CTs and PTs Explained: Measuring High Voltage and Current Safely

You can't clamp a multimeter onto an 11 kV busbar, and you can't push a 60 A motor feeder through a 10 A meter input. Every panel that measures voltage or current at real-world levels solves this the same way: a current transformer (CT) steps the current down, and a potential (voltage) transformer (PT/VT) steps the voltage down, into safe, meter-friendly quantities. Your panel meter, energy meter and protection relay all read the scaled-down replicas — never the live circuit directly.

This guide explains how CTs and PTs work, how to read their nameplates, and how to wire them into a metering panel without the classic mistakes.

The one-line job of CTs and PTs

• A CT goes in series with the conductor whose current you want to measure. A 100/5 A CT turns 100 A in the feeder into 5 A at its secondary — safe for meters and relay inputs.
• A PT goes in parallel across the lines. An 11 kV/110 V PT turns 11,000 V on the busbar into 110 V at its secondary — safe for voltmeters and meter voltage inputs.

Both are transformers in the ordinary sense (magnetic coupling, ratio set by turns), designed for measurement accuracy rather than power transfer.

Safety note: The theory here does not qualify you to touch an HV panel. Any hands-on work — opening panels, landing CT secondaries, changing PT fuses — happens de-energised and under qualified supervision.

How a CT works

A CT's primary is the feeder itself (sometimes literally one conductor passed through a window) and the secondary is a wound coil with many turns. Current ratio follows turns ratio: a 100/5 A CT steps current down 20:1. The secondary is designed to drive a low-impedance load — a current input of a meter or relay — and under normal conditions the secondary voltage stays small (a few volts at most).

Because the ratio is a turns ratio, CT accuracy is excellent while the core operates in its linear region — which it does as long as the secondary stays connected through its burden.

The one rule: never open-circuit a live CT secondary

This is the most safety-critical fact about CTs. With the primary carrying current and the secondary open, the core has no opposing secondary ampere-turns to cancel the primary flux. The core saturates, and the secondary voltage spikes — easily into the kilovolt range on a CT whose secondary normally sits at a few volts. The result is an insulation-breakdown and arc-flash hazard at the open terminals.

Practical consequences:

• Short the secondary before removing a meter. Meter replacement on a live panel uses terminal blocks with built-in shorting links — never lift the secondary wire first.
• Unused CT cores get shorted. A spare core on a multi-core CT ships with a shorting link across its secondary; leave it on.
• PT secondaries are the opposite: a PT is a voltage source and must never be short-circuited — which is why PT secondaries are fused.

If you remember one thing from this guide, remember this: an open CT secondary is dangerous; a shorted PT secondary is dangerous.

How a PT works

A PT (also called VT, voltage transformer) is a small step-down transformer wired across the lines — line-to-line or line-to-neutral depending on the connection. Standard secondaries are 110 V or 100 V, so an 11 kV/110 V PT gives your meter 110 V when the busbar is at 11 kV. PTs feed the voltage inputs of energy meters, voltmeters and under/over-voltage relays.

Two PT rules:

1. Fuse the secondary. A PT behaves like a voltage source; a shorted secondary draws fault current. A fuse or MCB on each secondary line is standard.
2. Mind the burden. Like CTs, PTs are rated for a VA burden (often 15–100 VA). Hanging too many parallel meter inputs on it degrades accuracy.

Reading the nameplate: ratio, class, burden

A typical CT nameplate reads something like: 100/5 A, Class 0.5, 15 VA, 50 Hz. Each number matters:

• Ratio (100/5 A): primary rated current to secondary rated current. A 5 A secondary is the global standard; 1 A exists too, and is better for long secondary runs because wiring losses (I²R) and voltage drop are 25× smaller at 1 A than at 5 A.
• Accuracy class: for metering, 0.2, 0.5, 1.0 — the number is the percentage ratio error at rated current (Class 0.5 ≈ ±0.5%). Protection cores use classes like 5P10: ±5% composite error at 10× rated current (the "accuracy limit factor" 10). A metering CT saturates gracefully near rated current; a protection CT stays linear far into fault current. Don't swap their jobs.
• Burden (15 VA): the rated secondary load. The connected meters plus wiring should total near this VA — too little or too much both move the operating point off the accuracy curve. For a 5 A secondary, wiring burden = I²R of the loop; a few tens of metres of thin wire can eat several VA.
• Rated voltage / insulation level: the system voltage the CT's primary insulation is built for (e.g. 12 kV equipment on an 11 kV system). Never under-rate this.

PT nameplates read similarly: 11000/110 V, Class 0.5, 50 VA — the same three ideas, voltage-flavoured.

Polarity marks: the little dots that matter

CTs and PTs carry polarity marks (dots, or P1/P2 and S1/S2 terminals). The dot means: current entering the dotted primary terminal corresponds to current leaving the dotted secondary terminal. For a single ammeter it doesn't matter; it matters the moment direction or phase relationships matter:

• Energy meters (import vs export),
• differential and directional protection,
• power and power-factor measurements, where a reversed CT flips the sign of measured power.

Wire P1 toward the source, S1 to the meter's corresponding input, and keep the convention identical across all three phases.

Grounding the secondary

Standard practice: ground one point of each CT and PT secondary (usually the S2/common terminal). This keeps the secondary near earth potential — a fault between primary and secondary then blows a fuse or trips protection instead of floating the whole metering circuit at line voltage. One ground point only; two create circulating paths through the ground system.

CT construction types you'll meet

• Bar type: the primary is a solid bar through the CT body — common on panel busbars.
• Wound type: the primary is a wound coil — used at low ratios where a single pass can't give enough ampere-turns.
• Window/ring (toroidal): the feeder cable passes through the ring — the standard retrofit on existing cables.
• Core-balance (zero-sequence): all three phase conductors pass through one large ring; it measures the residual (unbalanced) current — the basis of earth-fault protection.

Multi-core CTs bundle a metering core and one or two protection cores in one housing, each with its own secondary terminals — wire each core to its own circuit.

Worked example: metering a 3-phase feeder

Feeder: 415 V, expected load up to 80 A. Energy meter with 5 A current inputs and 415 V direct voltage inputs (many meters accept direct voltage up to ~500 V, so no PT is needed at LT level).

1. CT ratio: pick the next standard size above max load — 100/5 A, Class 0.5, 15 VA, bar type on the busbar.
2. Burden check: meter burden ~1 VA/phase + wiring (say a 20 m loop of 2.5 mm² ≈ 0.14 Ω → I²R = 25 × 0.14 ≈ 3.5 VA) — total ≈ 4.5 VA per phase, comfortably under 15 VA.
3. Polarity: P1 toward the incoming supply on all three phases; S1 to the meter's current inputs, S2 commoned and grounded at one point.
4. Leave the shorting links on the CT terminal block closed until the meter is wired; open them only when the meter circuit is complete.
5. If this were an 11 kV incomer instead, add 11000/110 V PTs (Class 0.5) for the meter's voltage inputs, fused on the secondary, secondaries grounded at one point.

At 11 kV and above, CTs and PTs are part of the switchgear itself (often built into the breaker or cable box) — your panel design just specifies the cores and ratios.

Common mistakes

1. Opening a CT secondary on a live circuit — the dangerous one. Always short first.
2. Shorting a PT secondary — the mirror-image mistake; PT secondaries are fused, not shorted.
3. Using a metering CT for protection (or vice versa) — a metering core saturates during faults exactly when the protection relay needs a faithful replica.
4. Ignoring secondary wiring burden — long thin runs on a 5 A secondary eat the VA budget and wreck Class 0.5 accuracy.
5. Mixed-up polarity on one phase — single-phase checks pass; the energy meter's power readings go wrong.
6. Two ground points on one secondary — circulating currents through earth add measurement error.
7. No fuse on the PT secondary — a fault in the voltage wiring becomes a fire instead of a blown fuse.

Quick checklist

• CT ratio ≥ max expected primary current (next standard size up)
• Metering cores Class 0.5/1.0, protection cores 5Pxx as required
• Total burden (meters + wiring I²R) within the VA rating
• 1 A secondaries considered for long secondary runs
• Polarity consistent P1→source, S1→meter, all phases
• Secondary grounded at exactly one point
• Shorting links on CT terminal blocks before any meter work
• PT secondaries fused; insulation level ≥ system voltage

FAQ

Why are CT secondaries 5 A and not something smaller? 5 A is the historical standard that meters and relays are built for; 1 A secondaries exist and are better for long runs (secondary wiring losses scale with I²). Either way, the ratio on the nameplate tells the meter how to scale the reading back up.

Can I use one CT for both metering and protection? Only with a multi-core CT — one core per function. A single core can't be both accurate at 1× current (metering) and linear at 20× current (protection).

What does 5P10 mean on a protection CT? ±5% composite error at up to 10 times rated primary current (accuracy limit factor 10). It guarantees the relay sees a faithful-enough waveform during faults.

Do I need a PT for a 415 V panel? Usually not — most panel meters accept 415 V directly on their voltage inputs. PTs enter the picture at medium voltage (3.3 kV, 11 kV, 33 kV) where the meter can't touch the busbar voltage.

Why is the secondary grounded if it's only magnetically coupled? Precisely because it's only magnetically coupled — without a ground reference it floats, and a primary-to-secondary insulation fault could raise the whole metering circuit toward line voltage.

Can a CT measure DC? No — an ordinary CT works on changing flux. DC measurement needs a Hall-effect sensor or a zero-flux (DCCT) device.

Limitations

• This guide covers measurement fundamentals, not switchgear selection: insulation coordination, creepage, partial discharge and HV clearances are a separate subject.
• Accuracy classes and burden figures here are the steady-state picture; transient performance (CT saturation during asymmetrical faults) needs protection-study tools.
• Always work from the CT/PT manufacturer's datasheet — standard sizes and classes vary by region and standard (IEC 61869 vs IEEE C57.13).

Where to go from here

• Energy Meter Types Explained — what the CT/PT secondaries connect to.
• MCB vs MCCB vs ELCB Selection — protection on the circuits your CTs monitor.
• Three-Phase Wiring Basics — the system the metering hangs off.
• How to Test Earth Pit Resistance — the grounding the secondary reference depends on.
• More on the Electrical branch hub.