New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
Current Transformer Accuracy Classes: Metering vs Protection Selection
Quick answer: Metering CT classes such as 0.2, 0.2S, 0.5 and 0.5S prioritize accuracy around normal load. Protection classes such as 5P10 or 5P20 must reproduce fault current up to an accuracy-limit factor. PX/Class X is specified from excitation and knee-point requirements for special protection schemes. |
A current transformer scales primary current to a standardized secondary value, commonly 5 A or 1 A. A complete CT specification includes ratio, secondary current, accuracy class, rated burden, frequency, insulation level, short-time current, core purpose and physical construction. The class cannot be selected independently of the connected meter or relay and its wiring.
Metering and protection classes
Marking | Purpose | Interpretation | Typical application |
0.2 / 0.2S | Precision metering | Tighter ratio and phase error limits; S class extends specified performance at low current | Revenue or high-accuracy energy measurement |
0.5 / 0.5S | General metering | Common monitoring accuracy; S class supports a wider low-current range | Building/industrial energy monitoring |
1.0 | Indication | Lower precision than 0.5 or 0.2 | Ammeters and non-critical monitoring |
5P10 | Protection | Composite error limit 5% at 10 x rated current at rated burden | Overcurrent/earth-fault protection |
5P20 | Protection | Composite error limit 5% at 20 x rated current at rated burden | Higher fault-current reproduction requirement |
10P10 | Protection | Composite error limit 10% at ALF 10 | Protection where this accuracy is acceptable |
PX / Class X | Special protection | Defined by knee-point voltage, excitation current and winding resistance | Differential, restricted earth fault and special schemes |
Do not interpret Class 0.5 as a universal +/-0.5% statement under every current and burden condition. IEC accuracy limits depend on current percentage, burden and class. Use the standard and the manufacturer's certificate for the specific CT.
Burden: the part most specifications miss
CT burden is the total secondary load in volt-amperes at rated secondary current. It includes the meter or relay input plus both conductors between the CT and device. Excess burden increases ratio and phase error and can cause a protection CT to saturate earlier.
Example: A 5 A secondary with 0.08 ohm total loop resistance creates I^2R = 5^2 x 0.08 = 2 VA of wiring burden. If the meter adds 0.5 VA, the total is 2.5 VA. A CT rated 5 VA has margin; a 2.5 VA CT would be at its nominal burden before connector and temperature effects. |
A 1 A secondary reduces wiring burden by a factor of 25 for the same loop resistance because burden scales with current squared. It is often preferable for long cable runs, provided the relay or meter accepts a 1 A input.
Model pairing example: Socomec DIRIS A10
ASBEAM lists Socomec DIRIS A10 references 48250400 and 48250401 as compatible with 5 A CT inputs; 48250401 is presented with RS485/Modbus communication. When pairing a CT with either model, specify a 5 A secondary, the primary ratio (for example 200/5 A or 800/5 A), a metering class such as 0.5 or 0.5S where appropriate, and enough VA burden for the instrument plus leads. The meter model does not determine the CT ratio; the maximum load current does.
Complete CT selection process
1. Define purpose: revenue metering, general monitoring, overcurrent protection, differential protection or a dual-core combination.
2. Select primary ratio so normal maximum current uses a useful portion of the CT range without chronic over-ranging.
3. Match the secondary current to the meter or relay input: 1 A or 5 A.
4. Choose the accuracy class from the purpose and governing standard; use S-class metering where low-load accuracy matters.
5. Calculate total burden from instrument input, lead length/gauge, terminals and test switches, then choose a rated burden with suitable margin.
6. For protection CTs, select ALF such as 5P10 or 5P20 from maximum fault current and relay requirements; confirm saturation performance.
7. For PX/Class X, specify knee-point voltage, excitation current limit and secondary winding resistance from the protection study.
8. Confirm window/busbar size, mounting, insulation level, frequency, short-time thermal current and dynamic current ratings.
9. Check secondary earthing, terminal shorting facilities and safe test procedures. Never open-circuit an energized CT secondary.
Common specification examples
Specification | Meaning | Typical use |
400/5 A, 5 VA, Class 0.5 | 400 A primary gives 5 A secondary; 5 VA rated burden | General metering with short leads |
800/5 A, 10 VA, Class 0.5S | S-class metering over a wider current range | Energy monitoring with variable load |
600/1 A, 10 VA, 5P20 | Protection CT, 5% composite error at ALF 20 | Long lead run to overcurrent relay |
1200/1 A, PX, Vk >= project value | Special protection core defined by excitation data | Differential or REF protection |
Frequently Asked Questions
What does 5P10 mean?
It is a protection CT class with a 5% composite-error limit at an accuracy limit factor of 10, at rated burden and stated conditions.
Is 0.5S always better than 0.5?
It provides specified accuracy over a wider low-current range, which is valuable for variable-load metering. Cost, burden and the measurement objective still matter.
Can one CT core feed both a meter and a protection relay?
Sometimes, but separate metering and protection cores are often specified because their saturation objectives differ. Follow the protection and metering design.
Can I connect a 1 A CT to a DIRIS A10 listed for 5 A CT input?
Not unless the exact meter/reference supports and is configured for 1 A. The cited ASBEAM product pages describe 5 A CT compatibility.
Bottom line
CT accuracy is a system property involving the CT, burden, wiring and connected device. Specify ratio, 1 A/5 A secondary, class, VA burden and protection saturation performance together. A model such as DIRIS A10 48250401 helps define the instrument interface, but not the entire CT specification.
For a CT and meter pairing review, send ASBEAM the one-line diagram, maximum load, fault current, lead length/gauge, required meter or relay model and desired accuracy class.
Technical references
• ASBEAM - Socomec DIRIS A10 48250401
• ASBEAM - Socomec DIRIS A10 48250400
New industry Technology regarding to Bussmann fuse, ABB breakers, Amphenol connectors, HPS transformers, etc.
TAG: current transformer accuracy class CT accuracy class accuracy limit factor protection CT