Phase Academy Interactive Power Systems Lab

CT / PT and Relay Measurement Simulator

Normal Educational instrument-transformer lab
📡 Primary → Secondary Measurement Chain
How the primary quantity is scaled down to what the relay actually measures.
Primary
400
A phase
→
CT Ratio
120:1
turns
→
Ideal Secondary
3.333
A sec
→
After Saturation
3.333
A sec
→
Relay Sees
3.333
A measured
⚡ Protection Status
Element 51 — Time Overcurrent
Primary Phase Current
400A
Load only
Primary Ground Current
0A
Residual 3I₀
Primary Voltage
69.0kV
System L-L
Ideal Sec. Phase
3.333A
Primary ÷ turns ratio
Actual Sec. Phase
3.333A
After saturation model
Sec. Ground Current
0.000A
Residual through CTs
PT Secondary Voltage
120.0V
L-L; L-N 69.3 V
Relay Measured Current
3.333A
Element 51
Time to Trip
—s
Below pickup
Required Sec. Voltage
6.67V
Iₛₑₓ × Zₑ₂ₕₖ₃ₖ
Excitation Voltage
7.67V
I × (R₋ₜ + Z₋)
Burden VA
22.2VA
I² × Z
Saturation Onset
100A
Secondary amps at knee
Saturation
None
Core in linear region
87 Operate / Restraint
—
Select element 87

💡 Insight

Normal operating conditions. All measurements within expected range.

CT Secondary Waveform
Ideal (dashed) vs. delivered secondary current.
Illustrative. Clipping uses the same saturation factor as the calculation engine; real saturation is driven by the excitation curve and DC offset.
CT / PT Phasor View
Secondary current relative to the PT voltage reference.
Voltage is drawn at a fixed 0° reference; current angle reflects CT polarity only (load angle is not modeled).
📋 Event Log
Trip decisions, saturation transitions, and configuration changes
🧮 Current Scaling — Step by Step
Worked with your current settings.
⚡ Voltage Scaling — Step by Step
PT ratio applied to the selected primary voltage.
📏 Accuracy Class & Burden Check
IEEE C57.13 relay accuracy classes. The classic field check: the CT will not saturate while Isec × Zburden stays below the C-rating.
ClassTerminal V at 20×Standard burdenMax sec. current before kneePrimary equivalent
Normalized Current Comparison
Each bar is a percentage of its own rated value, so primary and secondary quantities can be compared on one axis.
The active CT path (phase or residual) follows the selected relay element.
Saturation Distortion
Secondary waveform as the core enters saturation.
Illustrative clipping only. A saturated CT loses fundamental content, so the relay under-measures and may delay or fail to operate.
Pickup Threshold Gauge
Measured quantity against the operating threshold.
—
🔧 Element Detail
Element 51 — Time Overcurrent
Measured—
Threshold—
Multiple of threshold—
Operating time—
Verdict—
Time-Current Characteristic — 51 / 51G
IEEE C37.112 inverse curve at the selected time dial. The marker shows the present operating point.
Conceptual curve only — manufacturer curves include tolerance bands, relay reset characteristics, and breaker clearing time.
87 Operating vs. Restraint Characteristic
Single-slope percentage differential. Points above the characteristic operate; points below restrain.
Real transformer differential relays add second-harmonic (inrush) and fifth-harmonic (overexcitation) restraint plus a second slope for CT saturation on through faults.
⇄ Polarity Comparison
Primary (solid) and secondary (dashed) instantaneous current for correct and reversed CT connections.
Normal Polarity
H1→H2 primary flow produces X1→X2 secondary flow. Through-current cancels in a differential zone and directional elements see the correct quadrant.
Reversed Polarity
A 180° error makes through-current add instead of cancel — false differential operate, and directional elements look the wrong way. NETA ATS 7.10 requires a polarity check per IEEE C57.13.1.
Measurement & Decision Chain
From primary quantity to relay decision.
Scenario Library
Eight guided conditions. Loading a scenario configures every control and jumps to the Dashboard so you can trace the result through the Scaling and Relay tabs.
Knowledge Check
Twelve questions on instrument transformers, burden and saturation, relay elements, and the code rules that govern CT secondary circuits.
Quiz Score
0 / 12
🧪 Engine Self-Tests
These verify the calculation engine independently of the UI. They run automatically on load (results in the browser console) and are callable from the console as runSelfTests(). Running them here does not disturb your settings or quiz progress.

⚠ Never open an energized CT secondary

With primary current flowing, an open secondary forces all primary ampere-turns into magnetizing the core. The resulting voltage spikes are lethal and can destroy insulation. Short the secondary with a shorting screw or test switch before breaking the circuit, and treat the work as energized electrical work under NFPA 70E.

What a current transformer does

A CT reproduces a large primary current as a small, standardized secondary current — 5 A or 1 A at rated primary. The nameplate ratio is the turns ratio: a 600:5 CT has 120 secondary turns per primary turn, so it divides by 120.

600:5 CT with 400 A primary → 400 ÷ 120 = 3.333 A secondary. Relay settings are made in these secondary amps, which is why a pickup of “5 A” on this CT means 600 A of primary current.

Metering vs. protection CTs

  • Metering (0.15 / 0.3 / 0.6 / 1.2 accuracy class) — specified for ratio and phase-angle accuracy at and around rated current into a standard burden (B-0.1 through B-1.8). Many are built to lose linearity not far above rated current so instruments are protected during faults; the exact knee is design-specific. This simulator models a metering CT as departing from linearity near 4× rated secondary current.
  • Protection (C class) — specified to stay within 10% ratio error up to 20× rated secondary current (100 A on a 5 A CT) while delivering the class voltage to its standard burden.

Burden, class voltage, and saturation

Burden is the total impedance the CT drives: relay input, leads (both directions), meters, test switches. To push its secondary current through that burden the CT must develop a terminal voltage V = Iₛₑₓ × Z₋₟ₕₜₑₖ. The core also has to supply its own winding drop, so the internal excitation voltage is Vₑ = Iₛₑₓ × (R₋ₜ + Z₋₟ₕₜₑₖ).

A C200 CT delivers 200 V at 100 A secondary, so its standard burden is 200 ÷ 100 = 2.0 Ω. The everyday application check is simply: keep Iₛₑₓ × Z₋₟ₕₜₑₖ below the C-rating at maximum fault current and the CT will reproduce the fault faithfully.
  • Doubling the burden doubles the required voltage and halves the fault current at which saturation begins.
  • Long secondary leads are burden. Sizing conductors up is the usual field fix.
  • Burden VA = I² × Z, which is why burden matters far more during a fault than at load.
  • Secondary winding resistance is estimated here at 0.0025 Ω per secondary turn — the common rule of thumb for bushing CTs from IEEE C37.110.
  • Real saturation is governed by the excitation curve and by DC offset in the fault current; it is progressive, not the hard threshold used in this model.

A saturated CT clips its output, so the fundamental component the relay measures is lower than the true scaled current. Overcurrent elements slow down or fail to pick up, and differential elements can produce false operate current when one CT saturates on a through fault.

Potential transformers

A PT scales system voltage to a standard secondary — typically 120 V line-to-line, or 69.3 V line-to-neutral (120 ÷ √3) for wye-connected sets. A 69 kV : 120 V set has a ratio of 575:1.

This lab treats the PT ratio as a line-to-line ratio and scales linearly. Real installations use line-to-ground rated units (69 kV / √3 : 120 / √3) for wye-broken-delta schemes, and PT fusing and ferroresonance are not modeled.

Relay elements (IEEE C37.2 device numbers)

  • 50 — Instantaneous overcurrent. Operates with no intentional time delay as soon as the measured phase current reaches pickup; real relays add roughly one cycle of operate time.
  • 51 — Time overcurrent. Pickup must be held for a time set by an inverse curve. IEEE C37.112 defines t = TD × (A / (Mᵦ − 1) + B) where M = I / Iₚₛₘₖₙₛ. Time dial shifts the whole curve; curve shape (moderately, very, extremely inverse) sets how aggressively time collapses as current rises.
  • 50G / 51G — Ground overcurrent. Fed from residual current (3I₀) or a separate ground CT. Because balanced load cancels in the residual, ground pickups can be set far below load current — that sensitivity is the whole point.
  • 87 — Differential. Compares current entering and leaving a zone. Operate = |I₋ + I₊| with correct polarity conventions, restraint = (|I₋| + |I₊|) / 2, and the element operates when operate ≥ pickup + slope × restraint. Through-current cancels; internal faults do not.
Polarity errors are most damaging to 87 and directional elements. With one CT reversed, through-current adds instead of cancelling: 4 A in and 4 A out becomes 8 A of operate current, which is why the reversed-polarity scenario trips instantly.

Open secondary vs. ungrounded secondary

  • Open secondary — the current path is broken while primary current flows. The core drives toward saturation each half cycle and the secondary develops very high peaky voltages. This is a personnel hazard and an insulation hazard. The voltage cannot be predicted from a simple formula, so this simulator reports the condition instead of inventing a number.
  • Ungrounded secondary — the loop is closed but has no reference to ground. Capacitive coupling from the primary can float the whole secondary circuit to a dangerous potential. The fix is one, and only one, ground point per secondary circuit; two grounds create a parallel path that corrupts the measurement.

Codes and standards

ReferenceWhat it governs
NEC 250.170Instrument transformer circuits: CT and PT secondary circuits shall be grounded where the primary windings connect to circuits of 300 V or more to ground, and on switchboards regardless of voltage (with the code’s stated exceptions).
NEC 250.172Instrument transformer cases or frames shall be connected to the equipment grounding conductor where accessible to other than qualified persons.
NEC 250.174 / 250.176Grounding of instrument, meter, and relay cases — below 1000 V and 1000 V or over respectively.
NEC 250.178Instrument equipment grounding conductor: not smaller than 12 AWG copper or 10 AWG aluminum.
NEC 230.95Ground-fault protection of equipment on solidly grounded wye services over 150 V to ground and not exceeding 1000 V phase-to-phase, 1000 A and larger: maximum 1200 A setting and maximum 1 s time delay at 3000 A.
IEEE C57.13Requirements for instrument transformers — accuracy classes, standard burdens, and the C-class definition used throughout this lab.
IEEE C57.13.1 / C57.13.3Field testing of relaying CTs (ratio, polarity, excitation) and grounding of instrument transformer secondary circuits.
IEEE C37.110Guide for the application of current transformers used for protective relaying — burden, saturation, and DC offset.
IEEE C37.112Inverse-time characteristic equations for overcurrent relays — the curve constants used here.
IEEE C37.2Standard device function numbers (50, 51, 50G, 51G, 87).
ANSI/NETA ATS & MTS §7.10Acceptance and maintenance testing of instrument transformers: insulation resistance, polarity, ratio, and burden verification against nameplate.
NFPA 70EWork practices for energized work — shorting CT secondaries, test switch use, and PPE for relay and metering work.

Code text is paraphrased for teaching. Always work from the adopted edition of the code and the manufacturer’s instructions for the actual installation.

Field practice checklist

  1. Verify the CT nameplate ratio and tap actually in use — multi-ratio CTs are routinely landed on the wrong tap.
  2. Measure the real burden, including lead resistance both ways, and compare it to the class standard burden.
  3. Check polarity at every CT with a polarity test per IEEE C57.13.1, not by wire colors.
  4. Confirm exactly one ground per secondary circuit and that the ground is at the panel, not at the CT.
  5. Run an excitation (saturation) test on relaying CTs and compare the knee to the maximum expected fault duty.
  6. Short the secondary before disturbing any CT wiring, and leave shorting devices in place until the circuit is restored.