Wrong‑mounted SPD Equals No SPD at All

Many users focus heavily on parameters such as discharge current capacity, protection level and product class when selecting a Surge Protective Device (SPD). Yet they tend to overlook an equally critical issue: where should the SPD be installed and how should it be wired?

In fact, even a well‑performing qualified SPD may fail to deliver full protective performance if fitted in an improper location, with excessively long connecting conductors or inadequate earthing. It is fair to say that an SPD installed in the wrong position can see its protective effect drastically reduced.

Installing SPD inside a distribution box alone is not sufficient

An SPD works by rapidly discharging surge energy and clamping over‑voltage to a level tolerable by end‑equipment when surges arise from lightning induction, lightning electromagnetic pulses or switching over‑voltages.

Nevertheless, surges are extremely transient high‑energy events. Excessive distance between the SPD and protected equipment, or overly long connecting cables, will introduce additional voltage drop across the conductors themselves.

This means the nominal protection level printed on the SPD nameplate does not equal the actual voltage experienced by the protected equipment.

Poor installation can compromise real‑world protection performance, no matter how favourable the SPD’s technical parameters are.

Why should SPD be installed as close as possible to the protected point?

SPDs shall be mounted at critical points where surges may couple into the system, and kept as near as practicable to either the protected equipment or the incoming point of the power distribution system.

Typical bad‑practice examples include mounting the SPD at one end of a distribution cabinet while routing its connecting wires to the opposite end, or fitting the SPD far away from target equipment merely for construction convenience. Such layouts lengthen the surge‑current path and raise residual over‑voltage at the equipment terminals.

Accordingly, SPD installation is not simply “fit wherever space permits”. The core principles are proximity, short wiring and direct connection.

Subject to applicable codes and site constraints, connecting conductors for SPDs shall be kept as short as possible, with minimal unnecessary bends and detours. In engineering practice, the total length of SPD connecting leads is commonly optimised to preferably no more than 0.5 metres. Implementation shall comply with relevant standards, product datasheets and actual site conditions.

SPD placement shall comply with multi‑level protection requirements

A complete lightning‑protection system cannot rely on a single high‑discharge‑capacity SPD to handle all surge risks.

From the power service entrance through downstream distribution branches to the front‑end of critical loads, different locations undertake distinct protective tasks. Front‑stage SPDs primarily dissipate large‑magnitude surge energy, while downstream units further suppress residual over‑voltage to deliver refined protection for sensitive hardware including PLCs, DCS systems, servers and communication apparatus.

Mismatch between SPD mounting location and system‑protection hierarchy will break the progressive‑protection chain, even if the SPD model is correctly specified.

Therefore, SPD placement is more than choosing which cabinet to house the device. What matters most is whether it sits at the correct electrical protection node.

Do not ignore earthing and connection details

Once triggered, an SPD must discharge surge current via a sound conductive path. Apart from installation position, reliable PE bonding and earthing systems are equally essential.

Issues such as over‑long earthing conductors, poor joint quality and loose terminals will degrade surge‑discharge performance. All connections shall be mechanically secure during installation, with superfluous cable length and sharp bends minimised.

A high‑quality SPD can realise its full protective potential only with correct installation.

Reminder from Techwin

In real‑world projects, equipment damage is not always caused by missing SPD hardware. Frequently, it results from incorrectly‑installed SPDs.

Proper model selection is merely the first step. Mounting position, wiring method, conductor length and earthing workmanship all directly determine final protection effectiveness.

From Techwin’s perspective, lightning protection is far more than “installing one surge protector”. It represents holistic protection covering product selection, system design, mounting layout and code‑compliant site execution.

An SPD is not effective just by being fitted. Correct positioning and standardised wiring ensure reliable surge mitigation every time.

When inspecting lightning‑protection measures for your power‑distribution system, start with one question:

Is this SPD really installed in the right place?