Read measurement and protection paths
Learn how CTs and PTs/VTs supply measurements to a relay, and how a trip command differs from the primary power path.
Educational reference only. Examples and calculator results are not approved installation designs, equipment selections or operating instructions.
Learning goal: Separate the power circuit from measurement signals and the relay’s breaker trip command; explain CT, PT/VT, relay and breaker roles.
Explore the diagram
A protection overview combines two different stories: where electrical power travels and how a protection system observes it. Dashed lines here are functional signals, not secondary terminal wiring. Some relay functions use current only; others also use voltage. This example shows both without prescribing a protection scheme.
Click or tap equipment to learn its purpose. Keyboard: Tab then Enter or Space. On small screens, scroll the drawing sideways or use the equipment selector.
- Solid: primary power connection
- Dashed: measurement signal → relay
- Dash-dot: trip command → breaker
Read it in order
- Trace the solid primary circuit first. The feeder current passes through Q1 and the CT primary, then reaches L1.
- Read the measurement paths separately: CT1 observes current; VT1 observes voltage. Their dashed links indicate information delivered to R1, not complete secondary circuits.
- Follow the trip-command arrow R1 → Q1. Detection and interruption are different jobs. The diagram shows no actual fault, settings or breaker state.
All equipment, explained
Read every explanation here without using the interactive controls.
B1 — Supply busbar
The common supply connection for the illustrated feeder and voltage-measurement branch.
How to read it here
The solid path runs B1 → Q1 → CT1 → L1. VT1 observes a voltage on a branch; it does not carry the feeder’s load current.
What you cannot infer
The primary reference/return connection of VT1 is omitted. This is not a complete instrument-transformer circuit.
Supporting reference: Schneider Electric — MV substation functions and CT/VT metering.
Q1 — Circuit breaker
Interrupts the primary circuit when its mechanism operates, within its rated breaking capability.
How to read it here
The orange dash-dot path from R1 ends at Q1: it represents a trip command, not a power conductor.
What you cannot infer
A relay command alone does not prove the breaker operated. Auxiliary supplies, trip coils and feedback are not drawn.
Supporting reference: Schneider Electric — Protection of transformer and circuits.
CT1 — Current transformer (CT)
Provides a scaled current measurement for metering or protection. Its primary senses the current in the feeder; a ring-type CT surrounds a primary conductor.
How to read it here
The line through the ring is the primary conductor. The blue dashed link to R1 represents its measurement, not the feeder power flow.
What you cannot infer
A conventional CT secondary can develop dangerous voltage if open-circuited while primary current flows. This lesson gives no connection or servicing instructions.
Supporting reference: Schneider Electric — Open-circuit CT hazard (updated 7 October 2024).
L1 — Protected feeder load
Represents the downstream load observed by the illustrated protection system.
How to read it here
The load is on the primary circuit, not supplied by R1’s signal lines.
What you cannot infer
The protected zone and fault coverage cannot be determined from this overview alone.
Supporting reference: Schneider Electric — Protection of transformer and circuits.
VT1 — Potential / voltage transformer (PT/VT)
Provides a scaled voltage measurement for instruments or protection. PT (potential transformer) and VT (voltage transformer) name this role.
How to read it here
Its primary observes voltage across defined points, unlike a CT’s series current measurement. The blue dashed link is its measurement signal to R1.
What you cannot infer
The reference point, fuses, secondary earthing and terminals are omitted. Never use this functional drawing as wiring guidance.
Supporting reference: ABB — Indoor current and voltage instrument transformers.
R1 — Protection relay
Evaluates measurements against configured protection logic and can issue a trip command when its criteria are met.
How to read it here
Blue dashed inputs come from CT1 and VT1; the orange dash-dot output goes to Q1. R1 does not interrupt the main power circuit itself.
What you cannot infer
No pickup values, timing, coordination, CT accuracy or saturation assessment is provided. A relay is not a substitute for a rated interrupting device.
Supporting reference: Schneider Electric — Protection of transformer and circuits.
Check your understanding
Think through each question, then reveal its explanation. This is practice, not a competence assessment.
Which device detects conditions, and which interrupts primary current?
R1 evaluates protection criteria; Q1 is the interrupting device. The relay’s trip output is a control command, not the primary power path.
Are the dashed lines enough to connect a CT or PT/VT?
No. They are functional signal paths. Polarity, burden, reference points, secondary connections, protection and safety requirements are not specified.