Switchgear Sensor and Instrument Transformer Accessories: Ratio, Class, Burden, Wiring, and Fit

All images are generated illustrative technical visualizations, not XIYA POWER factory, product, metering, protection-operation, installation, calibration, test, commissioning, maintenance, or field photographs.

A switchgear sensor instrument transformer accessories review starts with the circuit function, not the outline shape. Ratio, class, burden, wiring, duty, and cabinet fit must close against one approved design revision. A photo can support identity, but it cannot release an electrical or mechanical interface.

Build a Function-First Accessory Selection Matrix

Identify the bay, circuit, and sensing purpose before comparing products. State whether each core or channel supports measurement, protection, control, or monitoring. This prevents a visually similar accessory from inheriting an unverified duty.

The sensor and instrument transformer accessories range provides the product-owner route for CTs, VTs, sensors, and related parts. Its drawings can support dimensions, mounting, terminal direction, and body form. They do not establish an unstated ratio or application rating.

The wider Switchgear Components pillar places these devices within metering, feedback, monitoring, and protection interfaces. Use both pages for routing and product context, then return to controlled project evidence for release.

Use a selection matrix so every decision has a required input and release evidence.

Decision	Required input	Evidence source	Offered return	Release evidence	Hold condition
Circuit and function	SLD, bay ID, purpose per core or channel	Approved SLD and schedule	Identified accessory position	Position and purpose map to one circuit	Circuit ownership is unclear
Ratio or output	Primary quantity, secondary quantity, taps, signal type	Metering or protection schedule	Nameplate and datasheet values	Values match the schedule	Ratio, tap, or output is open
Class and burden	Class per core, connected load, lead data	Project specification and calculation	Class and rated burden	Controlled calculation fits the rating	Inputs or class are unresolved
Fault or voltage duty	System and study inputs	Short-circuit study and specification	Declared withstand or voltage factor	Offered duty covers the requirement	Duration or rating differs
Wiring	Polarity, terminal map, connector, grounding and shield route	Wiring and terminal drawings	Terminal diagram and pinout	Every terminal maps across revisions	Cross-reference is absent
Physical fit	Aperture, body, base, holes, orientation, access	Cabinet overlay and dimensioned drawing	Controlled outline drawing	Tolerances and access close	Any interface remains assumed

Treat the matrix as a decision record. A missing source, owner, or revision is itself a hold condition, even when the offered item looks familiar.

Release Ratio, Class, Core Assignment, Burden, and Duty

Ratio defines the relationship between primary and secondary quantities. Available taps also need an explicit schedule match. A nearby ratio or an undocumented tap position cannot substitute for the specified value.

Assign every core or channel to one approved purpose. Metering and protection cores can carry different classes and burdens, so they cannot be exchanged silently. The nameplate confirms offered data; the circuit schedule confirms whether that data is suitable.

For a conventional instrument transformer, compare rated burden with the calculated connected burden. Include each meter or relay input, secondary lead resistance, terminal losses, and test-link losses. Lead length, conductor cross-section, and calculation revision must be controlled.

A low-power or digital sensor may use a rated analogue output or a digital interface instead of a conventional VA burden model. Verify signal type, level, connector, protocol, and receiving input together. Never convert that interface into a conventional burden check without applicable manufacturer data.

Keep frequency and duty separate from ratio and class. CT short-time and dynamic withstand must be compared with the project fault input. VT voltage factor and duration must follow the approved system and earthing requirements.

The complete MV switchgear RFQ data guide owns the upstream SLD, protection schedule, metering responsibility, and fault-current inputs. COMP-08 consumes those approved inputs; it does not create relay settings or a complete metering schedule.

The official IEC 61869-1:2023 publication page defines general requirements for newly manufactured instrument transformers above 1 kV AC or 1.5 kV DC. It covers analogue or digital secondary signals used for measurement, protection, and control. The relevant product-specific standard is also required. This public scope does not provide project ratios, classes, burdens, or fault duties.

A technical release therefore needs both sides of the comparison. Project evidence states the required duty, while current manufacturer data states the offered capability.

Map Polarity, Terminals, Wiring, Grounding, and Interfaces

Terminal designations vary across manufacturers and drawing conventions. Primary and secondary labels must be cross-referenced explicitly. Similar-looking notation cannot prove equivalent polarity or circuit direction.

Map each device terminal to its cabinet terminal block, destination, function, and drawing revision. Multi-core devices also need unique core identifiers that agree across the nameplate, terminal drawing, and circuit schedule.

Treat grounding and shielding as project-defined interfaces. The approved wiring design must identify the grounding point and shield route. This article does not prescribe a universal point, terminal, or energized work procedure.

Active and digital sensors may add auxiliary supply, polarity, connector, pinout, address, or communication requirements. Verify those items against the exact receiving device. A compatible physical connector does not prove electrical or protocol compatibility.

CT secondary circuits also need a defined shorting and test boundary on the approved drawing. Equipment-specific and site-specific instructions govern handling. COMP-08 records required interfaces but does not provide calibration, installation, or testing procedures.

Use a cross-reference table during technical review:

Wiring item	Project return	Manufacturer return	Release evidence	Hold condition
Primary polarity	Drawing labels and direction	Device labels and polarity mark	Approved notation map	Mapping is assumed
Secondary terminal	Core, terminal block, destination	Core and terminal labels	End-to-end schedule	Destination or core is unclear
Ground and shield	Approved circuit route	Permitted terminal interface	One coordinated drawing	Route or owner is absent
Active interface	Supply and receiving input	Pinout, output, protocol	Matched interface schedule	Level, pin, or protocol differs
Test boundary	Shorting and test-link drawing	Device terminal requirements	Equipment-specific scheme	Boundary is undocumented

This table documents compatibility. It does not claim that an accessory has been wired, energized, calibrated, or tested.

Confirm Aperture, Mounting, Orientation, and Cabinet Fit

Electrical suitability cannot release a part that does not fit the cabinet. Compare the device drawing with the same cabinet revision used for the circuit review.

For a ring device, check the complete primary conductor or cable envelope. Include insulation, supports, tolerances, and assembly clearance. Nominal aperture alone is insufficient when the remaining margin is unknown.

Compare the mounting base, hole pattern, fixing hardware, device weight, and support arrangement. A small hole-pattern difference creates a design change rather than an automatic fit. Any adapter or drilling change needs a controlled disposition.

Orientation controls polarity, terminal access, cable exit, and removal space. Show adjacent conductors, terminals, shutters, baffles, partitions, and moving parts on the overlay. COMP-08 checks accessory fit but does not own their movement or segregation acceptance.

The KYN28 switchgear specification provides cabinet-family context for CT and VT positions, compartment arrangement, and accessory locations. A cabinet-family page cannot replace the project general arrangement or the offered accessory drawing.

Record dimensions and tolerances in one overlay. Keep an interface on hold when body clearance, terminal reach, cable bend, fastener access, or removal path remains unresolved.

Freeze the RFQ and Technical-Release Data Pack

Turn the RFQ checklist into a revision-controlled data pack. Separate identity evidence, electrical evidence, wiring evidence, and dimensional evidence so one document cannot answer a different question.

Data pack item	Required input or return	Technical release condition
Circuit package	SLD, bay ID, function, core or channel schedule	One approved circuit revision
Ratings package	Ratio, taps, class, burden or output, frequency, duty	Requirements map to offered data
Calculation package	Connected load, leads, terminals, fault inputs	Inputs, method, and revision are controlled
Wiring package	Polarity, terminal map, connector, supply, ground, shield	End-to-end cross-reference closes
Product package	Nameplate, current datasheet, applicable evidence	Exact offered model is identified
Fit package	Outline, aperture, holes, tolerances, orientation, access	Cabinet overlay closes every interface
Control package	Revision register, deviations, owners, dispositions	No silent assumption remains

The switchgear component acceptance checklist owns incoming inspection after procurement. COMP-08 owns the earlier selection and approval data. Receiving inspection cannot repair an incomplete technical release.

Require a clear applicability statement for each manufacturer document. Generic brochures and unrelated evidence should not be used to close the exact model. Record open deviations with an owner and required return.

Hold the Representative CT Accessory at Four Decisions

Representative engineering review, not a XIYA POWER customer, supplier, factory, product, metering, protection-operation, installation, calibration, test, commissioning, maintenance, failure, or field case. The system values, ratings, terminal notations, dimensions, findings, and decisions are illustrative and non-universal.

The synthetic review concerns one 12 kV, 50 Hz feeder CT accessory. The request states 600/1 A with two cores. One metering core needs class 0.5 at 10 VA. One protection core needs 5P20 at 15 VA.

The synthetic connected-burden return states 7.6 VA for metering and 13.4 VA for protection. Lead length, conductor size, terminal losses, and calculation revision are uncontrolled.

The project fault input states 25 kA for 3 seconds. The offer states 25 kA for 1 second. The project uses P1/P2 and S1/S2, while the offer uses K/L and k/l without a cross-reference.

The primary conductor and support envelope is 78 mm. The offered aperture is 80 mm, but tolerance and assembly clearance are missing. The existing mount is 140 mm by 140 mm, while the offer shows 145 mm by 145 mm. Its secondary cable exits opposite the cabinet overlay.

Synthetic illustrative review table:

Decision	Observed return	Release state	Required evidence	Hold condition
Ratio, cores, and class	Requested values appear in the return	Hold	Core-purpose map, terminal map, and applicable evidence	Schedule mapping is incomplete
Connected burden	7.6 VA and 13.4 VA estimates	Hold	Controlled leads, conductor, terminal losses, and revision	Calculation inputs are uncontrolled
Fault duty and polarity	25 kA durations differ; notation is unmapped	Hold	Duty comparison and approved cross-reference	Duration and polarity remain open
Physical fit and exit	Aperture margin, hole pattern, and cable exit conflict	Hold	Toleranced overlay and approved disposition	Fit is not demonstrated

No statement is made that the offered CT is accepted, rejected, accurate, saturated, installed, calibrated, tested, commissioned, or operating. Each hold requires separate evidence before closure.

Frequently Asked Questions

What data is needed to select a switchgear CT or sensor accessory?

Provide the SLD, bay ID, purpose per core or channel, project specification, ratings schedule, fault input, burden or interface calculation, wiring package, manufacturer data, and dimensioned cabinet overlay.

Is matching the CT ratio enough for replacement?

No. Core assignment, class, burden, fault duty, polarity, terminal mapping, aperture, mounting, orientation, and cable exit are independent decisions. Every applicable interface must close.

How should instrument-transformer burden be reviewed?

Add device loads, lead resistance, terminal losses, and test-link losses using controlled inputs. Compare that total with the rated burden and required class for the same core.

Can K/L and k/l be assumed to match P1/P2 and S1/S2?

No. Compare the project wiring drawing with the manufacturer terminal diagram. Document polarity and terminal equivalence explicitly before technical release or wiring.

Which physical dimensions must match inside the cabinet?

Check aperture, conductor and support envelope, tolerances, mounting base, hole pattern, outline, orientation, terminal access, cable exit, adjacent clearances, and removal space on matched revisions.

When should an accessory remain on hold?

Keep it on hold whenever a rating, calculation input, document revision, terminal map, duty, tolerance, clearance, or owner remains unresolved. Visual similarity never closes missing evidence.