How does an insulation monitoring device work?

01 Jul 202252

According to the standards, the monitoring of ungrounded networks with insulation monitoring devices is mandatory.

An insulation monitoring device recognizes insulation faults and immediately reports that the value has fallen below the minimum. This prevents operational interruption caused by a second, more severe insulation fault.

This is the basic setup of the three types of electric systems. To understand the function of the insulation monitor, we can compare the effect of an earth fault in a device connected to the system. The fault closes an electrical loop via the PE conductor.

In a TN system each short circuit current trips an MCB (miniature circuit breaker). This system is normally used in industrial and residential buildings.


In a TT system, the fault current closes a loop through the earth of both generator and consumer. Here the resistance of the fault path back to the supply can be too high, so the short circuit current will not necessarily trip an MCB. A more reliable protection device is needed.

In such a case a residual current detector (RCD) is installed to detect the current leaking to the ground and to interrupt the circuit.

IT systems are designed for higher reliability. This means a single short circuit to earth will not necessarily trip a protection device since there is no closed short circuit loop. But the protection device is needed to react to a second failure.

To detect the first fault, different technology is needed. The only reliable way to determine an earth fault in an ungrounded system is to use an insulation monitoring device.

How does an insulation monitoring device work?

This is an IT system. It is supplied by an insulation transformer or an independent voltage source, such as a battery bank or generator. In IT-N systems the star point of the secondary side of the transformer is used as the neutral conductor.

Loads can be connected between the phases or between phase and neutral. The loads are connected to the earth while the power source is ungrounded. A three-phase motor can be connected to the phases and earth.

Another connected device is a water heater connected to the same supply system. If there is an insulation fault in the water heater, (in other words, a short circuit between a phase and the housing), no standard protection device will trip in an insulated system.

The failure current to earth in IT systems depends on the resistance and capacitances between one phase of the supply system and earth. After a first insulation fault, the system still works properly, because the relatively low failure current does not affect the system.

But if a second insulation fault occurs, (for example a short circuit between one phase and earth), the protection device will trip.

This can be prevented by an insulation monitoring device that monitors the first insulation fault. The insulation monitoring relays measure the resistance of the whole system between the phase conductor and earth. When the resistance drops below the threshold, usually around 50 kOhm, the device triggers an alarm.

Advantages of insulation monitoring device

The following are the benefits of an insulation monitoring device:

  • Operation continuity – in case of the first fault (the connection between IT power supply system and ground-earth fault) the network is still operational.
  • Higher safety of operation.
  • Immediate overview of network status, continuous monitoring of the insulation level to earth.
  • Early detection of faulty devices by immediate signalization by the insulation monitoring device.
  • Less risk of electric shock for the operator and higher fire safety.
  • Prevention of production losses and shutdowns, operations can continue in case of a first earth fault.

Applications of insulation monitoring device

Ships are ungrounded applications and have IT systems onboard.

The solar sector is growing rapidly. In PV applications, for example, the efficiency of the cells is improving constantly, so PV applications are becoming increasingly attractive. All DC circuits in solar plants are ungrounded.

Another example of an ungrounded application is a wind turbine. Depending on the technology, both  3-phase mains and DC mains must be monitored for insulation faults. The actual trend is to increase the DC voltage level to reduce the inverter losses.

Transportable construction cranes are very often electrically isolated and many insulation monitoring relays are used in crane applications.


Other applications of insulation devices are:

  • Machines and generators
  • Emergency power supplies
  • Railway applications
  • Mining
  • Hospitals
  • Mobile power generators (airplanes)
  • Industrial IT systems
  • Printing applications
  • Data centers