Instrument Transformer Accuracy Class and Burden: ESIT Guide

A revenue meter is only as accurate as the current transformer feeding it. Specify a CT one accuracy class too loose, or load it past its rated burden, and the meter under-reads at part load - the error compounds every billing cycle into lost revenue or a disputed K-Electric or DISCO bill. On the protection side the opposite mistake - a metering-class core where a protection core belongs - saturates during a fault, and the relay never sees the current it needs to trip.

ESIT Elektrik manufactures measurement and protection current and voltage transformers for LV and MV systems, from resin-cast toroidal CTs to 30 kV outdoor units [1]. This guide covers the two parameters a Pakistani specification has to get right - the accuracy class and the burden - both governed by the IEC 61869 series.

What CT Burden Means Under IEC 61869-1

The burden is the impedance the CT secondary drives - meter, connecting leads, and any transducers - expressed in VA at rated secondary current. IEC 61869-1 sets the rated-burden range over which a CT holds its declared accuracy [2]. Drive less than the rated burden and the CT stays in class; drive more and it falls out of its accuracy band.

Both extremes cause error. An over-burdened CT - too much lead resistance on a long switchroom run, or too many instruments on one core - pushes the secondary toward saturation and the meter under-reads. A lightly loaded CT can read high. So the burden is specified to the actual installed load, not guessed: the Ziegler meters and the lead length both count toward it, as the revenue-metering accuracy-class guide sets out.

Accuracy Classes for Metering and Protection (IEC 61869-2)

IEC 61869-2 splits current-transformer accuracy into two families that cannot be interchanged [3]:

Core type	IEC 61869-2 class	Where it goes
Revenue metering	0.2S, 0.5S	Utility and tariff metering, check meters
Industrial metering	0.5, 1	Sub-metering, panel instruments
Protection	5P, 10P (e.g. 5P20)	Overcurrent, earth-fault, differential relays
Combined	dual-core (0.5S + 5P20)	One CT body, separate metering and protection cores

Measurement classes are tight at and below rated current - the "S" classes hold accuracy down to 1% of rated current, which is why revenue metering specifies them: load rarely sits at full rated current. Protection classes are defined by their accuracy limit factor (ALF), the multiple of rated current up to which the core stays linear. A 5P20 core holds 5% error up to 20 times rated current, so the relay sees a faithful fault current. The instrument security factor (ISF) does the reverse for a metering core, capping how early it saturates to protect the connected meter during a through-fault.

Why One Core Cannot Do Both

Under IEC 61869-2, metering and protection ask opposite things of the magnetic core, which is why a single core tuned for one starves the other:

Parameter	Metering core	Protection core
Class examples	0.2S, 0.5S, 1	5P, 10P
Accuracy emphasis	tight at and below rated current	linear far above rated current
Saturation	saturates early (low ISF)	stays linear to a high ALF
Limiting factor	instrument security factor (ISF)	accuracy limit factor (ALF)

Where a feeder needs both, use a dual-core CT: one resin-cast body carrying, say, a 0.5S metering core and a 5P20 protection core on separate secondaries. ESIT's MV indoor (CTM, CTB, CTD) and outdoor (CTH) series and the LV MES series are built this way [1]. Asking for "a Class 0.5 CT" on a feeder that also runs protection is an incomplete spec - name both cores.

Voltage Transformers Under IEC 61869-3

Voltage transformers follow the same logic under IEC 61869-3 [4]. Measurement VTs carry classes such as 0.2, 0.5, and 1; protection VTs use 3P and 6P. The extra parameter is the voltage factor - how far above rated voltage the VT must run without overheating, and for how long: typically 1.2 continuously, with 1.5 or 1.9 for a defined time depending on earthing. On an unearthed or resonant-earthed network a healthy phase can sit well above nominal during an earth fault, so the voltage factor is part of the VT spec, not an afterthought.

Common Specification Errors

The recurring errors under IEC 61869 come from quoting half a specification:

• An accuracy class with no burden. A class figure is only valid across the rated burden range per IEC 61869-1 [2]; "Class 0.5S" alone does not say whether it holds at the installed VA.
• One core for metering and protection. Their saturation requirements are opposite - specify a dual-core CT, not a single core stretched across both duties.
• A measurement core where protection belongs. A 0.2S or 0.5S core saturates early by design; behind a relay it blinds the protection during the fault it exists to clear.
• Inventing ratios and burdens. Ratio, rated burden, ALF/ISF, and thermal and short-circuit ratings come from the ESIT datasheet for the specific series, not a generic assumption.
• Ignoring the VT voltage factor. On a network that can drive a phase above nominal during an earth fault, a VT specified only by ratio and class overheats [4].

What to Specify When Enquiring

An ESIT enquiry that can be quoted against IEC 61869 without back-and-forth states:

1. System voltage and location - LV or MV (10/20/30 kV), indoor switchgear or outdoor line
2. Ratio - primary and secondary rated currents, or voltages for a VT
3. Accuracy class per core - e.g. 0.5S metering, 5P20 protection - and the number of cores
4. Rated burden (VA) - sized to the connected meter, leads, and transducers
5. ALF / ISF - accuracy limit factor for protection cores, instrument security factor for metering
6. Thermal and short-circuit ratings - continuous thermal current and short-time rating to the prospective fault level

Sourcing and Support in Pakistan

Pacific Engineering & Automation is the authorized ESIT Elektrik reseller in Pakistan. We supply the full instrument-transformer range - MV indoor and outdoor CTs and VTs, LV and resin-cast toroidal CTs to 20 000 A, plus insulators, reactors, and power capacitors - and help match accuracy class, core count, and burden to the scheme before tender. ESIT CTs and VTs land in the same panels as Tempa Pano boards (see the LV distribution board IP and form guide) and Sifam Tinsley panel instruments, feeding the adjacent Ziegler meters.

To specify a CT or VT chain for your site, 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. ESIT Elektrik - official product portal
2. IEC 61869-1:2023 - Instrument transformers, Part 1: General requirements
3. IEC 61869-2:2012 - Instrument transformers, Part 2: Additional requirements for current transformers
4. IEC 61869-3:2011 - Instrument transformers, Part 3: Additional requirements for inductive voltage transformers