Which device is used to provide a reduction in current or voltage so that power can be measured?

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In power measurement, you need to bring large signals down to a level that measurement equipment can handle. A current transformer does exactly that for current: it produces a secondary current that’s proportional to the primary line current, but at a much smaller, safe value. This allows meters or protective relays to read the current without handling the full load, and it also provides electrical isolation from the high-power circuit.

Once you have the current scaled down, you can determine power by combining it with the voltage information (often scaled down in parallel with a potential transformer for safe voltage measurement). The device’s job is specifically to reduce current, which is why a current transformer is the best answer here. A potential transformer reduces voltage for measurement, but doesn’t address current in the same way; a power transformer is a general device for moving power around, and a voltage regulator isn’t used for measurement scaling.