Cable shield connections and groundloops
There are sometimes lots of talk about how the cable shields should connected whn the equipments are interconnected. Some arguments say that to avoid ground loops is a good idea to connect the cable shield only at one end to avoid large ground currents on cable shield. On the other hand EMC books and text talk about how important is to connect the cable shield on both ends to get good shielding against interference trying to get out from the cable and into the signal in the cable from external sources. Cable shield connections are still controversial. There are strong, valid arguments both ways.
For cable shield connections there are four options, with advantages and disadvantages:
1. Shield grounded at both ends: Good r.f. shielding but susceptible to ground loop currents that can be VERY large - up to at least 100 A in bad cases.
2. Shield grounded at both end, with large-area parallel bonding wire: Good at r.f. and now the ground current flows mainly through the bonding wire, but the intense magnetic field that may result is not good news. The bonding wire size needs to be, for example, 10 mm^2.
3. Shield grounded at one end only: No ground current but not good at radio frequencies for which the cable is more than 1/8 wavelength long. R.F. interference may actually be worse than for an unshielded cable.
4. Shield grounded at the sending end and grounded through a capacitor (may be two in parallel, for effectiveness from about 100 kHz up to 1 GHz and beyond) at the receiving end: Good at r.f. if the capacitor type and positioning are correctly-designed, and no low-frequency ground current. One capacitor needs to be of the type with an integral spark- gap so that it is not damaged by transient high-voltage spikes that may be induced on to the shield.
What wiring practices are used in practice
The professional lighting and sound industries generally favour solution 4, based on practical experience of what works and is reliable. There are also 'ground both ends' people in sound industries who remain unconvinced of this and use option 1. Video industry generally favours option 1.
When considering the wiring for other application you need to consider following things:
- What are the chances of large earth currents?
- What are the chances of excessive emission from cables whose screens are not earthed at low frequencies at both ends?