Motor protection relay current measurement is governed by IEC 61869-2 protection-class CT designations, with 5P20 (5% composite error at 20x rated current) and 10P20 (10% composite error at 20x rated current) serving as the baseline for standard overcurrent and motor thermal protection [S3][S4].
The class designation encodes three parameters: the letter P confirms protection application, the leading number sets the composite error ceiling under fault conditions, and the trailing Accuracy Limit Factor (ALF) specifies the multiple of rated current at which that error ceiling still holds [S2]. A 5P20 CT therefore guarantees no more than 5% composite error up to 20 times its rated primary current, the typical working range for an induction motor locked-rotor or bolted-fault event [S4].
Why Protection CTs and Metering CTs Cannot Be Swapped
Metering CTs (Classes 0.1, 0.2, 0.2S, 0.5, 0.5S) are wound on high-permeability cores that saturate quickly under fault current, a deliberate design that protects downstream revenue meters but destroys the linear signal a motor protection relay needs to discriminate between inrush, overload, and short circuit [S1][S5]. Grain-oriented silicon steel cores in metering CTs operate at 1.5 to 1.8 Tesla maximum flux density, while nanocrystalline cores reach roughly 1.2 T with lower losses, and neither can hold ratio accuracy once the primary current exceeds roughly 5 to 20 times rated [S2].
Protection CTs are designed to resist that saturation. A Class 5P20 CT must still hold composite error inside ±5% at 20x rated primary current, exactly the band where a 480 V induction motor draws 6 to 8x Full-Load Current (FLC) at start and where an electrical measurement channel must remain linear for the relay to compute a valid negative-sequence or thermal model value [S2][S4]. Swapping a 0.5S metering CT into a motor protection circuit therefore risks relay misoperation, delayed trip, or complete failure to clear a winding fault.
Decoding 5P20, 10P20, and the ALF Suffix
The "P" confirms IEC 61869-2 protection designation; the leading number (5 or 10) is the composite error ceiling in percent at the Accuracy Limit Factor; the trailing number (10, 15, 20, or 30) is the ALF, the ratio of maximum permissible fault current to rated current [S3][S6]. Common motor protection selections sit at 5P10 for smaller motors (under 75 kW) and 5P20 or 10P20 for larger units where the available fault current can exceed 20x FLC [S3].
Class 10P permits a 10% composite error at the same ALF and is acceptable where the protective device is an inverse-time overcurrent relay with a wide operating band, but it is not suitable for high-impedance differential or distance schemes where transient replication must be precise [S4]. Class PX (IEC) or Class C (IEEE) CTs are specified instead, with the knee-point voltage (Vk) and secondary winding resistance defined explicitly so the relay engineer can calculate the asymmetric CT performance during the first cycle of a fully offset fault [S3][S4].
Burden, Lead Resistance, and Why the Relay Coil Matters
Protection CT accuracy is only valid when the connected burden stays below the rated burden on the nameplate. Rated burden is expressed in VA at the rated secondary current, with standard values from 2.5 VA to 30 VA, and covers both the CT secondary wiring loop and the relay current-input impedance [S2][S4]. A modern microprocessor-based motor protection relay such as the SEL-751 or ABB REF615 draws under 0.1 VA per phase at 5 A, so 12 AWG or 10 AWG THHN copper runs of 50 to 100 m typically dominate the burden budget [S4].
Exceeding the rated burden pushes the core toward saturation earlier than the ALF predicts, and under fault conditions the ratio error climbs beyond the 5% or 10% envelope the relay was coordinated against. Standard CT secondary wiring practice is to use 12 AWG (3.3 mm²) or 10 AWG (5.3 mm²) conductors, keep the loop resistance under roughly 0.1 ohm, and never install fuses on a CT secondary, since a blown fuse creates the open-circuit hazard (induced voltages above 2 kV) that shorting terminal blocks are designed to prevent [S4].
Selection Matrix: Metering vs Protection CTs for Motor Circuits
The table below lines the four common CT classes against the criteria that drive motor protection relay design: ratio accuracy at normal load, composite error at fault current, knee-point behaviour, and typical application. [S2]
For a typical 400 V, 75 kW induction motor drawing roughly 140 A FLC with a 6x start current, a 200/5 A 5P20 CT delivers 5% composite error up to 4000 A primary, well above the 840 A locked-rotor current and inside the panel's let-through [S3].
Failure Modes: Saturation, DC Offset, and Open-Circuit Hazard
Three failure modes dominate motor protection CT problems. First, AC saturation under high primary current: the core leaves its linear region and the relay sees a clipped waveform, so the negative-sequence element over-reads or the instantaneous element under-reads [S5]. Second, DC offset saturation: a fully offset waveform with a 100% time constant of 45 to 120 ms drives the core into half-cycle saturation, and even a 5P20 CT can lose 30 to 50% of its signal during the first cycle if the X/R ratio of the upstream network exceeds 15 [S4].
Third, the open-circuit secondary hazard: opening a CT secondary under load removes the counter-magnetomotive force (MMF), drives the core into deep saturation, and can induce terminal voltages above 2 kV across the open points, well inside the arc-flash boundary of a typical 480 V motor control centre [S4]. Standard practice is a shorting terminal block with a test switch, never a fuse, and the relay engineer should verify the short is applied before any relay is unplugged. A microcomputer protection relay's internal CT shorting switches handle this automatically when the relay is withdrawn, but panel retrofits without that feature rely on the shorting block alone.
Specifying a Protection CT for a New Motor Feeder
Three questions drive the CT class on a new motor feeder: what is the maximum available fault current at the motor terminals, what is the relay's overcurrent pickup ratio, and does the protection scheme include differential or restricted earth fault? For across-the-line motors with simple thermal overload, a 5P20 CT with rated burden at least 1.5x the actual loop burden is the standard specification, and the rated burden should cover wire resistance plus the relay input impedance plus a 25% margin for future relay replacement [S2][S4].
For large motors (above 1 MW) or where high-impedance differential protection is applied, Class PX or IEEE C-class CTs are specified, and the engineer must state the knee-point voltage Vk, the 1 mA or 5 mA secondary excitation current at Vk, and the maximum secondary loop resistance, since the relay coordination study depends on these parameters directly [S3][S4]. For motor feeders where revenue metering is also required, a dual-core CT with one 0.5S metering core and one 5P20 protection core on the same primary is the typical configuration, and the cores must be wired to physically separated terminal blocks to keep the metering and protection grounds isolated [S1][S5].
Verification and Maintenance Signals
Two trackable signals confirm a protection CT is still operating inside its class. First, an annual primary injection test with at least 1x, 5x, and 20x rated current, comparing the measured ratio and phase angle against the factory test certificate; a drift of more than 1% at 20x rated current is the practical alert threshold for a 5P20 CT [S5]. Second, a thermal drift check: high-quality CTs hold ratio inside roughly ±0.005% per °C and annual drift below 0.02%, so any feeder where the relay starts reporting a 0.5% or larger bias between redundant CTs has likely picked up a wiring, burden, or connection problem rather than a CT ageing issue [S5]. Field engineers coordinating 1 kHz vs 100 kHz data logging for transients on motor starts should also confirm the protection CT bandwidth exceeds the recorder's, since a CT that clips the first-cycle DC component will also clip the recorded waveform.