Loop power and signal conductors run from the DCS cabinet to the safe-area field panel, pass through an intrinsically safe barrier, and continue to the hazardous-area instrument. The cable screen follows the same physical route, but its termination path serves interference control rather than carrying the measurement signal. Follow that path first: DCS cabinet, interpanel cable, insulated screen terminal, barrier panel earth system, field cable, and instrument enclosure.
Where can the screen and earth paths stop?
There are two proposed screen arrangements. Option 1 maintains screen continuity through insulated terminals beneath the three barriers and bonds the screen to instrument earth only in the DCS cabinet. Option 2 terminates the intrinsically safe cable screens on an insulated instrument earth bar in the barrier panel and connects that bar to the DCS instrument earth with a dedicated 16 mm2 conductor.
Neither arrangement can be assessed by counting earth connections alone. Trace three separate functions:
- Screen path: drains induced high-frequency and electrostatic interference without becoming a normal signal conductor.
- Barrier protective path: carries fault energy where an earth-referenced barrier depends on earth for intrinsic-safety protection.
- Panel protective bonding: bonds exposed conductive parts for electrical safety. This is not automatically the same connection as instrument earth.
A screen can be continuous through a panel without being bonded there. Conversely, a barrier can require a local earth connection even when the cable screen is bonded elsewhere. Combining these functions without checking the barrier documentation can create either an ineffective fault path or an unintended screen loop.
How do the two screen-earthing options compare?
| Decision point | Option 1: screen bonded at DCS | Option 2: screen bonded at barrier panel |
|---|---|---|
| Screen route | Continues through insulated terminals to the DCS instrument earth | Terminates on an insulated instrument earth bar in the field panel |
| Dedicated earth conductor | None proposed for the screen path | 16 mm2 conductor from the field-panel bar to DCS instrument earth |
| Number of intended screen bonds | One, at the DCS source end | One, at the barrier-panel earth bar, provided the DCS end and field instrument end remain isolated |
| Barrier earth | Not established by screen continuity alone | Can provide a local connection point, subject to barrier requirements |
| Main risk | A long barrier earth path or no qualifying barrier earth where the barrier relies on one | Parallel bonds through the screen, panel metalwork, armor, or another earth conductor |
| Best fit | A barrier that does not require an earth-referenced protection path, with the screen intentionally grounded at the DCS | An earth-referenced barrier installation where a short local connection and controlled common instrument earth are required |
For the described layout, use Option 2 when the installed barriers require an earth-referenced protective path. It places the screen termination and barrier earth reference at the barrier panel and avoids making the interpanel cable screen perform the barrier’s safety-earth function. Option 1 remains an EMC arrangement, not proof that a required barrier earth connection is adequate.
What determines whether the barrier needs local earth?
Start with the exact barrier type. A passive shunt-diode barrier depends on a defined earth path to divert fault current. A galvanically isolating barrier uses electrical separation and may have different functional-earth or protective-earth instructions. The word barrier does not identify which behavior applies.
Read the barrier marking, certificate documentation, installation instructions, and terminal diagram. Identify the terminal designated for intrinsic-safety earth, functional earth, or protective earth. Do not substitute a screen terminal merely because both eventually connect to the site earthing system.
The proposed 16 mm2 conductor states cross-sectional area, not installed impedance. Length, routing, joints, terminal resistance, corrosion, and parallel paths affect performance. Where the barrier requires a short safety-earth connection, route its specified earth conductor directly to the designated local bar with minimal loop area and without removable or fused devices in the path.
IEC 60079-14 is the installation document identified for checking this arrangement. Compliance also depends on the applicable Australian adoption, the edition required by the project, the barrier certification conditions, and the manufacturer’s instructions. Verify the permitted earthing arrangement and required earth-path characteristics from those controlled documents; the two sketches alone do not establish compliance.
How should the recommended arrangement be installed?
- Record the part number and protection method of each of the three barriers. From each installation diagram, identify every barrier earth, functional-earth, and screen terminal.
- Install the insulated instrument earth bar in the safe-area barrier panel. Its insulation prevents an uncontrolled second bond through the panel backplate or enclosure.
- Connect the earth bar to the DCS instrument earth using the proposed 16 mm2 conductor. Route it directly and protect its terminations from loosening and corrosion.
- Connect each hazardous-area instrument cable screen to the insulated bar at the barrier panel. Use a termination that provides continuous circumferential contact where the cable and gland system are designed for it; avoid long screen pigtails because their inductance reduces high-frequency performance.
- At the hazardous-area instrument, isolate the screen unless the instrument certificate or manufacturer’s diagram requires another termination. Check cable armor and drain wires separately because either can create an unplanned parallel earth path.
- At the DCS end, isolate the corresponding interpanel screen if bonding it would create a second screen-earth connection. If the design requires screen continuity between cables, document exactly where continuity stops and where the single intentional bond occurs.
- Connect any barrier safety-earth terminals to the earth bar using the barrier manufacturer’s specified arrangement. Do not route barrier fault current through a cable screen.
- Label the bar, conductors, and isolated screen terminals so later maintenance cannot convert an insulated termination into a chassis bond.
How do I find unintended screen bonds?
Layer one first. De-energize the affected circuit under the site’s approved isolation procedure, disconnect sensitive equipment where the resistance test could place voltage on electronics, and examine every metallic path before measuring.
| Check | Desired result for Option 2 | Fault indicated |
|---|---|---|
| Screen to barrier-panel earth bar | Continuity at the intended termination | Open terminal, broken drain wire, or poor clamp contact |
| Insulated earth bar to panel enclosure | No direct bond except through the documented earthing path | Mounting hardware, gland plate, or jumper bypassing insulation |
| Screen to instrument enclosure | Isolated unless the approved instrument diagram requires a bond | Conductive gland, spare drain wire, or internal terminal creating a second bond |
| Screen to DCS cabinet earth | No separate parallel screen bond | DCS terminal, surge device, or cable accessory grounding the remote end |
| Barrier earth terminal to designated bar | Direct documented connection | Barrier protection path omitted or routed through the screen |
Test one cable at a time with both ends identified. A low resistance reading can arrive through another loop, armor, panel bond, or connected device, so separate parallel branches until the measured path matches the drawing.
How is the completed installation verified?
- Compare every termination against the barrier and instrument installation diagrams.
- Measure continuity of each intended screen and barrier-earth path with equipment disconnected as required by the test method.
- Measure isolation between the insulated earth bar and panel enclosure, then investigate any path not shown on the approved drawing.
- Reconnect the loop and confirm correct DCS indication across the operating range available for commissioning.
- Record the earth-path measurements, test instrument, conductor routing, terminal identities, and document revisions in the commissioning record.
Frequently Asked Questions
How do I choose between DCS-end and barrier-panel screen earthing?
Read the barrier installation diagram first. If the barrier requires an earth-referenced protective path, use the local insulated earth bar arrangement and keep the screen’s EMC function separate from the barrier fault path.
How do I use the proposed 16 mm2 earth conductor?
Connect the insulated barrier-panel instrument earth bar directly to the DCS instrument earth, subject to the controlled design and barrier instructions. Verify the completed path by measurement because conductor area alone does not establish terminal resistance or installed impedance.
How do I verify that the screen is earthed at one point?
Isolate the loop, disconnect equipment as required, and measure from the screen to the earth bar, panel enclosure, DCS earth, instrument enclosure, armor, and drain wires. Reconnect the loop only after the single intended bond and every required barrier-earth connection match the approved drawing.