why we use ground wire

This should help. Just download and read it.

[PDF] Electrical grounding techniques

Yes, the ground wire helps keep interference from affecting the signal were wanting. By only terminating the ground on one end only preferably at the receiving end. You prevent ground fault loop, That is a voltage can be formed on the ground and signal wires because of the different in ground potential from the transmitter to the receiver.

Abdul, as a Measurement Engineer of 34 years, several years have been involved in Research for "Grounding" Theories.

With the Instrumentation cables, I believe you are referencing the drain wire for the shield. The connection of this 'drain' or 'ground' wire, is dependent on the type of signal flowing through it. Shielding & Guarding can be a very complex subject in certain situations, environments & especially if "mixed" signals (analog & digital) are involved.

Here are some simple 'Rules-of-Thumb' with the reality that this subject falls in the category of an 'Art & Science'.

Ground termination on 1 side is typical of Low Voltage Level, Low Frequency Analog Signals (LLLFA). The main reason is to shield the the low level analog from EMI or crosstalk from other signal or voltages. If the shield is connected on both sides in this situation & there is a voltage potential difference between the 2 grounds, a current will flow through the shield & induce a crosstalk error voltage into the analog signal. This is ground current flowing through the shield is commonly referred to as a 'Ground Loop' & usually has random noise frequencies associated with it.

For digital & high frequency analog signals the shield is normally connected on both ends for best effectiveness & to minimize any EMI radiating from it.

For the shielding of Low Voltage Level, Low & High Frequency Analog Signals (LLHFA), I ground 1 end of the shield (as in the LLLFA case) & the other end I ground through a capacitor of selected value, such that the low frequencies see an open sided shield & the high frequencies see a a shield grounded on both ends.

This is an oversimplied, short explaination of a very complex subject, without the introduction of Standing Wave patterns & the use of Smith Charts. I hope this helps. Paul

This thread may be additional help:

http://cr4.globalspec.com/thread/50876

- MS