Signature management systems
Passive Shaft Grounding (PSG) system
EMS offers integrated and monitored Passive Shaft Grounding (PSG) systems that communicate shaft voltages back to the integrated AICCP system. The advanced ICCP system calculates the shaft resistance and determines whether the PSG needs maintenance, repair or brush replacement, sending the PSG status to the ship's communications.
The Passive Shaft Grounding (PSG) system reduces the shaft resistance during rotation by by-passing the shaft bearings electrically using metal conductive straps (slip rings) with conductive brushes between the shaft and a ground cable connected directly to the hull. The PSG system is designed to reduce the resistance between shaft and hull to as close to zero as possible. However, it should be noted that as the brushes wear from use, the resistance increases dramatically. Typical measurements on ships from shaft to ground using PSG that are well maintained, are 0.05 ohms. The rotation of the shaft will vary the resistance from 0.05 ohms to 0.001 ohms. This modulation in resistance causes a modulation in shaft current, which, in turn, causes a modulation in electric field and CRM signatures. The magnitudes of these alternating signatures will be relatively low compared to a shaft with no PSG system. Thus, there will be an improvement over no shaft grounding.
If the PSG system is not regularly maintained, as well as worn brushes, high wear spots will increase the resistance between shaft and hull to as high as no grounding or 500 kilohms. The rotation of the shaft will vary the resistance from 500 kilohms to 0.05 ohms. This modulation in resistance causes a modulation in shaft current which, in turn, causes a modulation in electric field and CRM signatures, just as in the situation of no grounding. All of these modulations will have frequencies of the shaft rate and harmonics of that rate. The magnitudes of these alternating signatures have the potential to be quite high and detectable and can be comparable to a shaft with no PSG.
Modulating current flow through the shaft bearings, rather than the shaft grounding, which can be currents typically of 150 amps or more (up to 500 amps), will very quickly cause wear and damage to the shaft bearings.