Transformer Maintenance

Three Class II tasks in the transformer block are maintenance, not code: check transformer oil for contaminants, clean a power transformer, and test a transformer for output and performance under load. The public-domain Army/Navy facilities manuals (TM 5-684 for liquid-filled and substation units, TM 5-683 for dry-type units under 600 V) are the source; Article 450 (lesson nec-07-02 of this section) supplies the installation rules.

1. What is being maintained (TM 5-684 7-2, 7-3)

• Distribution transformers: 500 kVA and below (low-tension winding under 600 V); power transformers: above 500 kVA. Constant-voltage units hold roughly a 2 % voltage drop from no load to full load; taps on a de-energized manual tap changer adjust for the primary voltage; load-tap-changing (LTC) units adjust ±10 % under load with a motor-driven switch.
• Insulation systems: liquid-immersed — mineral oil, or less-flammable fluids (silicone, high-fire-point hydrocarbon) where Article 450 requires them indoors (450.23); askarel/PCB units removed; and dry-type — ventilated, nonventilated or sealed, air or gas insulated.
• Temperature limits: air-cooled units are rated for a 40 °C maximum ambient (30 °C average over 24 h); liquid-immersed units are rated 65 °C or 55/65 °C rise, dry-type 150 °C, 115/150 °C or 80/115 °C rise — the lower rise rating gives less capacity from the same core and coil. Operating above rated temperature, voltage or current, or below rated frequency, shortens insulation life; every 8–10 °C over the limit roughly halves it (the standard rule).
• Cooling classes: self-cooled (ONAN/AA), forced-air (ONAF/FA — fans add capacity, typically a 133 % rating), forced-liquid; dielectric strength of air-insulated units falls with altitude.