CBCT
Core-balance current transformer
A core-balance CT measures residual current from the net magnetic effect of conductors passing through its window.
SLD / Schematic
Symbols · K
Earth-fault current relay
Measuring relay form that operates on current flowing to earth, fed from a residual connection or core-balance CT.
In plain English
What does Earth-fault current relay mean?
The earth-fault current relay form identifies a relay whose operating quantity is current flowing to earth. It may be fed from a core-balance CT around all live conductors, from the residual connection of three phase CTs, or from a CT in the star-point earth lead, and the source changes its achievable sensitivity.
A simple example
The relay box shows the quantity with an earth additional drawing mark but not the setting, characteristic or whether it is directional. In resistance-earthed and isolated networks earth-fault current is small and often needs a sensitive or directional element that must be stated separately.
What the drawings help you do
Identify the CT arrangement that supplies the residual current. Check whether the element is directional or sensitive. An earth-fault current relay measures current to earth; an earth-fault voltage relay measures neutral displacement voltage.
This particular reference
Measuring relay form that operates on current flowing to earth, fed from a residual connection or core-balance CT. An earth-fault current relay measures current to earth; an earth-fault voltage relay measures neutral displacement voltage.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Phase
- One of the related AC waveforms in a power system.
- Terminal
- An identified point where a conductor connects.
Read the source: AutoCAD Electrical: Current monitoring relays · Residual current and neutral-earthing protection context
Schematic / SLD · K
What it represents
The earth-fault current relay form identifies a relay whose operating quantity is current flowing to earth. It may be fed from a core-balance CT around all live conductors, from the residual connection of three phase CTs, or from a CT in the star-point earth lead, and the source changes its achievable sensitivity.
The relay box shows the quantity with an earth qualifier but not the setting, characteristic or whether it is directional. In resistance-earthed and isolated networks earth-fault current is small and often needs a sensitive or directional element that must be stated separately.
How to read it
- Identify the CT arrangement that supplies the residual current.
- Check whether the element is directional or sensitive.
Information to show or reference
- Show residual CT source
- State setting and characteristic
- Reference system earthing method
A distinction that matters
An earth-fault current relay measures current to earth; an earth-fault voltage relay measures neutral displacement voltage.
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 Ground Failure Current Protection block; the illustration is independently drawn.
Residual current and neutral-earthing protection context
Public context for residual current and neutral-earthing protection.
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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