During recent discussions with a customer on surge protection strategies for CNC and machining applications, questions arose regarding the use and purported benefits of transformers in mitigating transients.

A persistent misconception in electrical systems is that transformers, especially isolation or shielded isolation types, provide meaningful surge protection.

Many assume their galvanic isolation 'blocks or attenuates' high-energy transients from lightning, switching surges, or utility disturbances.

The reality? Transformers are not surge protective devices (SPDs). They excel at power delivery, voltage conversion, and sometimes noise isolation but they offer no inherent clamping or diversion of surges.

IEEE standards make this clear.

Per IEEE Std 1159-2009 (section 5.5.1), impulsive transients pass straight through transformers via interwinding capacitance. These high-frequency components bypass the magnetic core entirely, appearing on the secondary side with little or no reduction by the turns ratio. Even shielded designs provide minimal attenuation for normal-mode (line-to-line) events.

When tested with IEEE C62.41 waveforms, like the 100 kHz ring wave, surges often propagate unchanged or can even amplify slightly on the output side.

Worse, transformers can generate their own problems. Sudden current interruptions (e.g., during breaker switching or fault clearing) cause the magnetic field to collapse, producing damaging voltage spikes via L di/dt inductive kickback.

These self-induced transients can stress insulation, arc across contacts, or create oscillatory ringing issues that IEEE surge testing consistently highlights.

Relying on a transformer for surge protection creates a false sense of security.

True mitigation requires dedicated SPDs in a layered approach: Type 1 or 2 at the service entrance, additional units at sub-panels, and point- of-use protection for sensitive loads, all per IEEE recommendations.

Key Takeaway:

Transformers are vital for distribution and isolation in many applications, but pair them with proper SPDs to safeguard electronics. Don't let outdated assumptions expose your systems to unnecessary risk.

When protecting sensitive equipment and electronics in today’s surge environments, make sure to install the Right SPD in the Right Location to get the Right Results.

Get the Right Gear!

The Staff @ best-surge-protector.com