What You Should Know About Insulation Monitoring Devices

July 29, 2024

Maintaining the integrity of electrical insulation in railway signalling power systems is critical for safety and reliability, performance optimisation, and controlling costs. An insulation monitoring device or IMD is important to detect faults that could undermine this integrity, thereby supporting the smooth running of railway services.

This guide explains what insulation monitoring devices are, how they work, their benefits for the rail industry, and how to select the most appropriate device or system.

 

Watch the video to learn more about CableGuardian- an Insulation Monitoring Device

What is an Insulation Monitoring Device, or IMD? 

Insulation monitoring devices, or insulation monitors, are safety components that detect insulation faults within electrical systems. They are required in an IT earthing system where all conductors are isolated from the earth. An insulation monitoring device works by continuously monitoring the insulation resistance of an electrical system and alerting system operatives.

In the railways, an IMD enables signalling system engineers to detect insulation failures early. This prevents potential electrical shocks, fires, or damage to lineside equipment, supports signalling system uptime, and avoids disruption to train services.  

Electrical engineers have been monitoring insulation resistance for many decades, and manual testing has evolved into the use of electronic devices that provide continuous, real-time monitoring and diagnostics. Signalling power system IMDs are these devices, as they continuously monitor insulation resistance between conducting elements and the earth.  

To do this, they inject a constant, low-frequency signal into the system and measure the response to detect any decrease in insulation resistance, which could indicate a potential fault.  

If the resistance falls below a certain threshold, an alarm will sound, and maintenance can be carried out.  

How does an Insulation Monitoring Device work? 

The exact components of an insulation monitoring device system can vary depending on its complexity. However, a railway signalling power IMD will likely consist of sensors, which measure insulation leakage current between the live supply and the earth, a signal processing unit (SPU), which calculates insulation resistance from the sensor leakage current signals, alarm units, which generate an alarm if the resistance falls below a certain threshold, and communication units, which transmit the alarm signal to a central monitoring system. 

Insulation Monitoring Devices: measurement techniques 

 Insulation monitoring devices use three main measurement techniques. These are the:  

  1. Direct current measurement – the most common method measures insulation resistance as outlined above
  2. Alternating current measurement – this measures the impedance of the insulation itself by combining resistance and capacitance. 
  3. Frequency domain measurement – defects in the insulation can be detected by measuring its frequency response.  

The benefits of Insulation Monitoring Devices for signalling power cables 

Insulation monitoring devices are not only beneficial for signalling power cables, but they are also vital for performance and safety. Insulation monitoring is mandated by BS 7671 (IET Wiring Regulations). In addition, Network Rail’s Insulation Monitoring and Fault Location Systems for Use on Signalling power and other electrical systems standard NR/L2/SIGELP/27725 recommends monitoring at three levels: the network level (Tier 3), feeder level (Tier 2), and sub-network cable section level (Tier 1). 

Continuous monitoring, using insulation monitoring devices, removes the need for cables to be disconnected during testing. As well as meeting regulatory compliance, insulation monitoring offers the following benefits to rail operators: 

  • Improves safety – when faults are detected early and quickly, electric shocks, electrical fires or damage to equipment can be prevented, and time spent by maintainers at the trackside is minimised. 
  • Reduces downtime: Performance is optimised, planned maintenance is carried out, and potential issues are closely monitored, preventing unexpected outages.  
  • Saves costs – equipment damage and unplanned downtime are avoided, repair and maintenance costs are limited, and schedule 8 penalties related to signal failures are reduced 

How to choose the right Insulation Monitoring Device 

Insulation monitoring devices (IMDs) for signalling power and other electrical systems must meet several basic requirements, such as handling specific voltage levels, adhering to industry standards, and surviving extreme temperatures, humidity, and exposure to dust and vibration. IMDs must also withstand these conditions with minimal, preferably zero, scheduled maintenance.  

Integration with existing railway signalling and monitoring systems is another key consideration. Insulation monitoring devices for railway power systems display other characteristics, such as: 

  • Sensitivity and accuracy – effective IMDs must detect the most minute insulation degradations to prevent signalling failures 
  • Real-time monitoring and diagnostics – proactive maintenance is achieved through ongoing monitoring resistance, diagnostics and fault location detection, which is vital to limit downtime 
  • Simplicity of operation and clarity of the user interest mean that non-specialists can operate the system 
  • Combined solutions – consider systems that combine insulation resistance monitoring with other methods of fault detection, such as conductor monitoring (SSTDR

CableGuardian for Insulation Resistance Monitoring 

CableGuardian, an advanced insulation and conductor monitoring system developed by Viper Innovations, is the first to proactively detect and locate faults in live, low-voltage railway power systems. Originally developed from extensively tested oil and gas technology, it has been adapted for and further tested on rail. It combines insulation resistance and SSTDR for continuous fault diagnosis without requiring system power-down or extra ‘boots on the ballast’. 

Multiple sensor units monitor electrical parameters across the network and transmit data to a cloud-based analysis system, enabling real-time fault location and reducing lineside maintenance. The system can be adjusted to specific rail maintenance needs and fully complies with Network Rail standards, enhancing network reliability. 

CableGuardian is playing a key role in keeping trains operating across the UK network.  Find out more about CableGuardian 

To learn more about how CableGuardian complies with the latest standards, read our CableGuardian Guide.  

Viper Innovations.

Insulation Resistance Monitoring

& Recovery Specialists.