What voltage should a meter indicate when measuring the line side and load side of a B fuse inside a 600-volt disconnect when the disconnect is on and the system voltage is 480/277?

What voltage should a meter indicate when measuring the line side and load side of a B fuse inside a 600-volt disconnect when the disconnect is on and the system voltage is 480/277?

• A. 480
• B. 208
• C. 277
• D. 0

Answer: (D)

When the disconnect is closed, the line side and load side of the fuse are connected through a conducting path, so they sit at the same electrical potential. The meter across the two points therefore sees essentially no difference in voltage, reading about 0 volts. The system voltage of 480/277 is the potential relative to other conductors (line-to-line or line-to-neutral), not the drop across a closed fuse. In practice, the fuse and connections have a small resistance, so you might see a tiny, but negligible, voltage drop, effectively still 0 volts on a typical meter.

When the disconnect is closed, the line side and load side of the fuse are connected through a conducting path, so they sit at the same electrical potential. The meter across the two points therefore sees essentially no difference in voltage, reading about 0 volts. The system voltage of 480/277 is the potential relative to other conductors (line-to-line or line-to-neutral), not the drop across a closed fuse. In practice, the fuse and connections have a small resistance, so you might see a tiny, but negligible, voltage drop, effectively still 0 volts on a typical meter.