Separate Earthing Requirements for Instruments

Isolated or separate earthing system for instrumentation is not recommended in many codes and standards. Electrical Earthing System and Instrumentation Earthing System finally should be connected to each other. You can find more about this matter in IEC 61000-5-2 1997 and also IEEE Std 142 1991.

In many installations, the service power ground is not well maintained. I have found this to be especially true in manufacturing facilities, where the process machinery is the focus of all attention from the maintenance staff. Another potential problem is that, in large facilities, the service ground may be a hundred or more meters from the instrument, and the ground wire from the instrument (which is usually fairly small) will have significant resistance due to the length.

To ensure their instrumentation is not affected, some instrument manufacturers want a separate, local, earth connection for their devices. However, if you have a good grounding system throughout the entire installation, you won't have any problems. The standards mentioned in the earlier post both include information on proper design and maintenance of the overall grounding system.

Amen to that. In our facility we had a terrible time with RTD's - the readings would shift 5-6 degrees when equipment was turned on and off. Ultimately, the culprit was was poor grounding systems. There was as much as 23V difference in potential between a wind tunnel running 460V and the ground of my temperature sensors - connected to a 110V separate source (we have 3 primary power feeds into the building). Installing a separate grounding rod to bring the two sources in question to the same point didn't help. In the end, running a new control transformer from the wind tunnel power feed brought them to the same level and eliminated the ground loop errors. I'm sure that the system can be improved, but the point is be wary of tying all ground systems together when you have poor ground quality to begin with. Good Luck, AndyC.

It depends what you're doing. If the equipment you're trying to make the measurements on is earthed to "dirty" mains you may well be better off earthing your instrument to the same point. I'm not familiar with other "instrumentation" but if you try to earth an oscilloscope in a "bench" socket whilst the equipment your monitoring is powered from elsewhere your readings can be swamped with noise (even with a short earth connection on the probe).

We just this past month moved our operations to a new plant. We have a Mitutoyo CMM that was among the items moved. When we had our CMM calibrated this week, the Mitutoyo Calibration Tech recommended that we establish a separate ground to "earth" to the controller. There is a stud provided for that purpose. This is a protection to the controller in case of lightning strike, which can fry the controller. If the controller is damaged during a lightning strike, and the controller is not grounded to "earth" Mitutoyo will not warrant the device. The controller is worth mucho $$$!

We are going to "earth" this puppy!

I also feel that separate earthing is required for instruments. Generally all the AC/DC machines or heating appliances will generate the noise in the form of emmitted radiations (electromagnetic) and conducting radiations (noise on the power lines). The emmitted radiations captured by the body of machines/appliances and inducts the colleted noise (electrical form) on the common bus bar ground. This reverse ground noise affects the readings of the ground sensitive measurement devices.

Bye

Shivashankar Deginal

I also feel that separate earthing is required for instruments. Generally all the AC/DC machines or heating appliances will generate the noise in the form of emmitted radiations (electromagnetic) and conducting radiations (noise on the power lines). The emmitted radiations captured by the body of machines/appliances and inducts the colleted noise (electrical form) on the common bus bar ground. This reverse ground noise affects the readings of the ground sensitive measurement devices.
Bye
Shivashankar Deginal

Dear Mr.Prasanna1283,

It is correct to have a seperate/dedicated earthing for instruments.

In the power circuit earthing there will be always some unbalanced current flow which will be significant and will affect the Instruments.

Inductance effect will be felt in the power circuit earthing which will have impact on the instrument earthing.

DHAYANANDHAN.S

<...In the power circuit earthing there will be always some unbalanced current flow which will be significant and will affect the Instruments...>

The purpose of the earth conductor is to operate the circuit protective device(s) in the event of a fault, thereby preserving life and equipment. So, that sentence doesn't make sense: ordinarily no current flows in a properly-installed earth conductor.

It is done to preserve the <... Instrument manufacturers ...>' reputations as most instruments cannot survive a fault current of a substantial number of amperes flowing into them.