EHT Capacitance Test Procedure: Troubleshooting Heat Trace Cables

This comprehensive procedure outlines the capacitance testing process used by EHT Group to troubleshoot and verify electrical heat trace cables. It covers testing for both Mineral Insulated (MI) and Self-Regulating (SR) cables.

1. Introduction

The EHT Capacitance Test Procedure is a crucial step in maintaining the integrity and functionality of electrical heat trace (EHT) systems. Whether you're dealing with Mineral Insulated (MI) cables or Self-Regulating (SR) cables, this procedure provides a clear path to identifying and rectifying issues within your heat trace system.

2. Purpose and Scope

This procedure serves the purpose of troubleshooting and verifying electrical heat trace cables, specifically:

• Using capacitance values to locate breaks in MI cables (ratio method).
• Verifying the length of SR cables.

3. Definitions and Abbreviations

Before we dive into the procedure, let's clarify some key terms:

• EHT: Electrical Heat Trace
• QMS: Quality Management System
• IR: Insulation Resistance (also referred to as megger)
• MI: Mineral Insulated
• SR: Self-Regulating
• EHT Controller: Equipment that controls one or more EHT circuits
• EHT Circuit: A control point of an EHT controller, often controlling multiple cables or zones.
• EHT Zone: An EHT cable or set of cables associated with an EHT Isometric, which can be part of an EHT circuit.

4. Procedure

Safety is paramount when dealing with EHT systems. Before beginning the capacitance test, ensure that all safety protocols, including Electrical Safety and Lockout Tagout (LOTO) procedures, are strictly adhered to. Field electricians should follow these steps:

4.1 Verification and Preparation

• Verify the EHT installation in the field, ensuring it aligns with ISO standards, including power boxes and splice boxes.
• Confirm zero energy in the power box.
• Isolate unterminated heat trace circuits in the power box.
• Perform initial tests for resistance and megger as a reference point.

4.2 Capacitance Testing SR Cables - Confirming Length

Before testing SR cables, take note of these considerations:

• Always test from the SR power box.
• Ensure no power cable connections between power or splice boxes.
  • Test separately if there is a power cables between any boxes.
  • Verify IR results of cables are good (minimum value shall be greater than 1MΩ @500V), the capacitance value becomes meaningless for a very low megger SR trace.
• Switch the meter to Capacitance Testing Setting, zero testing by connecting two leads to ensure the meter is working properly.
• One lead connects with cable conductors and another lead connects with shield of the cable.
• Apply test button and record the value.
• Calculate the trace length
  • Trace length (ft) = [Recorded capacitance (nF)] x [capacitance factor (ft/nF)]
  • Capacitance factor bases on SR trace capacitance factor table from different manufacturer.
• As an alternative, capacitance values from either end can be used.
• The ratio of one capacitance value taken from one end (A) divided by the sum of both A and B (A+B) and then multiplied by 100 yields the distance from the first end, expressed as a percentage of the SR trace length.
• Reconnect traces in the box if required.
• Remove LOTO and re-energy the circuit if required.

Raychem (nVent) Cables

Thermon Cables

4.3 Testing MI Cables

Testing capacitance for MI cables is primarily for troubleshooting, and the capacitance value is not meaningful. Length ratio is the critical value needed for troubleshooting. Follow these steps:

• Before Testing Note:
  • Different reference trace can not be used to compare.
  • Will only identify the location of open circuit condition when the insulation resistance is > 1MΩ.
  • Note cable type (A,B,D,E) as it will be important for calculation.
• Switch the meter to Capacitance Testing Setting, zero testing by connecting two leads to ensure the meter is working properly.
• Connect one test led to the conductor A and one to the sheath.
• Apply test button and record the value.
• Connect one test lead to B conductor and the other to the sheath.
• Apply test button and record value.
• Ratio of two conductors of the same trace will identify failure point sometimes.
• Calculate fault location:
  • Divide each capacitance reading by the sum of the two capacitance readings
    • Reading x = value from conductor A to sheath
    • Reading y = value from conductor B to sheath
    • L conductor length
  • Multiply each result by actual cable length (ensure to note cable type). (Multiply L by 2 for D type cables)