SPD Grounding, Equipotential Bonding and Surge Current Paths Explained
When people talk about lightning protection for industrial instruments, many simply think:
“Just install a surge protection device (SPD) and connect a grounding wire.”
However, engineers who have worked in real industrial plants know that lightning protection is far more complicated than that.
Industrial instruments are often connected with long power cables, signal cables, and communication networks. Inside these instruments are sensitive electronic components with limited withstand voltage capability.
During a lightning event, even if lightning does not directly strike the instrument, surge voltage can still enter the control system through:
- Power cables
- Signal cables
- Communication lines
- Cable trays
- Metal conduits
- Electromagnetic induction
The consequences can range from:
- Unstable measurement signals
- Communication interruption
- Instrument restart
- Damaged transmitters
- Burned PLC/DCS I/O modules
- Complete control system failure
Therefore, effective instrument lightning protection is not only about whether grounding exists, but about two fundamental questions:
- Where will the surge current flow when lightning occurs?
- Will dangerous transient voltage differences appear between connected equipment?
1. Why Does a Surge Protection Device (SPD) Need Proper Grounding?
A surge protection device (SPD) is commonly installed on:
- Instrument power supply lines
- 4-20mA signal circuits
- Communication networks such as RS485, Modbus, Ethernet, etc.
Under normal operating conditions, an SPD does not affect the circuit.
However, when a transient overvoltage occurs, the SPD rapidly becomes conductive and provides a low-impedance path to divert surge current away from sensitive equipment.
You can think of an SPD as a temporary “pressure relief channel”.
Normally, it remains closed.
When a surge arrives, it opens instantly and directs the excess energy into the equipotential bonding system instead of allowing it to pass through the instrument electronics.
However, one critical point is often ignored:
An SPD only works effectively when its grounding connection is properly designed.
A grounding wire that is:
- Too long
- Looped or coiled
- Poorly connected
- Installed far away from the grounding bar
can significantly reduce protection performance.
Even if the SPD operates correctly, the long grounding path can create additional voltage due to the high-frequency characteristics of lightning current.
Therefore, when checking SPD installation, do not only ask:
“Is it grounded?”
Also check:
- Is the grounding conductor as short as possible?
- Is the routing direct?
- Are unnecessary bends avoided?
- Is it connected close to the equipotential bonding bar?
- Are terminals tight and reliable?
A properly installed SPD is not simply connected to earth — the entire current path must be carefully designed.
2. Why Is Equipotential Bonding Important?
Many instrument failures during lightning events are not caused by lightning current directly entering the device.
Instead, they are caused by large transient potential differences between different metal structures and equipment.
During a lightning strike, the potential of many objects can rise rapidly:
- Building structures
- Grounding systems
- Cable trays
- Protective conduits
- Instrument housings
- Control panels
If these conductive parts are not properly bonded together, a dangerous voltage difference can develop.
This voltage difference must find a discharge path.
If a proper equipotential path does not exist, the surge may travel through:
- Signal terminals
- Communication ports
- Instrument circuits
causing:
- Insulation breakdown
- Electronic component damage
- Control system malfunction
Therefore, lightning protection grounding is not simply about “sending current into the ground”.
The more important purpose is:
Keeping connected equipment at nearly the same electrical potential during transient events.
The better the equipotential bonding system is designed, the lower the risk of destructive voltage differences.
3. Grounding of Cable Trays and Protective Conduits
Lightning current changes extremely quickly and creates strong transient electromagnetic fields.
Long instrument cables can pick up induced surge voltages between conductors.
Metal cable trays, conduits, and protective pipes can provide:
- Cable shielding
- Electromagnetic protection
- Equipotential bonding paths
However, this requires continuous and reliable metallic connections.
Common problems include:
- Loose connections between tray sections
- Paint preventing electrical contact
- Corrosion at connection points
- Missing bonding jumpers at expansion joints
- Insulating joints interrupting continuity
When the metallic path is interrupted, the shielding and equipotential protection performance can be greatly reduced.
During inspection, engineers should check:
- Are all cable tray sections electrically connected?
- Are connection points free from paint or corrosion?
- Are expansion joints equipped with bonding jumpers?
- Are protective conduits electrically continuous?
- Are cable trays bonded to control cabinets and junction boxes?
Only a continuous metallic path can provide effective lightning protection.
4. How Should Cable Armor and Shielding Be Grounded?
Cable armor and cable shielding are often mentioned together, but their purposes are different.
Cable Armor
The primary purpose of armor is mechanical protection.
However, it can also provide:
- Electromagnetic shielding
- Equipotential bonding
For outdoor armored cables entering buildings or control rooms, the armor is usually connected to the equipotential bonding system according to project requirements.
Cable Shield
The main purpose of a cable shield is reducing electromagnetic interference (EMI).
The grounding method depends on:
- Signal frequency
- Cable length
- Shield structure
- Equipment interface
- Ground potential differences
- Electromagnetic environment
For example:
Low-frequency analog signals
Such as traditional 4-20mA instrument signals:
Single-point grounding is often used to reduce ground loop currents.
High-frequency signals and digital communication
Such as:
- RS485
- Ethernet
- Fieldbus systems
Multi-point grounding or equipotential bonding may provide better high-frequency shielding performance.
Therefore:
There is no universal rule that all cable shields should always be grounded at one end or both ends.
The correct method depends on the actual application and system design.
5. Should Spare Cable Cores Be Grounded?
For unused cores inside multi-core cables, the first priority is not grounding.
The correct practice is:
- Keep them insulated
- Mark them clearly
- Secure them properly
Unused cores should never:
- Be left exposed
- Be randomly connected to terminals
- Replace dedicated grounding conductors
- Be used as SPD discharge paths
Whether spare cores should be grounded depends on:
- Cable length
- Cable construction
- Electromagnetic environment
- System design requirements
General recommendations:
- If there is no specific requirement, keep spare cores insulated.
- If grounding is required, connect them only at the specified location.
- Avoid random grounding at both ends.
- Do not create unnecessary interference loops.
6. Common Lightning Protection Grounding Mistakes
1. Only Checking Ground Resistance
Ground resistance is important, but it is not the only factor.
Lightning is a high-frequency transient phenomenon.
The following factors strongly affect protection performance:
- Ground conductor length
- Cable routing
- Bending radius
- Connection quality
A low resistance ground connection may still provide poor lightning protection if the current path has high inductive impedance.
2. SPD Ground Wire Too Long
A very common field problem:
The SPD is installed inside the control cabinet, but the grounding bar is far away.
A long grounding wire is installed with several loops.
Although it appears “grounded”, the surge protection capability may be significantly reduced.
The best practice:
Keep SPD connection leads as short and straight as possible.
3. Completely Separating Different Grounding Systems
Some projects separate:
- Instrument grounding
- Electrical grounding
- Lightning grounding
to avoid interference.
However, during lightning events, these systems may develop large transient voltage differences.
The correct approach is not necessarily complete separation.
Instead:
Different grounding systems should be coordinated through a proper equipotential bonding design.
4. Grounding Cable Shields Using One Fixed Rule
Statements like:
“All shields must be grounded at one end.”
or:
“All shields must be grounded at both ends.”
are both incomplete.
The correct grounding method depends on:
- Signal type
- Frequency
- Cable length
- Installation environment
Conclusion
Industrial instrument lightning protection is not simply about connecting a wire to earth.
A reliable protection system must achieve two goals:
- Provide a short and reliable path for surge current discharge.
- Minimize transient voltage differences between equipment and metallic structures.
SPD installation, cable tray bonding, protective conduit grounding, cable armor connection, and shielding practices must all work together.
A properly designed lightning protection system ensures that when a lightning surge occurs, the current follows the intended path — instead of passing through sensitive instruments and control systems.