TIDUEZ8C december   2022  – june 2023

 

  1.   1
  2.   Description
  3.   Resources
  4.   Features
  5.   Applications
  6.   6
  7. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Impact of Parasitic Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  8. 2System Overview
    1. 2.1 Block Diagram
    2. 2.2 Highlighted Products
      1. 2.2.1 TPSI2140
      2. 2.2.2 AMC3330
      3. 2.2.3 TPS7A24
      4. 2.2.4 REF2033
      5. 2.2.5 TLV6001
    3. 2.3 Design Considerations
      1. 2.3.1 Resistive Bridge
      2. 2.3.2 Isolated Analog Signal Chain
        1. 2.3.2.1 Differential to Single-Ended Conversion
        2. 2.3.2.2 High-Voltage Measurement
        3. 2.3.2.3 Signal Chain Error Analysis
      3. 2.3.3 Loss of PE Detection
      4. 2.3.4 Insulation Monitoring on AC Lines
      5. 2.3.5 PCB Layout Recommendations
  9. 3Hardware, Software, Testing Requirements, and Test Results
    1. 3.1 Hardware Requirements
      1. 3.1.1 Connectors
      2. 3.1.2 Default Jumper Configuration
      3. 3.1.3 Prerequisites
    2. 3.2 Software Requirements
    3. 3.3 Software
    4. 3.4 Test Setup
    5. 3.5 Test Results
  10. 4Design and Documentation Support
    1. 4.1 Design Files
      1. 4.1.1 Schematics
      2. 4.1.2 BOM
    2. 4.2 Documentation Support
    3. 4.3 Support Resources
    4. 4.4 Trademarks
  11. 5About the Author
  12. 6Revision History

2.3.3 Loss of PE Detection

In many systems, testing the connection of the PE to the insulation monitoring device is mandatory. The reason is that if the PE connection is lost, there is no current flowing through the switched-in measurement branch, resulting in no voltage at the input of the AMC3330. This is interpreted as an infinite insulation resistance, and possible insulation breakdowns can no longer be detected as shown in Figure 2-12.

GUID-20221013-SS0I-P5LC-CH8T-B6CHTKGQ0M1N-low.svg GUID-20221013-SS0I-QVJG-PDLW-LXB6GBRX3GT9-low.svg Figure 2-12 Problem of Lost PE Connection

To avoid a lost connection, implement a method to detect a loss of PE connection.

Such a method can be implemented by adding two known resistors between DC+ and PE and DC– and PE in the low MΩ range as shown in Figure 2-13.

Note: These additional resistors must have a separate connection to PE.
GUID-20221013-SS0I-QVSJ-TFJC-BXZ59WFB5CBF-low.svg GUID-20221013-SS0I-SQKW-B9NN-ZMQ7BVWJZ5HP-low.svg Figure 2-13 Equivalent Circuit With Added Resistors for Detecting Loss of PE

These resistors are in parallel to the parasitic insulation resistances, which limits the upper value of the insulation resistance. With a fixed limited insulation resistance, the input voltage at the AMC3330 can only drop below a certain value if the PE connection is valid.

With the additional resistors across DC+ to PE and DC– to PE, the calculations of the remaining insulation resistance must be adjusted accordingly. The original Equation 13 and Equation 14 for RisoN and RisoP are used now to calculate the parallel resistance of RisoP || RPE,P and RisoN || RPE,N. The values for RisoN and RisoP can be calculated by using Equation 21 and Equation 21.

Equation 21. RisoN =RPE,N × (RisoN||RPE,N)(RPE,N - (RisoN||RPE,N))
Equation 22. R isoP   = R PE , P   ×   ( R isoP | | R PE , P ) ( R PE , P   -   ( R isoP | | R PE , P ) )

If the design cannot feasibly have a high-omic resistor permanently connected between the DC lines and PE, there is the option to add another pair of TPSI2140 isolated switches which can also disconnect the additional resistors for loss of PE detection.