PH LOSS
Phase-loss monitoring relay
A phase-loss relay supervises the presence of the specified phases in a three-phase system.
SLD / Schematic
Symbols · PSI
Phase-sequence indicator
Instrument that shows whether three-phase supply rotation is positive or reversed before motors are run or sources paralleled.
In plain English
What does Phase-sequence indicator mean?
A phase-sequence indicator connects to all three phases and shows whether rotation is L1-L2-L3 or reversed. It is used when commissioning motors, generators and supply changes, because wrong rotation reverses motors and prevents correct paralleling.
A simple example
The instrument gives a check at one moment and does not protect anything on its own. If permanent protection against reversed rotation is needed, a phase-sequence monitoring relay with contacts must be shown instead of, or as well as, the indicator.
What the drawings help you do
Confirm all three phases are connected in the stated order. Check whether a protective relay is also required. A phase-sequence indicator only displays rotation; a phase monitoring relay acts on incorrect rotation or loss of phase.
This particular reference
Instrument that shows whether three-phase supply rotation is positive or reversed before motors are run or sources paralleled. A phase-sequence indicator only displays rotation; a phase monitoring relay acts on incorrect rotation or loss of phase.
- Normally open / normally closed
- The contact state in the normal condition defined for the drawing.
- Coil
- A wound conductor that can produce a magnetic field when energised.
- Phase
- One of the related AC waveforms in a power system.
Read the source: IEC graphical symbols for diagrams
Schematic · PSI
What it represents
A phase-sequence indicator connects to all three phases and shows whether rotation is L1-L2-L3 or reversed. It is used when commissioning motors, generators and supply changes, because wrong rotation reverses motors and prevents correct paralleling.
The instrument gives a check at one moment and does not protect anything on its own. If permanent protection against reversed rotation is needed, a phase-sequence monitoring relay with contacts must be shown instead of, or as well as, the indicator.
How to read it
- Confirm all three phases are connected in the stated order.
- Check whether a protective relay is also required.
Information to show or reference
- Show phase connections
- State expected rotation
- Reference commissioning check
A distinction that matters
A phase-sequence indicator only displays rotation; a phase monitoring relay acts on incorrect rotation or loss of phase.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
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.
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