Insulation monitoring is a technique used to continuously monitor the insulation resistance between the live conductors of an electrical system and ground, especially in IT systems, where the first fault does not interrupt the power supply. This monitoring allows for anticipating electrical faults and taking action before they compromise the safety or availability of the system.
An insulation monitoring device operates by connecting between the system’s live conductors (phase or phases) and ground potential. Its operating principle is based on injecting a low-magnitude test voltage (Um) that is superimposed on the electrical system.
When an insulation fault occurs—that is, a significant drop in resistance between a live conductor and ground—a circuit is closed through that fault resistance (RF). This allows a measuring current (Im) to flow, the intensity of which is directly related to the level of insulation deterioration.
This current causes a voltage drop across an internal measuring resistor (Rm), which is analyzed by the device’s electronic system. If this voltage drop exceeds a preset threshold—equivalent to a minimum acceptable insulation resistance—the device activates a warning signal.
The international standard IEC 61557-8 defines the technical requirements that these devices must meet. In particular, it states that the measuring principle used must be capable of detecting both symmetrical faults (affecting all live conductors equally) and asymmetrical faults (affecting one or more conductors in particular).
Thanks to this capability, insulation monitoring devices become key prevention tools: they do not stop the system from working, but they do provide early warnings about insulation degradation, allowing maintenance to be carried out in a scheduled and safe manner, without unexpected service interruptions.
Technical Notes
- Symmetrical deterioration of insulation is considered to exist when the insulation resistance of all conductors in the system under monitoring decreases by approximately the same proportion. In contrast, asymmetrical deterioration is said to exist when the insulation resistance of one or more conductors drops significantly compared to that of the other conductors in the system
- Devices called ground fault monitors, which detect voltage imbalances caused by a ground fault as the sole method of measurement, are not classified as insulation monitoring devices according to the criteria established in this regulation.
- In certain circumstances of the electrical system, it is possible to employ a combination of different measurement methods, including imbalance monitoring, to improve insulation monitoring.
- Symmetrical insulation faults are particularly common in direct current (DC) systems and control circuits. When the insulation resistance of the affected conductors is similar, devices based on superimposed voltage measurement may not be able to detect these faults. Therefore, IEC 61557-8 requires the use of continuous monitoring devices that ensure the reliable detection of this type of deterioration.
DC voltage superposition
One of the most common methods for insulation monitoring is the superposition of a direct current (DC) voltage between the phase and the protective conductor (PE). This procedure is particularly effective for conventional alternating current (AC) systems and three-phase systems with neutral (3(N)CA), such as those supplying motors.
However, when applied to AC or 3(N)AC systems that include components with galvanic DC connections, the DC currents present can affect the accuracy of the measurement. This causes insulation faults in the DC section to be detected more sensitively, which can influence the interpretation of the results.
The system leakage capacitances (Ce), common in these systems, are simply charged with the measurement voltage and do not affect the measurement after a brief initial period.
AMP measurement method
The AMP method, developed and patented by Bender, uses a microprocessor-controlled, timed measurement signal that automatically adapts to the specific conditions of the electrical system under analysis. Thanks to a sophisticated software-based evaluation algorithm, it is possible to distinguish actual leakage currents from interference that could distort the measurement, thus ensuring an accurate determination of insulation resistance expressed in ohms.
This method minimizes the impact of broadband interference, such as that generated during the operation of converters, ensuring reliable results even in complex electrical environments.
AMP Plus measurement method
The evolution of the AMP method, called AMP Plus, offers even more advanced interference suppression. Devices using this technology can be used universally in AC, DC, and mixed (AC/DC) systems, including those with variable voltages or frequencies, high leakage capacitances, and DC components.
This versatility makes the AMP Plus method ideal for modern electrical distribution systems, especially those subjected to demanding electromagnetic conditions and intensive use of converters and high-frequency technology.
| System Type | Generation | Remarks | Measurement Principle |
| Pure AC System | Transformer Generator | Single-phase Three-phase | Direct Current (DC) |
| DC System | Battery Rectifier Solar Cell Fuel Cell | Without AC components With AC components derived from half-wave or full-wave rectifiers | AMP Method |
| AC System with DC Components | Transformer Generator with rectifiers sharing a common electrical connection | Single-phase Three-phase | AMP Method |
| AC system with electric actuators | Thyristor Triac GTO | High harmonic content Presence of DC components | AMP Method |
| Variable frequency AC system | Frequency converter | Wide frequency range Presence of DC components | AMP Method |
Summary of measurement methods
There is a close relationship between the type of electrical system, its configuration, its components, and the measurement method used by the insulation monitoring device. Therefore, selecting the appropriate device based on the system parameters is crucial to ensuring effective monitoring.
Maintenance Optimization
- Early detection and signaling of insulation deterioration
- Automatic location of faulty sections.
- Better planning of time and human resources.
- Centralization of information on the state of the electrical system.
- Possibility of remote diagnosis via Internet or Ethernet.
Greater operational reliability
- Continuity of power supply in the event of phase-to-ground faults
- Absence of false disconnections due to insulation failures.
- High operational availability of the facilities.
- Monitoring capabilities even in offline mode.
Improvement in economic efficiency
- Prevention of unexpected and costly disruptions.
- Reduction of maintenance times and costs.
- Identification of critical points in the installation.
- Support in the strategic management of investments.
Increased accident prevention
- Reduced contact currents in small and medium-sized installations.
- Protection of control circuits in equipment and machinery to prevent failures.
Advanced fire prevention
- Early detection of progressive insulation failures.
- Significant reduction in the risk of failures due to electrical arcs, the main cause of fires.
- Isolation and monitoring of areas at risk of explosion or fire using isolation transformers