Issue:
Different Line-To-Ground Voltages on Isolated Power System
Product Line:
Hospital Isolated Power Systems
Environment:
NFPA 99, NEC 517.160
Resolution:
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.
Different Line-To-Ground Voltages on Isolated Power System
Product Line:
Hospital Isolated Power Systems
Environment:
NFPA 99, NEC 517.160
Resolution:
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.
Released for: Schneider Electric USA
