Long-Overdue Update Re: Isolation Transformers on Legacy Steamship, Plus a Question...

[crshears]

Hello all,

For the background to this post see the thread at

http://www.eng-tips.com/viewthread.cfm?qid=356842.

Of particular interest from that thread was the very first response where itsmoked strongly suggested I check the polarities of the lamp sockets to see which ones had live shells as opposed to live centre buttons.

I finally was able to get a good start on this task with the assistance of a couple of helpers; what I've found is very discouraging, namely that the polarities are completely inconsistent, right down to the individual components within each branch circuit. At least all of the branch wiring is relatively modern Pyrotenax; unfortunately it was fished into the original conduit and boxes, many of which have been painted shut for years; correcting all of the polarities would therefore literally involve thousands of hours of work.

I'v been re-reading Transport Canada's Marine Safety Ship's Electrical Standards, also known as TP127E, as well as the Canadian Electrical Code, and if I'm interpreting these correctly, not only are ungrounded two-wire single phase distribution systems a recognized and acceptable standard type of vessel electricity distribution system, but the Canadian Electrical Code does not apply aboard ships registered with Transport Canada.

What is causing the issues aboard the old girl is that the existing wiring is tied into a 120/240VAC three-wire system with a solidly grounded neutral. Since the prospect of undertaking all that re-wiring is such a daunting task, and seeing how the interposition of an isolating transformer is still hanging fire, I am considering the installation of two separate single-phase isolating transformers, both with a 240VAC input. The first would be a unity bank with a standard three-wire 120/240V output, grounded neutral, and would be connected to the newer three-wire loads aboard the vessel, including the VFD that supplies the main engine turning gear; I'm estimating that a unit rated at ~50 kVA would do nicely for these loads. This transformer's 240V input winding would have a 200 amp breaker interposed in its supply circuit.

The second IT I'm contemplating using, if I can find such a beast, is one with a 2:1 voltage ratio, with either a single output winding or a pair of 120VAC output windings which could be parallelled; these would be used to supply all of the ship's original distribution boxes in an ungrounded configuration. Ground detection could be provided either by the old tried-and-true "two lamps in series across the supply with the midpoint grounded and look for one to go dim" method, or by a more modern detection system that would not suffer from the limitation of having a blind spot to two grounds of equal severity occurring on the live legs simultaneously.

If I'm reading the regulations correctly, this could be done in conformity with TP127E, one provision being that equally-rated Edison fuses would have to be used in both legs of all the branch circuits. I'm guesstimating that a transformer rated at ~30 kVA would be required to supply these loads. This transformer's 240V supply side would be protected with a 125 amp breaker.

Any thoughts as to what else needs to be considered along the way?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[waross]

Thoughts only;
1 Polarity;
Can you correct the polarity at each lamp holder?
2 Wondering about the isolating transformer for the grounded circuits. If this is to avoid grounding the supply transformer on board the ship so as to avoid possible hull currents it is probably a good idea.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Hi Bill,

Re your first query: like I said, "At least all of the branch wiring is relatively modern Pyrotenax; unfortunately it was fished into the original conduit and boxes, many of which have been painted shut for years; correcting all of the polarities would therefore literally involve thousands of hours of work." So yes, the individual polarities could be corrected, but it would be a huge task, hence the idea of putting all of the original wiring on to an isolating transformer of its own with an ungrounded secondary, other than for perhaps the connection to ground between a pair of detector bulbs.

Your second point is exactly where I was going when it comes to hooking up the second IT. There is a diagram in TP127E that clearly illustrates how a 120/240 centre-tapped "split phase" secondary of such a beast is to be wired; see Figure 11-1 near the bottom of the page at this link:

http://www.tc.gc.ca/eng/marinesafety/tp-tp127-distribution-141.htm

My thoughts are to go with that scheme, but with the addition of a pair of galvanic isolator diodes between shore ground and ship ground [physically, this would be between the IT core and case] so as to eliminate as much as possible any hull currents flowing while precluding the development of a step potential difference between the vessel and the shore.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[MikeHalloran]

I have no solutions, but I'm gland to hear that you haven't given up, electrocuted yourself, or sunk the ship.


Mike Halloran
Pembroke Pines, FL, USA

 

[crshears]

I'm glad too!

One thing that occurred to me since my OP is that all of the wiring presently operating at near-zero potential difference from ground would have to be meggered to confirm the integrity of the insulation...which is not a big issue, unless it tested bad...

A further notion: any knowledge out there about how 'grandfathering' of old equipment works? As near as I can tell nothing is actually written down about it in either the Code or TP127E...after all, this is all existing stuff; there's no new build involved.

Do any of you have experience in dealing with your AHJ in idiosyncratic situations such as this?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]