SPD Engineering, Site Inspection and OEM Selection Guide
A low earth-resistance reading is helpful, but it does not by itself prove that an SPD has an effective surge-current path. Engineers must also check PE continuity, transient impedance, equipotential bonding, earthing-system type and the relationship between the SPD and the protected equipment.
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Réponse rapide
High grounding resistance can reduce the installed protection performance of an SPD by increasing transient voltage along the earth, PE and bonding path. However, there is no universal 1Ω, 5Ω or 10Ω requirement for every SPD installation. The applicable project standard, earthing system, equipotential bonding and complete surge-current path must all be checked.
The SPD may operate normally while the protected equipment still experiences excessive voltage caused by conductor impedance or a transient potential difference between separate grounding references.
Good SPD protection requires a coordinated, short and low-impedance current path—not only a low reading on an earth-resistance tester.
Can an SPD Work Without Grounding?
The technically correct answer is: it depends on the SPD protection mode and the system design.
An SPD limits voltage between the conductors connected to its terminals. A protection mode between line and neutral can limit a differential-mode surge without using a local earth electrode as the direct current path. By contrast, L–PE, N–PE, positive–PE, negative–PE and many signal-to-earth protection modes depend strongly on a correct PE or equipotential-bonding connection.
| Mode de protection | Dependence on PE or Earth | Engineering Meaning |
|---|---|---|
| L–N or pole-to-pole | May limit differential-mode voltage between the two connected current-carrying conductors. | This does not prove that common-mode protection or the complete installation is safe without PE. |
| L–PE or pole-to-PE | Requires a suitable protective-earth or equipotential-bonding path. | A missing, loose or high-impedance PE connection can seriously reduce the installed protection result. |
| N–PE | Depends on the neutral-to-earth protection path defined by the earthing system and SPD topology. | Particularly important in many TT-system 1+1 and 3+1 arrangements. |
| Signal-to-earth or shield | Requires coordinated reference bonding between the signal SPD, equipment and cable screen. | Separate earth references can force current through communication ports or equipment insulation. |
Even where one differential protection mode can conduct, the installation may still lack safe common-mode protection, protective bonding or the connection required by the SPD manufacturer. Always follow the wiring diagram and the applicable electrical design.
Why Does Grounding Matter to an SPD?
When a transient exceeds the SPD operating threshold, the SPD changes to a conductive state and creates a lower-impedance path between its connected terminals. Surge current then follows the electrical system path through live conductors, neutral, PE, bonding conductors, cable screens, structural metal or the earth-termination system.
The purpose is not simply to “send all surge energy into the soil.” The more important objective is to keep connected equipment and conductive systems at controlled relative potentials during the surge event.
Four Grounding Terms That Must Not Be Treated as the Same Thing
Earth Electrode Resistance
The measured resistance between an earth-electrode system and the surrounding soil under the selected test method.
PE Continuity
Whether the SPD earth terminal, cabinet, PE bar and main earthing terminal have a continuous and mechanically reliable metallic connection.
Surge-Path Impedance
The transient opposition created by resistance, inductance, conductor geometry, joints, routing and the wider bonding network.
Equipotential Bonding
The coordinated connection of conductive systems to reduce dangerous voltage differences during fault, lightning and surge events.
A site can show a relatively low earth-electrode resistance but still have poor SPD performance because of a long PE route, corroded joint, isolated earth network or large separation between the SPD reference point and the protected equipment.
What Happens When Grounding Resistance or Path Impedance Is Too High?
| Potential Problem | What Happens During a Surge | Possible Project Consequence |
|---|---|---|
| Higher path voltage | Surge current passing through resistance and inductance creates additional transient voltage along the current path. | Equipment may experience a voltage higher than the SPD’s declared Up alone would suggest. |
| Ground-potential rise | Current entering an earth-termination system raises its local potential relative to other reference points. | Current may flow through data cables, screens, pipes or another structure’s grounding network. |
| Different equipment references | The SPD and protected equipment do not rise to the same transient potential at the same time. | Damaging voltage can appear across equipment terminals, communication ports or insulation. |
| Unintended current path | The surge uses a parallel path with lower transient impedance than the intended bonding route. | Cable shields, PLC I/O, RS485, Ethernet or inverter communication ports may carry part of the current. |
| Protection coordination failure | Power and signal SPDs reference different earth or bonding points. | Different circuits connected to the same device can be clamped to different transient potentials. |
The main problem is normally not slower internal SPD operation. The problem is additional voltage outside the SPD, along the conductors and between different grounding references.
Grounding Resistance vs Surge-Path Impedance
A normal earth-resistance test provides useful information about the earth-electrode system under the selected test conditions. Lightning and surge currents, however, can rise very quickly. During these transient events, conductor inductance, bends, loops, spacing and bonding geometry can become major parts of the current-path impedance.
Keep the surge-current loop compact, use the intended equipotential bonding point, avoid uncoordinated reference systems and protect all conductive services entering the protected zone.
Does an SPD Require 1Ω, 5Ω or 10Ω Grounding Resistance?
There is no universal SPD-only resistance limit that applies to every installation. A numerical requirement may come from a national electrical code, lightning-protection design, utility rule, telecom operator specification, project owner, insurer or equipment manufacturer.
The measured result must therefore be compared with a clearly identified governing document—not copied from a generic online article.
| Project Situation | How to Treat the Ohm Value | What Must Also Be Verified |
|---|---|---|
| General LV building | Follow local electrical-safety and earthing requirements. Do not invent a separate SPD limit without a defined basis. | Main earthing terminal, PE continuity, bonding, SPD location, protection mode and conductor routing. |
| Free-field PV plant | Use the project’s PV and lightning-protection design, including its specified earth-grid acceptance criteria. | Meshed earth network, interconnected structures, inverter stations, cable routes, corrosion and seasonal conditions. |
| Telecom or tower site | Use the operator’s earthing specification and the site lightning-protection design. | Tower, cabinet, AC SPD, DC SPD, data SPD, antenna feeder, screens and common earth bar. |
| Critical facility | Treat the value as a complete system-design requirement, not a universal SPD product parameter. | Earth-grid geometry, touch and step voltage, fault current and protection coordination. |
| OEM distribution board | The panel builder cannot determine final site earth resistance from the SPD model or panel drawing alone. | Correct PE terminals, conductor requirements, installation diagram and site commissioning instructions. |
A site may show a satisfactory electrode result while the SPD is connected through a long conductor, corroded joint or separate reference from the protected equipment. The complete path must be inspected.
How Should SPD Grounding Resistance Be Measured?
There is no special instrument that measures a universal “SPD grounding resistance.” The test method depends on whether the engineer needs to assess an earth electrode, one branch of a parallel grounding network, a complete ground loop or only the continuity of the local PE conductor.
| Test Method | Utilisation typique | Main Limitation | What the Buyer Should Request |
|---|---|---|---|
| 3-point fall-of-potential | Testing an earth electrode or electrode system using separate current and potential test stakes. | Requires sufficient test distance and suitable soil access. Parallel paths may need to be considered or isolated. | Electrode tested, probe positions, test curve or validation points, measured result and site conditions. |
| Selective earth test | Measuring one section of a parallel grounding system using test stakes plus a current clamp. | The tester must understand the parallel network and place the clamp on the correct conductor. | Branch measured, network diagram, clamp position, test-stake layout and result. |
| Clamp-on or stakeless test | Quick testing where multiple grounding paths create a complete conductive loop. | It is not suitable for every isolated single electrode because a valid return loop is required. | Confirmation that a closed parallel loop exists and an explanation of what resistance the instrument measured. |
| PE continuity test | Checking whether the SPD earth terminal is continuously connected to the PE bar or main bonding point. | Does not measure earth-electrode resistance or prove low high-frequency impedance. | Test points, conductor identification, measured continuity and inspection of terminals and joints. |
| Visual and torque inspection | Detecting loose terminals, corrosion, broken conductors, sharp loops and incorrect reference connections. | Cannot replace an electrical measurement. | Photographs, conductor size, route, terminal condition and commissioning record. |
Minimum Measurement Record for an EPC or Maintenance Team
- Exact electrode, grid or conductor tested
- Instrument model and calibration status
- Test method used
- Probe or clamp positions
- Measured value and unit
- Whether parallel paths were connected
- Soil and weather condition
- Test date and seasonal condition
- Applicable acceptance limit
- Name or role of the qualified tester
- Photographs of the connection points
- Corrective action and retest result
A multimeter can help identify an open or obviously poor connection, but it cannot replace an appropriate earth tester or demonstrate the complete transient impedance of the SPD current path.
What Should Be Done If Grounding Resistance Is Too High?
Corrective action should address the cause of the result and the complete protection system. Replacing the SPD with a higher-kA model does not repair a poor earth or bonding network.
- Verify the measurement Confirm that the method is suitable for the electrode configuration and that parallel paths, probe spacing and seasonal conditions were considered.
- Inspect accessible connections Check clamps, lugs, bars, buried joint access points and conductors for looseness, corrosion, breakage, moisture or mechanical damage.
- Repair defective joints Restore the designed conductive path using suitable materials, connection methods and corrosion protection.
- Review the electrode design A qualified engineer may need to modify electrode depth, spacing, quantity, ring conductors or the site earth-grid design.
- Interconnect systems where required Avoid adding an unbonded SPD-only earth rod. Separate structures and services must be coordinated according to the project design.
- Improve local SPD connections Keep connections to live conductors and the PE or bonding bar short, direct and free from unnecessary loops.
- Coordinate power and signal protection Check AC, DC, RS485, Ethernet, coaxial, shields and metal services so that one unprotected path does not bridge two transient potentials.
- Retest and document Record the corrective action, repeat the appropriate test and update the as-built drawing and maintenance file.
An uncoordinated isolated electrode can create a dangerous transient voltage difference between the SPD, the equipment PE and communication systems. A low value at one isolated rod does not guarantee safe equipotential protection.
How Earthing-System Type Changes the SPD Assessment
| Système | Relevant SPD Path | Grounding and Bonding Check |
|---|---|---|
| TN-S | L–PE, N–PE or L–N paths depending on the SPD topology. | Confirm that the SPD connects to the panel PE bar and common metallic PE network through a short and direct path. The local electrode result alone may not describe this path. |
| TT | Commonly L–N protection plus a coordinated N–PE path in a 1+1 or 3+1 arrangement. | Confirm the installation earth electrode, RCD arrangement, N–PE protection mode and local requirements. Do not create an unauthorised downstream N–PE bond. |
| TN-C-S | Depends on whether the SPD is positioned before or after PEN separation. | Identify the PEN split point. Downstream N and PE must be treated according to the installation design and local rules. |
| TN-C | Phase conductors may be protected to PEN at the applicable point. | Confirm the permitted installation position and conductor arrangement. Do not assume that separate N and PE exist at that point. |
| TI | Project-specific phase-to-earth or pole-to-earth arrangement. | Confirm system voltage to earth, first-fault behaviour, insulation monitoring, equipment requirements and suitable SPD Uc. |
For additional topology guidance, review the Guide de câblage du SPD triphasé et le 3P+N SPD vs 4P SPD Guide .
Grounding Checks for Solar, Telecom and Industrial Projects
Systèmes photovoltaïques
- Confirm whether an external lightning-protection system exists.
- Check module-frame and mounting-rail bonding.
- Identify array, inverter and building earth networks.
- Review DC, AC and communication cable entry points.
- Check corrosion and seasonal soil variation.
- Confirm whether separate structures are correctly interconnected.
Telecom and Base Stations
- Coordinate AC, −48V DC, data and antenna-line protection.
- Inspect tower, cabinet and feeder-screen bonding.
- Connect power and signal SPDs to the intended reference.
- Check external joints for corrosion and water ingress.
- Review separate indoor and outdoor bonding levels.
- Follow the telecom operator’s earthing specification.
Industrial and Outdoor Cabinets
- Check incoming power and field-signal cable routes.
- Bond enclosures, doors and gland plates where required.
- Separate protected and unprotected conductors.
- Review PLC, VFD, RS485 and Ethernet interfaces.
- Inspect terminal torque, lugs and conductor routing.
- Identify nearby structures using another earth reference.
When two buildings, PV arrays, cabinets or field instruments are connected by copper cables but use separated earth networks, lightning can create a large potential difference between them. Power and signal protection must be coordinated at the relevant zone boundaries.
Information Buyers Should Provide Before Ordering an SPD
An SPD supplier cannot determine the complete grounding design from system voltage and kA rating alone. EPCs, panel builders and project buyers should provide the site information below before final product confirmation.
- Country and applicable project standard
- System voltage and frequency
- AC, DC, signal or combined protection requirement
- TN-S, TT, TN-C-S, TN-C or IT system
- Single-line diagram showing N, PE and PEN
- Position d'installation du SPD
- Main board, sub-board, combiner box or control cabinet
- External lightning-protection system: yes or no
- Overhead or underground incoming supply
- Measured earth resistance and test method
- Measurement date and seasonal condition
- Number and location of electrodes or grids
- Whether structures use separate earth networks
- Distance between SPD and protected equipment
- Distance from SPD to PE or bonding bar
- Required Type 1, Type 1+2 or Type 2
- Required Uc, Ucpv, Up, In, Imax and Iimp
- RCD and backup-protection arrangement
- Power and communication cable entry points
- Remote signal-contact requirement
- Required certificates and documentation
- OEM label, colour and packaging requirement
“We need an SPD for a 400V three-phase outdoor distribution cabinet. Earthing system: TT. The measured earth resistance is 12Ω using the fall-of-potential method. The site has an external LPS and an RCD. The SPD-to-PE-bar connection is approximately 300mm. Please confirm the suitable Type 1+2 configuration, N–PE protection mode, Uc, Iimp, backup protection and wiring diagram.”
Common SPD Grounding Mistakes
| Erreur | Why It Is Risky | Better Approach |
|---|---|---|
| Using an isolated earth rod only for the SPD | It can create a large voltage difference between the SPD, equipment PE and other conductive services. | Use the project’s coordinated earthing and bonding design unless a qualified design explicitly requires another arrangement. |
| Accepting the site from one ohm reading | The reading does not reveal long conductors, poor geometry, reference separation or unprotected services. | Combine measurement, drawing review, visual inspection and current-path verification. |
| Increasing the SPD kA rating to compensate | Higher Imax or Iimp does not remove voltage developed along a poor connection or isolated bonding network. | Correct the grounding and bonding system, then select the SPD for the actual exposure and installation point. |
| Daisy-chaining multiple SPD earth connections | Shared connection length can add impedance and allow one SPD current to affect another circuit. | Connect each SPD to the intended common bonding point using the project-approved arrangement. |
| Ignoring communication lines | Data cables can become the current path between equipment at different transient potentials. | Coordinate power and signal SPDs at the same protection-zone boundary and bonding reference. |
| Calling every value above 10Ω an SPD failure | It applies a generic number without considering the governing standard, earthing system or complete current path. | Compare the result with the project requirement and inspect the full transient-protection system. |
How Is This Different from the SPD 0.5m Connection-Length Rule?
0.5m Connection-Length Topic
Focuses on the local conductors between the supply, SPD and PE or bonding point inside the distribution board.
Its main concern is the additional inductive voltage caused by unnecessarily long SPD connection leads.
Grounding-Resistance Topic
Focuses on the wider PE, bonding, earth-electrode and site grounding network beyond the SPD terminals.
Its main concern is whether the SPD and protected equipment share a coordinated reference and complete surge-current path.
A short SPD lead cannot repair a badly separated grounding network. A low electrode-resistance reading cannot cancel voltage added by an unnecessarily long SPD connection.
Questions fréquemment posées
Does an SPD work if the grounding resistance is high?
The SPD may still conduct, but the installation may not achieve the expected protection level. Resistance and transient impedance can create additional voltage outside the SPD.
Can an SPD work without a ground connection?
Some line-to-neutral protection modes may limit differential voltage, but protection modes using PE or earth will not have the required path. An installation without the required PE connection should not be considered fully or safely protected.
Is 10Ω mandatory for every SPD installation?
No. A 10Ω value may appear in a particular project or grounding specification, but it is not a universal SPD-only requirement for every country and installation.
Is lower grounding resistance always better?
A lower electrode resistance is generally beneficial, but it does not replace correct bonding, short conductors, suitable geometry and coordinated SPD placement.
What is the best method for measuring earth resistance?
There is no single best method for every site. The fall-of-potential method is widely used for electrode testing, while selective and clamp-on methods can be useful where parallel paths exist. The tester must choose a method suitable for the actual grounding network.
Can a clamp meter test an isolated earth rod?
A clamp-on earth tester normally requires a complete conductive return loop. It may not provide a valid result for a single isolated electrode without another parallel grounding path.
Can an SPD use a separate dedicated earth rod?
A separate unbonded earth rod can create dangerous potential differences. The SPD should normally be integrated into the coordinated earthing and equipotential-bonding design.
Does a higher-kA SPD solve poor grounding?
No. A higher discharge-current rating does not remove voltage created by poor conductor routing, corroded joints, separated references or an inadequate earth network.
Why can equipment fail while the SPD indicator is green?
A green indicator normally shows that the SPD module has not reached its internal end-of-life condition. It does not confirm correct grounding, conductor length, bonding or protection coordination.
What grounding information should an EPC send to the SPD supplier?
Send the earthing-system type, single-line diagram, measured resistance, test method, external LPS information, SPD position, distances to the PE bar and equipment, incoming cable routes and applicable project specification.
Confirm the SPD and Grounding Path Before Bulk Ordering
Send LEEYEE your single-line diagram, earthing-system type, site test record, SPD installation position and project requirements. We will help confirm the appropriate AC, DC or signal SPD configuration for your panel, solar, telecom or industrial project.
Authoritative References
- DEHN, FAQ Power Supply Systems . Explains that no single earth-resistance value is required specifically for every SPD installation and highlights equipotential bonding.
- NEMA Surge Protection Institute, Grounding and Bonding . Discusses low-resistance and low-impedance paths, short direct SPD grounding leads and conductor self-inductance.
- DEHN, Lightning Protection Systems and Equipotential Bonding . Explains the use of lightning equipotential bonding to reduce potential differences caused by lightning current.
- Fluke, Four Methods to Conduct Earth Ground Testing . Introduces fall-of-potential, selective, stakeless and related earth-test methods.
- Fluke, Fall-of-Potential Measurement . Describes probe positioning and validation considerations for fall-of-potential testing.
- Megger, Clamp-On Earth Testing . Explains that clamp-on testing requires a complete conductive loop.
- Megger, A Guide to Earth Testing . Covers fall-of-potential electrode testing and periodic earth-system checks.
- DEHN, Lightning and Surge Protection for Solar Parks . Discusses local and remote equipotential surfaces in free-field PV systems.
