Most of the newer earth leakage monitors for 24v DC are solid state reference type, the older two lamps in series with a resistor down to the hull are known to impress a small current on the hull and can in some circumstances actually promote corrosion.

The modern unit functions by measuring the current flow to earth
through a potential divider and comparing it to a fixed
reference using two amplifiers. If the current signal
voltage falls below or rises above the reference voltage,
one of two amplifiers switches from high to low, which
triggers a RC timer. Once the timing period (pre-set) has
elapsed, the normally energised PCB mounted relay de-energises
and the voltfree contacts connected to the
eleven pin socket change state. Reverse polarity
protection is provided using a number of high voltage
diodes.

These leakage detectors are vital on the 24 - 36 volt DC circuits on a vessel as DC is much more corrosive than AC.