Ship electrical earth resistance value of 0.1ohm 1

[Engineer2021]

In many class rules for ship electrical systems requires the earth resistance between equipment and earth plate ( or ship's hull) to be 0.1ohm. Just wondering does this take into account the entire earthing installation connected with the equipment ( including length of conductor from the equipment all the way to the earth plate, keeping in mind also there are other earth conductors connected sharing the same earth plate). Surely if all these parameters are taken into account, the resistance of the earthing system would be more than 0.1ohm.

Any thoughts or ideas please is greatly appreciated?

 

[waross]

Surely if all these parameters are taken into account, the resistance of the earthing system would be more than 0.1ohm.

Additional ground paths will be in parallel.
Please review the formula for combining resistance in parallel.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Engineer2021]

Thanks Waross for replying. That actually makes sense. Path connected on the same earth terminal will be in parallel. Is it accurate to assume earth conductors A and B will be in series then if for example: earth conductor A is Connecting equipment to earth terminal 1 and conductor B is connecting terminal 1 and terminal 2?

 

[waross]

Two grounding cases:
Equipment.
Grounded by being mounted to vessel structures that are welded to the hull.
Grounded by an equipment grounding conductor.
In the event that both methods are used, the hull connection will generally be the lowest resistance. The grounding conductor will serve to lower the overall resistance somewhat.
Supply.
The neutral of the supply may be connected to ground. In this case, the neutral grounding conductor will often be the highest resistance in the total ground path.
In the event that the supply is ungrounded, there will be no such conductor and the ground path resistance of equipment connected to the hull will be negligible.
Your ruling code may dictate an equipment grounding conductor independent of the mounting conditions. In such a case, the required ground path resistance may be the resistance independent of mounting connections.
Disclaimer: My marine work was in an area where basically no rules or codes were enforced. I cannot speak to regulatory requirements. My comments are based on basic principles and on-shore wiring conventions. Check your local marine codes.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Gr8blu]

Engineer2021: The combined resistance between a piece of equipment (let's say a propulsion motor) and the earth plate (let's say the hull) must be no more than 0.1 ohm. This includes all series and parallel paths from the equipment to the plate.

As waross stated, there's usually more than one path to ground for any given piece of equipment: a direct mechanical connection and a separate ground conductor of some sort. For the propulsion motor case, the mechanical joint path is going to be very low resistance - but it includes the contact between machined motor foot and machined bedplate, any welds used to construct the bedplate to get from the top surface to the bottom, and the contact between the bottom surface and the hull. A bad weld or an incorrectly-spaced support between plates (or even a painted surface where there shouldn't be one) is going to result in an increase to the measured resistance of the path. Adding one or more sizable ground conductors (AWG 4/0 or better) will help lower the circuit resistance by providing additional parallel paths, even though the conductors themselves will be individually higher than the mechanical connection.

The same argument holds true for any power electronics such as a drive, a UPS, or navigation and/or communication equipment: there are connections to ground planes inside the equipment that are "transferred" by some means to an exterior surface point (or points). A ground conductor from that exterior surface contact to the earth plate has to be no more than 0.1 ohm as well - which means using a sizable conductor and a short length.

The reason behind this low resistance path has to do with common sense and SOLAS (Safety of Life at Sea) regulations. Most of this equipment has a metal housing, and the confines on shipboard are fairly tight - which means the likelihood of coming in contact with a "live" surface is pretty high, particularly in rougher seas. So everything gets treated with a really good ground, to try to limit the touch potential for anyone wandering around the vessel or platform.

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