A brief look at the operating principle
Measuring insulation at a particular moment and monitoring it continuously during operation provide different information. The right approach depends on the system's lifecycle and distribution system configuration.
A single measurement represents one moment
Insulation resistance is an important indicator of cable and equipment condition. A measurement during maintenance or commissioning reflects the connections and environmental conditions at that time. Subsequent humidity changes, new loads or ageing mean the same value cannot be guaranteed to remain. Insulation withstand testing, resistance measurement and continuous monitoring are different processes; each answers a different question in the system lifecycle.
Continuous monitoring tracks change
In an IT system, an IMD monitors the insulation resistance to earth of electrically connected circuits during operation. In TN-S and TT systems, RCM provides condition information through residual current; this is not a direct Riso measurement. Offline insulation monitoring can be used for suitable disconnected loads. Continuous information helps investigate when deterioration began and which operating conditions may be associated with it.
Testing and monitoring complement the maintenance plan
Commissioning, normal operation, repairs and significant changes have different inspection needs. Installing a monitoring device therefore does not automatically eliminate all required tests. Test voltage, duration and connection conditions must be assessed alongside current equipment documents and applicable requirements. The method must be verified separately for circuits containing sensitive electronic components. Continuous monitoring data guides maintenance planning, while the scope of required measurements and tests depends on the system and the inspection purpose.
| Approach | What does it provide? | When is it assessed? |
|---|---|---|
| Insulation test | Assesses insulation behaviour under specified test conditions | According to the equipment and inspection purpose |
| Insulation resistance measurement | Provides the resistance value at the time of measurement | During commissioning, inspection or maintenance |
| Monitoring with an IMD | Tracks changes in an IT system's insulation resistance | During operation in a suitable IT system |
| Monitoring with RCM | Tracks residual current values and changes | During operation in suitable TN-S / TT systems |
| Offline monitoring | Monitors the insulation of a disconnected load | While the load is de-energised, using a specific connection arrangement |
Three different Bender examples for testing, IMD and RCM
These models are not interchangeable: equipment testing follows a defined test sequence, an IMD measures insulation resistance in an IT system, and RCM measures residual current in a suitable earthed system.
UNIMET® 610ST
Offers configurable test sequences for electrical equipment and machines. Measurements such as insulation resistance and protective conductor resistance can be managed with records and subsequent comparisons.
Not presented as a general-purpose device covering every installation test. Verify the test scope against the applicable standard and device software.Explore the model and documents
ISOMETER® iso685…
Monitors the insulation resistance of the electrically interconnected IT system during operation; history records and isoGraph support assessment of changes.
Continuous monitoring does not automatically replace required equipment testing or commissioning measurements.Explore the model and documents
LINETRAXX® RCM420
Monitors AC and pulsating DC residual current using a suitable measuring current transformer, with prewarning and alarm notification.
A Type A monitoring device; it does not measure Riso, cover smooth DC or independently fulfil an RCD protective function.Explore the model and documentsTest and monitoring methods are selected according to the distribution system and maintenance objective. Product images belong to Bender.
Bender sources for these models
Bender technical explanations
This English summary is based on the Bender technical sources listed below.
- Bender — Insulation resistance: testing, measuring, monitoring ↗
- Bender — Insulation resistance: testing, measuring, monitoring (technical article) ↗
Sources checked: 2026-10-19. Diagrams belong to Bender; explanations were summarised by ADEO Teknik and are presented here in English.