Ungrounded system: Application examples

Monitoring of a main circuit

Figure: Insulation monitoring in a main circuit with a pure AC load
Insulation monitoring in a main circuit with a pure AC load
Figure: Main circuit with variable-speed drive
Main circuit with variable-speed drive

Monitoring of a control circuit

Figure: Monitoring of an AC control circuit
Monitoring of an AC control circuit
Figure: Monitoring of a DC control circuit
Monitoring of a DC control circuit
Figure: Monitoring of an AC/DC control circuit
Monitoring of an AC/DC control circuit

Monitoring a disconnected load

Many loads are only switched on in an “emergency” (e.g. fire pumps, slides, all types of heating, etc.). Whilst they are offline, however, moisture or other factors can cause insulation faults in the associated supply cable or on the load itself. When these loads are switched on, the protective device will respond or fires will start, thereby preventing operation.

The insulation monitoring device, also known as the offline monitor, monitors the insulation resistance during offline periods and signals insulation faults immediately. When the load is switched on, insulation measurement based on the K contactor is deactivated. The fault memory is also able to detect transient insulation faults. If it can be assured that all poles on the load will be taken offline, this monitoring can also be used in TN and/or TT systems.

Response value of the insulation monitoring device: The insulation monitoring device must signal insulation faults ≤ 1 MΩ, this is because motors are frequently rendered non-operational at an insulation resistance of ≤ 500 kΩ.

Figure: Monitoring an offline motor
Monitoring an offline motor

Monitoring of mobile generators

Electrical safety is often difficult to assure on mobile power generators.

  • In very few cases does the ground/environment permit an earthing system to be installed (stones, rubble, rock, asphalt, etc. would have to be penetrated).
  • Time is of the essence, there is not enough time to measure and check the protective measure.
  • The relevant skilled person is lacking.

However, the ability to respond immediately and appropriately is vital in an emergency situation. It is for this reason that mobile power generators feature the protective measure (Protection by electrical separation with insulation monitoring and disconnection) in accordance with IEC 60364-5-51.

Figure: Mobile power generator with the protective measure “Protection by electrical separation with insulation
Mobile power generator with the protective measure “Protection by electrical separation with insulation