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An instrument earthing drawing shows how the instrumentation of a plant area is connected to earth. For each system cabinet, marshalling cabinet and field junction box it shows the earth bars and the earth each bar belongs to, where each cable screen and armour is terminated, and how the bars are joined to the earth grids and the earth pits beneath them. Deliverable lists name it among the instrumentation design documents, some as the instrument grounding layout or the earthing philosophy drawing.
The drawing records the earthing the design chose. Resistance targets, conductor sizes and the arrangement of the earth system come from the project's earthing philosophy, the control system supplier's requirements and the electrical design under the codes that apply.
In short
- It is a map of earths: each cabinet, junction box, screen and armour, and the bar it lands on.
- Most projects keep three earths apart through the cabinets and the field: the safety earth, the instrument earth and, where intrinsically safe circuits run, the intrinsically safe earth.
- Each cable screen is drawn with the one point where it is earthed, and the drawing is where that choice is written down.
- The conductors between the bars, and their joints to the earth grids, carry the sizes the design gave them.
- It has to agree with the cable schedule, the junction box and cabinet drawings, the loop diagrams and the electrical earthing layout.
The earths it keeps apart
| Earth | Also called | What lands on it |
|---|---|---|
| Safety earth | Protective, electrical, power or dirty earth | Cabinet and junction box bodies, cable armour, instrument housings, cable trays |
| Instrument earth | Clean, reference, signal or electronic earth | The screens of analog signal cables, at the end the design earths them |
| Intrinsically safe earth | IS earth | The screens of intrinsically safe circuits, carried to their own bar |
Each earth does a different job. The safety earth carries fault current away from people and equipment. The instrument earth gives sensitive signals a quiet reference. The intrinsically safe earth belongs to the protection of circuits that run into hazardous areas. In a system or marshalling cabinet the drawing shows a bar for each, with the instrument and intrinsically safe bars mounted on insulators, while a network, server or power distribution cabinet usually carries the safety earth bar alone.
Apart does not mean unconnected. Each earth runs on its own bars and conductors to a common bar in the cable cellar or under the false floor, and from there to its grid and the earth pits. The drawing shows where the earths join the plant earth, at the point the earthing philosophy chose.
What the drawing shows
| Element | What the drawing records |
|---|---|
| Earth bars | Each bar in each cabinet and junction box, its earth, and whether it stands on insulators |
| Common bars and grids | The common bar of each earth in the cable cellar or under the false floor, and the grid it joins |
| Earth pits | The pits or electrodes each grid ends at, as the electrical earthing layout places them |
| Cable screens | Where the individual and overall screens of each cable are terminated, and where they are left unearthed |
| Cable armour | Where the armour of each cable is terminated, at the gland of each end |
| Housings and structures | The earthing of instrument housings, junction box supports and cable trays to the nearest steel or earth boss |
| Conductors | Each earthing conductor between bars and grids, with its size and colour as specified |
| Notes and legend | The earthing philosophy the drawing applies, its symbols, and the documents it depends on |
A design review checklist for earthing and grounding layouts, published by Saudi Arabia's Government Expenditure and Project Efficiency Authority, asks whether a layout gives the type, size and number of electrodes and conductors, the interconnections and the terminations at equipment, and whether its design accounts for instrumentation and computer systems.
One junction box, screen by screen
In this invented example, junction box JB-3401 in Unit 34 gathers four transmitters, PT-101, FT-101, LT-103 and TT-104, and carries their signals to the marshalling cabinet CP-1 on one multipair cable. The project's earthing philosophy earths each screen at the control system end only.
| Element | At JB-3401 | At CP-1 |
|---|---|---|
| Box or cabinet body | Earthing stud bonded to the nearest steel structure, on the safety earth | Safety earth bar |
| Individual screens of the four transmitter cables | Landed on an insulated screen bar; at each transmitter, cut back and insulated | Instrument earth bar |
| Overall screen of the multipair cable | Landed on the same insulated screen bar | Instrument earth bar |
| Armour of each cable | Terminated at its gland and bonded to the box body | Terminated at its gland, on the safety earth bar |
| Link to the earth system | The earthing stud, to the structure | Each bar, by its own conductor, to the common bar of its earth |
Read along a row, the table is the path one screen or one armour takes to earth. A screen earthed at both ends, or at neither, shows up as a row that does not end at exactly one bar.
Where the drawing meets the other documents
| Document | What it shares with the earthing drawing | The mismatch to look for |
|---|---|---|
| Cable schedule | Each cable, its screens and armour, and its two ends | A screened cable whose screen the drawing earths nowhere, or at both ends |
| Junction box schedule and termination drawings | The terminals the screens pass through in each box | A screen terminal the earthing drawing does not show |
| Loop diagrams | The point where each loop's screen is earthed | A loop diagram that earths a screen at the field end |
| Installation details | The earthing detail for each device, box and cabinet | A detail that bonds a device in a way the drawing does not show |
| Cabinet layouts | The bars each marshalling cabinet and system cabinet carries | A cabinet with intrinsically safe circuits and no intrinsically safe earth bar |
| Electrical earthing layout | The grids, the earth pits and the points where the instrument earths join them | An instrument earth joined to the grid at a point the electrical layout does not have |
| Instrument location layout | The positions of the junction boxes and cabinets | A box earthed where the layout no longer places it |
| Hazardous area classification | Which areas are hazardous, and so which circuits are intrinsically safe | An intrinsically safe circuit drawn on the instrument earth |
Keeping it current
The drawing changes whenever the installation does, with a cabinet added, a junction box moved, or a cable rerouted or replaced by one of another construction. Each change is a revision under the project's change process, and the as-built issue records what was installed.
At installation the earthing is checked against the drawing, for continuity from each bar to its grid and for clean contact faces where conductors meet the bars. In hazardous areas, IEC 60079-14 adds initial inspection items for the earthing of conducting screens and the bonding of cable armour on intrinsically safe circuits. The drawing shows the earthing those checks expect to find.