# Partial discharge sensor

Canonical URL: https://eloragrid.ai/symbols/partial-discharge-sensor
Publisher: Elora Grid
Updated: 2026-10-28
Lookup label: PD
Drawing use: SLD, Schematic
Reference families: Manufacturer

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.

### Terms

- 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.

Concept diagram: Identify the device and form → Read the connections and state → Check the equipment reference
A guide to reading this reference. The original symbol illustration and its drawing notes appear below; these boxes are not a wiring diagram.

- [Secondary-distribution switchgear and ring main units](https://new.abb.com/medium-voltage/switchgear/secondary-distribution)
- [Electrical and physical sensing functions](https://www.ti.com/technologies/sensing.html)


## 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

## Important distinction

A partial discharge sensor assesses insulation condition; an insulation monitoring relay measures insulation resistance of an unearthed system.

## Facts

- Sensor types: TEV, UHF and high-frequency CT
- Purpose: Insulation condition monitoring
- Preview: Functional illustration of the device role; not a product footprint or certified standard graphic

## Public references

- [Secondary-distribution switchgear and ring main units](https://new.abb.com/medium-voltage/switchgear/secondary-distribution) — ABB. Public switchgear context in which insulation condition monitoring is applied.
- [Electrical and physical sensing functions](https://www.ti.com/technologies/sensing.html) — Texas Instruments. Public overview of electrical sensing functions.

## Graphic scope

Independent original educational concept illustration by Elora Grid. Not a certified standards graphic or manufacturer footprint. Unitless; not to scale. Original artwork licence: https://creativecommons.org/licenses/by/4.0/. Publisher material is linked only.