ISOM insulation monitoring
ISOM permanent insulation monitors continuously monitor the insulation resistance of isolated neutral (IT)…
A permanent insulation monitoring device (IMD) is a measuring and monitoring device that continuously checks the insulation resistance of an electrical network with an isolated neutral, known as an IT system. ISOM devices immediately signal the appearance of a first insulation fault via a visual alarm and an output relay, with no shutdown triggered. This function is essential for installations where continuity of service is critical, in line with the requirements of the IEC 61557-8 standard and NF C 15-100.
Operation and alarm thresholds of ISOM monitors
Continuous measurement and early detection of insulation faults
ISOM IMDs inject a measurement voltage into the monitored network and assess insulation resistance between phases and earth in real time. Our installation experience confirms that this permanent measurement identifies insulation degradation well before it reaches the critical threshold. Alarm-threshold settings adapt to the specifics of each installation, whether an operating theatre or a server rack.
Output relay and alarm reporting to supervision systems
Each ISOM device incorporates a dedicated output relay for reporting alarms to a building management system (BMS) or a PLC. Our commissioning tests show that this feature makes integration into existing supervision architectures easier. The DIN-rail polyamide housing provides fast, reliable fixing in electrical panels and distribution enclosures.
Typical applications and installation contexts
Continuity of service in healthcare facilities
Hospitals and clinics make extensive use of the IT system for hazardous locations, notably operating theatres and intensive-care units. In the field, we find that a correctly configured ISOM IMD allows technical staff to intervene within the time allowed between the first fault and the repair obligation, without ever interrupting the supply to sensitive equipment.
Safety of industrial processes and critical infrastructure
Process industries, data centres and telecommunications infrastructure also favour the IT network combined with permanent insulation monitoring. Our installation feedback shows that these environments tolerate unplanned shutdowns poorly, hence the value of detecting the first fault with no shutdown. A second fault would cause a direct short circuit and a forced interruption, which the IMD is specifically designed to prevent.
Compliance standards and regulatory requirements
IEC 61557-8 certification and application of NF C 15-100
The IEC 61557-8 standard defines performance requirements for permanent insulation monitoring devices on AC networks up to 1000 V or DC networks up to 1500 V. NF C 15-100, part 5-53, requires the mandatory use of an IMD to monitor IT networks in installations where a shutdown cannot be tolerated. Feedback from design offices confirms that traceability of these certifications is systematically required during installation handover.
Frequently asked questions
What is the difference between an IT network and a TN or TT network?
In an IT network, the neutral is isolated from earth or earthed through a high impedance, allowing continuity of service in the event of a first fault. TN and TT networks, by contrast, earth the neutral directly and necessarily cause a shutdown as soon as the first insulation fault occurs.
Does the ISOM monitor cut off the supply in the event of a fault?
No, the ISOM IMD does not command any shutdown. It only triggers a local alarm and activates its output relay, leaving the operator to intervene at their convenience before a second fault appears, which would be dangerous.
How is the alarm threshold set on an ISOM?
The alarm threshold is generally set between 10 kΩ and 100 kΩ depending on the network voltage and the specification's requirements. Our standard practice favours a threshold stricter than the regulatory minimum to anticipate gradual insulation degradation.
Can an ISOM IMD monitor a DC network?
Yes, provided the rated voltage does not exceed 1500 V DC, in line with the scope of the IEC 61557-8 standard. This capability is particularly useful for monitoring stationary batteries and photovoltaic installations.