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Legacy Shipboard DC System Now Operating on AC

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crshears

Electrical
Mar 23, 2013
1,818
Hello all,

In the past I've posted about my volunteer involvement aboard a historic vessel. For the interested, those posts can be found at:



and


Moving on...

I've recently agreed to take on the task of assessing said vessel's electrical system, principally because nobody more qualified can be found to do the job, and the need is desperate.

The vessel was originally powered with two steam-engine-driven dynamos, each rated to provide a 30 kW output at 125VDC. I'm in the very early stages of gathering information, but the electrical system appears to have been operated ungrounded; the three distribution panels I've looked at so far all have two bus bars running down the centre, with lateral double pole single throw switches supplying each branch circuit. The load sides of said switches are fused in both legs.

Since the vessel's retirement and transition into a museum, the electrical system has been converted to a 120/240 VAC configuration with a grounded neutral, which is where it gets interesting...

The aforementioned panels are thus now wired with one bus bar hot and the other at effective ground potential. However, in two of the panels the hot bus bar is on the left while in the third it is on the right...

In many cases I found fuses of too high a rating for the circuit they were protecting, and these I promptly replaced with fuses of the correct rating...with a caveat:

I was horrified to find that in numerous cases the 'neutral' fuse was of a lower rating than the 'hot' fuse, meaning that if a branch circuit 'neutral' fuse were to blow for any reason the circuit itself [depending on fault magnitude] could remain live via the higher rated 'hot' fuse. In these instances I placed a fuse of the correct rating in the 'hot' side with a deliberately higher-rated fuse in the neutral side in an effort to achieve some level of fuse co-ordination.

Another concern I have yet to investigate is whether any attempt was made to observe the correct polarity when converting the system to AC, in other words if for example the shells of all the lamp sockets are connected to neutral and all centre 'buttons' are connected to hot...or whether any attention was even paid to this when the vessel was first laid down, meaning that some lamp socket shells might now be hot while others are connected to neutral...

I'm posting this here because it was in this forum that the question of whether a distribution system should be grounded or ungrounded was once discussed. The vessel has essentially been put to bed for the [northern-hemisphere] winter, so nobody will be working on board until the spring. I'm wondering whether I should even consider going to an ungrounded neutral system [since this requires added vigilance which might not avail over the winter], or if it would be better to stick with a grounded neutral and swap branch circuit polarities as needed to clean up the safety issue of hot shells.

Based on what I've provided, I'd appreciate any advice on how to proceed in the short term to keep the old girl safe and from catching fire which, considering she has a supertructure with wood overlay, would be disastrous...

I'm also looking for a recommendation as to how best to proceed with insulation assessment; a hand-crank megger rated at 300 volts would probably be ideal, as well as historically authentic...if I can even locate such a beast; but I don't want in any fashion to let that stop me if employing more modern metering techniques would enhance my abilities to detect the presence of problems I might otherwise miss.

Thanks.
 
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I can't tell you grounded or ungrounded. Probably pretty hard to successfully force an ungrounded system.

Is there no good way to completely bypass the neutral breakers? Get a standard insulated stand-off neutral bar and mount it in the box?

Stop wondering about the shells. Get your voltmeter out, post-haste, and check the shells to the hull.


Keith Cress
kcress -
 
If you are subject to the on-shore codes, you must use a grounded system for voltages below 300 Volts. The neutral bar is a good idea. My old crank megger was switchable between 250 V, 500 V, and 1000 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
What about "slugging" the neutral fuse? It's done in corner-grounded three-phase systems (not a "neutral" wire, I know). You'd have no worries about the neutral side fuse blowing before the hot side, and you get to keep the existing panelboards. I would re-wire them to be consistent with each other. Clear labels on the panels would be a good idea too.

I'd second your plan for re-wiring field devices to conform with your newly modified neutral arrangement. That being said, I'm not sure bonding one side of your supply to hull "earth" would conform to best practices for ships. I used to work on cruise ships and we had a decidedly strange, at least by land-based standards, wiring scheme. All loads were supplied by delta-wired transformers with (I believe) no reference to ground. There was one set of transformers for 120vac distribution, and one for 240vac; all loads were phase-to-phase. All staterooms had both N. American as well as European receptacles, since our guests were from all over he world. The ground pin was connected to hull "earth," I presume to provide a place for ground noise to go, etc... There may have been zig-zag transformers to supply a center point to the delta that was tied to "earth" as well, but I'm not sure.

This resulted in fairly spectacular results when a surge protector was plugged in; the MOV's saw who-knows what voltage from line and neutral to ground, and promptly popped like firecrackers, usually with some smoke for added effect. Power line filters of any sort also met their untimely, smoky demise quite quickly.

I believe this wiring scheme was in place to prevent any issues with galvanic corrosion, though I'm not entirely sure how much of a problem that is with AC power. I can certainly understand why your old DC system was ungrounded though. You'd plate away the hull, or some other bit of metal contacting seawater if you had DC leakage. Did the old system have ground detectors?

If it's possible for you, I'd recommend contacting someone more familiar with AC shipboard power systems. I'm sure there are many considerations when converting an historic DC system to AC and an expert could help spot those issues before they cause problems.


SceneryDriver
 
Hi all, thanks for the replies so far.

'Slugging' the neutral fuseholders: excellent idea, and one that was only vaguely under consideration on my distant horizon...I'm just doing the fuse co-ordination thing short term.

Re bypassing the neutral breakers: I'm not sure there ARE neutral breakers...all I've seen so far are neutral fuses; but it's still early days...I'm planning to be there tomorrow, and will pop the covers off the main panels to see whether the breakers there are single or double pole. I'll also check as many lamp socket shells as I can for polarity...

Based on meter readings so far, the neutral is definitely grounded; but at this point I haven't been able to open the door on the main disconnect / transfer switch to visually confirm this due to having to interrupt all power to make the check [door is mechanically interlocked with switch, and I haven't [yet?] found a way to cheat the interlock - - plus all the other volunteers have work to do that needs power, blah blah]. Once I can confirm that there is neutral bonding to ground, I'll only need to shunt / 'slug' out all interposed neutral fuses as noted above.

To the best of my knowledge, the Canadian Electrical Code states [1] that neutrals are to be bonded to ground at only one point within any portion of a given distribution system, something I will have to confirm compliance with, and [2] that any vessel that is secured to shore for more than six months out of the year must be wired in compliance with the Code. The fun part will be figuring out how to intelligently grandfather this old stuff into the 21st century...after all, there is still knob-and-tube wiring in service out there, so how hard can it be? ;o)

I've already got feelers out for a 500 volt megger...we'll see how that works out...

As for consulting an expert on shipboard AC systems, I'll ask around for such a person locally; if we can get advice gratis, that would be all to the good, since our budget is more shoestring than hawser.
 
Hello all,

Here's the latest.

Power supply to the vessel is via a conventional single-phase service at 120/240VAC. Flow is from a 16 kV primary pole-mounted transformer through buried secondary cable to a post-mounted 200 ampere circuit breaker in a weatherproof box, immediately adjacent to the ship. Unfortunately someone has misplaced the key to the enclosure and I could not open it up to see whether the neutral-to-ground bond is located within...although I suspect it is; more on this in a moment.

From the supply breaker ashore, a cable brings power into a manually operated three-pole double-throw transfer switch that enables the onboard loads to be supplied either from ashore or from the onboard emergency backup generator. As a consequence the transfer switch transfers not only the two 'hots' but the neutral as well [I have yet to locate the bonding point between the generator's neutral and ground].

There is no connection on board between the shore cable's neutral and ground, but they're definitely bonded somewhere; after determining that there was no potential difference between the two, I took a deep breath, checked for resistance, and got a reading of 0.3 ohms.

At least three of the ship's original distribution panels are still in use; each is supplied by a double-pole 150A breaker connected so as to interrupt both the 'hot' and neutral connections to the panel [good call, itsmoked!]. In each of these panels I have swapped around fuses as needed to ensure that all the hot supplies have a lower-rated and correct sized fuse for the wires connected to them, and a higher-rated fuse in the neutral side [arranging to 'slug' out the neutral fuses will have to wait until I can devise a good means of doing this].

There are also some panels supplied by single-pole breakers wired with solidly connected neutrals directly from the splitter box from the transfer switch. What I suspect, however, even though I have not yet been able to determine so, is whether any of the newer distribution panels located throughout the ship are being fed from the hot side of one of the original distribution panels - - meaning that even though they have their own 'solid' neutrals, they are solid only insofar as the upstream 150A breaker doesn't trip.

As the end of my day approached I started looking at the branch circuits to check for correct polarity; all I can say about that is that so far I'm not encouraged by what I've found...I have no doubt that [1] I will end up having to check every branch circuit aboard for health, and that [2] I will have no recourse but to remove a number of them from service pending corrective measures. I'm certain to draw some flak over this, but I'll deal with it.

That's all for now; thanks for listening.
 
Thanks for coming back with detail.

One thing, I may be misinterpreting, is the new panel neutrals. If, indeed, they can have their neutrals interrupted you're looking at a disaster. You realize what will happen if just the neutral is lost, (ill conceived breaker trips), to a 120/240 panel right? You'll be letting smoke out of all sorts of things - and smoke on ships leads quickly to fires.

I still think you should deep-six the breakered neutrals for these things:
tn_4-22,1,22NeutralBar-300x300.jpg


They take almost no space and are fool proof. You would want to stand them off to prevent them from 'bonding' to earth. They probably sell the little insulation blocks here:
Neutral bars are us

Ebay is literally crawling with neutral bars and their standoffs.

Pull all the breakers, (you can reuse them appropriately), block off the panel board faces where they projected and use the freed-up space to mount neutral bars and reconnect the wires.. Duck-soup. :)


Keith Cress
kcress -
 
Hello again all,

Lots of good points, itsmoked; thanks! It's too bad there's only one of me and the amount of time I can spend aboard is limited...

Yes, I am acutely cognizant of the implications of open neutrals and live faulted components and the havoc such could create; nevertheless at this point I can only hope that the breakers within said 'remote' panels are correctly supplied from the hot side, and pray that they're not on the neutral side of the branch circuits...at least the upstream 150A breakers are ganged double-pole and will completely isolate all downstream equipment upon trip. A further point is that other than for the new VFD to the main engine turning gear, there are no 240VAC loads on board at all; since the original wiring was all 125VDC, all of the remaining loads are supplied from only one hot leg @ 120VAC, which will at a minimum somewhat limit the fault current infeed capability. Incidentally this is causing more unbalanced load than I care for, but seeing as how this doen't pose a hazard, addressing it is way down on my list of priorities.

I'm also going to have to confirm that the wiring supplying the remote panels is not undersized for its supply breakers...

I very much like your idea of bypassing the neutral contacts in the existing ganged double-pole 150A supply breakers, but I have my doubts there's room in the boxes for that; we may end up having to just replace all of the main panels. As near as I can tell they are 1960's vintage, not original, and they are in a location not normally seen by visitors, so replacing them wouldn't spoil the tour experience with distracting anachronisms.

SceneryDriver wondered about whether "bonding one side of [the] supply to hull "earth" would conform to best practices for ships." I don't think it would be a good idea either, and there are no plans to do this; if I'm correct, the only bond between neutral and ground is ashore, but it will take a complete interruption of the power supply to the ship to accommodate confirming this. If there is still negligible resistance between the hull and neutral even with the transfer switch in the open position, I'm going to be led on a merry chase tracking down the location of this errant additional bond...

SceneryDriver also recommended 'slugging out' the neutral fuses, and it certainly does seem the best approach...unfortunately I haven't found a source of screw-in Edison-thread slugs, so I'm hoping as mentioned earlier that having fast-acting Type W 15A fuses on the hot side and 25 or 30A Type T fuses in the neutral fuse holders will accomplish much the same thing on a provisional basis.

That is all...
 
crshears

As for consulting an expert on shipboard AC systems, I'll ask around for such a person locally; if we can get advice gratis, that would be all to the good, since our budget is more shoestring than hawser.

There is no connection on board between the shore cable's neutral and ground, but they're definitely bonded somewhere; after determining that there was no potential difference between the two, I took a deep breath, checked for resistance, and got a reading of 0.3 ohms.

You might want to locate a good marine electrical consultant quickly. If there is a metalic connection between shore neutral (grounded) and ships ground anywhere you will have galvanic corrosion issues. This can progress quite rapidly to the point that a through hull fitting will disintegrate and the ship will go to the bottom.

There are several schemes for isolating ship's systems. One might already be employed in some location that you have yet to examine. Or perhaps not.
 
After several days digesting the situation, may I make some observations and suggestions?
#1 Neutral bonding.
Leakage from the shipboard system whether from neutral current sharing between the neutral conductor or from high resistance leakage from the hot side will pass through the hull and through the water to return to the shore ground. This current passing from the hull to the water may cause rapid corrosion of the hull. One hull destroyed by corrosion from an electrical mistake and no-one will remember all the good things you did. You may find complimentary corrosion damage at a one or more shore side grounding electrodes.
Best practice, an on-board delta wye transformer with the secondary wye point grounded to the hull. Any and all currents to ground will return from the hull to the neutral and no current should pass through the hull to the shore based system.
#2 On the subject of hull corrosion: Is any one concerned with the state of the zincs? These passive corrosion inhibitors are often overlooked when a vessel is not in use for an extended period of time. The zincs are sacrificial and must be replaced from time to time. In salt water I undestand that it is good practice to inspect them at least annually.
Is there an active cathodic protection system or has an active cathodic protection system been considered?
#3 Switches and fuses: I doubt that there will be breakers, most likely switches and fuses. I doubt that old DC components will be rated to modern fault interrupting standards.
I would consider gutting the existing panels and buying new panels or panel interiors. The panel interiors would be installed in the existing tubs and the wire reconnected. You will have properly rated equipment complete with neutral busses. The neutral conductors may be covered with white heat-shrink tubing (subject to the wishes of the AHJ). I understand that all the circuits are two wire 120 Volt circuits. In the event that some switched neutrals are encountered, the conductors may be easily swapped at the panel.
#4 Shore feed: With a delta/wye transformer transformer installed on-board, the only source of leakage current will be from the transformer primary and the primary feeder cables. I would suggest an encapsulated or potted transformer and a new, good quality, three conductor feed cable from shore to the ship. The AHJ may require a fourth grounding conductor in the feeder cable for code compliance but this ground conductor must not be connected to anything on board the ship.



Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Hello again everyone,

Thank you, PHovnanian and waross, for your responses.

PHovnanian, I share your concerns about galvanic action / electrolytic corrosion; I wouldn't want to see the old girl sink either!

By way of assurance, I will digress, but only slightly, and not for long:

After having served for 55+ years with a 125VDC electrical system and then sitting permanently berthed for another 45+ years with a single-phase 120/240VAC system, [ultrasonic?] plate testing performed by divers in 2012 revealed that the original 3/4" plates had suffered approximately 1/16" of attrition from when the hull was first laid down. Based on this, whatever was done up to now has caused either very little loss of plate thickness or none at all.

I say 'up to now' because [based on the responses to date] I'm beginning to have questions about the way our present power supply is hooked up...

I know that when the ship was returned to her original home port in Canadian waters there was a dry-type transformer on board, just forward of the main power distribution nexus; this transformer has since been removed and gone I know not where. I didn't much concern myself with it at the time since the electrical system was not within my purview or area of contribution / responsibility, but I seem to recall hearing that its primary winding had been supplied at 480V, which rendered it of no use to us.

I'm now wondering if this transformer served to provide the isolating function mentioned by waross...and as always, Bill, your comments remain welcome; I'll address them now.

You suggested 'Best practice, an on-board delta wye transformer with the secondary wye point grounded to the hull.'

The power supply we have is single-phase 120/240VAC and is likely to remain so, therefore the delta/wye part doesn't quite apply...but I take your point: connecting in an isolating transformer [wound at unity] between the shore power supply and the ship's electrical system would sever any galvanic connection between shore ground and ship ground and serve to eliminate [a] undesired circulating currents and thus any risk of electrolytic corrosion...at least from that source.

I see now I have some spadework ahead of me, such as determining whether, among other things, the ESA [sorry, the Electrical Safety Authority, our provincial AHJ] [a] will insist on the use of an isolating transformer with a neutral tap on the supply side, something that to me would be of no value whatever, or whether just having the two 'hots' and a ground conductor will suffice, whether it will still be necessary to switch the neutral of the load side of the IT when transferring to the back-up generator in the event of power outages, and [c] whether the shell of the IT will have to be bonded to ship or shore ground, and, if the latter, whether, to maintain the desired isolation, what sort of isolating block, pad, stand-off insulators or whatever the IT would have to be placed on, etc., etc.

Re the 'zincs,' our Engineering Team has already had e-mail exchanges about this, and plans are afoot for the spring to have divers inspect not only the zincs but also the sea chests...but I digress; not electrical components.

If my recollections are any good, the original DC system live-front central switchboard has a pair of circuit breakers, one from each dynamo, and DPDT knife switches to each load to accommodate its being supplied from either dynamo; all of this equipment has been retired ever since the vessel was removed from active service. Incidentally, there is still vestigial switchgear ashore, adjacent to where the ship is tied up, that appears to have been used to supply DC from the harbour's isolated, meaning stand-alone, generating plant over the winter when the vessels were laid up; although sufficient steam was carried to keep the on-board heating systems operational, the pressure developed was not great enough to operate the dynamos. I suspect, although I can't prove, that this was done to obviate the need for additional coaling over the winter.

Much of the branch circuit wiring appears to have been replaced while the vessel was still in active service, as it has rubber insulation; only the supply conductors to the distribution panels themselves is still the original linen-covered oil-impregnated [or is that just years of dirt?] cable. Thankfully the Edison-thread sockets are perfectly accepting of today's fuses, so even if the guts of these panels are eventually swapped out, I'm not certain replacing them is of high priority provided they megger out acceptably...

For the longer term, we are also going to research the ways and means to provide a permanent impressed-current cathodic protection system for the hull. For what it's worth, I recently noticed someone had unearthed [from who knows where] weights shaped like long skinny milk bottles, made of what may well be zinc; I'm speculating that these may have been suspended from the vessel's sides during winter lay-ups, but I have no proof of this and have not yet had the opportunity to dig into it further.

One factor that has probably contributed to the longevity of the hull is that the only time her plates saw salt water was on her trans-Atlantic voyage from the shipyard in Scotland where she was constructed to her home waters in the Great Lakes.

Thanks for listening.

 
And isolation transformer, is definitely, the very best way to go. Could that other one have been a 480/480 that was being ably used as a 240/240?

Keith Cress
kcress -
 
What you don't want to have is a situation where a current returns to it's source through the hull and the water.
There is more than one possibility. If there is a path through the hull and the water and that path is in parallel with the neutral, then the neutral current may split and some current may pass through the hull.
If a shore source of power has the neutral grounded to the hull, another vessel fed from the same source may cause a voltage rise on the neutral. Then current may flow from the neutral in the on-board panel, through the bonding jumper to the hull and back to ground through the water.
I am happy to hear that the hull has suffered so little erosion over the years.
BUT
Changes have been made.
Apparently there was once an isolation transformer that has gone missing.
Changes by well meaning but uninformed people may lead to unexpected and rapid corrosion.
Changes on shore such as the addition of a nearby load drawing a heavy neutral current may lead to drastically increased corrosion if the ship wiring feed is such as to allow any neutral or ground current to flow from the hull and through the water.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
If I'm reading this correctly, you have zincs protecting the ship hull. The hull is bonded to the secondary neutral (either on ship or on shore). The secondary neutral is generally bonded to the primary neutral at the service transformer per code. The primary neutral of course is bonded to many other secondary neutrals, including those feeding other berths. Therefore your zincs are attempting to protect a huge and varying amount of metal. This is why the isolating transformer provides value, it keeps your cathodic protection local.
 
Hi stevenal, thanks for joining the thread.

I concur with all you've written, which is why I intend to aggressively pursue modifying the shore connection to include an isolation transformer not currently present.
 
Generally the zincs are cast with iron strips extending past each end. The iron strips are welded directly to the hull. The zincs act with the hull and sea water to form a battery that acts to plate zinc from the zinc to the hull. You don't really get any zinc on the hull, but the action is enough to prevent most corrosion of the hull. The issue with neutral or ground connections to shore is not a large area of protection but stray currents strong enough to overcome the galvanic action of the zincs.
Consider the following example:
The service is at the edge of the terminal facility. The neutral is properly grounded. There is a long feeder to the warves to provide shore power to ships alongside. A ship connects to shore power. The ship draws a heavy neutral current but has no connection from neutral to the hull. No problem.
However the neutral current may cause a voltage drop of several volts between the neutral at the distribution panel and true ground at the service.
Now any ship connecting to that distribution panel will have a neutral that is several volts above ground potential. If one of those ships has a connection from the neutral to the hull, the voltage difference between the distribution panel neutral and true ground will cause a current to flow from the hull to the sea.
You may check and test and determine that there is no voltage difference between the distribution panel and ground and no hull current, and erroneously assume that you are safe using a neutral connection to the hull.
WRONG.
Tomorrow a ship or a shore load that draws a heavy neutral current may be connected to the distribution panel and your hull will start flowing current to the sea water.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
If you investigate all the google searches on shipboard double insulated grounding, I think you would understand better what waross is trying to point out. Its not just the ground current flow from hull through sea to shore. But also, the double isolation of ground that is a requirment aboard ships. It is very easy to get electricuted versus on shore. The time i was in Navy onboard ships for 4-5 years I saw 3 people almost get killed from electricution versus hardly anyone almost dieing in my 25 years doing electrical projects.

Google
double isolation of grounding on ships
 
Nice link controlsdude.
I would add that a shore neutral connected to the hull is potentially even more damaging than a ground connection to shore.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
Back from radio silence; just love those extended days off!

I ditto Bill's comment: nice link, controlsdude! I have saved that file for future reference & refutation...

First good news: I now have a 1960's vintage hand-cranked 250/500/1000V Megger brand insulation tester available; I'll begin using it once the weather warms up.

Second good news: the powers that be have agreed that an isolation transformer is needed, and are budgeting for the acquisition and installation of same in 2014. I've already provided the specs for it to my supervisor.

Here's to hoping two years of direct connection won't haven't caused any hull damage...
 
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