No Possessions Required: Faster Insulation Resistance Testing In Signalling Power Systems

April 13, 2026

When a Network Rail Central Region signalling team needed to investigate low Insulation Resistance (IR) in its Signalling Power Systems (SPS), CableGuardian provided immediate real-time intelligence. By enabling feeder-level monitoring without disconnecting supply, the team identified faults faster, reduced reliance on possessions, and improved the efficiency of fault-finding operations.

The Challenge: Delays and Disruption in Traditional Insulation Resistance Testing

A signalling team in the Central Region was responsible for investigating signalling power systems (SPS) with low insulation resistance (IR), specifically where network values fell below the Tier 3 threshold of 250kΩ.

Identifying the root cause was often slow and operationally challenging. Traditional insulation resistance testing required:

Planning a possession to isolate the 650V signalling power supply

Disconnecting individual feeder cables

Manually testing insulation resistance across the network

This approach introduced several constraints:

Delays in accessing the network due to possession availability

Risk of cancellation after mobilisation

Operational disruption from switching off power

Extended fault-finding time, particularly where multiple feeders were involved

In many cases, teams often spent significant time isolating feeders, only to find the issue was not linked to a specific cable.

The Viper Solution: CableGuardian for Live Monitoring and Faster Fault Identification

To improve the speed and efficiency of insulation resistance investigations, the team deployed a rapid, non-intrusive monitoring approach using CableGuardian.

The objective was to:

Begin investigations without waiting for possessions

Monitor insulation resistance on live signalling power systems

Quickly determine whether faults were feeder-specific or network-wide

Reduce time spent on unnecessary isolation and manual testing

How the solution was applied in practice

Using live condition monitoring, the team was able to fundamentally change how signalling power system faults were investigated.

Immediate investigation without possessions
Monitoring could begin as soon as an issue was identified, removing dependency on planned access and avoiding delays of days or weeks.

Live testing on energised systems
Insulation resistance could be measured continuously without switching off the 650V supply, eliminating operational disruption and improving safety.

Feeder-level visibility (Tier 2 monitoring)
Each feeder could be assessed individually, allowing the team to rapidly determine whether:

A specific feeder was responsible, or

The issue originated within the Principal Supply Point (PSP)

This avoided unnecessary cable disconnections and reduced time spent on fault isolation.

Targeted fault location along feeders
Once a problematic feeder was identified, additional monitoring units could be deployed along its length, enabling teams to pinpoint the affected section before attending site.

New insight into network behaviour
Continuous monitoring provided visibility into how insulation resistance varied over time and under different environmental conditions—something not previously achievable with manual testing.

Results & Benefits: Reduced Delays and More Efficient Fault-Finding

Significant reduction in fault-finding time, with issues often narrowed down before site visits

Avoidance of planned possessions for initial investigation activities

More efficient use of maintenance resources, focusing effort only where required

Improved fault targeting, reducing repeat visits and unnecessary testing

Enhanced understanding of network performance, including environmental impacts on insulation resistance

CableGuardian let us stay ahead of faults without taking power off or stopping trains. They split the fault into clear sections during the day, so we already know where to go when we head out at night. That alone saves a huge amount of time.

They also give us true insulation values and let us track how the network behaves in different weather conditions, which is something we’ve never been able to do before.

– Programme Manager, Network Rail Central Region

Intelligence Installed

CableGuardian enabled a shift from reactive, possession-led insulation resistance testing to proactive, live monitoring. By providing continuous visibility of network performance, the signalling team was able to take control of fault investigations, reducing delays, improving accuracy, and minimising disruption to railway operations.