Generator Insulated from "Ground"

[Jereb]

G'day All,
I wanted to ask your opinions of a setup that is happening on the site i'm on. We currently have a 1500kVA Gen Set which is on top of a trailer (rubber tyres) being towed behind some mobile plant. What i'm interested to hear from you all about is the earthing of this generator. Because it's insulated from earth and the mobile plant's only connection to earth is through it's tracks, a good 15m away, there might be some nasty touch potentials for anyone climbing aboard this trailer whilst in operation. Recently they have potentially bonded the mobile plant and trailer which i suspect was a good idea on their part, but i'm still left worried about what may happen. I have been on other projects where a poor connection to true earth resulted in some 75V being raised between the mobile plant and scaffolding which resulted in an electric shock. Although this was minor our standard is to place them on an ECG for 24 hours to ensure nothing suspect has happened to there heart.

What are your thoughts on this?

Regards,
Jereb

 

[wolf39]

The grid or the consumer to which your genset is to be connected should be able to provide a solid ground connection to the trailer and/or to the genset.

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[waross]

If all the power is used on the mobile equipment, and the generator neutral is connected to the equipment frame it should be safe.
Did your description of the previous incident leave out a few details?
Was utility power being used on the scaffold?
For a touch potential to be present you need a connection between one structure and ground and a connection between a voltage above ground and the other structure.
If all the equipment on your mobile rig is bonded properly, any contact between a "hot" conductor and exposed metal should cause the protection devices to operate.
Now if you are taking power off the mobile equipment and a hot conductor contacts exposed metal that isn't grounded, you can expect a potential between that metal and your mobile equipment.
The good news is that Ground Fault Interruption devices will provide protection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Agree with the previous comments. The cable connecting the generator to your loads probably has a ground conductor, so there is likely a ground connection there.

 

[Jereb]

Well i'm not sure if i would call it a grid/consumer, i suppose you could look at the machine as an electric shovel that drags around it's own generator.

The incident we had on site was when power was being supplied to the areas of the machine via generators. I should mention this was a sizeable dragline that was being worked on. In any regards the machine had been placed up on blocks to remove the tub and only a single earth point remained which was (obviously) too small. The incident occured on the far end of the machine where a boilermaker had errected scaffold and had placed his earth connection onto the frame of the machine and received a shock whilst holding the scaffold and frame of the dragline (no power was being used on the scaffold as yet). Upon investigation we found that the resistance in the single earth cable was higher than what should be allowed, the touch voltage between the frame and scaffold was around 75 Volts. Our solution was to place further earth stakes connected to leads of a larger cross section around the machine (I think we placed about 4 more around the place).

 

[waross]

That sounds as if someone fell afoul of an improperly grounded electric welder. If this was a current to ground from the generator, you should have started tripping breakers on the faulted circuit after the extra grounds were added.
For you to get a voltage to earth from a machine, there must be a circuit to earth.
With your generator mounted on a structure that is connected to the machine that it is feeding, you have to have a connection between a hot wire on the structure and earth. That is the dirt, not the structure.
If your previous event involved portable generators placed around the machine and grounded, that is a different situation. For you to have 75 volts between the machine and the scaffold, there must be a hot conductor faulting to either the scaffold or to the machine frame.
I suspect that someone may have grounded a welding machine to the machine frame and then left the stinger in contact with something connected to the scaffold, or vise-versa.
The voltage is about right and the circumstances are abot right.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Jereb]

Yeah i think your probably right, probably was the welder. I think i heard something about a damaged lead or something on it but i just can't remember. We were using mobile generators to power instruments in the machine also, not the machine itself. Mostly a very large multi outlet board for the boilermakers again.
Well at least i feel better about the way the mobile plant is grounded anyway.

Thanks for the advice guys.

Jereb