involved devices and cables, without any gaps and interruptions, offers a superior performance refering to EMC! Andy said:
Now, if the ground differences between the driver and receiver are so great that you have "Excessive" shield current, then you're probably going to exceed the receiver's input voltage range anyways. In such cases the introduce of a PEC (Parallel Earth Cable) is recommended, which is to be connected and layed in parallel to the RS485 cable, concretely spoken to its shield. It drastically lowers the impedance of cable shield and by this reduces ground noise. Even with rather high ground currents, as they can be observed sometimes with improper ground connections, for instance in welding plants, the PEC can keep the ground noise minimal and can help to prevent an overheating of RS485 cable. In any case the PEC helps to enhance the saftey performance by keeping touch potentials of chassis minimal. Kai |
