For a 7% impedance transformer over 1,000 volts in a supervised location and without coordinated thermal overload protection, a primary fuse could be sized up to what percentage of the primary current without secondary protection?

For a 7% impedance transformer over 1,000 volts in a supervised location and without coordinated thermal overload protection, a primary fuse could be sized up to what percentage of the primary current without secondary protection?

• A. 200%
• B. 250%
• C. 225%
• D. 300%

Answer: (B)

The main idea here is protecting a transformer when there’s no secondary protection by sizing the primary fuse to balance normal startup surges with fault protection, using the transformer's impedance to understand fault current. With a 7% impedance, the available fault current on the primary is about 1 / 0.07 ≈ 14 times the rated primary current. You want a primary fuse that will not blow during normal operation or startup inrush (which can be several times rated, especially with a time-delay fuse) but will open quickly during a fault. A primary fuse rated at roughly 250% of the primary current achieves that balance: it passes normal load and startup surges, yet will clear a fault because the fault current greatly exceeds the fuse rating. The other percentages either provide too little headroom for startup surges or risk slower protection during a fault (larger ratings), or are too aggressive and could nuisance-trip in some startup conditions. Therefore, the best choice is the option representing 250% of the primary current.

The main idea here is protecting a transformer when there’s no secondary protection by sizing the primary fuse to balance normal startup surges with fault protection, using the transformer's impedance to understand fault current.

With a 7% impedance, the available fault current on the primary is about 1 / 0.07 ≈ 14 times the rated primary current. You want a primary fuse that will not blow during normal operation or startup inrush (which can be several times rated, especially with a time-delay fuse) but will open quickly during a fault. A primary fuse rated at roughly 250% of the primary current achieves that balance: it passes normal load and startup surges, yet will clear a fault because the fault current greatly exceeds the fuse rating.

The other percentages either provide too little headroom for startup surges or risk slower protection during a fault (larger ratings), or are too aggressive and could nuisance-trip in some startup conditions. Therefore, the best choice is the option representing 250% of the primary current.