The Short Answer

The HPD200 is a two-channel partial discharge detector built for metal-clad MV switchgear: TEV (3–100 MHz) measured on the enclosure, plus airborne acoustic at 40 ± 1 kHz. The HPD500 is a five-channel detector that adds acoustic emission (20–200 kHz), HFCT (3–30 MHz, ≤3 pC sensitivity) and UHF (300–1500 MHz) — the channels you need the moment the asset list includes cable terminations, transformers or GIS. The SC Series is a different shape of answer again: it places 384×288 or 640×512 thermal imaging and airborne acoustic plus TEV discharge screening inside one instrument, for teams that already inspect panels thermally and want a discharge check in the same pass.

The decision comes down to two questions: which insulation systems are on your asset list, and does your programme screen for activity or diagnose a defect. Both are answered with channel data below, followed by a selection checklist and a quotation checklist you can send straight to a supplier.

Selection rule: write the asset list first and the sensor list second. A detector is only the right one if one of its channels physically reaches every asset class you are responsible for.

What Each Detection Method Can Reach

Partial discharge shows itself in several different physical forms, and no single sensor sees all of them. That is the whole reason a multi-method instrument exists, and it is also the reason a switchgear-only detector should not be asked to sign off a cable or GIS programme.

Method	Band	What it detects	Main limitation	Unitech Tools models
Airborne acoustic (AA)	40 ± 1 kHz dynamic range −8 to 68 dBµV	Surface discharge, tracking, corona and arcing with a clear acoustic path to the sensor	Enclosure and background noise can block or mask the signal; it does not see discharge buried inside solid insulation	HPD200, HPD500, SC300 / SC600
Acoustic emission (AE)	20–200 kHz	Contact measurements through tank walls and enclosures, where airborne methods lose the signal	Requires contact with a suitable surface; a point measurement, not a wide-area scan	HPD500
Transient earth voltage (TEV)	3–100 MHz 0–60 dBmV on the SC Series	Internal discharge inside metal-clad switchgear, coupled to the enclosure and read on its surface	Needs a suitable accessible metal surface; it locates activity in the lineup, not the compartment	HPD200, HPD500, SC300 / SC600
HFCT	3–30 MHz sensitivity ≤3 pC	Discharge pulses travelling to earth — cable terminations, joints and grounding paths	Only useful where a sensor can be clamped around an earth path; interference control matters	HPD500
UHF	300–1500 MHz	GIS compartments and shielded switchgear, where acoustic signals are blocked by the enclosure	Needs a compatible sensor or window; not intended for open-air equipment	HPD500

A partial discharge detection kit assembled around switchgear — a TEV probe and an airborne acoustic sensor — covers the assets that account for most indoor MV panels. The same handheld device cannot hear a cable joint buried in a trench or a GIS compartment behind a welded enclosure, and it is not the right tool for those assets. The partial discharge detector category page summarises the same method-to-asset logic, and the article on PD detection methods for switchgear goes deeper on the switchgear case.

HPD200 vs HPD500: Channel by Channel

These two instruments share the same detection software, the same recording spectra and the same battery envelope, so the buying decision is entirely about channel coverage.

Specification	HPD200	HPD500
Detection methods	TEV, AA (airborne acoustic)	AA, AE, TEV, UHF, HFCT
Detection spectra	PRPS, PRPD, continuous, flight, pulse waveform	PRPS, PRPD, continuous, flight, pulse waveform
AA frequency band	40 ± 1 kHz	40 ± 1 kHz
AA dynamic range	−8 to 68 dBµV	−8 to 68 dBµV
AE frequency band	—	20–200 kHz
TEV frequency band	3–100 MHz	3–100 MHz
HFCT frequency band	—	3–30 MHz
HFCT sensitivity	—	≤3 pC
UHF frequency band	—	300 MHz–1500 MHz
Data transmission	WiFi	WiFi
Control	Mobile phone / tablet	Mobile phone / tablet
Battery life	>6 hours	>6 hours
Operating temperature	−20 to 55 °C	−20 to 55 °C
Weight	—	450 g
Dimensions	—	158 × 102 × 36.8 mm

What the table means in the field. Both instruments store phase-resolved data, so both let you build a baseline and trend a panel over successive visits. The difference is where they can be pointed. If every asset on your list is metal-clad MV switchgear inside a building, the HPD200 covers the job with the two methods that work on that equipment. If the list also contains cable terminations, power transformers or GIS, the extra three channels on the HPD500 are not an upgrade in convenience — they are the only way to measure those assets at all.

Both units record the same five spectra, including PRPD and PRPS, which is what makes a screening programme defensible: the second visit can be compared with the first in the same format. Pattern interpretation is covered in our guide to PRPD and PRPS pattern analysis, and the practical meaning of the numbers is explained in dBmV and pC readings.

Field rule: buy the channels your asset list needs, then train on the pattern analysis. A five-channel detector used as a one-number screening tool delivers less than a two-channel detector used with a written baseline and a follow-up interval.

Where the SC Series Fits

The SC Series answers a different question: can thermal inspection and discharge screening happen in one pass, with one instrument and one report? It carries a thermal detector plus the two switchgear-screening channels.

Specification	SC300	SC600
IR resolution	384 × 288	640 × 512
Thermal sensitivity (NETD)	<40 mK @ 30 °C
Spectral range	8–14 µm
Temperature range	−20 to +150 °C / 0 to +410 °C / up to +2000 °C (optional)
Temperature accuracy	±2 °C or ±2 % of reading
AA detection band	40 ± 1 kHz, dynamic range −8 to 68 dBµV
TEV detection band	3–100 MHz, dynamic range 0–60 dBmV
Display	4.3 inch / 5 inch sunlight-visible
Visible camera	5 MP with LED light
Storage	32 GB SD card
Battery life	>3 hours at 25 °C
Protection / weight	IP54 / 1.13 kg with battery

Choose the SC Series when a single technician walks a switchgear room and must leave with both a thermal record and a discharge screening result. Choose a dedicated multi-method detector when the programme also diagnoses cables, transformers or GIS, or when the discharge work needs HFCT and UHF evidence. Many programmes end up with both, and in that case the SC Series handles the routine indoor pass while the HPD500 is reserved for the assets that need diagnostic depth. The SC300 vs SC600 comparison covers the two thermal resolutions in detail, and thermal vs acoustic PD detection explains what each technology can and cannot see.

Screening vs Diagnosis

• Screening answers one question: is there discharge activity here, and does it stand out from its neighbours? A TEV reading on each panel door, or an airborne acoustic listen at the same points, does this in minutes. The output is a ranking, not a verdict.
• Diagnosis answers what type of defect it is, where it sits inside the asset, how it behaves with phase and voltage, and how it changes over time. That requires the right sensor for the asset, phase-resolved patterns, and repeated measurements under recorded load.

A partial discharge detection system built for diagnosis is a combination of instrument, method coverage and programme discipline — not simply a more expensive box. The HPD200 and the SC Series are perfectly good screening instruments. The HPD500 is the one that keeps the programme defensible when a repair decision has to be justified: HFCT for cable earth paths, UHF for GIS, AE for contact measurements on tank walls, and PRPD patterns that can be shown to a maintenance committee. Our article on TEV vs acoustic camera for partial discharge looks at the same split from the locating-instrument side, and PD vs corona vs arcing covers classification of the patterns themselves.

Asset-by-Asset Selection

Asset class	Practical method	Recommended instrument
Indoor metal-clad MV switchgear	TEV on the enclosure plus airborne acoustic at panel joints and vents	HPD200 for screening; SC300 / SC600 when thermal imaging is in the same pass
Cable terminations, joints and earth paths	HFCT clamped on the earth path, with airborne acoustic as a cross-check	HPD500
Power transformers	AE contact measurement on the tank wall; oil analysis remains a separate test	HPD500
GIS and shielded HV switchgear	UHF through a compatible sensor or window; AE as a second opinion	HPD500
Outdoor substations and overhead connections	Airborne acoustic and corona location from a safe distance, plus thermal imaging for hot spots	GSW Series or SW136 acoustic camera with a thermal camera

Two honest limits belong on this page. First, one PD method does not cover every asset — a clean TEV survey of a switchgear room says nothing about the cable trench outside it. Second, discharge screening flags suspicious assets but does not by itself confirm a defect type; confirmation needs patterns, a second method and repeated measurement. For locating an already-confirmed source in open equipment, an acoustic imaging camera such as the GSW Series with its 136-microphone array, 2–100 kHz bandwidth and 0.3–130 m detection distance can show the position directly, which shortens the repair window.

Six-Point Selection Checklist

1. Asset list. Write down every asset class the instrument must reach. This alone eliminates most wrong choices.
2. Methods in scope. Mark TEV, AA, AE, HFCT and UHF against that list. Any asset with no matching method is an asset you cannot inspect with that purchase.
3. Screening or diagnosis. Decide which the programme has to deliver in the first year, and be honest about the second year.
4. Thermal in the same instrument? If the same crew inspects panels thermally, the SC Series removes a second trip and a second report format.
5. Reporting and trending. Confirm the recording spectra, how readings are exported and stored, and who will own the baseline.
6. Environment and accessories. Operating temperature (−20 to 55 °C on both detectors), control by phone or tablet over WiFi, battery duration for a full shift, sensors, clamps and training.

A good fit for the HPD200	Not the right instrument
An indoor MV switchgear fleet, screened on a routine cycle by one technician	A programme that must also test cable earth paths with HFCT
A first PD programme that needs a defensible baseline before expanding	GIS or heavily shielded equipment where UHF is the only practical channel
Contractors who need TEV and airborne acoustic results in a single handheld unit	Teams that need contact AE measurements on transformer tank walls

If two or more items in the right-hand column describe your programme, the HPD500 is the correct purchase and buying the smaller instrument first simply adds a second purchase later. If none of them apply, the HPD200 or the SC Series will do the job, and the budget is better spent on training and a written baseline procedure.

What to Send for a Quotation

Suppliers cannot quote accurately from a product name alone. Send these seven items and the first quotation will be the right one:

1. Asset classes to cover (LV panels, MV switchgear, transformers, cables, GIS, outdoor equipment).
2. Methods required (TEV, AA, AE, HFCT, UHF) and whether any asset dictates a specific one.
3. Whether thermal imaging must be integrated, or a separate thermal camera is preferred.
4. How results are reported and trended, and whether patterns must be exported.
5. Quantity, sensor and accessory requirements, and spare batteries.
6. Country of delivery, local voltage and language requirements for the manual and interface.
7. Documentation needed: calibration or test reports, certificate scope, neutral or branded packaging.

Certificate scope and neutral packaging availability are confirmed per model and per order quantity, not for a whole product family — state the requirement up front and ask for it to appear in the quotation. Our procurement support page lists what we handle for importers and distributors, and the product selector is a fast way to narrow the category before you request a model.

Common Mistakes and Honest Limits

• Buying channels instead of coverage. Five sensors do not help if none of them reaches your dominant asset class.
• Assuming airborne acoustic finds internal discharge. It finds activity with an acoustic path; insulation voids usually have none.
• Comparing readings across different plants or load levels. The same dBmV value means different things on different equipment, and a reading taken at light load cannot be compared with one taken at rated current.
• Skipping the baseline. A first reading without a scheduled follow-up is an anecdote; the value of PD screening comes from the second and third visit.
• Treating a cool thermal image as a clean bill of health. Early discharge produces almost no heat, which is exactly why a PD detector and a thermal camera are complements rather than substitutes.

Honest limits to state in any tender document: TEV requires a suitable accessible metal surface; airborne acoustic can be blocked or masked by enclosures and background noise; HFCT depends on an accessible earth path and interference control; UHF requires a compatible sensor or window; IP54 protection applies to the model variants named in the relevant report and not to an entire family; and temperature measurement accuracy on thermal instruments is quoted at ±2 °C or ±2 % of reading, whichever is greater. A specification that acknowledges these limits is harder to challenge later.

Frequently Asked Questions

What is the difference between the HPD200 and the HPD500?

Sensor coverage. The HPD200 has two channels: TEV (3–100 MHz) on the metal enclosure and airborne acoustic at 40 ± 1 kHz. The HPD500 has five: AA (40 ± 1 kHz), AE (20–200 kHz), TEV (3–100 MHz), HFCT (3–30 MHz, sensitivity ≤3 pC) and UHF (300–1500 MHz). Both record PRPS, PRPD, continuous, flight and pulse waveform data, so both can be trended; the HPD500 simply covers the assets where switchgear-only methods cannot reach.

Can one partial discharge detector cover switchgear, cables and GIS?

Not with one method. TEV and airborne acoustic cover metal-clad switchgear. Cable terminations, joints and earth paths need HFCT because the discharge pulses travel to earth. GIS compartments are usually shielded, so acoustic signals are blocked and UHF (300–1500 MHz) is the practical channel. A programme that includes all three asset classes therefore needs a multi-method instrument such as the HPD500.

Do I need a thermal camera as well as a partial discharge detector?

They answer different questions. Thermal imaging finds resistive heating from loose connections and overloads; a partial discharge detector finds insulation activity that produces almost no heat until it is advanced. Teams that inspect panels thermally and also screen for discharge can use the SC Series, which combines 384×288 or 640×512 thermal imaging with airborne acoustic and TEV in one instrument.

Does partial discharge testing require a power outage?

No. TEV, airborne acoustic, AE, HFCT and UHF measurements are made on energised equipment. PD activity is load and voltage dependent, so the load at the time of the measurement must be recorded before any reading is compared with a baseline.

Is a partial discharge detector a replacement for an insulation resistance test?

No. A PD detector screens and, with pattern analysis, helps classify active discharge on in-service equipment. Insulation resistance, dielectric loss and contact resistance tests require the equipment out of service and measure different parameters. Treat PD testing as a screening and diagnostic layer that sits alongside, not instead of, offline electrical testing.

What information is required for a PD detector quotation?

Send the asset classes you test, the methods your programme requires (TEV, AA, AE, HFCT, UHF), whether thermal imaging must be in the same instrument, how findings are reported and trended, the quantity and accessory requirements, the country of delivery, and the documentation needed. Certificate scope and packaging options are confirmed per model and quotation.

The Bottom Line

The right partial discharge detector is the one whose channels reach every asset on your list. For an indoor switchgear fleet, the HPD200 covers the two methods that work there, and the SC Series adds thermal imaging to the same pass. For programmes that extend to cables, transformers or GIS, the HPD500’s additional AE, HFCT and UHF channels are not a luxury — they are the instruments those assets require. Write the asset list, mark the methods against it, decide whether the first year is screening or diagnosis, and send that sheet to your supplier. The quotation that comes back will be comparable, honest and complete.