In short. Surge protective devices divert transient overvoltage to earth and clamp it to a level the equipment can survive. Protection works in layers: at the service entry, at distribution boards and at the load. Earthing and lead length decide whether it works.
Sources of surges
- Lightning on or near lines.
- Utility switching and capacitor banks.
- Large internal loads switching on and off.
- Generator and ATS transfer events.
Layered protection
| Location | Role |
|---|---|
| Service entry or main board | Handles the largest surge energy |
| Sub-distribution boards | Reduces residual voltage further |
| At sensitive loads | Final clamping and local protection |
Installation rules that matter
- Keep connecting leads short and straight.
- Connect to a low-impedance earth and bond to the main earth.
- Coordinate stages so energy is shared as intended.
- Use correct overcurrent backup protection per the maker.
- Include indicators and plan replacement.
See earthing.
What surge protection does not do
- Fix sustained overvoltage.
- Cover outages.
- Remove harmonics.
Generators and ATS
Include SPDs on generator and changeover boards where electronics and controllers are present. See utility, ATS and DG architecture.
Illustrative example
A building loses a few drives each monsoon. The fix is a primary SPD at the main board with a short earth lead, secondary devices at the drive panels, and a periodic check of indicators.
Understanding SPD ratings
| Rating | Meaning |
|---|---|
| Maximum continuous operating voltage | Highest steady voltage the device tolerates without conducting |
| Voltage protection level | Residual voltage during a surge; lower is better |
| Nominal discharge current | Repeated surge capability |
| Impulse current (for devices at exposed points) | Capability for direct lightning currents |
| Short-circuit withstand and backup protection | Safe failure behaviour |
The device types are commonly grouped into classes by test waveform, with classes intended for service entrance, distribution and equipment points. Use the classification in the standard your market follows.
Why lead length matters
A surge current rises extremely quickly, and every metre of lead adds inductance. Voltage across a lead is inductance times current rate of change. A long lead can add hundreds of volts to the protection level. That is why short, straight connections matter more than the device price.
Coordinating stages
Between the service entrance device and a downstream device, cable length or decoupling is needed so the stages share energy as intended. Follow the maker’s minimum separation or use devices designed to coordinate.
Earthing and bonding
- Bond the SPD earth to the main earth bar with a short conductor.
- Bond all earths in the building to a common system.
- Bond incoming services such as telecom and data lines at the entry point.
Local conditions
Monsoon thunderstorms around Hyderabad and Secunderabad make lightning-related surges a seasonal risk. Review protection before the season and check status indicators after storms.
Inspection routine
- Check indicators monthly.
- Replace modules after major events.
- Re-test earth resistance annually.
- Keep a record of failures and dates.
The master engineering question
Where is the surge entering, and is the path to earth short and low-impedance enough?
What to send us
Send layout, earthing details and failure history. Send your application details.
Questions engineers ask
Is one SPD enough?
Usually layers are better.
Do SPDs wear out?
Yes, each surge consumes some capability.
Does lead length matter?
Yes, long leads add voltage and reduce protection.
Is lightning protection the same?
No. It is an external system for direct strikes; SPDs protect wiring.
Values are illustrative. Follow applicable standards.