I DIFF
Differential-current measuring relay
A differential-current relay responds to the relationship between specified current inputs.
SLD / Schematic
Symbols · K
Percentage-biased differential current relay
Differential relay form whose operating threshold rises in proportion to the through current, set as a relative value.
In plain English
What does Percentage-biased differential current relay mean?
This relay form represents differential protection where the operate quantity is the difference between currents entering and leaving a zone, and the threshold is a percentage of the through current. The bias keeps the relay stable when CT errors grow during heavy external faults or transformer tap changes.
A simple example
On a drawing the relay box is fed from CT sets at each boundary of the protected zone. The symbol does not show the slope settings, the harmonic restraint used for transformer inrush, or the vector-group and ratio compensation, all of which must be taken from the relay settings record.
What the drawings help you do
Locate every CT set that bounds the protected zone. Check CT polarity so through current cancels in the relay. A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.
This particular reference
Differential relay form whose operating threshold rises in proportion to the through current, set as a relative value. A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.
- Winding
- A coil of conductor inside an electrical machine or transformer.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Rating
- A value stating the conditions for which equipment is specified.
Read the source: AutoCAD Electrical: Current monitoring relays · IEC graphical symbols for diagrams
Schematic / SLD · K
What it represents
This relay form represents differential protection where the operate quantity is the difference between currents entering and leaving a zone, and the threshold is a percentage of the through current. The bias keeps the relay stable when CT errors grow during heavy external faults or transformer tap changes.
On a drawing the relay box is fed from CT sets at each boundary of the protected zone. The symbol does not show the slope settings, the harmonic restraint used for transformer inrush, or the vector-group and ratio compensation, all of which must be taken from the relay settings record.
How to read it
- Locate every CT set that bounds the protected zone.
- Check CT polarity so through current cancels in the relay.
Information to show or reference
- Show all zone CTs
- Mark CT polarity
- Reference slope and restraint settings
A distinction that matters
A biased differential relay raises its threshold with through current; a plain differential current relay uses a fixed threshold.
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 Differential Current Protection - Relative Value 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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