PDS Design T1689 Technical Manual Rev 07
Special Requirements for “IT” Networks MV3000 Air Cooled DELTA
Page 106
4.5 SPECIAL REQUIREMENTS FOR “IT” NETWORKS
An IT network is a network in which the neutral of the supply is not earthed.
This leads to two requirements:
• Protection against transients coupled from the primary of the supply transformer;
• Earth fault monitoring.
4.5.1 Protection Against Transients
In IT networks, the power circuit is effectively floating. A capacitive potential divider can exist between the
supply transformer primary, the motor and the system earth, with the power electronics of the DC link, being
the centre node. If switching transients are capacitively coupled from the primary of the transformer, the DC
link could move to a very high voltage with respect to earth. This could exceed the voltage rating of the
insulation between the power circuit and the control electronics on the DELTA modules.
To prevent this, the supply transformer must contain an earthed screen.
4.5.2 Earth Fault Monitoring
Local safety regulations may dictate the form of earth fault monitoring. Where these regulations conflict with
the information here, the safety regulations take precedence.
NOTE: Only a single Earth Fault Monitoring system should be incorporated into a drive system, using multiple
systems on the same drive may cause nuisance alarms and trips.
The enclosure designer should clarify whether earth fault monitoring is to be performed within the drive
enclosure or whether it is being performed elsewhere in the supply system.
Proprietary insulation monitoring devices operate by using an internal voltage source to inject a signal with
respect to earth and thereby to measure the insulation resistance of the system. Such insulation monitoring
devices should comply with an appropriate safety standard, such as IEC 61557-8, Insulation monitoring devices
for IT systems up to 1000V AC and 1500V DC. If such a device is used, this should be fitted between the supply
transformer and the drive.
Alternatively, one point in the supply system may be connected to earth via a high resistance. A current sensor
measuring any current to earth is then used to drive an earth fault alarm. If such a system is used, the
following points should be observed:
• If the neutral of the supply system is available, the resistor should be connected between the
supply neutral and earth. The voltage and power rating of the resistor must be able to withstand
the full line-to-line voltage in case of an earth fault on the output to the motor.
• If the neutral of the supply system is not available, two resistors may be connected in series
across the DC link. The midpoint is then connected to earth. This method is only possible when
there is only one drive connected to the supply transformer. Each resistor must have a voltage
and power rating sufficient to withstand the full DC link voltage in case of an earth fault on the
DC link.
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