Galvanic Isolator

A galvanic isolator is a device that uses diodes arranged in pairs to block low-voltage corrosion currents while still allowing safety-critical fault currents to pass. The key is how diodes behave electrically.

The Basic Idea

• Stray DC voltages (often < 1–1.5 volts) can exist between a boat's bonding system and shore ground.
• Those tiny voltages drive galvanic corrosion through the shore power ground wire.
• A galvanic isolator interrupts that path—but only for low voltages.

How the Diodes Are Arranged

• Inside the isolator, two diodes are wired in parallel but oriented in opposite directions.
• That pair is then duplicated in series.
• A single silicon diode has a forward voltage drop of about 0.6–0.7 V; two in series → about 1.2–1.4 V.
• Because they face both directions, the device works for either polarity of current.

What Happens in Practice

1. Low-voltage galvanic currents (corrosion-causing)
   • Voltage is typically below ~1.2 V
   • Diodes do not conduct — circuit is effectively open
   • Corrosion current is blocked
   • This is the main purpose.
2. Higher voltage (fault conditions)
   • If a dangerous fault occurs (ie, a hot wire touching the boat's metal), voltage jumps well above the diode
   • Diodes turn on and conduct
   • Ground path is restored, fault current flows back to shore, breaker trips as it should
   • This preserves safety grounding.

Why Multiple Diodes?

• Raise the blocking threshold above typical galvanic voltages
• Provide redundancy and current capacity
• Meet marine safety standards (ie, ABYC)

One Subtle but Important Detail

• Modern "fail-safe" isolators often include capacitors (to pass AC fault signals instantly) and monitoring circuits (to detect diode failure).
• If a diode fails shorted, you lose corrosion protection; if it fails open, you could lose safety grounding.

Simple Analogy

• Small pressure (low voltage) → valve stays shut → no flow (no corrosion)
• Big pressure (fault) → valve opens → flow allowed (safe grounding)