What a Survey Actually Covers

A plant electrical survey is a two-pass job. In the scan pass, an infrared camera is used to find temperature differences across every connection in a defined asset list while the equipment is energised and carrying load. In the verification pass, each anomaly is re-measured, compared with its phase siblings and its own history, and documented as a finding: temperature, ΔT, the load current at the time of the scan, the emissivity setting, and a recommended action with a priority.

The camera finds the anomaly. The asset list, the load data and the report format decide whether anyone can act on it — and that is why the workflow below matters more than the resolution on the datasheet.

Step 1 — Scope It Before the Camera Comes Out

The most common reason a thermal survey produces images nobody uses is that the scope was never agreed. Before an inspector opens a panel, six things should be on paper:

• Asset list and single-line diagram — the panels, bays, joints and transformers that are in scope, with IDs that match the site’s own labelling.
• Load data — typical and current loading of each circuit, so the survey is scheduled when the equipment is actually carrying current.
• History — previous reports, repairs and repeat offenders. A joint that was re-torqued last year should be treated as a known risk, not a new discovery.
• Access and permits — who opens panels, what is energised, what is behind covers, and the arc-flash and PPE rules that apply at each location.
• Severity criteria — the ΔT bands and response times the client wants applied, agreed before the first image is taken.
• Report format — what a “finding” must contain and who signs it off.

It also helps to agree, in advance, what the survey does not cover, so nobody assumes that a clean thermal pass is a full electrical test.

Covered by a standard thermal survey	Needs a separate scope
Energised equipment under load: connections, joints, cable terminations, busbars, fuses, breakers, transformer tanks and radiators	Insulation resistance, dielectric loss or contact-resistance tests, which require the equipment out of service
Surface temperature and ΔT documentation with the load recorded at the time of the scan	Load-flow, power-quality or protection setting studies
Partial discharge screening with airborne acoustic and TEV instruments when it is written into the scope	Diagnosis and trending of PD, which needs repeated measurements and, for some assets, HFCT or UHF methods
Priority ranking of findings with recommended actions	Repair work, torque verification and post-repair re-testing

Step 2 — Match the Camera Class to the Asset

Three numbers decide which camera goes on the survey: the size of the smallest target, the distance you must work from, and what the client needs out of the report (a picture, or a defensible measurement). A 15 mK sensitivity matters in a cool indoor panel; a laser rangefinder and a telephoto option matter in an outdoor substation; acoustic and TEV capability matter wherever switchgear is in scope.

Asset class	Typical target	Working distance	Camera class	Unitech Tools models
LV panels, MCC buckets, control cabinets	Terminals and screws, 6–12 mm	0.5–1.5 m	Tool-like 256×192 / 384×288, ≤15 mK, light body	EasIR Series (E3S / E4S)
MV switchgear, busbar joints, cable terminations	Lugs and joints, 20–30 mm	1–6 m	384×288 / 640×512 with laser ranging and interchangeable optics	Hammer II / Hammer II Pro
Outdoor substations, transformer bushings, overhead connections	Small targets at 8–30 m	8–30 m	High-resolution HD body with telephoto or TwinView DFOV optics	PT II Series (up to 1280×1024)
Switchgear that needs thermal + PD screening in one pass	Panel surface (AA) and enclosure (TEV)	0.5–3 m	Thermal imaging plus airborne acoustic (40±1 kHz) and TEV (3–100 MHz)	SC Series SC300 / SC600
PD verification on cables, GIS and transformers	Contact, proximity and directional targets	On contact / close	Multi-method detector: AA, AE, TEV, UHF, HFCT	HPD500 / HPD200

Reference points from the current lineup: the EasIR Series combines 384×288 @ 12 µm or 256×192 @ 12 µm detectors with ≤15 mK thermal sensitivity in a 0.6 kg body — useful when an inspector has to climb, crawl and open 60 panels in a day. The Hammer II Series moves to 640×512 (Hammer II Pro) or 384×288 (Hammer II) with a 5 MP visible camera and a 40 m laser rangefinder, so every image can carry its distance. The PT II Series is the long-range option at up to 1280×1024 with <20 mK sensitivity and 1-lens TwinView DFOV optics, and the SC Series SC300 / SC600 pairs 384×288 or 640×512 thermal imaging with airborne acoustic and TEV partial discharge screening in the same instrument.

Step 3 — The Five Passes in the Field

1. Access and safety pass. Permits checked, panels opened by authorised staff only, and the inspector working outside the boundaries the site defines. The route is planned so that the equipment with the highest load and the worst history is scanned first, while attention is still fresh.
2. Visual pass. Before switching the camera on: look for discoloured busbars, corrosion, dust trails, tracking marks, water ingress, loose covers and anything that has clearly been hot. Visual evidence strengthens every thermal finding and sometimes explains it.
3. Scan pass. Panel by panel against the asset list. Keep the lens perpendicular to the target, include the full width of the busbar, and capture all three phases plus both ends of every connection — a high reading on one phase only means something different from a high reading on all three.
4. Measurement pass. Measure only where the target covers at least 3×3 detector pixels. Adjust the emissivity setting or apply matte tape on shiny aluminium, record the load current at the time of the scan, and check by changing your angle whether a hot area follows the camera — if it does, you are looking at a reflection, not a hot spot.
5. Verification pass. Re-measure anything suspicious, compare it with its phase siblings and with the previous report, and where the thermal result is ambiguous — small, cool or intermittent anomalies — escalate to partial discharge screening (Step 5).

Distance example. Take a 25 mm cable lug at 5 m, viewed with a 384×288 camera and a standard 25° lens. The IFOV works out at roughly 1.3 mrad, so one pixel covers about 6.5 mm at that distance and the lug spans around 3.8 pixels — workable. The same lug at 10 m spans under two pixels, which is not enough for a defensible reading: at that distance you need a narrower lens or a higher-resolution detector. The lens and IFOV selection guide works through the calculation for any target and distance.

Step 4 — Turn an Image into a Finding

The number that goes in the report is not the absolute temperature on the display; it is the temperature difference (ΔT) between the target and a reference measured under the same load. The reference is either an identical phase on the same circuit or the same component recorded in a previous survey. Without that reference, a reading of 68 °C means very little: on a fully loaded busbar it may be normal, and on an unloaded circuit it may not.

ΔT above reference	Example response	Example priority
Under 10 °C	Monitor; re-scan at the next scheduled survey	Low
10–20 °C	Investigate at the next planned maintenance window; check torque and the condition of the joint	Medium
20–40 °C	Schedule corrective maintenance and screen the same circuit for partial discharge	High
Over 40 °C	Treat as an immediate risk: verify with a second method, report the same day, plan a shutdown	Critical

These bands are an example of how many inspection programmes rank findings. The criteria that govern your site come from your own maintenance standard, or from the standard the client specifies. Confirm the bands before the survey and apply them consistently — a report that uses a different scale from the client’s is a report that gets filed and forgotten.

Load correction. Heating at a resistive connection rises steeply with current, so a joint reading a modest ΔT at 40 % load can be a serious problem at rated current. Always record the load at the time of the scan, and never compare a finding taken at light load with one taken at full load.

Emissivity and reflections. Polished aluminium busbars, tinned terminals and shiny copper have low emissivity, so the camera reads a mix of the target and the surroundings. Fix it with a matte tape target or the correct emissivity setting, and read more in emissivity in electrical thermal inspections. Sunlight, hot neighbouring phases and warm enclosure surfaces also create reflections that can be mistaken for a hot spot; moving the camera a step to the side settles the question quickly.

Step 5 — When Thermal Alone Cannot Decide

Thermal imaging finds heating. Early partial discharge produces very little heating, and some discharge mechanisms produce no usable thermal signature at all until they are advanced. A clean thermal image is therefore not a clean bill of health for switchgear — it is one input. This is where acoustic and TEV detection come into the same survey.

Method	Band	Typical use in a survey	Models
Airborne acoustic (AA)	40 ± 1 kHz	Surface discharge, tracking and corona on energised equipment; localised from a safe distance	SC300 / SC600, HPD500, HPD200 (dynamic range −8 to 68 dBµV)
Transient earth voltage (TEV)	3–100 MHz	Internal discharge inside MV switchgear, measured on the metal enclosure	SC300 / SC600 (0–60 dBmV), HPD500
Acoustic emission (AE)	20–200 kHz	Contact measurements on tank walls and enclosures	HPD500
HFCT	3–30 MHz, ≤3 pC sensitivity	Cable terminations, joints and grounding paths where discharge pulses travel to earth	HPD500
UHF	300–1500 MHz	GIS compartments and shielded switchgear where acoustic signals are blocked	HPD500

A TEV reading that is high on one panel and normal on identical neighbours is a screening flag, not a diagnosis. The next step is a repeat measurement under the same load, plus a diagnostic method where the asset type calls for one — HFCT for cable paths, UHF for GIS. The detectors above record PRPD and PRPS patterns, continuous readings, flight and pulse waveforms, which is what makes trending possible; the guide to dBmV and pC readings explains how to read them, and thermal vs acoustic PD detection compares what each method can and cannot see.

Field rule: if one panel in a switchgear lineup reads clearly higher on TEV than its neighbours under the same load, book a repeat measurement before the next scheduled service — not after the next failure.

Step 6 — What the Report Must Contain

A finding without the load current and the calibration date is hard to defend in a maintenance meeting, and impossible to trend. Every line item in the report should carry:

• Asset ID, location and a paired image: infrared plus visible for the same spot.
• Measured temperature, reference temperature and the resulting ΔT.
• Load current at the time of the scan, and the ambient temperature.
• Emissivity setting, and whether a tape target was used.
• Field conditions for outdoor equipment: sun, wind, rain, time of day.
• Severity band, recommended action, priority and a suggested re-scan interval.
• Camera model plus its calibration date, and the inspector’s signature.

If your QA programme or your client requires a calibration certificate with the report, ask for the documentation that applies to the specific model being quoted — documentation and certificate scope are confirmed per model and quotation, not for a whole product family.

What to Send When You Specify a Camera or a Survey Kit

Vendors cannot quote sensibly without the application. Send these eight items and the quotation will be right the first time:

1. Asset classes to cover (LV panels, MV switchgear, transformers, cables, GIS).
2. Smallest target size to be measured, in millimetres.
3. Maximum working distance from the camera to the target.
4. Whether partial discharge screening is part of the same programme, and which methods are in scope (AA, TEV, HFCT, UHF).
5. The severity standard or ΔT bands the client applies.
6. Number of units, spare batteries and any accessory requirements.
7. Country of delivery plus any local standards or voltage requirements.
8. Documentation required: calibration report, test report, packaging and labelling conditions.

Neutral or branded packaging availability depends on the model and the order quantity, so state the preference up front. Our procurement support page lists what we handle for importers and distributors, and the product index is the fastest way to check which categories exist before you specify.

Common Mistakes and Honest Limits

• Scanning without an asset list. You will photograph the same three panels every visit and miss the ones that matter.
• Comparing findings across different load levels. The hottest-looking image in the folder is sometimes just the circuit that was busiest.
• Trusting the resolution number alone. Resolution without the right lens and working distance does not give you a measurable target.
• Treating a cool image as a clean bill of health. Partial discharge screening is a different measurement with a different instrument.
• Reporting images instead of numbers. A thermal photo without a ΔT, a load figure and a priority is decoration.

A survey is only as useful as its weakest field note. A 640×512 image of a hot joint with no load current written beside it is a picture, not a finding.

Honest limits: temperature measurement accuracy depends on emissivity, distance and target size, and our handheld models are quoted at ±2 °C or ±2 % of reading, whichever is greater; IP54 protection applies to the model variants named in the relevant report, not to a whole family; acoustic and TEV screening flags suspicious assets but does not by itself confirm a defect type; and acoustic imaging cannot detect every gas under every condition. A survey programme that states these limits up front keeps its credibility when the next report is questioned.

Frequently Asked Questions

What thermal camera resolution do I need for electrical inspection?

It depends on the smallest target you must measure and the distance you must work from. Aim for the target to cover at least 3×3 detector pixels. At 5 m, a 384×288 camera with a standard 25° lens gives a pixel footprint of roughly 6.5 mm, workable for a 25 mm cable lug; the same lug at 10 m needs a narrower lens or a 640×512 or higher-resolution detector.

Can an infrared camera detect partial discharge?

Often it cannot do it alone. Early partial discharge usually produces very little heating, so a clean thermal image is not proof that switchgear is discharge-free. Thermal screening should be paired with airborne acoustic and TEV detection, and with HFCT or UHF methods where cables or GIS are in scope.

Does an electrical thermal survey require shutting the plant down?

No. Thermal screening is normally done online, while the equipment is energised and carrying load, because the load current is what produces the heating you are measuring. The load at the time of the scan must be recorded, since a finding taken at light load cannot be compared with one taken at rated current.

What information should I send to get an inspection camera quotation?

Send the asset classes you inspect, the smallest target size, the maximum working distance, whether partial discharge screening is part of the same programme, the severity standard your client uses, the number of units and the country of delivery. Documentation and neutral packaging are confirmed per model and quotation.

Should one survey kit include more than one camera?

For indoor panels and MCC rooms, one tool-like 256×192 or 384×288 camera with 15 mK sensitivity covers most work. Programmes that also inspect outdoor substations at 8–30 m benefit from a higher-resolution or interchangeable-lens body, and programmes that include partial discharge screening need acoustic and TEV capable instruments as well.