Bibliotek

Tracing a ground fault on an isolated DC system can be simple or quite difficult depending on the nature of the fault and the characteristics of the DC system. This application note explores why a DC system’s characteristics may impede efforts to locate a ground fault and offers a simple solution.

When tracing ground faults on ungrounded DC systems, one should not assume that there will be no DC current flowing through the circuit. If a current clamp (CT) is placed on a line with DC current, the DC current will cause the CT, namely its iron core, to magnetize. The higher the DC current, the stronger the magnetization will be. This can make removing the CT quite difficult. The CT can be damaged in the process of removal. In some cases, the CT cannot be removed from the line.

Ungrounded DC battery strings can have voltages ranging from 130 V to 250 V and higher.

When searching for ground faults, a transmitter injects a current through the fault (fault current) and a receiver is then used to trace the path of the fault current. The injected current can be either an AC signal or it can be a pulsed DC output. A pulsed output often needs to be synchronised with the receiver. This way the receiver knows when to look for the pulse. This is required due to potential noise on the system. Random noise can look like a pulse. However, on very noisy systems this can still be problematic. In addition, if the sync is lost, then the receiver needs to be reconnected to the transmitter in order to acquire the synchronisation again. Capacitance can also be a large problem for pulsed outputs. There will always be some amount of capacitance on a DC system. Shielded cables have a capacitive property. They consist of 2 conductors separated by an insulator. This is, in essence, a capacitor.

Depending on the ODEN configuration, different max current/voltages output levels are possible. In total there are 46 ODEN combinations. This application note focuses on two different application examples of an ODEN setup. Test of Current Transformers and Low Voltage Breakers.

Estimating a load is important when setting up a system to test for example a low voltage breaker or anything else that may require a high current and high comply voltage. This application note guides you through how to determine the total load and from there determine an appropriate system configuration for the ODEN or INGVAR system.

Cable connections for minimum impedance

Ratio and polarity test is made to confirm the marking on the CT (primary and secondary) and to verify it is matching with the drawing.

Ratio and polarity test is made to confirm the marking on the CT(primary and secondary) and to verify it is matching with the drawing.

To use ODEN and INGVAR in the best possible way is to determine a suitable current cable for the actual application