Grounded on One or Both Ends

http://en.wikipedia.org/wiki/Ground_loop_(electricity)

sorry i still can't understand...

i see some shield gound on both, ans some gound more than two point (also in middle), what purpose is it? ^^'

Ground loops occur when the grounds at the two ends of the cable shield are at different potentials. Since a conductor, in this case the cable shield, which connects two points at different potential (voltage) will conduct a current I=V/R there will current flowing through the shield. This current will interfere with the signal on the shielded conductor. To prevent current from flowing through the shield one end of the shield wire is normally not grounded. If the grounds at both ends of the shield are at exactly the same potential, for example in a tabletop laboratory environment, then it is acceptable to ground both ends of the shield. Ground loops are most troublesome with long cable runs, between buildings for example.

thx for your detail explanation.

i found there are some shield cable ground more than one point at a electrical cabinet. i find some artical say the ground in one end can eliminate the electric field, ground in both end can eliminate the magnetic field. is it true?

This subject has also been discussed in depth on a number of threads on CR4. Try a CR4 search on the left hand side for more information.

Communication or signal cables should only be grounded on one side. If you have extension cables in between (bad idea) you would tie the ground on both ends of the cable leaving one of the devices. The other side should remain floating for the reasons posted by others.

does anybody know what cause the different voltage grounding in a building.

eg. the ground on topest floor is different with ground floor.

is it rely to weather?