In short. Relays see the system only through CTs and VTs. If a CT saturates, is wired with the wrong polarity or is open-circuited, protection can fail to operate or trip wrongly. Selecting the correct class, ratio and burden is as important as setting the relay.
CT basics
- Ratio: primary to secondary current, commonly 5 A or 1 A secondary.
- Protection class: specified by accuracy limit factor and burden (for example, 5P or 10P classes), or by knee-point voltage for class X CTs used in differential and REF.
- Burden: the load on the secondary, including relay and wire resistance. Excess burden drives the CT into saturation.
- Metering CTs are accurate near rated current but saturate early, to protect meters; do not use them for protection.
Saturation
During a fault with large current or DC offset, the CT core may saturate and the secondary current becomes distorted. Check that the CT will not saturate before the relay operates at the maximum fault current, with margin for DC offset.
VT basics
- Provide voltage for protection, synchronising and metering.
- Protected by fuses or miniature breakers; loss of VT signal must not cause false trips. Relays should detect VT failure and block voltage-dependent functions.
- Accuracy class and burden need to match the connected devices.
| Item | CT | VT |
|---|---|---|
| Connection | In series with the circuit | In parallel with the circuit |
| Secondary must not be | Open-circuited (dangerous voltage) | Short-circuited (overload and damage) |
| Main risk | Saturation, polarity, open circuit | Fuse loss, wrong phasing |
Selection steps
- Choose the primary ratio from load and fault current.
- Calculate the burden of relays and cabling.
- Check the accuracy limit or knee-point for the maximum fault.
- Specify class by function: overcurrent, differential, REF, metering.
- Define polarity and earthing of the secondary circuits (one point only).
Commissioning checks
- Ratio and polarity tests.
- Magnetisation curve for class X CTs.
- Insulation and burden measurements.
- Verification of secondary earthing and no open circuits.
- Primary injection to prove end-to-end connections. See injection testing.
Illustrative example
A generator panel uses 1000/1 A CTs with a relay burden and cable giving a total burden of a few ohms. Calculation shows the CT would saturate at the maximum fault current, so a CT with a higher knee-point or a shorter cable run, or heavier conductors, is chosen. Without this check, the relay might trip too slowly or not at all.
Why CTs decide whether protection works
A relay sees only what the current and voltage transformers deliver. Wrong ratio, polarity, class or saturation turns a good relay into a wrong one.
CT selection parameters
| Parameter | Meaning |
|---|---|
| Ratio | Primary to secondary current; primary rating above maximum load |
| Accuracy class | Metering classes for billing, protection classes (such as 5P or 10P) for relays |
| Accuracy limit factor | Multiple of rated current up to which accuracy holds |
| Burden (VA) | Load of relay plus wiring on the secondary |
| Knee-point voltage | For class X CTs used in differential schemes |
Saturation check (illustrative)
If a CT is rated 1000/1 A, 10P20, at 15 VA, and the actual burden is 20 VA, accuracy at 20 times rating can be lost. Reduce burden by shortening leads, using larger conductors or choosing a 1 A secondary, or select a larger CT.
Safety rules
- Never open-circuit a CT secondary while the primary carries current. High voltage can appear. Use shorting links.
- Earth each CT secondary circuit at one point only.
VT issues
- Fuse or MCB failure on a VT secondary can mimic undervoltage. Add VT supervision.
- Check ratio, burden and phase sequence.
- For generators, VT location relative to the breaker affects protection and sync functions.
Testing
Ratio, polarity, magnetisation curve and burden measurements are routine commissioning tests. See injection testing.
Common mistakes
- Metering CT used for protection.
- Secondary earthed at two points.
- Reversed polarity in differential schemes.
What to send us
Send the CT and relay data. Send your application details. See also differential protection.
Questions engineers ask
Why must a CT secondary never be open?
An open secondary can develop dangerously high voltage and damage the CT.
What is CT knee-point?
The voltage at which the core starts to saturate, important for differential and REF schemes.
Can one CT serve protection and metering?
Not reliably. Use separate cores or CTs.
What if a VT fuse blows?
Voltage-based protection may misbehave, so relays include VT supervision.
This guide gives general principles. CT specification needs relay and fault data.