CT RES
Residual connection of phase CTs
A residual CT connection combines secondary currents from phase CTs to obtain a residual-current input.
SLD / Schematic
Symbols · K
Neutral-current measuring relay
Measuring relay fed from a CT in a neutral conductor or star-point connection, responding to neutral current.
In plain English
What does Neutral-current measuring relay mean?
This relay form is used where a CT is installed in a neutral conductor or in the earthing connection of a transformer or generator star point. Current there indicates earth faults or, in four-wire systems, heavy unbalance and triplen harmonic loading, depending on where the CT sits.
A simple example
The symbol identifies the measured conductor but not the purpose. A neutral CT in the star-point earth lead gives standby earth-fault protection, while one in the four-wire neutral of a feeder sees load unbalance as well, so settings and grading differ greatly.
What the drawings help you do
Confirm whether the CT is in the neutral conductor or the earthing lead. Check grading against downstream earth-fault devices. A neutral-current relay measures one neutral path; a residual connection sums the three phase CTs to derive the same quantity.
This particular reference
Measuring relay fed from a CT in a neutral conductor or star-point connection, responding to neutral current. A neutral-current relay measures one neutral path; a residual connection sums the three phase CTs to derive the same quantity.
- Winding
- A coil of conductor inside an electrical machine or transformer.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Terminal
- An identified point where a conductor connects.
Read the source: AutoCAD Electrical: Current monitoring relays · IEC graphical symbols for diagrams
Schematic / SLD · K
What it represents
This relay form is used where a CT is installed in a neutral conductor or in the earthing connection of a transformer or generator star point. Current there indicates earth faults or, in four-wire systems, heavy unbalance and triplen harmonic loading, depending on where the CT sits.
The symbol identifies the measured conductor but not the purpose. A neutral CT in the star-point earth lead gives standby earth-fault protection, while one in the four-wire neutral of a feeder sees load unbalance as well, so settings and grading differ greatly.
How to read it
- Confirm whether the CT is in the neutral conductor or the earthing lead.
- Check grading against downstream earth-fault devices.
Information to show or reference
- Show neutral CT location
- State setting and delay
- Reference grading study
A distinction that matters
A neutral-current relay measures one neutral path; a residual connection sums the three phase CTs to derive the same quantity.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
AutoCAD Electrical: Current monitoring relays
Public AutoCAD Electrical relay listing that names the In Neutral block; the illustration is independently drawn.
IEC graphical symbols for diagrams
IEC context for graphical symbols used in diagrams; no IEC graphic is reproduced.
About this graphic
This illustration was authored independently by Elora Grid to explain the concept. It is not reproduced from a customer drawing, an official standard graphic or a manufacturer CAD file. Its correspondence to an exact IEC or IEEE symbol identifier has not been certified.
SVG and DXF downloads contain this original artwork. DXF uses unscaled drawing units; a GA envelope supplies no product dimensions, clearances or fabrication information. Original artwork is available under CC BY 4.0. Third-party references retain their own terms.
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