A stand-alone merging unit (SAMU) digitizes standardized analogue instrument-transformer signals; a process interface unit (PIU) brings status and commands between switchgear and IEC 61850 applications. They may share hardware, but measurement accuracy, time alignment and SV behavior are different assurance problems from binary input/output duty, trip-circuit integrity and GOOSE fail-safe behavior.
This guide applies IEC 61869-13, IEC 61869-9 and IEC 61850 to selection, architecture, wiring, redundancy, commissioning and lifecycle support for MV retrofit and new switchgear.
1. Definitions and scope
- SAMU: product covered by IEC 61869-13 for AC applications with standardized analogue inputs from conventional instrument transformers and digital output according to IEC 61869-9.
- Merging unit (broader): electronics that acquires/merges current/voltage sensor channels and publishes SV; may be integrated with LPIT or switchgear.
- PIU/process I/O: project/vendor term for intelligent binary input/output near the process, publishing status and subscribing to commands, usually via GOOSE.
- Combined unit: one chassis can contain MU, breaker I/O and switchgear-control functions; analyze common-mode impact.
IEC 61869-13 does not by itself standardize every PIU function or all LPIT-integrated MU arrangements. State applicable product, switchgear, relay, communication and cybersecurity standards separately.
2. Where a SAMU adds value
- Retrofit conventional 1 A/5 A CT and standard VT secondary circuits into an SV process bus.
- Share one measured source with several digital subscribers without adding relay burdens.
- Reduce long analogue copper runs between switchgear and centralized/remote protection.
- Create a staged migration: retain CT/VT primary assets while replacing protection communication.
- Provide a defined metrological boundary and calibration point.
- Enable centralized or duplicated protection architectures—subject to common-source analysis.
3. Analogue input engineering
- Match rated current/voltage inputs to CT/VT secondaries; wrong 1 A/5 A or phase-neutral/phase-phase scaling is dangerous.
- Calculate CT burden including leads, test blocks and SAMU input; digitization does not remove CT saturation.
- Maintain CT shorting/safe-open-circuit provisions and one-point secondary earthing.
- Coordinate VT fuses/MCBs, burden, grounding and isolation.
- Prove polarity, phase order and neutral/residual source.
- Specify protection-range accuracy, phase displacement, frequency response and saturation behavior.
- Keep analogue leads short, segregated and EMC-controlled.
4. SAMU measurement specification
| Requirement | Engineering content |
|---|---|
| Inputs | Number/type/rating, overload, insulation, burden and grounding |
| Accuracy | Amplitude/phase error over protection range, frequency and temperature |
| Dynamic behavior | Latency, step/transient, clipping, anti-alias filtering and channel skew |
| Digital output | IEC 61869-9/IEC 61850 profile, rate, dataset, scaling and quality |
| Time | PTP profile, accuracy, holdover and loss/recovery response |
| Network | Ports, PRP/HSR, VLAN/priority, stream count and diagnostics |
| Environment | Temperature, humidity, vibration, EMC, enclosure and auxiliary supply |
5. SV engineering
- Define svID, dataset member order, APPID, multicast MAC, VLAN, PCP and ConfRev in SCL.
- State sampling profile/rate, nominal frequency and ASDUs per frame supported by all subscribers.
- Trace Ia/Ib/Ic/In and Va/Vb/Vc/Vn from terminals to dataset indexes.
- Define scaling/primary rating ownership and calibration coefficients.
- Supervise smpCnt, smpSynch/time quality, data quality and stream timeout.
- Calculate continuous stream bandwidth plus redundancy forwarding.
- Test wrong stream/adjacent bay and configuration mismatch.
6. PIU binary inputs
- Specify wetting voltage/current and thresholds for dry contacts.
- Map breaker 52a/52b, disconnector, earthing switch, truck position, spring, pressure and local/remote.
- Define debounce and timestamp origin while preserving mechanism travel sequence.
- Use complementary contacts and detect impossible/intermediate states.
- Define input quality on module failure, DC loss or contact disagreement.
- Publish standardized XCBR/XSWI/other data semantics where possible.
- Include sufficient independent I/O channels and terminal isolation for claimed redundancy.
7. PIU outputs and trip circuit
- Output contact/electronic making and breaking duty at actual DC voltage/current.
- Pickup/release time, pulse/latched mode and behavior on reboot.
- Trip Coil 1/2 independence and separate supplies/modules where required.
- Trip-circuit supervision across breaker open/closed states.
- Fail-safe state for GOOSE timeout, invalid quality and configuration mismatch.
- Physical output isolation and test mode.
- Complete command-to-current-interruption timing including coil and mechanism.
8. Integrated versus separate MU and PIU
| Choice | Benefit | Risk |
|---|---|---|
| Combined SAMU/PIU | Fewer devices, supplies, ports and engineering interfaces | One chassis/firmware/power failure removes measurement and trip I/O |
| Separate units | Failure containment and independent maintenance | More hardware, space, cabling and network load |
| Duplicated combined units | Potential complete bay redundancy | Shared CT/sensor/terminals/configuration may remain |
| Hybrid hardwired backup | Independent critical trip path | More test/maintenance and possible logic divergence |
9. Time and redundancy
- Derive required differential timing error from the protection application.
- Use IEC/IEEE 61850-9-3 PTP profile where applicable and qualify every clock/network element.
- Define holdover, quality and subscriber response to loss/drift/step.
- PRP/HSR protects a network path, not the SAMU analogue input or sampling clock.
- Map sensor, input, unit, power, time, LAN and subscriber common modes.
- Monitor both network paths and clock state.
- Test failure and restoration without transient false differential current or output.
10. Installation and environmental issues
- Locate close enough to reduce analogue/I/O copper but within qualified temperature, humidity, vibration and arc-zone conditions.
- Provide accessible terminals, CT shorting/test links and safe isolation.
- Separate redundant DC/fiber and analogue routes.
- Control EMC, screen termination, grounding and fiber bend/cleanliness.
- Account for auxiliary-supply inrush, ride-through and selective protection.
- Provide local diagnostics without unsafe door opening.
- Use replaceable connectors/modules with clear bay/channel labels.
11. Failure modes
| Failure | Effect | Mitigation |
|---|---|---|
| Analogue input open/short/wrong ratio | Wrong SV values | Wiring safety, plausibility, primary injection |
| ADC/channel failure | One/all measurements corrupted | Quality/diagnostics, backup source |
| Time loss/error | Misaligned samples | Holdover/quality/fallback test |
| SV network loss | Subscriber data unavailable | Redundancy and function response |
| PIU input failure | Unknown position/interlock | Complementary contacts, safe inhibit |
| PIU output failure | Failure to trip/close | Supervision, duplicate path/coil |
| Common DC/firmware/SCL | Multiple functions lost/wrong | Independence, review and regression |
12. FAT/SAT
- Review type tests/certificates against IEC 61869-13/-9 and project requirements.
- Calibrate/verify every analogue channel over application range and temperature evidence.
- Prove ratio, polarity, phase, channel order, dynamic response and latency.
- Test SV profile, quality, stream loss, rate/mismatch and time failures.
- Operate every PIU input/output and contact discrepancy.
- Measure GOOSE-to-output and total breaker time.
- Fail power, LAN A/B/ring, time, publisher/subscriber and combined-unit functions.
- Test simulation/output isolation and restoration.
- At SAT, primary-inject and operate actual switchgear/trip circuits.
- Archive calibration, SCD/CID/settings/firmware and replacement package.
13. Lifecycle checklist
- Guaranteed firmware/support and vulnerability notification.
- Calibration interval/method and coefficient backup.
- Compatible spare hardware, connectors and tool licences.
- Proven replacement loading and identity/address control.
- Trending of self-diagnostics, clock, packet loss and analogue errors.
- Impact-based regression after any firmware/SCL/network change.
- As-built reconciliation and staff training for CT safety plus digital diagnostics.
14. Procurement data schedule
- Exact IEC 61869-13/-9/-1 and IEC 61850 editions, profiles and type-test evidence.
- Input rating/burden/overload, measuring/protection accuracy and phase error tables.
- Frequency/transient response, anti-alias filtering, latency/jitter and channel skew.
- SV rates, frames/ASDU, stream/resource limits, SCL ICD and conformance certificate.
- PTP profile, holdover/error and quality behavior during loss/recovery.
- Binary input thresholds/debounce/timestamps and output contact DC duty/timing.
- Auxiliary voltage range, interruption/ride-through, inrush, environmental/EMC and enclosure.
- PRP/HSR role, diagnostics, supported switches/RedBoxes and cybersecurity capabilities.
- Firmware/tool support, calibration, field replacement, warranty and obsolescence plan.
15. Replacement and maintenance scenario
- Identify failed unit and all protection/control functions it serves.
- Apply CT/VT and trip-circuit safety isolation; declare affected protection unavailable.
- Verify replacement hardware/firmware/options and calibration status.
- Load the target-specific approved file—never clone a neighboring bay.
- Set/verify IED name, stream addresses, PTP/redundancy ports and calibration coefficients.
- Secondary/primary inject every measurement channel and operate every I/O.
- Test SV/GOOSE quality, time, timeout and each redundant path.
- Remove all test/output blocks; compare as-loaded hash and update the asset register.
16. Acceptance evidence
- Type/routine/calibration reports tied to device serial and firmware.
- Analogue error/phase/latency results and channel map.
- SCL/CID/settings/switch/time configuration with hashes.
- GOOSE/SV captures and function timing.
- Power, clock, network and device-failure results.
- Trip-circuit and actual breaker SAT evidence.
- As-left modes, alarms, redundancy and maintenance baseline.
References and further reading
- IEC 61869-13:2021 — Stand-alone merging units
- IEC 61869-9:2016 — Digital interface for instrument transformers
- IEC 61850-9-2 consolidated edition — Sampled values mapping
- IEC 61850-8-1 consolidated edition — GOOSE/MMS
- IEC 61850-3:2013 — General requirements
- IEC TS 60255-216-1:2025 — Digital protection tests
- IEC 62351-6:2020 — IEC 61850 security
Engineering note: Do not specify “merging unit” as a generic black box. State the input technology, metrological class/range, SV profile, time, network, failure response and whether process I/O is in the same failure domain.