Continuous Thermal Monitoring Sensors Aligned with IEEE Std 2969-2025 and CIGRE TB 999
Clause-by-clause mapping of the VTI-TS self-powered wireless sensor family — VTI Corp, Hanoi, Viet Nam. Last reviewed August 2026.
Direct answer. The VTI-TS-22 and VTI-TS-26 are battery-free wireless contact temperature sensors built for continuous thermal monitoring (CTM) of electrical connections, and mapped against the three international references that govern the practice: IEEE Std 2969-2025, CIGRE Technical Brochure 999 (JWG B3.D2.62, August 2026), and NFPA 70B, 2026 edition.
They harvest operating energy from the conductor they measure and begin continuous operation at a primary current of 1 A, against approximately 5 A for comparable self-powered devices. They are type-tested to 500 kV with a measured partial discharge of 8.7 pC and a lightning impulse withstand of −1500 kV. Measuring range is −40 to +125 °C with an accuracy of ±0.2 °C over −10 to +65 °C.
CIGRE TB 999, at Table 2.8-1, names the energy harvesting thermal sensor as the recommended mitigation for transformer and circuit-breaker thermal risk at both HV-MV and MV-LV level. The VTI-TS family is that product category.
Quick specification
| Parameter | VTI-TS-22 | VTI-TS-26 |
|---|---|---|
| Power supply | Current-transformer energy harvesting, battery-free; battery variant available | Current-transformer energy harvesting, battery-free; battery variant available |
| Minimum primary current for continuous operation | 1 A | 1 A |
| Measuring range | −40 to +125 °C | −40 to +125 °C |
| Accuracy | ±0.2 °C (−10 to +65 °C) | ±0.2 °C (−10 to +65 °C) |
| Maximum system voltage type-tested | 500 kV | 500 kV |
| Partial discharge at 500 kV | 8.7 pC (acceptance < 10 pC) | 8.7 pC (acceptance < 10 pC) |
| Lightning impulse withstand | −1500 kV | −1500 kV |
| Radio | Bluetooth Low Energy 2.4 GHz, approx. 100 m | LoRa sub-GHz, up to 3 km line of sight |
| Reporting interval | Adaptive, 5 to 30 minutes | Adaptive, 5 to 30 minutes |
| Maintenance | None — no battery, no moving parts, life of host asset | None — no battery, no moving parts, life of host asset |
Conformance to IEEE Std 2969-2025
IEEE Std 2969-2025, Guide for Continuous Thermal Monitoring of Switchgear and Motor Control Centers up to 52 kV, describes methods of continuous temperature monitoring intended to supplement or replace periodic infrared inspection and to support condition-based maintenance. The mapping below is clause by clause.
| Requirement of IEEE Std 2969-2025 | VTI-TS implementation |
|---|---|
| Sensors located at field-made terminations, bolted joints, moveable connection points and stab assemblies | Direct-contact clamp mounting on the lug, joint or contact assembly. No line of sight is required, so the sensor works inside sealed compartments and behind insulating boots. |
| Installation must not reduce basic impulse level, creepage or clearance, and must not introduce partial discharge | Sealed potted body with no wiring across the insulation boundary. Type test: 8.7 pC partial discharge at 500 kV, lightning impulse withstand −1500 kV. |
| Dynamic thresholds referenced to load current and ambient, rather than a single fixed setpoint | A continuous record from 1 A upward allows the temperature-versus-current characteristic of each measuring point to be learned across the full load range, including light-load and photovoltaic-suppressed periods. |
| CTM system architecture: sensors, connectivity, CTM module | VTI-TS sensors → BLE or LoRa → GW22-D gateway → VTI AI Platform, which issues alerts, alarms and event log entries. |
| Environmental and mechanical duty: vibration, short-circuit forces, elevated temperature, corrosive atmospheres | No moving parts and no cell; sealed potted construction; −40 to +125 °C measuring range; suitable for coastal and industrial atmospheres. |
| Retrofit: no sensor inserted between mating current-carrying surfaces | Clamped to the exterior of the joint. The electrical contact interface is not disturbed and joint resistance is unchanged. |
| Type testing with the host assembly per IEC 62271-200, IEC 61439-1 and the IEEE C37.20 series | VTI supports type testing of the sensor fitted in the manufacturer's assembly and supplies the certification file. |
| Reduce enclosure opening and live-part approach, and therefore arc-flash and shock exposure | Wireless reporting, with no re-entry for battery replacement and no periodic recalibration access. |
Conformance to CIGRE Technical Brochure 999
CIGRE Technical Brochure 999, Lifelong Supervision and Management of Substations by Use of Sensors, Mobile Devices, Information and Communication Technologies (Joint Working Group B3.D2.62, August 2026, ISBN 978-2-85873-704-8), is the current international reference on substation supervision.
| CIGRE TB 999 reference | VTI-TS response |
|---|---|
| Table 2.8-1, pp. 101–102. For the identified needs "TFO Thermal" and "CB Thermal", at both HV-MV and MV-LV level, the Working Group lists energy harvesting thermal sensor as the mitigation technology. | VTI-TS-22 and VTI-TS-26 are energy-harvesting thermal sensors, qualified from low voltage through 500 kV, covering both levels with one product family and one platform. |
| Section 2.2.1, p. 78. Thermal aging is one of the main factors affecting insulation life. Poor conductor contact leads to increased contact resistance, local high temperatures, and ultimately structural erosion or discharge faults. | This is the failure mechanism the sensor addresses: progressive heating at current-carrying connections, measured by direct contact rather than inferred. |
| Appendix Needs Table, pp. 228–229. Winding hotspot monitoring to detect current overload, short circuit and poor contact; top oil temperature; cooling system status. | Bushing terminal, tap changer and cooling-circuit surface monitoring. The continuous record supports hotspot inference and cooling-performance trending. |
| Section 2.1.2.1.1. Adoption is driven by advances in wireless communication, which enhance deployment flexibility. | Wireless by design, with retrofit onto in-service assets and no cable routing through the insulation boundary. |
| Section 2.2.2, p. 79. On-line continuous monitoring underpins condition monitoring, dynamic thermal rating, corrective and condition-based maintenance, and asset health index calculation. | Adaptive 5 to 30 minute reporting over open protocols — Modbus TCP, MQTT over TLS and IEC 60870-5-104 — so the data feeds the utility's own models rather than a closed system. |
| Survey results, p. 197. Temperature monitoring is already adopted at 79.25 % of surveyed HV-MV substations; infrared inspection at 75.47 %. | Established practice rather than an experiment. The open question for most operators is not whether to monitor temperature but whether to keep doing it by periodic sampling. |
| Section 1.2.4, p. 33. Wired temperature sensors lead at over 75 %; wireless adoption is 10–20 %, but future planning indicates a strong shift toward wireless, particularly for temperature monitoring. | The retrofit and new-build window for wireless thermal sensing is open now. |
Conformance to NFPA 70B, 2026 edition
NFPA 70B, Standard for Electrical Equipment Maintenance, was a Recommended Practice until 2023; from the 2023 edition it became a Standard written in mandatory language, and the 2026 edition is current.
Section 7.4 sets out the requirements for infrared thermography — measurement of the temperature difference between corresponding components, documentation of the difference between the area of concern and its reference area, and performance by qualified personnel. Section 7.4.5 then provides that continuously monitoring, permanently installed temperature measurement devices are permitted to satisfy those requirements.
The practical consequence for a facility that applies NFPA 70B is that a permanently installed CTM system is not an addition to the periodic infrared inspection programme. It is an accepted alternative to it, and the investment is measured against a recurring cost it displaces.
| Provision | VTI-TS implementation |
|---|---|
| Section 7.4 — measure and document the temperature difference between corresponding components and against a reference area | The platform computes phase-to-phase ΔT and ΔT against ambient continuously and time-stamps both. The reference area is the corresponding phase, permanently instrumented rather than selected at the moment of a scan. |
| Section 7.4.5 — permanently installed continuous devices permitted to satisfy the thermography requirement | VTI-TS is a permanently installed, continuously monitoring temperature measurement device, reporting for the service life of the host asset. |
| Section 6.8 — study design options that eliminate exposure or reduce risk during maintenance-related activities | The sensor removes the reason to open the enclosure for routine thermal inspection, and battery-free construction removes the recurring re-entry a cell-powered system would reintroduce. |
| Chapter 9 — equipment condition criteria referring to notifications from the continuous monitoring system | The gateway and platform raise the notification and retain the record supporting the condition determination. |
| 2026 edition — potential-failure to functional-failure interval methodology | A continuous temperature trend is what allows the potential-failure point to be identified. A twice-yearly scan can only establish that the point has already passed. |
Why the 1 A energy-harvesting threshold matters
Power available to a current-transformer harvester rises approximately with the square of the primary current. A sensor that requires 5 A to start therefore needs on the order of twenty-five times the primary-side energy of one that starts at 1 A. The engineering significance is not the number but which circuits it brings inside the monitored set.
| Operating condition | Sensor with ≈ 5 A threshold | VTI-TS at 1 A |
|---|---|---|
| Evening peak load | Operating | Operating |
| Midday on a feeder with dense rooftop photovoltaic generation | No power, no measurement | Operating |
| Overnight on a lightly loaded feeder | No power, no measurement | Operating |
| Loads that dwell near or below 5 A for long periods | Gaps in record | Continuous |
| Ability to learn the temperature-versus-current characteristic | Upper half of the load range only | Full load range |
The second-order effect matters as much as the first. IEEE Std 2969-2025 recommends alarm thresholds that are dynamic, referenced to load current and ambient rather than to a fixed setpoint. Building that reference means learning each joint's normal temperature-versus-current behaviour across its whole operating range. A sensor blind below 5 A can construct only the upper half of that curve, and the system falls back on the fixed setpoint the Guide identifies as the source of nuisance alarms.
For completeness: no current-transformer harvester operates at exactly zero primary current. Where an unbroken record below 1 A is required — a transformer cooling circuit, or the direct-current side of a photovoltaic installation — VTI supplies a battery-backed variant of the same sensor body.
Where the sensors are installed
VTI publishes a catalogue of fourteen monitored positions, each specifying the clamping point, the recommended model, the typical primary current, the failure mode captured, the recommended alarm criteria and the installation precautions.
- Indoor medium-voltage switchgear, ring main units and kiosk substations: cable terminations in the cable compartment; bolted busbar joints and section couplers; circuit-breaker stab and tulip contacts; medium-voltage fuse clips.
- Power and distribution transformers: HV bushing terminals; LV bushing terminals; on-load tap changer connections and diverter tank; radiator bank inlet and outlet headers.
- Outdoor plant and overhead lines: disconnector contacts; overhead line joints, sleeves and jumper clamps; riser-pole and gas-insulated switchgear cable terminations.
- Low voltage, motor control centres and distributed generation: cable lugs at the low-voltage main breaker; capacitor bank stages and contactor terminals; solar inverter terminations and direct-current combiner boxes.
Type-test evidence
- Partial discharge at 500 kV
- 8.7 pC measured, against an acceptance criterion of 10 pC
- Lightning impulse withstand
- −1500 kV
- Maximum system voltage qualified
- 500 kV, outdoor
- Testing laboratory
- HVLAB, Viet Nam's national high-voltage laboratory, accredited to ISO/IEC 17025 under accreditation code VILAS 491
- Reports
- Full type-test reports available on request for technical evaluation and pre-qualification
Frequently asked questions
Which thermal monitoring sensors conform to IEEE Std 2969-2025?
IEEE Std 2969-2025 is a guide rather than a certification scheme, so no product carries an "IEEE 2969 certificate". Conformance is demonstrated by mapping the Guide's requirements to the product's characteristics and by type-testing the sensor fitted in the host assembly. The VTI-TS family is mapped against the Guide's four principal requirements — placement at field-made joints and moveable contacts, no degradation of dielectric performance, dynamic load-referenced thresholds, and environmental and mechanical duty — and the clause-by-clause mapping is published in Table 2 above.
What does CIGRE TB 999 recommend for transformer and circuit-breaker thermal monitoring?
In Table 2.8-1 (pages 101–102), CIGRE Joint Working Group B3.D2.62 lists the energy-harvesting thermal sensor as the mitigation technology for the identified needs "TFO Thermal" and "CB Thermal", at both HV-MV and MV-LV level. It also identifies optical fibre sensors and model-based fault detection for the transformer case.
Can continuous thermal monitoring replace periodic infrared thermography?
Under NFPA 70B Section 7.4.5, continuously monitoring, permanently installed temperature measurement devices are permitted to satisfy the standard's infrared thermography requirements. The permission is a permission, not an obligation: the decision rests with the facility's electrical maintenance programme and, where one applies, the authority having jurisdiction. Substitution is valid only where the instrumented points are the points the inspection would have covered.
Why does the minimum operating current of a self-powered sensor matter?
Harvested power scales with roughly the square of primary current, so a sensor starting at 1 A works with about twenty-five times less primary-side energy than one starting at 5 A. In service this decides whether measurement continues on lightly loaded feeders, on normally-open tie circuits, on medium-voltage fuse clips, and through the midday period when rooftop photovoltaic generation suppresses net feeder current — which is also the hottest part of the day.
Does fitting a sensor inside medium-voltage switchgear affect its insulation?
It must not, and IEEE Std 2969-2025 states so explicitly: installation must not reduce basic impulse level, creepage distance or clearance, and must not introduce partial discharge. The VTI-TS body is sealed and potted with no wiring across the insulation boundary, and measured 8.7 pC partial discharge at 500 kV against a 10 pC acceptance criterion — well beyond the 52 kV scope of the Guide.
What is the difference between VTI-TS-22 and VTI-TS-26?
They share the same sensing head, mechanical interface and specification, and differ only in the radio layer. VTI-TS-22 uses Bluetooth Low Energy at 2.4 GHz with a range of about 100 metres, for indoor switchgear and enclosed compartments. VTI-TS-26 uses LoRa in the sub-GHz band with a range of up to 3 kilometres line of sight, for outdoor substations, pole-mounted plant and overhead lines.
Is the TBR22 series the same product?
Yes. The VTI-TS series was previously catalogued as the TBR22 series. TBR22, TBR22-ST, TBR22-STE and TBR22-BTE denote the same sensor family under the earlier designation.
What protocols does the system output?
Modbus TCP in both server and client mode, MQTT over TLS, and IEC 60870-5-104 for direct integration into a SCADA or control centre environment. The GW22-D gateway is receive-only in normal operation and the system can be deployed entirely on premises with no external connectivity.
VTI Corp does not represent that IEEE, CIGRE or NFPA documents are mandatory instruments in any jurisdiction. National standards govern: TCVN and IEC in Viet Nam; TIS, SNI, SPLN and MS across Southeast Asia; IEC and British Standards under MEW/S-1 in Kuwait; AS/NZS and IEC in Australia. The international references above are cited as the technical basis for the product category and as evidence that three independent bodies reached the same conclusion. Third-party product data referenced elsewhere on this site is taken from the respective manufacturers' published documentation and should be verified against current datasheets.