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The passage around this formula

The instrumentation baseline for a new site starts with IEEE 519, the North American reference for harmonic control, which frames the problem around the point of common coupling — the interface between the utility and the customer — and assigns the utility responsibility for background voltage distortion while assigning the customer responsibility for the current distortion its own equipment injects [ 7 ] . The standard’s working measure of current distortion, total demand distortion, is the root-sum-square of harmonic currents up to the fiftieth order expressed as a fraction of the maximum demand current $I_L$ rather than of the instantaneous fundamental: $\mathrm{TDD} = \frac{100}{I_L}\sqrt{\sum_{h=2}^{50} I_h^2}\ \%$. a normalization…

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