In vendor drawing of Smart junction Box, vendor have connected instrument earth with safety earth. Is it ok ?
Connection Between Instrument and Safety Earth Explained
Yes, Probably it is okay. But The only segregation in earthing is in electrical (dirty) and in instrument (clean).
You can combine PCS earth with SIS as long as both are connected with plant clean earth, which in most of the cases we ensure.
It means final connection shall be to clean earth ? These cabinets are in fields and contractor have not made provision for clean earth in field. Plant / dirty earth is available.
Connection Between Instrument and Safety Earth As per Code IEC 60364
I suggest looking at IEC 60364, and you will get an idea of how earthing installation is made within the building. You can also check BS 7671.
Eventually, everything is going to be earth, either HV, LV electrical equipment, or a 24V circuit. Earthing is very important, so you need to define a criteria or follow existing criteria if you are working in brown fields. Usually, it is considered that a clean earth resistance should be <1 Ohm.
One question arise. In the drawing both should be connected to safety earth. There is no field wiring shield connection shown ? Both are AC earth.
Agree and thanks for detailing. The definition is clear, clean earth is less than 1 ohm and safety earth is less than 10 ohm. Field Instrument Enclosure building have both provisions. I think need to check with vendor again , they linked safety earth bar and instrument earth bar so eventually it will have same potential (no difference in Instrument/clean earth and Safety Earth), which raises the concern to check.
Regarding values first perform soil test as well.Because on majority of the places due to soil difficult to achieve 1ohm and 10 ohm.
As per standard and General Practice current always follow shortest/less resistive path, so earthing value adjust accordingly.
This practice is usually done in offshore where you can merge both the ground or after the panel both the grounds goes to same grounding network. But in onshore, both networks need to be segregated.
With clean soiled earth, you can achieve 1 ohm by installing 3 earth bars in triangle arrangements within 1 meter distance each, and it is better to have 3 meter deep penetration.
Grounding triads 3 individual pits joins and form triads (triangular joined network of earth-pits). Considering domestic small installation base, single earthing electrode (pit) may be sufficient but considering industrial complex and large infrastructure installation base to achieve idealistic equipotential base, it is recommended to form earth-pit triad with almost at 1 to 3 meter distance (distance varies depending of resistivity if earth soil, we need to achieve 1 ohm for instrumentation system grounding less than 50V instrument / systems).
Total number of individual earth-pits, earthing triads depends on overall infrastructure installation base of the facilities/plant, earth soil resistivity etc… Finally all these pits are connected together to form a unique grid (to act as a single point of same potential – Equipotential), called as “Plant Grid System”.
Connection Between Instrument and Safety Earth As per Code IEEE 81
Measurements of earth resistivity and ground impedance shall be made in accordance with IEEE 81 or by a non-contacting ground resistivity mapping instrument.
Sensitive electronic equipment should be grounded in accordance with the recommendations of IEEE 1100.
Each instrument signal common shall be connected to the isolated instrument DC and Shield ground bus with copper wire sized to carry the expected fault current or 12 AWG, whichever is larger using a screw-type compression ring lug. Two insulated copper conductors, 25 mm² minimum, shall be connected from the instrument DC and Shield ground bus within each cabinet to a single tie point on the Master Instrument Ground bar (MIG) within the control building in a closed loop configuration. The resistance from the isolated instrument DC and Shield ground bus to the plant ground grid shall be less than 1 ohm. Exception:
For offshore platforms, AC and DC & Shield ground bus bars within standalone cabinets can be directly connected to dedicated and separate points on the platform overall grounding grid.
Connection Between Instrument and Safety Earth As Per Code NEC Article 250
Both safety ground and instrumentation DC & Shield ground must conform to NEC, Article 250.
I have experienced the above somewhere around in 2003, and had difficulty to achieve 1 ohm, i tried reducing the distance and bring it close to 1 meter and i got 1 ohm.
That was for ethylene flow metering skid instrumentation grounding. We have used copper wires not galvanized, 25 mm.sq.