CT Accuracy Class 0.2S vs 0.5S: Which Revenue Metering Needs

Two current transformers can carry the same ratio, sit in the same panel, and disagree by whole percentage points about how much energy passed - because one is a Class 1 CT reading a feeder that spends its life at 20% load, and the other is a 0.2S unit built to stay accurate exactly there. On a tariff connection, that difference is not academic: the CT's error is applied to every kilowatt-hour, every billing cycle, for decades.

The class printed on the nameplate is a precise claim defined by IEC 61869-2 [1]. This guide unpacks what 0.2S and 0.5S promise for ratio and phase error, why the "S" matters at part load, how burden can void the class, and which class a Pakistani metering point needs.

What the Class Number Promises (IEC 61869-2)

A measurement CT's class is its maximum ratio error at rated current: a Class 0.5 CT may err by ±0.5% at 100% of rated current, a Class 0.2 by ±0.2% [1]. The promise weakens as current falls - which is where the "S" (special) classes come in: guaranteed accuracy extends down to 1% of rated current, where ordinary classes stop at 5% [1].

Load point	Class 0.5	Class 0.5S	Class 0.2S
1% of rated current	not specified	±1.5%	±0.75%
5% of rated current	±1.5%	±0.75%	±0.35%
20% of rated current	±0.75%	±0.5%	±0.2%
100-120% of rated current	±0.5%	±0.5%	±0.2%

The error limits are IEC 61869-2's own accuracy tables [1], and the pattern is the whole argument: at the 1-20% loads where real feeders spend most of their hours, an ordinary class is either unspecified or several times looser than its S counterpart.

Why the S Classes Exist: Life Below 20% Load

A revenue meter on an industrial connection rarely sees rated current. Night shifts, weekends, and oversized CT ratios push the operating point down the curve - a 400/5 CT on a feeder averaging 80 A runs at 20% of rating, and idles below 5% overnight. An ordinary Class 0.5 CT has no guaranteed error below 5% [1]; the S classes hold a defined error down to 1% of rating, which is why tariff and check metering specify them.

Phase Displacement: The Second Limit in Minutes (IEC 61869-2)

Ratio error is only half of each class. The same tables limit phase displacement - how far the secondary current leads or lags the primary - in minutes of arc [1]:

Load point	Class 0.5	Class 0.5S	Class 0.2S
1% of rated current	not specified	±90′	±30′
5% of rated current	±90′	±45′	±15′
20% of rated current	±45′	±30′	±10′
100-120% of rated current	±30′	±30′	±10′

The minutes matter because active energy is voltage times current times the cosine of the angle between them; a CT that shifts the angle corrupts the power factor the meter computes. Thirty minutes of displacement is worth roughly 0.65% of active-energy error at power factor 0.8, and about 1.5% at 0.5 [1] - the reason reactive-heavy industrial loads are where the 0.2S limit of 10 minutes earns its price.

Burden: The Fine Print That Voids the Class (IEC 61869-1)

The class is only valid within the CT's rated burden range - the VA load of the meter, the leads, and any transducers on the secondary, defined under IEC 61869-1 [2]. Overload the secondary with long lead runs or too many instruments and the CT drifts out of class even though the nameplate still says 0.2S; run it far below the rated burden and accuracy can also leave the specified band. The instrument transformer accuracy and burden guide works through the arithmetic; the short version: class and burden are one specification, never two.

Which Class for Which Duty: 4 Cases

The selection logic for Pakistani installations:

• Tariff (billing) metering - 0.2S or 0.5S per the utility's metering code and the meter class it feeds; an IEC 62053-22 [3] class 0.5S energy meter deserves a CT that does not throw the accuracy away.
• Check metering - match the main metering class, or one class looser where the scheme allows.
• Sub-metering and cost allocation - Class 0.5 or 1 is usually adequate; the loads are compared against each other, not billed.
• Panel indication - Class 1 serves; paying for 0.2S on an ammeter circuit buys nothing.

ESIT Elektrik manufactures measurement CTs across these classes, and Ziegler covers the switchboard metering side - the revenue metering accuracy classes guide treats the meter-and-CT chain end to end.

Common Specification Errors

The recurring errors under IEC 61869-2 all come from reading the class as one number:

• Specifying Class 0.5 where 0.5S is meant. Below 5% of rated current the ordinary class promises nothing [1]; the S suffix is the part-load guarantee, not a typo.
• Quoting a class without the burden. The class holds only within the rated burden range [2]; "0.2S" with an overloaded secondary is a fiction.
• Oversizing the ratio. A CT chosen two sizes up "for growth" pushes normal load down the accuracy curve, exactly where errors grow.
• Pairing a precise meter with a loose CT. A 0.5S meter [3] behind a Class 1 CT delivers Class 1 accuracy; the chain is only as good as the transformer.
• Buying metering and protection accuracy in one core. Measurement classes saturate early by design; protection needs its own core - a dual-core CT, not a compromise.

What to Specify When Enquiring: 6 Items

A CT enquiry that quotes cleanly against IEC 61869 states:

1. Duty - tariff, check, sub-metering, or indication
2. Class - 0.2S / 0.5S for revenue points, with the S deliberate
3. Ratio - chosen against the real load profile, not the switchgear rating
4. Rated burden - computed from the meter, leads, and transducers actually connected
5. Meter class - the IEC 62053-22 [3] class of the energy meter the CT feeds
6. System details - voltage, frequency, and thermal/short-circuit ratings from the fault study

Sourcing and Support in Pakistan

Pacific Engineering & Automation is the authorized reseller for ESIT Elektrik instrument transformers and Ziegler metering instruments in Pakistan. We help match CT class, ratio, and burden to the meter class and the load profile before the enquiry goes out - the difference between a metering chain that reads true at 3 a.m. and one that is only accurate at full load.

To specify CTs for a metering point, request a catalogue or quotation or contact our engineering team.

Field-derived case studies will be added to this post as Pacific Engineering & Automation accumulates engagement records. The current version is grounded in published specifications, regulatory documentation, and standards body references.

Sources

1. IEC 61869-2:2012 - Instrument transformers, Part 2: Additional requirements for current transformers
2. IEC 61869-1:2023 - Instrument transformers, Part 1: General requirements
3. IEC 62053-22:2020 - Static meters for AC active energy, classes 0.1S, 0.2S, 0.5S