PV Earth Resistance Testing: Method Selection and Common Pitfalls

A low reading on a PV site's grounding conductor can be reassuring for the wrong reason. Depending on the test arrangement, the instrument may be seeing an electrode and its return paths, a parallel metallic loop, or a bonding connection. Those are different measurements. The first task in PV earth resistance testing is to define which one is required.

Conventional earth testing methods do not become invalid simply because a site has solar panels. Problems arise when the method, network topology and interpretation do not match. This guide explains that selection process for commissioning and maintenance teams. It is not an instruction to disconnect grounding conductors or perform energized work. Qualified personnel must follow the site safety plan and the relevant instrument and equipment instructions.

Separate four questions that are often confused

Put the intended question on the test sheet. Without it, results from different methods can be compared as if they measured the same thing, creating false trends or an unjustified pass.

When fall-of-potential testing is the right starting point

Fall-of-potential testing uses auxiliary electrodes to establish a measurement arrangement around the electrode or system under test. It can be appropriate where the test objective and available space permit a valid setup. Its practical difficulties on a PV site include the extent of buried conductors, restricted access, underground services and interaction between the test arrangement and the grounding system.

Do not solve these difficulties by accepting the first convenient stake position. The responsible tester needs to follow the selected method, assess spacing and geometry, and verify that the readings support a valid interpretation. A large grid or complicated site may require specialist planning rather than a simplified rule copied from a small-rod example.

The connection state also matters. An electrode that remains connected to a wider network may produce a different result from an isolated electrode. Whether any disconnection is permitted is a safety and engineering decision, not a shortcut for obtaining a preferred number. Never lift a protective connection on an operating installation just to simplify a test.

What a clamp earth tester actually needs

A clamp earth resistance tester needs a suitable closed return path. In an appropriate multi-grounded system, it measures a loop containing the path under test and the return network. It does not make an isolated earth rod measurable merely by closing the jaw around its conductor.

Before selecting the clamp method, identify the available return paths and ask what the displayed loop resistance represents. A very low reading can come from a parallel metallic path rather than the intended path through earth. Megger explains both limitations in its earth/ground testing Q&A.

The EK-B200X clamp earth tester is an equipment option to review when the network supports the clamp method. Confirm conductor access, the expected resistance range and the precise job requirements against the supplied documentation. For a fuller explanation of the method boundary, see the EK-B200X ground resistance testing guide.

Why PV sites produce misleading comparisons

Parallel connections change the measured system

Module frames, mounting structures, inverter protective conductors and the wider earthing network can create multiple paths. A result taken before final interconnection is not necessarily comparable with one taken after the complete network is connected. Record the actual topology and connection state rather than relying on the asset name alone.

Soil and weather are part of the record

Moisture, seasonal conditions and changes around electrodes can affect results. Note recent weather and visible site conditions. A difference between two surveys is a reason to investigate; it is not automatically evidence that an electrode has deteriorated or that an instrument is inaccurate.

Current and interference need their own assessment

An operating PV installation may have currents and electrical noise that affect test conditions. Use the earth tester's stated interference checks and operating limitations. If conditions fall outside those limits, stop and revise the test plan instead of treating an unstable display as a valid resistance result.

A leakage current clamp such as the EK-G668A serves a different measurement purpose: its published range table specifies AC current measurement. A current reading is not an earth resistance test, and it does not replace verification of protective bonding or an electrode test. Confirm suitability for the waveform and installation before selecting a current instrument.

Do not turn a commissioning reference into a product certification claim

IEC 62446-1 addresses documentation, commissioning tests and inspection for grid-connected PV systems. Its scope does not establish that an individual tester is “IEC 62446 certified,” nor does mentioning the standard prove that a site has passed every applicable requirement.

Define acceptance criteria from the approved design, applicable requirements and equipment instructions for the particular installation. Do not apply one universal resistance threshold to every PV site. The report should identify the criterion and its source, not simply state “good grounding.”

A field record that makes the result usable

Use the following structure for each measurement point. It is a record template, not a substitute for the approved test procedure.

1. Asset and purpose: site, electrode or conductor identifier, and the question being tested.
2. Method and boundary: fall-of-potential, an appropriate clamp method, continuity or another specified method; include the network connection state.
3. Safety and operating state: reference to the approved work controls and whether the relevant plant was operating or isolated.
4. Instrument: model, serial number, relevant calibration status, accessories and settings.
5. Arrangement: a sketch or photograph showing test points, accessible return paths and auxiliary electrode positions where applicable.
6. Conditions: date, soil/weather observations, interference indications and any access limitation.
7. Results: readings with units, repeated checks required by the method and any excluded or unstable results.
8. Assessment: the applicable acceptance criterion, conclusion, unresolved limitations and corrective action owner.

Repeat measurements after corrective work under sufficiently comparable conditions. If the method or connection state changes, explain why the before-and-after figures cannot be interpreted as a simple percentage improvement.

Frequently asked questions

Can I use a clamp earth tester on a single isolated rod?

Not without the valid return circuit required by that method. Select an appropriate electrode test method instead; do not invent a return path by treating an arbitrary metal connection as remote earth.

Does a low resistance reading prove adequate lightning protection?

No. The result documents a particular measurement under stated conditions. Lightning protection also depends on the designed paths, bonding, geometry, surge protection and other applicable requirements. An ordinary resistance measurement is not a simulation of a lightning strike.

Which instrument should a PV maintenance team buy first?

Start with the tests required by the site's commissioning or maintenance plan. Bonding continuity, electrode resistance, insulation resistance and leakage current are separate tasks. Send EK the network arrangement, expected ranges, access constraints and reporting requirements so instrument suitability can be reviewed before purchase.