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Why do I have different Line-To-Ground Voltages with my Isolated Power System?


Product Line:
Hospital Isolated Power Systems

NFPA 99, NEC 517.160

Except for rare occasions, the measured line-to-ground voltage quantities will not be equal for several reasons: 

1. The proximity of each (L1 and L2) conductor to ground will be different when running the wires in conduit; therefore, the line-to-ground capacitance will also be different. Running twisted pairs of conductors (which are twisted) at roughly 2-foot intervals is a good approach for achieving symmetrical line-to-ground capacitances. 

2. Not all circuits have loads that can be turned ON and OFF with DPST (double pole single throw) switches; therefore, loads with SPST (single pole single throw) switches that are in the OFF state add to the line-to-ground capacitance between only one of the isolated power conductor and ground. 

3. Loads are often designed to be powered from a grounded, instead of an isolated, supply. A line-to-ground filter is often added between the HOT wire and ground. This approach for attenuating electrical noise further adds to the asymmetry between line-to-ground capacitances.
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