How to Select Protective Current Transformers: 5P10 vs 5P20 Explained
Choose a 5P10 protective current transformer when its accuracy limit factor is sufficient for the maximum fault current your protection scheme must measure; choose 5P20 when the relay must receive a usable current signal at a higher multiple of rated current. Both classes have a specified composite error limit of 5% at their rated accuracy limit current and rated burden. The key difference is the multiple: 5P10 is rated to 10 times rated primary current, while 5P20 is rated to 20 times. These ratings are not interchangeable with the CT’s thermal or dynamic short-circuit ratings, which must also suit the installation.
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At a glance: What 5P10 and 5P20 mean
- 5P: A protection CT class with a maximum specified composite error of 5% at the rated accuracy limit current and rated burden.
- 10 or 20: The accuracy limit factor (ALF), indicating the multiple of rated primary current up to which the specified protection accuracy applies under stated conditions.
- Practical choice: Select the lowest ALF that meets the protection study’s fault-current and relay requirements, while checking burden, wiring, and short-circuit withstand ratings.
What the protection class tells you
A protective current transformer reproduces current for protective relays and related equipment. During normal operation, the secondary current is proportional to primary current within the CT’s specified accuracy. During a fault, the primary current can rise sharply, so the CT must maintain suitable performance long enough for the protection system to detect and respond.
In a designation such as 5P10, “5P” identifies the protection class and its composite-error limit; “10” is the ALF. For a 5P20 CT, the ALF is 20 instead. The ALF is specified in relation to the rated burden, so it should not be treated as a guarantee independent of the connected relay, wiring, and other secondary-circuit loads.
Illustrative current example
Consider a hypothetical CT rated 100/5 A. At rated burden, its nominal accuracy limit current is 1,000 A for a 5P10 rating (100 A × 10) and 2,000 A for a 5P20 rating (100 A × 20). These figures illustrate the ALF calculation; they do not establish the CT’s thermal withstand, actual fault response in a specific circuit, or performance at a burden different from its rating.
5P10 vs 5P20: Key differences
| Specification | 5P10 | 5P20 |
|---|---|---|
| Protection class | 5P | 5P |
| Specified composite-error limit | 5% at rated accuracy limit current and rated burden | 5% at rated accuracy limit current and rated burden |
| Accuracy limit factor | 10 | 20 |
| Nominal accuracy limit current | 10 × rated primary current | 20 × rated primary current |
| Typical selection consideration | May suit schemes where the calculated fault-current multiple is lower | May suit schemes requiring a higher fault-current multiple before the specified limit is reached |
How to select the right ALF
1. Establish the protection duty
Start with the protection function and the maximum prospective fault current at the CT location. Confirm which fault conditions the relay must detect, how quickly it must operate, and whether the scheme depends on current accuracy during high-current events. Use the project’s protection study and relay requirements rather than selecting an ALF based only on a general preference for a larger number.
2. Compare fault current with rated primary current
Calculate the relevant fault-current multiple by dividing the maximum primary fault current by the CT’s rated primary current. Compare that multiple with the proposed ALF, allowing for the engineering requirements and assumptions specified for the installation. If the required multiple exceeds 10, a 5P10 rating may not meet the stated accuracy-limit requirement at rated burden; a 5P20 may be worth evaluating.
3. Verify the actual secondary burden
Include the relay input, secondary wiring, terminals, and any other connected devices in the burden assessment. A CT’s ALF is associated with its rated burden, and a higher connected burden can reduce the effective accuracy limit factor. Ask the manufacturer or supplier to confirm suitability for the actual burden rather than assuming that the nameplate ALF applies unchanged in every circuit.
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4. Check ratings beyond 5P10 or 5P20
Verify the rated primary and secondary currents, rated burden in VA, frequency, insulation requirements, installation conditions, and applicable short-circuit withstand ratings. Thermal short-time current and dynamic peak current address withstand capability; they are distinct from the ALF. A higher ALF alone does not prove that a CT is suitable for every fault level or protection arrangement.
When each option may be suitable
Consider 5P10 when
A 5P10 CT may be appropriate when the protection calculation shows that the required fault-current multiple is within its specified accuracy limit at the relevant burden. It can be a practical choice where the project does not require the higher ALF of 5P20 and all other electrical and mechanical ratings are satisfactory. Confirm the complete specification against the relay and system design before ordering.
Consider 5P20 when
A 5P20 CT may be appropriate when the protection scheme requires the specified protection accuracy at a higher multiple of rated primary current. This can matter where calculated fault current is high relative to the CT ratio or where the burden and relay requirements make a higher ALF necessary. It is not automatically the better choice: the final selection still depends on the system study, burden, CT construction, and relevant withstand ratings.
Common selection mistakes
- Choosing by ALF alone: A 5P20 rating does not replace checks of ratio, burden, insulation, frequency, and short-circuit withstand.
- Ignoring wiring burden: Long secondary leads or additional connected equipment can change the burden seen by the CT.
- Confusing protection and measurement requirements: Protection CT classes and metering CT accuracy classes serve different purposes; specify each core’s function clearly when a CT has multiple cores.
- Assuming a higher ALF is always safer: The correct rating is the one that meets the documented duty and fits the complete design, not simply the largest available number.
Information to prepare before requesting a quotation
To help a manufacturer assess a protective CT requirement, provide the system voltage and frequency, CT ratio, required 5P class and ALF, rated burden, installation type, and any applicable insulation or short-circuit withstand requirements. Include the relay model or its relevant input burden where available, plus the calculated maximum fault current and the protection application. Clear requirements make it easier to compare proposals on the same technical basis and identify missing specification details before procurement.
Conclusion: Make the choice from the protection study
The difference is direct: 5P10 has an ALF of 10, while 5P20 has an ALF of 20; both specify a 5% composite-error limit at their rated accuracy limit current and rated burden. Select 5P10 when the calculated protection duty is covered, and consider 5P20 when the scheme requires the higher current multiple. In either case, verify the actual burden and all separate electrical and short-circuit ratings before final approval.
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