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P1 is the only product here that is a solar string tester. The listing describes open-circuit voltage measurement up to 1000 V DC at module or string level, short-circuit current up to 20 A DC, on-location I-V curve capture with comparison against manufacturer curve data on the analyzer screen, and a claim of meeting IEC 62446-1 for Category 1 and Category 2 tests. P2, P3 and P4 are combiner boxes: they gather strings, fuse them and protect against surge, but they do not measure a string and cannot produce a test result. P5 is also a combiner box. If you need a string tester, buy P1 or an equivalent instrument from another supplier; if you need to combine strings, the combiner boxes are the right product but they belong on a different page. The rest of this page explains what the string tester specification means, where the listing is silent, and the maintenance and safety points that decide whether a test result is worth recording.
Why four of the five listings cannot test a string
A combiner box is a junction. It takes several PV strings, fuses each one, provides a surge protective device and a DC breaker, and sends a single pair of cables to the inverter. The listings for P2, P3 and P4 describe exactly that: pre-wired strings, fuses rated 15 A or 20 A, a 63 A breaker, a lightning arrester, an IP65 enclosure. None of them measures open-circuit voltage, none measures short-circuit current, and none produces a record you can put in a commissioning file. They are protection hardware, not instruments.
This matters because the search term ‘solar string tester’ is used loosely. Some buyers mean the instrument. Some buyers mean the box that terminates the strings. If you arrive at this page wanting to prove a string is healthy, a combiner box will not do it. If you arrive wanting to combine four or six strings safely, P1 will not do it either, because it is a measuring instrument with no array input terminals.
For the measurement job, the relevant comparison pages are the I-V curve tracer comparison and the PV commissioning tester round-up, which cover instruments that do the same work as P1 in different configurations.
What the string tester listing actually publishes, and what it does not
Four figures appear in the P1 listing. Open-circuit voltage at module or string level up to 1000 V DC. Short-circuit current at module or string level up to 20 A DC. An I-V curve function that runs on location and compares measured data against manufacturer curve data on the analyzer screen. And a standards claim: IEC 62446-1, Category 1 and Category 2 tests.
The listing does not publish irradiance accuracy, voltage measurement accuracy, current measurement accuracy, the number of I-V curve points captured, the sweep time, the maximum string power it can sweep, or the operating temperature range. Those are the numbers that decide whether the instrument suits a particular array, and they are simply absent. Where the listing is silent, this page says so rather than guessing.
The 1000 V DC ceiling is the figure to read carefully. On a modern utility array running at 1500 V DC, a 1000 V instrument is not usable for string-level measurement. The listing states 1000 V, so treat it as a 1000 V instrument. The 1500 V DC multimeter page covers what changes at the higher voltage, and the utility array tester page deals with that class of work directly.
| Feature | P1 | P2 | P3 | P4 |
|---|---|---|---|---|
| Measures VOC | Yes, to 1000 V DC | No | No | No |
| Measures ISC | Yes, to 20 A DC | No | No | No |
| I-V curve | Yes, on location | No | No | No |
| Strings accepted | Not applicable, measures one at a time | 6 | 4 | 4 |
| Fuse rating | Not stated in the listing bullets | 20 A | 15 A | 20 A |
| Breaker | Not stated in the listing bullets | 63 A | 63 A | 63 A |
| Enclosure rating | Not stated in the listing bullets | IP65 | IP65 | IP65 |
The table makes the point in one glance: the combiner boxes publish fuse, breaker and enclosure data because that is what they are; the tester publishes measurement data because that is what it is. Comparing them on a single axis would be misleading.
The one real string tester, and the four listings that are not
Fluke SMFT-1000 Solar Tools Kit
The only listing here that measures a string. It publishes VOC to 1000 V DC, ISC to 20 A DC, on-location I-V curve tracing with comparison to manufacturer data, and an IEC 62446-1 claim for Category 1 and Category 2 tests. The kit also includes an irradiance meter and a 100 A AC/DC clamp, which matters because an I-V curve without irradiance is hard to interpret.
Elnuimex 6 String Metal Combiner Box
Not a tester. A six-string combiner with 20 A fuses, surge protection and a 63 A breaker in a metal IP65 enclosure. Included here only because it was supplied for this topic, and it is excluded from the string tester comparison.
LiuTaoWMX 4 String Solar Combiner Box
Not a tester. A four-string plastic combiner with 15 A fuses, a 63 A breaker and a lightning arrester, IP65. Excluded from the string tester comparison.
Elnuimex 4 String Metal Combiner Box
Not a tester. A four-string metal combiner with 20 A fuses, a 63 A breaker, 9 AWG cable and a 32 A per-input maximum. Excluded from the string tester comparison.
The I-V curve, and why irradiance travels with it
A current-voltage curve is the shape a PV string draws as the load across it is swept from short circuit to open circuit. A healthy string has a characteristic knee. A string with a cracked cell, a bypass diode stuck on, or a mismatched module shows a step or a shoulder in the curve that a single VOC reading will never reveal. That is the reason to pay for curve tracing rather than a multimeter: the multimeter gives you one point, the tracer gives you the shape.
The catch is that the shape moves with irradiance and cell temperature. The same string at 400 W/m2 and at 1000 W/m2 produces different current, and if you compare a measured curve against a manufacturer curve without knowing the light level, you are comparing two different things. The P1 listing includes an irradiance meter in the kit, which is the correct way to handle this: measure the light at the same moment as the curve, then correct before you judge. The listing does not publish the irradiance meter’s accuracy or its measurement range, so those remain unknown. The irradiance meter comparison covers that instrument class in more detail.
Temperature correction is the second half of the problem. The listing does not mention a temperature sensor or a temperature input, and it does not publish a correction method. If you intend to compare curves against nameplate data, ask the supplier how temperature is handled, because the answer changes the interpretation.
Safety: what a string tester does not do
A string tester measures a live DC circuit. It does not make the circuit safe. Isolating the string, proving dead, and applying lockout are separate procedures that the instrument does not perform for you. The P1 listing claims IEC 62446-1 Category 1 and Category 2 coverage, which is a statement about the test regime the instrument is designed to support, not a statement that the instrument protects the operator from a live array.
Two practical points follow. First, DC arcs do not self-extinguish the way AC arcs do, so any work on string terminations carries a risk that a tester cannot remove. Second, the instrument’s voltage ceiling is a hard limit: 1000 V DC on this listing. A 1500 V array exceeds it, and no test result from an over-range instrument should be trusted. If your array runs above 1000 V, this is not the instrument.
The combiner boxes in this list carry their own safety role: they fuse each string, provide surge protection and include a breaker. That is protection hardware, and it is worth having, but it does not substitute for a tester and a tester does not substitute for it.
Maintenance and record keeping nobody mentions
MC4 connectors are the weak point in most string test setups. They corrode, they loosen, and a poor contact adds resistance that shows up as an odd curve shape. Clean and inspect the mating contacts before a commissioning sweep, and replace any connector that has been opened repeatedly. The tester cannot see a bad connector as a connector fault; it sees the electrical result, which you then have to interpret.
Record the irradiance and the module temperature alongside every curve. A curve without those two numbers cannot be compared against a previous visit or against the manufacturer data, and the comparison is the whole point of paying for a tracer. Keep the files with the string identifier and the array location so a later visit can be checked against the first.
Finally, calibrate. The listing does not publish a calibration interval, and no listing ever does. Ask the supplier for the recommended interval and the traceable calibration route, because an uncalibrated tester produces numbers that look authoritative and are not. This is the same discipline that applies to insulation resistance testing on PV, where a drifting instrument quietly invalidates a whole commissioning file.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Is P1 a string tester or a combiner box?
P1 is a string tester. The listing describes VOC measurement to 1000 V DC, ISC measurement to 20 A DC, on-location I-V curve tracing and an IEC 62446-1 claim. It has no array input terminals and does not combine strings.
Can I use P2, P3 or P4 to test a string?
No. They are combiner boxes. They fuse, protect and combine strings, and none of them measures voltage, current or an I-V curve. They were supplied for this topic but do not belong in a string tester comparison.
What voltage ceiling does P1 have?
The listing states open-circuit voltage measurement up to 1000 V DC at module or string level. That is the published ceiling. It is not suitable for a 1500 V DC array.
Does the P1 listing publish measurement accuracy?
No. Voltage accuracy, current accuracy, irradiance accuracy, sweep time and the number of I-V points are all absent from the listing bullets. Ask the supplier for a datasheet before relying on the numbers for a formal commissioning record.
Why does an I-V curve need an irradiance reading?
Because the curve shifts with light level. Comparing a measured curve against manufacturer data without knowing the irradiance at the moment of measurement compares two different conditions. The P1 kit includes an irradiance meter, which is the correct approach.
Is a string tester a safety device?
No. It measures a live DC circuit. Isolating, proving dead and locking out are separate procedures. The IEC 62446-1 claim describes the test regime the instrument supports, not operator protection.
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