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Current Flow for Insulation Failure Onboard Ship

09/30/2010 7:45 PM

On board a ship when there is falure in insulation resulting in a live conductor touching the hull some current will flow as the hull of the ship is brought up to the same potential as the live conductor touching it. The ships hull acts like a capacitor after the hull is at the same potential as the live conductor the current ceases to flow.

Does anyone know what amount of current can flow and what sort of duration would it last? How about if it was not a good connection to the hull, say 1Kohm?

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#1

Re: Current Flow for Insulation Failure Onboard Ship

09/30/2010 10:08 PM

The ships hull acts like a capacitor after the hull is at the same potential as the live conductor the current ceases to flow.

Are we talking a metal hull sitting in a conductive solution of salt water with a direct potential connection to ground through the conductive liquid medium?

I don't believe the ship will operate like a capacitor as you have described it. Even with voltage grading through the conductive water to ground I don't think the hull will charge with a corresponding drop in (hull charge) current to a level equaling the leakage current between the hull and earth through the sea water.

Remember the (metal?) ship hull is connected to all the rest of the metalwork on and around the ship and hence at the same potential.

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#8
In reply to #1

Re: Current Flow for Insulation Failure Onboard Ship

10/08/2010 2:23 PM

What you say makes sense. GA.

The potential will be brought down to the level of the medium (seawater at 0 volts).

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#2

Re: Current Flow for Insulation Failure Onboard Ship

10/01/2010 12:17 AM

On a ship all electrix is connected to a singe point earth and it is a mandatory code requirement to interconnect all metal parts for an equipotential bonding. Use OHM's law if the resistance is 1K ohm.

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#3
In reply to #2

Re: Current Flow for Insulation Failure Onboard Ship

10/01/2010 10:58 PM

If you have a place with bad insulation that shows a 1000 ohm path to ground, get it fixed ASAP, or the leak will get worse and there may be heating which will accelerate the process.

This can be very dangerous. Does the ship have an electrician or is it a small ship. For now, pull the breaker or fuse for that line, if you can

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#4

Re: Current Flow for Insulation Failure Onboard Ship

10/02/2010 1:29 AM

hi

you need to be carful if you do actually have a current leak as you could be turning the hull into part of a cathodic protection circuit.

I'm not 100% up to speed with detailed principals but you COULD find little holes appearing in the hull where the isulation has failed/broken down.

Try these links to get a better idea

http://en.wikipedia.org/wiki/Cathodic_protection

http://www.cathodicprotection101.com/

As previously in the other replies... if you have a current leak.. find it and repair it

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#5

Re: Current Flow for Insulation Failure Onboard Ship

10/02/2010 7:33 AM

Typically ship power is delta with a floating neutral. Current would have to have a return path thru other equipment. A 1000 path would do little and possible nothing. Your inside a metal shell, a faraday shield and thus would notice nothing. The answer is dependent on the supply voltage and power levels involved. 480 volts is a common voltage on older vessels and 1000 ohms on one leg would not be dramatic overall

THere would be no effect to the sea water, the power system is not earth referenced and would have no effect unless the hull current was high enough to create a potential drop to cause corrosion due to electrolysis.

Woody

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#6

Re: Current Flow for Insulation Failure Onboard Ship

10/02/2010 7:35 AM

To the best of my knowledge . . . the electrical systems on a ship are ISOLATED, thus ONE insulation failure does not result in any catastrophe. As the second insulation failure will result in a catastrophe, there are GROUND DETECTORS used to detect this FIRST grounding. Such ground detectors typically ground the "floating system" with a high value resistor . . in the order of 1,000 . . . 100,000 ohms. . . . . Oleh

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#9
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Re: Current Flow for Insulation Failure Onboard Ship

10/08/2010 2:26 PM

You and everyone else are forgetting fuses and breakers with over current detection.

They trip and blow!!

Been there and done that and had the T-Shirt!!

You all need to think outside the box more often!!

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#7

Re: Current Flow for Insulation Failure Onboard Ship

10/03/2010 11:06 AM

This is the schematic of an earthing lamp for a single phase 240v system, the mid point earth is the ships hull.

When the test button is pushed if all is OK both lights will glow at half brilliance, if a small earthing problem exists one light will glow slightly brighter that the other, if a bad earth exists one light will be out and the other will glow at full brilliance.

It is similar for a three phase 415v system except you will have three lamps one each for the three lines (red, white, blue phases). The globe connected to the phase that the earth is in, will glow dimly or be out, the other two lamps will either glow brighter or be at full brilliance.

The fault is easily found by isolating circuits, sub-circuits and section breakers in turn and testing the systems until the earth is found. It can then be isolated, tagged and locked out and the circuit repaired.

Three reasons for detecting earth faults are crew safety, prevention of stray current corrosion and continuity of supply.

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#10
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Re: Current Flow for Insulation Failure Onboard Ship

10/08/2010 2:27 PM

GA

You bring common sense into being again. Thanks.

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#11

Re: Current Flow for Insulation Failure Onboard Ship

10/09/2010 12:35 AM

Next question; onboard our vessel we have the configuration as I have drawn. The neutral is floating with no reference to the hull. How is there a voltage measurable between the active conductors and the hull? How is it possible for someone to get a shock when coming in contact with the hull and a live conductor?

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#12
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Re: Current Flow for Insulation Failure Onboard Ship

10/09/2010 4:44 AM

On all RN ships with three phase, neutral was always left floating......as 3 phase loads do not need it......

For single phase equiment, there were local transformers to produce single phase outputs, also with no neutral.

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#13
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Re: Current Flow for Insulation Failure Onboard Ship

10/09/2010 7:03 AM

What they do with the neutral conductor (3 phase 415v) is use for domestic loads. Connecting any phase (red, white or blue) with the neutral phase will give 240v single phase, e.g. lighting, domestic power, etc...............or sometimes it is done by using step down transformers.

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#14
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Re: Current Flow for Insulation Failure Onboard Ship

10/09/2010 11:15 AM

On all the AC ships I served on, that was simply not possible, due to the fact that the 3 Phase was set at 440 Volts. Which is NATO Standard since the 50's.....

Shore supplies for ships supply the same back so that a ship can shut its own generators down. I would be surprised if 380 VAC 3 phase was also on offer.....

To get nominal 220VAC 1 Phase, you need to have the 3 Phase at around 380 Volts....

Which is why generally speaking, 380V 3 Phase to 3 x local single phase transformers are used on ships to achieve either 220 or 120 Volts AC..

I do believe they were Delta/Wye transformers, but don't quote me.....

I do not remember what voltage can be measured between a single phase of 440 volts 3 phase and the neutral.....more than 240 I am pretty sure.....

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#15
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Re: Current Flow for Insulation Failure Onboard Ship

10/10/2010 8:06 AM

This is a problem that Australian ships have (415v 3 phase 50 Hertz), however if you google "Cold Ironing" (a fl;ash name for shore power), its quite interesting.

The ship to shore power connex can be adapted to any power, frequency, voltage and phase..........at this point in time there are only a few ports where this is available........hopefully this will increase...........ships also require a retrofit of their shore power connection.

This is one of the Princess ships in Vancouver.

An illustration of one of the "Cold Ironing" units

To get nominal 220VAC 1 Phase, you need to have the 3 Phase at around 380 Volts....

Yep, that would be about right..........therefore we get 240v single phase from 415v 3 phase supply (any phase) via the neutral.

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