single phase 220 and three phase 480 using same ground in mobile unit??

[ikunat33]

Hi,

We have converted an old livestock trailer to a mobile mushroom dryer.
One of the things we must have is LOTS of airflow over the mushrooms to ensure they dry out a fast as possible.
We have a large fan to help with this.

Because we will be out in the boonies running this thing we have the following setup:

Generator with single phase 220 volt outlet. This runs through a line reactor to clean up the power then connects to a VFD where it is converted to 480 volt three phase.

Our electrician is talking about running the hot and the common to the line reactor and connecting the ground to the frame of the trailer.
The two outputs from the line reactor (which is grounded to the frame) then run to the VFD.
From the VFD he is taking the Protective Earth and connecting that to the frame.
He is also running the Protective Earth connection from the motor to the frame.

The frame of the trailer will not be hooked up to a grounding spike or plate as we move around far to much for that.

This seems.... Questionable to me.

First of all I am concerned that having the 220 single phase and the 480 triple phase both grounded to the frame will throw the ground fault in the generator all the time.
If that is not a problem what about current leakage and the possibility of someone grounding out when stepping up into the trailer and getting kicked out of the gene pool :?

anyone see a problem with this?

Sean.

 

[kwired]

You want the two systems bonded together so there will not be any potential between them.

You don't want current carrying "neutral" conductors interconnected between the two systems though, or you will have stray neutral currents flowing on bonded objects, bonding each system at only one point is necessary to prevent this from happening.

 

[drcampbell]

Seems entirely reasonable to me. Connecting everything to the metal frame of the trailer minimizes the possibility of someone receiving a shock in the event of an insulation failure.

Automobile tires are electrically conductive. The reason behind this is to dissipate any static-electricity charge that might be present before refueling. Otherwise, the static charge would be dissipated when the gasoline pump's filler nozzle touched the vehicle's filler neck, which is not the best place for an arc.

Unfortunately, I don't know how well they conduct or whether that's also true of trailer tires.

 

[kwired]

Seems entirely reasonable to me. Connecting everything to the metal frame of the trailer minimizes the possibility of someone receiving a shock in the event of an insulation failure.

Automobile tires are electrically conductive. The reason behind this is to dissipate any static-electricity charge that might be present before refueling. Otherwise, the static charge would be dissipated when the gasoline pump's filler nozzle touched the vehicle's filler neck, which is not the best place for an arc.

Unfortunately, I don't know how well they conduct or whether that's also true of trailer tires.

They conduct well enough to eliminate significant static charge to make an arc the voltage needs to be thousands of volts but is low current when it happens. Those tires are still primarily insulators when talking about under 600 VAC.

 

[ikunat33]

You want the two systems bonded together so there will not be any potential between them.

You don't want current carrying "neutral" conductors interconnected between the two systems though, or you will have stray neutral currents flowing on bonded objects, bonding each system at only one point is necessary to prevent this from happening.

Thanks for the response kwired. I have attached a PDF of a basic text drawing that should make what I am talking about clearer. I think it is problematic but I am not sure what to suggest as an alternative... other than insulating the line reactor from the frame and installing some sort of GFCI between the VFD and the frame of the trailer which I would treat as PE for the motor. Would this accomplish what you are suggesting?

 

[Johnnybob]

Looks right to me:happyyes:

 

[Ingenieur]

Thanks for the response kwired. I have attached a PDF of a basic text drawing that should make what I am talking about clearer. I think it is problematic but I am not sure what to suggest as an alternative... other than insulating the line reactor from the frame and installing some sort of GFCI between the VFD and the frame of the trailer which I would treat as PE for the motor. Would this accomplish what you are suggesting?

you have L1 and L2 not L1 and N
any single phase 120 loads?
if so N needs pulled and only bonded to G at the genset
if no 120 looks fine
since the genset is the source and the trailer is its gnd reference there should be no frame potential to earth/ground, ie, touch potential or shock hazard

 

[kwired]

I maybe misunderstood what you had first time around.

Looks like generator - line reactor (which I question if even needed) - vfd which apparently has internal step up to 480 volts?

Whether this was connected to generator or to utility power you would ground it the same way. The 480 output of the drive doesn't really have a solid ground like a transformer output would, but the drive does sense if there is a ground fault but has no impact on or from the supply circuit.

 

[ikunat33]

Thanks Everyone Who responded,

It is all hooked up and everything works wonderfully. We have a great mini wind tunnel for our mushrooms to dry in. Now we just have to put in some ducting and a wood burning stove and some racks and build some screens and ....

But this was the big concern

:bye: