Glossary

Fault current

Unit mA

What is Fault current?

Current that does not flow back via the intended return conductor but takes a different path — via the protective earth, via conductive parts or through a person.

What matters about Fault current in practice?

On the AC side, the residual current device is the answer: it compares outgoing and returning current and disconnects at a difference of 30 milliamps — long before a circuit breaker would respond.

On the DC side, the counterpart is a leakage current monitor. It reports currents flowing away via hull, water or trim — the cause of many cases of unexplained consumption and rapid anode wear.

How does a residual current device protect?

It compares the current flowing out via the line conductor with the current returning via the neutral. If 30 mA or more are missing, current is taking another path, for example through a person, and the device disconnects within a fraction of a second.

How often should it be tested?

Regularly via the test button as specified by the manufacturer, and at the start of every season. If it does not trip when pressed, it must be replaced. A device that has never been tested can stick.

What if it keeps tripping?

  • Switch loads on one by one to find the cause.
  • Check sockets, lights and cable glands for moisture.
  • With several devices with mains filters the leakage currents add up; splitting them across several devices helps.

More on the 230 V side on board on the page shore power on board.

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