Dockside Distribution Assemblies with Insulation Monitoring Device to Protect IT Supplies to Vessels

Overview

Electrical protection on many vessels is achieved by adopting an IT distribution system with an Insulation Monitoring Device (IMD) constantly monitoring the integrity of the installation. If a deterioration in insulation levels is detected, IMDs can operate a pre-alarm, which highlights the need for corrective action to be taken and avoids any interruption to the supply. If there is a more significant reduction in insulation levels, the IMD operates an associated switching device, to isolate the supply. Clearly, the loss of the electrical supply on any vessel has serious implications, particularly so in a military application, and taking appropriate action at the pre-alarm stage is critical.

When a vessel is in port, electrical power can be provided from a shore supply, and in those instances it may be necessary for the IMD to be incorporated in the shore based distribution equipment, which was the case on a recent project. The shore supply was provided by a Blakley Dockside Distribution Assembly, which received a plug-in, 400A, 440V, 60Hz supply, feeding a 400A 4P MCCB with under volt release (UVR). On the outgoing side, 3 no. 63A, 440V, 3P+N+E, switched and interlocked sockets were fitted, each protected by a 63A 4P MCCB.

An ISOMETER insulation monitoring device monitored the overall 400A 440V supply and if a deterioration in insulation levels is detected, the pre-alarm contact causes a beacon to flash and a sounder (with mute facility) to operate. If a further deterioration in insulation levels is detected, the alarm contact causes the UVR in the 400A MCCB to drop out, opening the MCCB. If the vessel’s on board IMD device is compatible with the shore supply, the IMD protection within the Dockside Distribution Assembly can be disarmed.

All other aspects of the design of the Dockside Distribution Assembly recognised the harsh installation conditions. The body of the enclosure was made from grade 316L stainless steel with a painted finish, providing ingress protection to IP55. The enclosure was housed within a robust crash frame with fork lift pockets and overhead lifting facility. Indicators show when there is a supply to the live side of the 400A MCCB and a separate set of indicators show when there is a supply to the load side. An anti-condensation heater was also incorporated.