For a transformer with 4% impedance that feeds a fused safety switch, the maximum fuse rating in the switch is 150 A. Which set of options correctly reflects this configuration?

For a transformer with 4% impedance that feeds a fused safety switch, the maximum fuse rating in the switch is 150 A. Which set of options correctly reflects this configuration?

• A. 100
• B. 125
• C. 200
• D. 150

Answer: (D)

The key idea is protection coordination from a transformer with a given impedance. A transformer with 4% impedance can deliver a fairly large fault current because fault current is roughly the transformer’s full-load current divided by its impedance in decimal form. That means you must choose a protective device rating that will open on a fault without being so large that it leaves the transformer unprotected or causes nuisance trips during normal operation. In this scenario, the switch has a defined maximum fuse rating it can accommodate, which is 150 A. To respect that limit and still provide proper protection, you select the fuse rating at that maximum allowable value. Using a smaller rating would still protect, but it wouldn’t reflect the switch’s stated maximum; using a larger rating would exceed what the switch can handle. So the configuration that aligns with both the transformer’s capability and the switch’s limit is the 150 A fuse rating. The 4% impedance underscores why protection must be coordinated carefully, since the transformer could present a high fault current, and sizing the fuse to the switch’s allowed maximum ensures appropriate protection without overstepping the equipment’s rating.

The key idea is protection coordination from a transformer with a given impedance. A transformer with 4% impedance can deliver a fairly large fault current because fault current is roughly the transformer’s full-load current divided by its impedance in decimal form. That means you must choose a protective device rating that will open on a fault without being so large that it leaves the transformer unprotected or causes nuisance trips during normal operation.

In this scenario, the switch has a defined maximum fuse rating it can accommodate, which is 150 A. To respect that limit and still provide proper protection, you select the fuse rating at that maximum allowable value. Using a smaller rating would still protect, but it wouldn’t reflect the switch’s stated maximum; using a larger rating would exceed what the switch can handle. So the configuration that aligns with both the transformer’s capability and the switch’s limit is the 150 A fuse rating.

The 4% impedance underscores why protection must be coordinated carefully, since the transformer could present a high fault current, and sizing the fuse to the switch’s allowed maximum ensures appropriate protection without overstepping the equipment’s rating.