It is wrong to ground(earth) a cable at more than one point !because ground loop currents may generate hundreds amps of current in the sheild,because that there is a capacitance between the sheild and wire cores of cable a large noise will be induced on the sensors and weak signals .

Hallo Parviz,

please don't fell offended by me, but loop currents in the range of hundreds of amps is totally unrealistic!
Two main mechanisms can produce a ground current in the loops, (1) highly unsymmetrical load situation in an three-phase application and (2) induced voltage, which forces a current through ground loop.

(1)
If in a three-phase application one phase breaks or load of three phases is unsymmetrical, a certain amount of current is flowing through N (neutral) back to mains supply. If earth is connected to N at more than one place in your factory a certain amount of current can also flow through earth connection. BUT: Whole earthing concept of factory must be realized in such a way, that in any case between two different positions earth potential difference must never be greater than a few volts. Exact data depends on country and protection class.
In many countries, additionally, machine is stopped immediately, means is disconnected from mains, when some current, erroneously flowing through earth connection, is detected!

Assume that there is such a potential difference between two earth potentials of about 1V. Assume further that you connect these two points by a long cable with total resistance of about 1Ohm, then only 1A would flow through the cable and heat dissipation would be about 1W.
But, it's very improbable that through earth 1A is flowing. That's so much earth current that earth current protection switch would immediately stop this failure.
If actually 'hundreds of amps' would flow, as you told, this would cause a voltage drop accross the cable of also 'hundreds of volts'. But this would be absolutely deadly! It would kill every technician, which comes in contact with.

So, in a factory, where proper grounding rules are obeyed, you will not find high earth potential differencies between two points. And earth currents are also very small.

(2)
Assume that our cable is part of a ground loop showing a resistance of 1Ohm. If actually hundreds of amps would flow through the loop, caused by induced voltage, cable would have to dissipate a power of tens of kW (!), according to P = I x I x R. But where should such a high power come from? Magnetic stray field of a three-phase motor? No, that's abolutely unrealistic! If this would be true, everything metallic in your factory would heat up due to induced voltage of magnetic stray fields.

Concluding: Whenever you see in a factory earth potentials bigger than 10V, then something is wrong with earthing concept. Situation becomes dangerous for technicians and earth routing problem must be fixed as soon as possible. One methode of fixing (when there is no other big mistake left, of course!) is to route an additional cable of much lower resistance parallel to signal cable. This additional cable is called PEC (parallel earth cable). It decreases resistivity between to earth points further, so that earth potential difference at cable is also decreased.


In one point you are right. Ground noise in the range of a few volts will couple into signal via cable capacitance. That's why the use of symmetrical signal routing is highly recommended: Capacitive coupling adds ground noise to signal to same amount and polarity and because at reciever both signals (+line and -line) are subtracted form each other, signal doubles, while ground noise vanishes!
By the way, when driver output is rather low ohmic, which is nearly always the case, then capacitive coupling results in a very small capacitively added ground noise to signal. Let's have some estimation:
Assume that cable is hundred meters long, resulting in 10nF capacitance between shield and each signal line (assuming 100pF/m). Driver output resistance shall be 100Ohm. Then capacitive coupling can be estimated by the following formula:

Coupling = -20 / LOG(10) x LOG (SQRT((1 / 2 / pi / f / R / C )**2 + 1))

Assume you have 1Veff ground noise at 50Hz, then formula gives Coupling = -70dB. Means, 1Veff ground noise at 50Hz results in 316µVeff interference added to signal. Assuming a further common mode rejection of at least 40dB, ground noise appears at signal behind substracting stage of reciever with less than 3µVeff!

So, 'disadvantage' of capacitive coupling to signal is totaly negligible. On the other hand, an additional advantage arrises, because introducing of grounded shield, with shield connected to earth at both ends, is a very good screen against cell phone radiation, electrostatical charge, etc. You then will profit from full benefit of Faraday cage concept.

Kai