IMD
Insulation monitoring relay
Relay that continuously checks insulation resistance of an unearthed (IT) system to earth and alarms on a first fault.
Schematic / SLD
Symbols · PD
Partial discharge sensor
Sensor on switchgear, cables or transformers that picks up signals from partial discharge for insulation condition monitoring.
In plain English
What does Partial discharge sensor mean?
Partial discharge sensors detect small electrical discharges inside insulation before it fails. Common types include transient earth voltage sensors on metal-clad switchgear, UHF antennas in gas-insulated equipment and high-frequency CTs on cable earth straps, each feeding a monitoring unit that records and trends the activity.
A simple example
The symbol shows the sensor location but not its type or frequency range, and results from different sensor types are not directly comparable. Monitoring is used for maintenance decisions rather than protection, so the sensor output normally goes to alarms or a condition monitoring system rather than a trip circuit.
What the drawings help you do
Identify the sensor type at each location. Confirm outputs go to monitoring rather than tripping. A partial discharge sensor assesses insulation condition; an insulation monitoring relay measures insulation resistance of an unearthed system.
This particular reference
Sensor on switchgear, cables or transformers that picks up signals from partial discharge for insulation condition monitoring. A partial discharge sensor assesses insulation condition; an insulation monitoring relay measures insulation resistance of an unearthed system.
- Signal
- Electrical information passed between devices, rather than necessarily the main power supply.
- Rating
- A value stating the conditions for which equipment is specified.
- Schematic
- A drawing showing function and connections rather than physical appearance.
Read the source: Secondary-distribution switchgear and ring main units · Electrical and physical sensing functions
SLD / Schematic · PD
What it represents
Partial discharge sensors detect small electrical discharges inside insulation before it fails. Common types include transient earth voltage sensors on metal-clad switchgear, UHF antennas in gas-insulated equipment and high-frequency CTs on cable earth straps, each feeding a monitoring unit that records and trends the activity.
The symbol shows the sensor location but not its type or frequency range, and results from different sensor types are not directly comparable. Monitoring is used for maintenance decisions rather than protection, so the sensor output normally goes to alarms or a condition monitoring system rather than a trip circuit.
How to read it
- Identify the sensor type at each location.
- Confirm outputs go to monitoring rather than tripping.
Information to show or reference
- State sensor type
- Show location on equipment
- Reference monitoring unit
A distinction that matters
A partial discharge sensor assesses insulation condition; an insulation monitoring relay measures insulation resistance of an unearthed system.
Sources you can follow
These public references support the explanation and provide further detail. Their graphics and downloadable files remain with their publishers.
Secondary-distribution switchgear and ring main units
Public switchgear context in which insulation condition monitoring is applied.
Electrical and physical sensing functions
Public overview of electrical sensing functions.
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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