Electrical stuff

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griff
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Electrical stuff

Postby griff » Fri Apr 06, 2012 2:00 pm

How to disconnect a motor that's needing opened up for survey?

You're on watch and a circuit breaker will not open, what do you do?

Any help greatly appreciated.

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The Dieselduck
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Re: Electrical stuff

Postby The Dieselduck » Sat Apr 07, 2012 10:40 am

How to disconnect a motor that's needing opened up for survey?

    Check usage profile - are you going to need it, is it going to affect safety of vessel?
    Identify and lock out, tag out unit's power supply.
    Isolate mechanical system, lock out tag out.
    Verify leads / motor terminals for actual power shut off.
    Label terminals and wires. Label motor.
    Remove and cap wires, secure out of the way.

That's how I would do it anyways... but then again this is how I just did it in drydock, last month.

    Check usage profile - are you going to need it, is it going to affect safety of vessel?
    Label motor.
    Isolate mechanical system, lock out tag out.
    Call certified electrician to attend.
    Show electrician where unit power supply is.
    Supervise yard Mechanical crew's removal of motor
    Send same to approve workshops.
    Prod same for detailed report for Lloyd's
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D Winsor
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Re: Electrical stuff

Postby D Winsor » Sat Apr 07, 2012 10:55 am

There are usually many ways to isolate motor so it can be safely disconnected besides opening the breaker in the main panel.

At the Motor Starter there may be a secondary breaker, disconnect switch or fuses. Opening the breaker or disconnect or pulling the fuses will isolate the power source. The breaker or disconnect will also have some means to be locked in the Open position. If the fuses are removed they should be removed from the starter box. The Load wires can also be disconnected from the main contactor inside the starter box, these wires are usually connected at the base of the contactor and should be labeled to insure proper connection when the work is completed. It is also recommended that the fuse for the motor control circuit be pulled when using these methods to isolate the motor

The stater box should then be labeled to indicate work is being done on the motor and what was disconnected or modified on the starter.
Troubleshooting 101 "Don't over think it - K.I.S.S. it"

griff
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Re: Electrical stuff

Postby griff » Sat Apr 07, 2012 12:29 pm

Cheers for the info.

Any answers for the second question? Unrelated to the first, maybe should have made the question a bit clearer. The question in whole is:

The 4th Engineer calls you in the middle of the night, a main circuit breaker on the main switchboard wont open. What would you do.

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D Winsor
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Re: Electrical stuff

Postby D Winsor » Sat Apr 07, 2012 2:52 pm

If the breaker is a main breaker for a generator you should do the following.
If the breaker is electrically controlled from a remote position or an automatic control system switch the breaker to manual control and try tripping the breaker with electrical control system or the mechanical trip on the breaker.
If this does not work it is highly likely that the fixed and moving contacts have welded together and it will be necessary to remove the breaker from the main buss.
In this situation I would attempt the following
Switch the load/speed control of the generator to Manual
Reduce the electrical load on the affected generator to 0 kw/kva but do not shut down or de-clutch the generator else power will feed back through the faulty breaker and motorize the generator.
Check for potential energy (Voltage) between the frame of the breaker and ground with a meter.
If the voltage between the breaker frame and ground is 0 and while wearing the proper personal protective equipment (insulated gloves, boots and safety glasses), use the hand crank to withdraw the breaker and disconnect the breaker from the main buss
If there is voltage between the frame of the breaker and ground or if there no means to withdraw the breaker the only way to safely disconnect the breaker from the buss will be to Black the ship out.
If the faulty breaker is a feeder breaker to other equipment located on separate Motor Control Center you will have to trip the tie breaker to affected Motor Control Center and remove the faulty breaker
Troubleshooting 101 "Don't over think it - K.I.S.S. it"

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JK
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Re: Electrical stuff

Postby JK » Sun Apr 08, 2012 5:46 am

As said by a co-worker electrician when confronted by a main breaker that wouldn't close: more fat little electricians are killed by faulty switchgear then anything else.

I immediately left the breaker room and went back to the MCR, to avoid witnessing any such accident leaving the Chief and electrician to it.

Main breakers should be serviced regularly by the OEM to lower the chances of such incidents.

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Re: Electrical stuff

Postby griff » Mon Apr 09, 2012 3:18 pm

Cheers folks.

Another question relating to dry docking requirements.

4 things you need to know about the shore supply? I've got voltage, frequency and power available, can't think of a fourth.

Also, the drydock supply is 380v, 50hz and your ship is 440v, 60hz. Do you accept it and what effect would it have on machinery and electrical equipment?

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D Winsor
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Re: Electrical stuff

Postby D Winsor » Mon Apr 09, 2012 5:03 pm

The 4th item you have to verify before connecting to shore is the proper phase sequence. If the vessel is not fitted any sort phase sequence indicator it is permissible to go on shore power temporarily and run a 3 phase motor long enough to verify proper rotation
If at all possible you avoid operating a vessel with a 440 Volt 60 Hz Buss on 380 Volts 50 Hz. Operating 440V 60 Hz motors and other induction equipment such transformers, coils and fluorescent light starters will overheat and become a fire hazard when operated on 380V 50 Hz.
At the lower buss voltage and frequency the motors will run slower and draw a higher current and the current draw may exceed the current limit for the shore supply circuit. In order to minimize the risk to motor damage I would try and have the yard supply a 440V 60 Hz generator or a transformer to raise the supply voltage up to 440V and the 50Hz frequency should not be an issue.
If the yard is unable to supply power at the proper voltage you may be forced to operate the ship on the emergency generator for the time the vessel is on the Dry Dock in order to avoid unnecessary damage to the ship's electrical system.
Troubleshooting 101 "Don't over think it - K.I.S.S. it"

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Big Pete
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Re: Electrical stuff

Postby Big Pete » Mon Apr 09, 2012 11:48 pm

A lot of good stuff here.

One omission I noted is that no one made it clear that motors often have ancilliary electrical connections as well as the main power supply.

Motors often have electrical heaters inside them with an independant power supply and you should make sure any heaters are disconnected as well.
Some motors are heated by passing a low voltage directly through the main windings rather than through a seperate heating coil.
Both types of heater will usually be fed through auxiliary contacts on the contactor which will be open when the motor is running, closed when the motor is stopped.

Some motors have electromagnetic brakes built in, which will again have a power supply to the motor. These are again fed by auxliliary contacts on the contactor, but in this case will be energised when the motor is.

Large motors (and Alternators) also often have Thyristors included in the winding to provide temperature monitoring/protection. These also have to be disconnected.

Increasingly the ship's ISM system will require permits to work, and a Risk Assessment before the work is carried out.

BP
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JK
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Re: Electrical stuff

Postby JK » Wed Apr 11, 2012 3:06 am

Instead of running the emergency, you can consider flooding the sea bay with an external connection and running the ship service to supply power.
Also, some shore transformers have changeable taps for different voltages.

Big Pete also points out the safety side. If you are doing exams remember that aspect, lock and tag outs for the motors and planning.

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Big Pete
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Re: Electrical stuff

Postby Big Pete » Wed Apr 11, 2012 4:24 am

As JK says it is possible to run the ship's generators in Drydock if the shore supply voltage or frequency are too far out of spec.

Some ships, mainly smaller ones that are designed to sit on the bottom in small tidal Ports, will be able to circulate water from a Ballast tank, usually the After Peak, through the SW cooling system, and provide cooling that way.

On other ships it will be possible to weld a "T" into the SW inlet pipe to the cooler and connect a fire hose to this, fed from the shipyard's fire fighting supply.

BP
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JK
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Re: Electrical stuff

Postby JK » Thu Apr 12, 2012 5:24 am

On the ship I was on, we took the deck seal SW supply line off on deck and back fed thru it to the sea bay. The disadvantage is having the diesel running and maintaining a watch.

The other comment I want to make on the faulty breaker is that you can rack it out and possibly replace it with the shore power breaker or another breaker,BUT, the trips have to be adjusted on the breaker to correspond to the breaker you are replacing.


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