A sacrificial anode produces a small potential difference between itself and the metal it is protecting. To measure the protection current, a resistive shunt is placed in the series with the return connection. Any voltage across the shunt reduces the protection current so it must be kept to a minimum. The measured voltage is then amplified and converted to a 4-20mA signal which is then fed through a 4-20mA galvanic isolation unit. The power for the complete instrument is entirely derived from the external 4-20mA loop so it is intrinsically safe. Monitoring the current allows the operator to estimate the rate of anode depletion and when it will need replacing.