The distance associated with each shock protection approach boundary to exposed energized electrical conductors or circuit parts is determined by the nominal phase-to-phase system voltage range for ac systems and the nominal potential difference for dc systems.

• A. Alternating current / Direct current
• B. Direct current / Alternating current
• C. Low voltage / High voltage
• D. Single-phase / Three-phase

Answer: (A)

The main idea is that protection boundaries are set by the voltage that could appear between exposed conductors. In AC systems, there are multiple phases, so the highest exposure is between two different phases, i.e., the phase-to-phase (line) voltage. The boundaries are therefore determined by that AC line voltage range. In DC systems, there aren’t rotating phases; what matters is the voltage difference between conductors, the nominal potential difference, so the boundaries are based on that DC voltage. That’s why alternating current is the correct match for the AC part and direct current for the DC part. The other options don’t fit because they mix up the type of current with a concept (like phase count or voltage level) that isn’t what governs the boundary distances.