Grounding in ships

[power2020]

Dear Folks,
In one of the ships, all 3-ph AC generator neutrals (9NO.,) are solidly connected to Ship hull. Pl. suggest me, is it require to calculate Touch and step potential?
Generator rating:
110kVA, 415V AC 3ph, 60Hz
Advanced thanks
Rajesh.T

 

[waross]

I hope this ship never goes on shore power. It is common practice to run ungrounded on ships that may ever go on shore power. Connecting a ship with a grounded system to shore power may lead to severe hull corrosion. Search this site.
respectfully

 

[power2020]

In this case, it is a grounded/earthed system i.e. generator neutrals are solidly connected to ship's hull. And there is no shore power supply. Here hull acts as ground return path if any ground/earth fault occurs.
Is it require / possible to calculate Step/touch potentials of the ship?

 

[HCBFlash]

This isn't really a "Motors and Controls" topic.

Please select forums carefully.

 

[lansford]

Step and touch potential are mostly calcuated in a substation where you have a certain amount of resistance to true earth and where you have spacing between conductive ground grides.
Here you esentially have an infinte conducting plane which in analogy to the substation, you are stting on a large big conducting plate.
In esence, and with the fault current you can encounter and assuming the groundig conductors are relatively large, I see no need for any calcs of step and touch potential.
You have the best groudnging field you can have.
JIM

 

[rovineye]

You have no neutral earthing resistors installed. Most 4160V ships I have seen don't have them either, but neither do they ground the neutral. Are you using some sort of E/F relay to monitor current in that neutral connection?

 

[NEWFIEBOY]

The electrical system you are describing sounds like what we refer to as a hull return system. The generators would be connected in a Y or star pattern and the neutral point would be connected to the ships hull. The phase to phase voltage would be as you state 415V. The phase to hull voltage would be about 240v which would be used to power your lighting circuits and power outlets, similar to the way a 12 dc system is connected in automobiles. This system is never installed in modern ships and is mostly found in older ships maybe 40 or 50 years old.

 

[waross]

A suggestion:
Given that iron oxides usually form on the support rails for machinery after a few years and grounding of machinery through the base may not be dependable, I suggest grounding all large machinery with a separate copper conductor. A calculation based on the impedance of the supply conductors and the grounding conductor should give you a worst case touch voltage after several years of service.
respectfully

 

[power2020]

Thank you for all replies.
Basically this is a Barge mounted desalination plant. It is not for sail.This barge contain 9-AC generator sets and all neutrals are solidly connected to ship hull without neutral breaking during parallel operation.

Mr.rovineye: All generator neutrals have individual neutral O/C relays. If any spl.protection needed for this application, pl. advice me. Advanced thanks

 

[rovineye]

Good that you have individual OC relays.

I use a high resistance ground on the switchboard bus to monitor and LIMIT ground faults, and where I have large motors isolated from that bus we use neutral earthing resistors connected at the star point to limit voltage and current during ground fault. But we are at much higher voltages.

Ships 4160v and below I have always seen and used completely ungrounded installations. Ground monitors are required then, not to limit ground faults but to prevent them. Superimposed DC or ground detection transformers are typicaly used, both sending a signal to notify that one phase is grounded and you better fix it before another grounds and causes a bolted fault.

I guess I still don't understand the reason for the Y ground on a 415 volt vessel when you are not using the star connection for distribution.