A brief look at the operating principle
A standby motor or emergency load can be monitored for insulation condition before it is needed. Offline monitoring gives early warning of deterioration in disconnected loads and cables.
Why can an unused load fail?
Fire pumps, emergency valve motors and standby generators may remain idle for long periods. During this time, moisture, water ingress, mechanical influences or ageing can deteriorate insulation in motor windings and supply cables. If the fault is discovered only when the load starts, a protective device may trip and the required function may be unavailable. Offline monitoring is a specific approach used to make the problem visible before a start command is issued.
Measurement takes place while the load is disconnected
An ISOMETER designed for this purpose monitors the de-energised load's insulation resistance to earth. The motor's normal supply may be TN, TT or IT; the key requirement is the correct isolation and connection arrangement for offline measurement. The manufacturer's principle diagram relates the contactor state to the measuring connection. This must not be confused with connecting an ordinary IMD to an energised TN or TT system.
Early warning and permission to start are separate decisions
On suitable models with two alarm levels, the first threshold can give the maintenance team time to investigate. At a lower insulation level, the second output can be used in the designed control chain to prevent the faulty load from starting. The measured value can be read from a display or model-dependent communication interface. Tracking changes over time is particularly helpful when investigating moisture-related problems. Supply, measuring range and connection accessories must be checked separately for solutions such as IR420-D6 and isoNAV685-D-B. The interlock is a task within a suitable control arrangement, rather than a standalone guarantee of safety for the entire installation.
Bender offline models for de-energised loads
For offline monitoring, select models the manufacturer specifically defines for this task. Distinguish the standby motor or load's normal supply type from its de-energised state during measurement.
ISOMETER® IR420-D6
Monitors the insulation resistance of de-energised loads normally supplied from TN, TT or IT systems. Two independent alarm thresholds provide separate information for early warning and the control chain.
The load must be de-energised during measurement; assess the isolation/connection arrangement and any required coupling device using product documents.Explore the model and documents
ISOMETER® isoNAV685-D-B
Designed for de-energised TN, TT or IT systems. AMP Plus measurement, isoGraph and history records support monitoring via Modbus TCP.
isoNAV685-D-B is not an example of connecting a general-purpose IMD to an energised TN/TT system; its offline connection conditions apply.Explore the model and documentsOffline measuring connections and interlock conditions differ from energised IT monitoring. 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 — Continuous monitoring of de-energised loads and conductors ↗
- Bender — Offline Monitoring (January 2026) ↗
Sources checked: 2026-10-19. Diagrams belong to Bender; explanations were summarised by ADEO Teknik and are presented here in English.