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On board a cargo ship, our UPS has an earth indicator that is on and we have been looking where the problem is. Those lines related to these circuits are mostly sensors and hydraulic valves and actuators switches. I have isolated/disconnected some shorted wires due to sea water flooding. Please tell me more how to diagnose.

Hi Roman,

Thank you for your question regarding Uninterruptable Power Supplies (UPS) and Shipboard Grounding.  It is our pleasure to help.

Shipboard Grounding can present some fairly difficult and complex issues as you are using the ship itself as the ground source (ultimately grounding through the hull and into the sea water).  The United States Navy has developed an excellent standard for shipboard grounding: MIL-STD-1310H.  This standard has some very specific requirements for ensuring proper grounding on board sea-going vessels. 

The basic principle of shipboard grounding is to use the ships structure as a ground plane, and then allow the accumulated charge to exit the ship via conductive salt water.  Often the hulls of ships are painted with non-conductive materials, as direct metal contact with salt water is generally bad for corrosion.  To remedy this, most ships have multiple copper plates installed at the lowest parts of the hull, these plates are in turn bonded together inside the ship using copper wire.  This establishes a common low-impedance (Z) ground plane that can be used as a reference point.  Testing of this ground plane is certainly possible, however it is not generally done.  The standards only have resistance and impedance goals for specific individual ground connections, and not for the entire grounding system.  However, if you were to test your ground system one would certainly expect to see a Direct Current (0 Hz) resistance from any point in the ground plane to any other point of less than 1-ohm.

It will be important to ensure that your ships ground system is valid.  Has someone painted over the copper plates on the hull?  Have the copper plates corroded?  When you bond your ground wire to the hull, did you scrap away the paint first to ensure metal-to-metal contact? (Note: the entire connection area should then be further protected by anti-corrosion products)

Another issue could be Hull-Generated EMI.  When was the last time your ship went through a degaussing process?

Now, as far as your ground light indicator goes, you may have to check with the manufacturer of the UPS, but generally speaking these “Open Ground” tests work by sending a small amount of electrical energy down the ground wire, it the energy doesn’t go (generate an imbalance), it is considered ‘open’ and the light comes on.  Any of the above items could be causing this problem.

It is of course hard to diagnose this problem from an email.  Please feel free to contact us and we will gladly speak with you about your problem, free of charge.

Best regards,

The Engineering Team at E&S Grounding Solutions

Photo Credit:  http://www.flickr.com/photos/georgio/2398167215/sizes/o/in/photostream/

2 Responses

  1. Hi Rajen,

    Snubbers are electronic circuits designed to dissipate the electrical energy left in an inductive load (such as your motor) when there is a sudden interruption of power. Often when the power is disconnected from the motor, the electrical energy left in the windings will generate what is called an “Inductive Flyback Voltage”. This voltage can be twice has high as the voltage incoming power and can damage the device.

    Good grounding is important for the proper operation of the Snubber, as an ineffective ground could cause the Snubber to fail. But if the circuit was designed to have a snubber, even the best of grounding will not eliminate the sharp rise in voltage that will occur across the inductive load of the motor.

    Here is some info on Snubber circuits: http://en.wikipedia.org/wiki/Snubber

    The bottom line is that you need to keep the Snubber circuit in place AND have good grounding.

    Best of Luck!
    David

  2. We have yarn winding machine and this winding machine is having two 5 HP motors connected to drive drum shaft and these two motors are connected to 3 phase supply via mechanically came driven contacs and cam is driven by a third motor, which makes and breaks contacts 01 times in a minute breaking time is around .5 seconds.. when we have snubber circuit on motor our electronic system works fine but we wish to make our snubber circuit removed and make our system work.
    Is it due to motor insulation making large current flow when motors are disconnectd from supply, if we make make our grounding solid with motor than will this problem will go away?

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