Transformers

A combined instrument transformer puts CT cores and PT windings in one casting. Arrangement, ratings, burden planning, anti-tamper sealing and the RFQ items that decide it.

Metering CT vs protection CT comes down to saturation behaviour, not accuracy class. Decimals versus P classes, FS and ALF, class PX, burden math and the spec mistakes that fail on site.

A sealed oil immersed transformer absorbs expansion in its corrugated walls; a conservator breathes through silica gel and carries a Buchholz relay. Rating, moisture and maintenance decide.

CT and PT matching for KYN28 cubicles and ring main units: fault duty before ratio, two cores in one body, PT connection choice, and the lead burden that eats a 10 VA core.

PT fuse selection is two requirements pulling against each other: survive the inrush, interrupt a winding fault. Rating, E characteristic, I2t coordination, and the ferroresonance no fuse can fix.

Contact wear in a vacuum contactor can only be measured, not seen. Overtravel limits, loop resistance, vacuum integrity and the duty mix that turns operation counts into a maintenance interval.

A mechanical latch keeps a vacuum contactor closed with no coil current, an electromagnetic hold needs continuous power, and that decides what happens when the control supply dips.

An auto recloser interrupts fault current and recloses itself, a sectionalizer cannot interrupt it at all, and a breaker waits for a command. The rules that make them work as a scheme.

An embedded pole VCB is only as good as its casting: the APG cycle, the silicone buffer that absorbs thermal expansion, and the partial discharge record that reveals a void nobody can see.