IT unearthed marine (offshore) elect systems

[ekuegler]

Hi all,

Happy New Year......!

Would appreciate your advice.

Can someone please indicate the correct procedure to adopt in proving an unearthed IT electrical systems dead. (Offshore Oil/Gas Platform)

With no theoretical earth I would assume that to test for no volts/a dead system you would use a live line 2 pole tester, testing between phases on a 3 phase system having your indicative dead stick, 1 of 2 bonded to the sub frame....?

Is there any other requirement in proving dead prior to working on elect conductors/applying a CME.

Is there a difference in proving HV to LV systems of this nature? i.e. using a martindale test meter on LV systems in the same way proving no volts between phases or is additional testing required.

It is a system (IT) that I am unfamiliar with having only ever worked with resistive earthed systems i.e. generators earthed through an NER.

I would appreciate your thoughts on this matter.

Best regards

 

[7anoter4]

From "Solution To Overcurrent Protection" See:

http://www.lehmanengineering.com/quiz/quiz1sol.html

"The ungrounded delta is not truly ungrounded. Rather it is considered to be "capacitively grounded." Notice in the schematic figure, that these stray capacitances are actually Y connected to the delta system, and their neutral point is connected to the ground"
The working platform [or the approaching bridge] stands on columns on the actual earth. For the Medium Voltage system the usual test stick will be enough. The low voltage PE is bonded to the metallic platform considered "earth". Between live phase to "earth" will be some thousands megohm insulation resistance but less capacity so the low voltage tester could work.
If there are in vicinity other systems supplied from remote source the column may be considered an actual earth.
See also [for instance]:

http://www.hubbellpowersystems.com/catalogs/lineman/2450_Instrum.pdf

 

[ekuegler]

Thanks,

Still a little unclear - to summarise, your thoughts are that a single test stick is adequate on an MV system?

My current approach is to prove no volts on an unearthed system is by using a live line 2 pole tester - testing between the 3 phases (R-Y, R-B, Y-B) proving dead stick before/after.

Regards LV systems, I use a martindale test lamp again proving between phases, and also each P-E, proving device before/after.

Is this correct in respect to unearthed systems.

 

[7anoter4]

The safety rule states to consider the conductor still live until the Short-circuiting device will not connect the conductor to the earth. So, you have to check each phase with the tester prior the Short-circuiting Devices are in place and as soon as possible to mount it and then to pass to the next phase in the same way. See [for more information]:

http://www.pppl.gov/eshis/ESHD_MANUAL/safety/es15.0.pdf

I think it has to be a British Standard also as the follows:
"Code of practice for Earthing" BS 7430 ART 28.2 Safety earthing