Are isolated neutral (IT) LV distributions systems allowed on FPSOs

[davva]

A bit of help required from anyone working in the offshore industry and working with IEC codes.

The question relates to an FPSO (floating production storage & offloading) vessel which need to comply with IEC 61892.

I have read IEC 61892 Part 2 sections 5.3 and 5.4 and it is not clear to me if an IT (totally insulated neutral - not high resistance neutral earthing) distribution system is allowed for an LV network.

We are at the design stage and would like to use a 690V IT (totally insulated neutral) 3 phase 3 wire distribution system. Is this permitted?

Thanks in advance

 

[Marmite]

For such a critical application I wouldn't rely on the answer you get from an internet tips forum, if I were you. No disrespect to the many knowledgeable and experienced contributors intended.
Regards
Marmite

 

[davva]

Yes thanks marmite.

I do intend to contact and get confirmation from DNV (the classification society), but was hoping also to get some feedback from forum members who have implemented such a distribution system.

 

[sberbece]

Davva,
If you are going to ask marine authorities such as DNV or ABS they will answer you that both LV distribution systems are recognized.
However, in marine industry/ships the IT distribution system is overwhelmingly used, because of its distribution reliability advantage against TN system (i.e. the power supply is still available after a first fault to earth). A short description of IT distribution system & its use in the attached document.

The FPSO is a special type of vessel, firstly you have to comply with the requirements of standards concerning electrical installation in hazardous areas, which imposes the NT system LV distribution system, with the neutral directly connected to earth.

I suggest you seek the advice of an electrical engineer competent on hazardous areas electrical design.

http://files.engineering.com/getfil...&file=ECT178_The_IT_earthing_system_in_LV.pdf

Regards,
Stefan

 

[SudionoSu]

IEC 61892 recognised Isolated system (IT system) as one of the neutral earthing methods. The reason being quoted from the standard as below:-

"Although not intentionally connected to earth, the so called "unearthed" or "isolated" system is in fact
capacitively earthed by the distributed capacitance to earth of the conductors throughout the system together with
any interference suppression capacitors."

The only exception is Where phase to neutral loads shall be served, systems shall be directly earthed.