# Neutral CT directional-reference interface Canonical URL: https://eloragrid.ai/symbols/neutral-ct-directional-reference-interface Publisher: Elora Grid Updated: 2026-11-03 Lookup label: NCDI Drawing use: Schematic Reference families: Manufacturer Representation: Functional block Installation focus: substations Input: Neutral CT polarity Output: Directional current reference Provides the defined polarity and direction reference for a neutral-current protection input. ## In plain English ### What this block does Provides the defined polarity and direction reference for a neutral-current protection input. ### A simple example In a substation protection panel, separate CT cores can supply metering and protection circuits with different duties. A test boundary must preserve the secondary arrangement required by the selected CT and test device. For this reference, the drawing identifies “Neutral CT polarity” at the input and “Directional current reference” at the output. Directional logic also needs its selected polarising quantity. ### What the drawings help you do Start at the named input: Neutral CT polarity. Identify its source device or document and the state represented on the drawing. Follow the named output: Directional current reference. Cross-reference its destination and separate the command from confirmation that the equipment responded. ### Terms - Input: Neutral CT polarity - Output: Directional current reference - Application reference: An explanation of a named drawing function or circuit relationship; the project documents define the equipment and its connections. Concept diagram: Neutral CT polarity → NCDI → Directional current reference A reading guide. The entry's original illustration and input/output facts describe this application. - [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) ## What it represents Provides the defined polarity and direction reference for a neutral-current protection input. A current-transformer secondary arrangement carries a measuring current from a defined primary location to metering or protection inputs. Core identity, polarity and the connected circuit determine the measurement's meaning. In a substation protection panel, separate CT cores can supply metering and protection circuits with different duties. A test boundary must preserve the secondary arrangement required by the selected CT and test device. For this reference, the drawing identifies “Neutral CT polarity” at the input and “Directional current reference” at the output. Directional logic also needs its selected polarising quantity. Trace every secondary connection and the assigned earthing point against the actual connection diagram. These illustrations explain circuit roles and never prescribe a live CT switching or disconnection procedure. The functional block is an independent explanation of this named application. Equipment-specific ratings, terminal connections and operating conditions must be established in the project documentation. ## How to read it - Start at the named input: Neutral CT polarity. Identify its source device or document and the state represented on the drawing. - Follow the named output: Directional current reference. Cross-reference its destination and separate the command from confirmation that the equipment responded. - Directional logic also needs its selected polarising quantity. - Trace every secondary connection and the assigned earthing point against the actual connection diagram. These illustrations explain circuit roles and never prescribe a live CT switching or disconnection procedure. ## Information to show or reference - Label the source and connection reference for neutral ct polarity. - Identify the destination and defined state of directional current reference. - Record the equipment or control-document revision supporting this functional block. - Check this application limit in the project design: Directional logic also needs its selected polarising quantity. ## Important distinction Directional logic also needs its selected polarising quantity. This reference describes the named application; it does not establish a new IEC graphical symbol, an ANSI device-function number or an exact manufacturer connection diagram. ## Facts - Representation: Functional block - Installation focus: Substations - Input / boundary: Neutral CT polarity - Output / connection: Directional current reference - Lookup label: NCDI is a library lookup label, not a standard device designation. ## Public references - [Indoor current and voltage transformers](https://www.abb.com/gb/en/areas/electrification/medium-voltage/grid-components/core-components/indoor-instrument-transformers) — ABB. Public technical context for current-transformer secondary arrangements. Current and voltage measurement transformers and their secondary interfaces. The application text and educational graphic are independently authored; the publisher does not certify this representation. ## Graphic scope Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.