At 17:43 10/04/02 +0000, chris@neitzert.com wrote:

I'm in the process of managing cabling for a large install (500-ish runs) and a vendor came to me with a story about the creation of ground loops in running sheilded+gounded cat-5e in large installations. ...

A perspective from the audio/video world ... ... we tie shields down at BOTH ends to the local mass/frame ground, the price paid for this can be LARGE currents flowing in the braids. This is BAD(tm) because no matter how good the common mode rejection is, there is always enough un-balance to inject noise. It's also rather dangerous to man and machine. The remedies are designed at an overall system level. Firstly a radial earth scheme is employed (referencing to an equi-potential point). This works well within an installation, but is a little cumbersome across larger distances where hum-bucking hardware (transformers et al) are used. Secondly "sacrificial earths" are used to cope with those circulating currents that just can't be avoided, i.e. we place highly over-dimensioned cables electrically in parallel with the bundles of individually shielded pairs, but locate the run away from where the fields can impact cables carrying signals. In summary, STP earthing is tricky and needs to be considered in conjunction with the overall earthing scheme in the installation, but not tying down both ends is an unacceptable and unsafe practice. p.s. there are a number of other issues relating to the Electrical Safety Standards and Safety Earths that have to be fed into the mix.