CT & PT (Current / Potential Transformer)

• Faraday's law of electromagnetic inductionEMF = −N × dΦ/dt

  A CT or PT is a transformer used as a measuring instrument, so the same EMF = −N·dΦ/dt sets its ratio.

• Ampère's circuital law∮ B · dl = μ₀ × I(enc)

  A CT's core sees only the field circling the primary conductor, which is how it reads thousands of amps without touching the circuit.

• Lenz's lawthe minus sign in EMF = −N × dΦ/dt

  The secondary current opposes the core flux — and if you open-circuit a live CT nothing opposes it, so the flux and secondary voltage rise to lethal levels.

Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.

Nameplate carries ratio, class, burden and terminal marks: P1/P2 (primary), S1/S2 (secondary) — polarity dots matter for power metering.

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.

Pick ratio so your normal load sits mid-range, accuracy class for the job (billing vs indication), and burden ≥ actual meter + lead resistance.

• Split-core CT + burden resistor into an energy-monitor ADC
• Panel ammeter fed from a 200:5 CT
• Protection relay watching a motor feeder

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.

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.

For hobby monitoring, a hall-effect sensor module (ACS712 style) or Rogowski coil covers many CT uses non-invasively.