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Wednesday, July 28, 2010

Testing for grounded DC supply in an Ungrounded DC System

In an ungrounded DC System, the positive and negative terminals are not directly connected to ground while in the Grounded DC System, one of either terminal is connected directly to ground. Usually, this DC System is created when the configuration of the AC voltage source connected to the rectifier is in delta-connection and the junction between the rectifiers is not connected to ground.
The main advantage of Ungrounded DC System is when one of the terminals is accidentally connected to ground, the DC Supply is still in normal operation. But one disadvantage of Ungrounded DC System is when line-to-ground fault occurs on one terminal, and another single line-to-ground fault occurs on the other while the previous fault is not yet corrected, then the DC supply is already faulted line-to-line.
In Grounded DC System, the positive and negative to ground voltage measurements must be at least equal to half the value of the total DC supply. In order that this method can be applied to the Ungrounded DC System, a ground reference must be created. This can be done by two high resistance precision resistors connected in series across the DC system and the junction is grounded. By doing this ground reference, the Ungrounded DC System is converted to Grounded DC System and the method of testing will now be applicable.

To convert our Ungrounded DC System to Grounded DC System, we connected two resistors that were previously used as dropping resistor of annunciator lights. The resistors are rated 125VDC and have 1.25 Kilo-ohm value. Then we connected the junction to the ground and the other ends to the positive and negative supply, respectively. Then we measured the positive to negative terminals and it yielded us 121.5 VDC. We measured the positive to ground and the measured value was 58.4VDC. Lastly, we measured the negative to ground and 63.4VDC was obtained. The result is not that satisfactory since we only used ordinary resistors. Higher value of resistors must be used in order to obtained a better result.

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