Current / Potential Transformer (CT/PT)
Transformer
Transformers built for measurement, not power. A CT (current transformer) steps a large current down to a measurable one; a PT/VT steps a high voltage down. They let meters and relays watch big circuits safely.
Schematic symbol
Transformer
How current / potential transformer (ct/pt) appears in a circuit diagram.
Types & variants
Key specs
Ratio
e.g. 100:5 A CT, or 11 kV:110 V PT.
Accuracy class
0.5/1.0 for metering; 5P10 style for protection relays.
Burden
VAHow much load (meter + leads) the CT can drive accurately.
Rated primary
A / VThe full-scale primary quantity.
Markings
Nameplate carries ratio, class, burden and terminal marks: P1/P2 (primary), S1/S2 (secondary) — polarity dots matter for power metering.
Standard values
CT secondaries are standardised at 5 A or 1 A; PT secondaries at 110 V. Hobby energy monitors use 100 A:50 mA split-core CTs.
How to choose
Pick ratio so your normal load sits mid-range, accuracy class for the job (billing vs indication), and burden ≥ actual meter + lead resistance.
Pinout & package
CT: a window the conductor passes through + 2 secondary terminals. PT: 4 terminals (HV in, LV out).
Example circuits
- Split-core CT + burden resistor into an energy-monitor ADC
- Panel ammeter fed from a 200:5 CT
- Protection relay watching a motor feeder
Common failures
The classic and dangerous one: an open-circuited CT secondary while primary current flows generates lethal voltage spikes — always short an unused CT. Insulation breakdown in aged PTs.
How to test
With the primary DEAD: winding resistance and insulation test. Live checks (ratio) are for qualified personnel with the secondary properly burdened — never open a live CT secondary.
Substitutes
For hobby monitoring, a hall-effect sensor module (ACS712 style) or Rogowski coil covers many CT uses non-invasively.