Current transformer

01

Choose the class for metering or for protection

ABB's instrument transformer guide explains that metering transformers need high accuracy at normal operating current, while protection transformers need linearity over a wide range so relays see faults correctly. For CTs, ABB states that "typical output levels of instrument transformers are 1-5 amperes". [1]

Under IEEE C57.13, as ABB summarises it, metering classes 0.3, 0.6 and 1.2 set the permitted ratio and phase-angle error at a stated burden; twice the error is allowed at 10% of rated current, and the 100% limit applies up to the current allowed by the thermal rating factor, which ABB describes as "a multiplier of 1.5 or 4.0 applied to many transformers". [1]

For relaying, ABB explains C and T classes based on a 10% error limit from 1 to 20 times rated current. The number is the secondary voltage the CT can deliver without saturating: a C200 CT can drive 100 A (20 × 5 A) into a 2 Ω burden, 200 V, within 10% error. ABB lists standard relaying classes of 50, 100, 200 and 800. [1]

Suppliers selling to IEC markets quote classes such as 0.2S or 0.5S; GFUVE, for example, lists a 2000/5 A medium-voltage CT in class 0.2S or 0.5S. IEC 61869-2 was not read for this page, so ask the supplier which standard each class refers to. [2]

Tell the supplier what the CT feeds: revenue meter, panel meter, protection relay or monitoring device. One core rarely does both metering and protection well; ask whether a multi-core CT is offered.

02

Add up the real burden of the secondary circuit

ABB notes that CT accuracy holds only at the burden it is rated for, and that the test burden must include the secondary leads. IEEE C57.13 standard burdens for 5 A secondaries run from B-0.1 (2.5 VA) to B-1.8 (45 VA) for metering and B-1 (25 VA) to B-8 (200 VA) for relaying; for other secondary ratings the ohmic values scale by (5/I)². [1]

As an editorial calculation example only: a lead loop of 0.7 Ω carrying 5 A absorbs 0.7 × 5² ≈ 17.5 VA, more than a B-0.5 metering burden of 12.5 VA on its own; at a 1 A secondary the same loop absorbs only 0.7 VA. Long lead runs are one reason to specify 1 A secondaries.

List every device on the secondary with its burden, plus the cable length and cross-section, and ask the supplier to confirm the class at that total burden.

03

Never open a loaded secondary

ABB explains that if the secondary is opened while primary current flows, all of it becomes exciting current, producing very high voltage peaks at the secondary terminals as the current passes through zero. These peaks may not show on an ordinary voltmeter but can break down insulation and are dangerous to personnel; ABB treats an open secondary as a serious accident. [1]

ABB states CTs are insulated for emergency secondary peaks up to 3,500 V; for larger CTs where opening is probable, a protective device should be permanently connected across the secondary. An open circuit or a fault can also leave the core magnetised, which affects later accuracy. [1]

Specify shorting terminal blocks so meters and relays can be removed safely, and ground the secondary at one point; ABB notes that "grounding of the secondary circuit is most important, but in complicated three-phase connections, the best point to ground is not always easily determined". [1]

04

Check thermal rating and short-circuit withstand

ABB describes the continuous thermal current rating factor as a multiplier of rated current the CT can carry continuously at 30 °C ambient, and gives a loading guide for 55 °C rise CTs: loading to that guide is not safe if average ambient exceeds 30 °C or overloads occur more than once a day. [1]

Ask for the rated short-time thermal current and dynamic (peak) current against your prospective fault level, and the insulation level for the system voltage. Lugao, for example, lists oil-immersed CTs up to 330 kV and epoxy-cast ring CTs for low voltage. [3]

05

Window, busbar, cast-resin, oil-immersed and split-core builds

HEAG, Lugao and GFUVE between them list low-voltage window and busbar CTs, medium-voltage cast-resin CTs, oil-immersed high-voltage CTs, zero-sequence CTs and split-core CTs. [4] [3] [2]

HEAG, for example, lists 0.66 kV indoor CTs and cast-resin instrument transformers from 3 to 36 kV; CNC Electric lists RCT indoor CTs for circuits up to 0.5 kV at 50 Hz. [4] [5]

06

Core, windings, insulation and terminals

Materials come from the data sheet of the CT you order.

07

Ratio, class, burden and short-circuit ratings for the quotation

Ratio, burden and fault level come from your own circuit. Where ABB or a supplier publishes a value, it is in the middle column.

08

Send the circuit, class and burden data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Circuit: [system voltage; frequency; normal and maximum load current; fault level]
Use: [revenue metering; panel metering; protection relay; monitoring]
Ratio: [primary/secondary; multi-ratio taps; number of cores]
Class: [per core; standard (IEEE or IEC)]
Burden: [devices and VA; lead length and cross-section]
Mounting: [window size; busbar or cable; indoor or outdoor; split-core]
Ratings: [short-time thermal current; dynamic current; insulation level; thermal rating factor]
Documents: [routine test report; accuracy test at stated burden; type test certificates]

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