TIDUEZ8 May   2021


  1.   Description
  2.   Resources
  3.   Features
  4.   Applications
  5.   5
  6. 1System Description
    1. 1.1 Insulation Monitoring
    2. 1.2 Isolation Capacitance
    3. 1.3 IEC 61557-8 Standard for Industrial Low-Voltage Distribution Systems
    4. 1.4 Key System Specifications
  7. 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. Differential to Single-Ended Conversion
        2. High-Voltage Measurement
        3. Signal Chain Error Analysis
      3. 2.3.3 PCB Layout Recommendations
  8. 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 Test Setup
    4. 3.4 Test Results
  9. 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
  10. 5About the Authors

Isolation Capacitance

In an unearthed power distribution system, the isolation barrier protects the user and components sitting on the low-voltage side by preventing high currents flowing to protective earth. The isolation barrier is expected to be of a resistive nature. Nevertheless, some factors such as improper earth connection or humidity may increase the isolation capacitance to earth of the system.

Figure 1-9 Isolation Barrier Capacitance Effect on Insulation Monitoring Device

In this system, under proper operation or asymmetrical fault of the isolation barrier, this static capacitance to earth forces a delay in the settling time of the isolation voltage when the resistive branch is switched in. A certain measurement time is expected.

By allowing higher currents across the isolation barrier through our switched resistive branch, faster settling times are expected and smaller errors on the isolation barrier resistance calculations are achieved. Assuming an asymmetrical fault on the negative resistor, where RisoN is small and RisoP big, the settling time of the isolation voltage is given by Equation 11:

Equation 11. τ = ( R i s o P / / R s t P ) × C i s o P   ( s e c o n d s )

Specify the maximum isolation capacitance to protective earth, normally in the order of 2 μF to 5 μF.

As per standards, currents of up to 12.5 mA are allowed for less than one second through the isolation barrier into the switched resistive branch. Hence, consider the tradeoff between faster settling times and power dissipation when designing a resistive divider branch. Further details on the implementation in this reference design are found in Section 2.3.