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IT systems - lets restrict this to LV systems derived from either the secondary winding of a delta star HV / LV three phase transformer or from an LV three phase alternator with a star winding.
In both cases then we have a star point - just like a normal TN distribution system. The difference is that the neutral is not generally distributed and the star point is referenced to earth via a fault limiting impedance. This would apply to shore based systems, or to marine systems in steel hulled vessels.
There are other possibilities such as might be found in ships with non conducting hulls (fibre glass), medical systems and so on - but these will just muddy the waters
. There is an LV IT distribution network used in parts of Norway - but again let's leave that out.
Some of the main advantages of an IT system are that 'earth' fault currents are considerably reduced and reliability is potentially improved provided the system is supervised.
The major disadvantage is the need for supervision. This is needed to watch for a first fault which could connect the system solidly to earth. It then becomes a TN system and this could occur unnoticed. This could create danger as the system is not designed to provide protection against high earth fault currents and the resulting fault voltages associated with TN systems.
Now on a ship we have staff who should be capable of supervising the system - this generally means monitoring an alarm system for indications of a first fault. We might benefit from the improved security of supply that arises because faults to earth cannot automatically cause a circuit to disconnect. Such an occurrence could cause danger if an essential system goes out without warning.
I usually use this example - imagine you are on a ship in a violent storm. If a fault to earth on, say, the electric steering gear were to automatically shut it down the ship may broach, roll over and sink. With an IT system if the fault occurs in the steering gear motors an alarm would sound. There is a chance that the motors might catch fire but there is also a good chance that they won't - so tell the bridge to switch to emergency steering and then its down to the steering gear flats with a bucket of water.
Anyway why worry - if the motors catch fire it will go out when you sink.
This all sounds good in theory but in practice many vessels sail for long periods with 'first faults' that are on the list of 'things to do'
In both cases then we have a star point - just like a normal TN distribution system. The difference is that the neutral is not generally distributed and the star point is referenced to earth via a fault limiting impedance. This would apply to shore based systems, or to marine systems in steel hulled vessels.
There are other possibilities such as might be found in ships with non conducting hulls (fibre glass), medical systems and so on - but these will just muddy the waters
. There is an LV IT distribution network used in parts of Norway - but again let's leave that out.Some of the main advantages of an IT system are that 'earth' fault currents are considerably reduced and reliability is potentially improved provided the system is supervised.
The major disadvantage is the need for supervision. This is needed to watch for a first fault which could connect the system solidly to earth. It then becomes a TN system and this could occur unnoticed. This could create danger as the system is not designed to provide protection against high earth fault currents and the resulting fault voltages associated with TN systems.
Now on a ship we have staff who should be capable of supervising the system - this generally means monitoring an alarm system for indications of a first fault. We might benefit from the improved security of supply that arises because faults to earth cannot automatically cause a circuit to disconnect. Such an occurrence could cause danger if an essential system goes out without warning.
I usually use this example - imagine you are on a ship in a violent storm. If a fault to earth on, say, the electric steering gear were to automatically shut it down the ship may broach, roll over and sink. With an IT system if the fault occurs in the steering gear motors an alarm would sound. There is a chance that the motors might catch fire but there is also a good chance that they won't - so tell the bridge to switch to emergency steering and then its down to the steering gear flats with a bucket of water
. Anyway why worry - if the motors catch fire it will go out when you sink.
This all sounds good in theory but in practice many vessels sail for long periods with 'first faults' that are on the list of 'things to do'
