When certain military and mercantile vessels are moored at pontoons or quaysides, an earth free IT supply is provided to power the vessel’s electrical system, enabling the on board generator to be shut down. An insulation monitor (IMD) is required to ensure the integrity of the IT shore supply. The IMD provides an alarm if a “first fault” is detected and isolates the supply if there is a further deterioration in insulation levels. Some vessels may incorporate an IMD to monitor the on board generated supply and the presence of multiple IMDs within the same installation can cause conflict between devices, which can undermine the integrity of the protection provided by the IMDs.
The Blakley Projects Team has recently been involved in developing 2 and 4 way assemblies to supply multiple vessels at a Royal Naval Dockyard. The assemblies address the concerns associated with providing Shore supplies to vessels and provide IT supplies with the highest level of integrity.
Assemblies are fed from TP, 440V, 60Hz, IT supplies. An incoming isolator feeds up to 4 no. 100A TP MCCBs (set at 63A) with under volt release (UVR), each feeding a 50 kVA 1:1 ratio transformer. The transformer outputs are earth free and each feeds a 63A 440V interlocked socket, which provides the supply to one vessel.
The incoming supply to the assembly and the secondary of each transformer is fitted with a separate IMD. Each IMD offers a pre-alarm and an alarm function. The pre-alarm operates: a local flashing beacon, a sounder (with mute facility) and a pair of volt free contacts wired to terminals. The alarm works in conjunction with the UVR fitted to the associated MCCB, to disconnect the supply. Each IMD is fitted with an ON/OFF switch enabling individual monitors to be switched ‘OFF’ when connected to a vessel fitted with an onboard IMD. The monitor and ON/OFF switch are positioned behind a clear, hinged window and the beacon and sounder are fitted above each MCCB / associated socket outlet. The incoming IMD is fitted on the side of the panel, adjacent to the main switch.
A GA drawing of a 4 way assembly is available on the pdf version. If you have a similar requirement for high integrity IT supplies, please contact our Projects Team.
Download pdf version:
SP14_IT_Supplies_for_Vessels.pdf (946.73 KB)