As with all transformers, the ____________ of an instrument transformer must be known and marked on the transformers.

• A. coil designation
• B. polarity
• C. air temperature range
• D. pH level of insulation

Answer: (B)

Polarity marking is essential because instrument transformers must maintain the correct phase relationship between their primary and secondary. In devices like current transformers and voltage transformers, the secondary signal used by meters and protective relays must align in phase with the primary so that a current or voltage increase produces a corresponding, correctly directed response. The polarity marks (such as dots or labeled terminals) show which ends are in the same polarity, ensuring proper connection. If polarity isn’t known or marked, wiring the secondary in the wrong orientation can reverse the signal, causing erroneous meter readings or incorrect relay tripping. The other options don’t serve this purpose: coil designation, ambient temperature range, or insulation pH are unrelated to the required phase relationship and proper functioning of the protection and metering system.