CT / PT and Relay Measurement Simulator

Trace how current and voltage transformers scale primary quantities for relay inputs — then inject load, fault, and ground fault currents to see how overcurrent, differential, and directional relays respond in real time.

PREVIEWAUTOMATED CHECKS ONLYSTANDARDS-AWAREADVANCEDIEEENETA ATSNETA MTSWHAT DO THESE STATUSES MEAN?

CT · PT SCALING5 RELAY FUNCTIONS8 FAULT SCENARIOSSATURATION MODEL

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Overview

Instrument transformers are the sensory system of every protection scheme, and misunderstanding their behavior — burden, saturation, polarity, open-secondary danger — is a leading source of protection failures in the field. This simulator builds the link between primary-side quantities and relay-terminal values by letting you set CT ratio, class, and burden, PT ratio and primary voltage, then inject load current, fault current, and ground fault current and watch scaled secondary quantities, relay pickup status, and operating point update in real time. The Relay tab supports five ANSI function types: 50 (Instantaneous OC), 50G (Ground OC), 51G (Time Ground OC), and 87 (Differential). The Scenarios tab provides eight guided fault conditions — from normal load to CT saturation, reversed differential polarity, high burden, and the open CT secondary danger condition. The Scaling tab shows the math step-by-step. A Self-Test tab quizzes knowledge. Targeted at protection engineers and NETA technicians preparing for commissioning or maintenance testing.

App preview

A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

Inside the lab

Example data — not a real field report

Dashboard with primary/secondary metrics, relay state, waveform, and phasor view — Dashboard · Primary and secondary quantities, relay pickup state, burden VA, saturation factor, waveform, and phasor view — all live as you move sliders.

Scaling View showing step-by-step CT and PT math — Scaling View · Step-by-step CT and PT math from primary quantities to relay-terminal values, with accuracy-class context.

Relay View with 50/50G/51G/87 function selector and operating chart — Relay View · Pick 50, 50G, 51G, or 87 and watch the operating vs. restraint characteristic and time-to-trip respond to your inputs.

Scenario library with eight guided fault conditions — Scenarios · Eight guided cases — normal load, phase and ground faults, CT saturation, differential polarity, high burden, and the open-CT-secondary danger condition.

Learn tab covering CT/PT theory, burden, saturation, and open-secondary danger — Learn · Concise theory on burden, saturation, polarity, and why an open CT secondary is a life-safety hazard.

Self-Tests knowledge check — Self-Tests · Quick knowledge checks to confirm you can predict what the relay will see before you inject the next scenario.

What you'll do

01Explain how CT ratio and burden affect secondary current accuracy and relay input
02Describe the danger of an open CT secondary and identify the protective action required
03Identify CT saturation conditions and explain how saturation affects protection performance
04Configure 50, 50G, 51G, and 87 relay functions and verify pickup against injected currents
05Trace the differential operating vs. restraint characteristic for through-fault and internal-fault conditions
06Select appropriate CT class (C100, C400, C800) for a given protection application

Try a guided scenario

Load a CT and watch accuracy degrade.

CT and PT measurement · Protection & Relay Coordination

No sign-in required. Progress is saved on this device.

What this lab is not

Not a design document, stamped calculation, or approval record.
Not a substitute for engineering analysis by a licensed professional.
Not an accredited test report — it is a NETA-aligned educational field workflow, and acceptance decisions stay with the responsible test organization.
Not a relay setting file and not a coordination study of record.
Not connected to live equipment and not a safety device.

Calculation basis

References IEEE, NETA ATS, NETA MTS. Results are computed from the entered inputs and cited back to the referencing clause or table number — no code text or table is reproduced verbatim.

Simplifying assumptions are stated in the lab itself; where a model is intentionally conceptual, the lab says so rather than implying study-grade accuracy.

Standards basis: Standards-aware. Validation state: Automated checks only (last checked 2026-08-15).