Ship protection and monitoring relays ensure safety and a high level of ship electrical system availability by monitoring electrical power (voltage, current, and frequency).
An important function of ship monitoring relays is early error detection. This makes it possible to respond to irregularities before it is necessary to switch off part or all of a ship’s electric system.
Ship monitoring relays
Ship monitoring relays are not intended to substitute fuses or motor protection switches.
Instead, they signal irregularities earlier before the corresponding protective devices are tripped.
Trips from a ship’s thermal relays are linked to:
- Ship power system current imbalance
- Abnormal temperature rise of the PTC (Positive Temperature Coefficient) thermistor probes
- cosfi measurement dropping too low
- Locked rotor and over-torque detection
- Earth fault currents
- Phase rotation reversals
- Prolonged starting
Only a few of the trips listed in the control circuits are used onboard ships and are restricted to essential electrical equipment:
- Current monitoring relays
- Voltage monitoring relays
- Phase imbalance and phase sequence relays
- PTC relays
Some other terms commonly used for this subject are:
Starting override – Measured values outside the tolerated range are ignored during startup. This is when the starting override is active. The purpose is to tolerate brief fluctuations, such as motor inrush currents.
Response delay -Threshold value violations are accepted within the delay time. A signal is only generated if the threshold value violation continues beyond the delay time.
Threshold value – If the measured value passes the set minimum or maximum, the relay will trip.
Reset value -The value where the relay picks up again.
Hysteresis – The range between the threshold value and the reset value.