An instrument-transformer screen may be connected or intentionally floating. ABB’s insulation discussion includes shielding for local field control [1]. Preserve the actual references and separations instead of applying a blanket “earth all screens” rule.
The objective is to preserve the designed connections and separations. A blanket instruction to earth all screens would change the insulation system rather than verify it.
Give every screen an electrical identity
Assign the drawing identifier, intended reference and physical connection route to each accessible screen or screen group. For an intentionally floating layer, record the separation that must be preserved. For a connected screen, show where its connection reaches the reference; a visible metal overlap is not necessarily a qualified electrical joint.
| Screen state in the design | Observation to verify | Fault interpretation |
|---|---|---|
| Connected to a specified reference | Continuous intended route and correct endpoint | An open route can leave a different potential distribution |
| Connected to another screen or electrode | Correct pair and connection geometry | A wrong endpoint changes the grading arrangement |
| Intentionally floating | Absence of unintended bridges to adjacent parts | A bridge can clamp the layer to an unintended potential |
| Segmented for a defined purpose | Intended separation and identified connecting features | A bypass can create an unwanted conducting path |
These categories are a review framework. The actual manufacturer’s drawing decides which applies to a particular part.
Preserve connection evidence before wrapping hides it
Record joint identity, routing and inspection at the manufacturing stage where the connection is accessible. Later paper layers can hide a wrong joint or a displaced connection tail. A final terminal-level measurement may show an aggregate path while leaving the internal endpoint uncertain.
Connect each observation to the relevant drawing revision. If a screen is renumbered or its reference changes, update the inspection record rather than interpreting an old continuity entry under the new schematic.
An unintended bridge is a different failure from an open route
An open connected screen can become capacitively floating. An unintended bridge can force two designed potentials together. Both alter the electric field, but they require different investigations. A conductive burr, folded foil or incorrectly routed tail may create a bridge that a check of intended continuity alone never tests.
Conducting loops also deserve design-specific attention. A screen arrangement that accidentally forms a closed path linking changing flux may support induced current and heating. Do not infer the presence or absence of that path solely from a low resistance reading between two terminals.
Assume a screen is intended at earth through a 0.10 Ω connection. After disconnection, let its capacitances be 20 pF to 100 kV RMS and 80 pF to earth. With zero net node charge and negligible leakage, ΣC(Vscreen−Vnode)=0 from Q=CΔV [2], giving 20 kV RMS. The physical screen remains present while its potential changes. Hold the connection discrepancy and verify the endpoint/continuity after assembly, then analyse the actual fault-state field; 20 kV is not a withstand rating.
An accessible terminal can confirm a conducting path while leaving its internal endpoint uncertain. The electrical identity map and observations made before wrapping need to resolve that endpoint, with inaccessible regions retained as limits. Otherwise a plausible resistance reading can conceal the wrong grading potential.
References
- ABB, Instrument Transformers: Technical Information and Application Guide, 1VAP420003-TG, revision A, December 2004.
- U.S. Department of Energy, DOE-HDBK-1011/2-92, Electrical Science, Volume 2, 1992.