Hi,
I have a 7000W portable generator with a apparent problem. With the generator running, but not a cable connected to the 240V outlet on generator, I measure 122V hot to neutral on both legs, as it should be.

However I measure 122V to Ground on one leg, and 40 volts hot to ground on the other leg.

It should be 122V hot to ground on both legs, just like hot to neutral.

The electrician measures similiar readings at the transfer switch with the cables connected to the 240V Generator output.

Is there a problem with the generator? We do not get 122V between hot and neutral on one side when measured inside the house. The electrician thinks the generator is "bad". However we get proper voltage from hot to nuetral.

Do generators come with floating grounds? Is this generator bad or do I have an issue in the wiring going into the house?

 

I would suspect a shaky electrical connection, though that would likely produce very visible symptoms when loaded...
What make / model / country of origin are we dealing with ??

Are the windings lacquered or otherwise immobilized ?

 

The generator windings are floating to ground.
All ungrounded generators measure like this.
You can verify this by measuring the voltages after the cables are connected to the utility.
All a/c windings inn gen set or motor will burn out if there is even 1 turn shorted.

regards,
Maanga

 

Maanga,
so your saying the generator is operating as designed and a floating ground is normal?

 

Think about it. You have not tied the generator to ground, therefore it is "floating". The generator output is 240 split to provide 2 120 outputs. Many times the generator manual will specify grounding the generator frame. If yes, do it properly as per code, usually 10ft 3/4 inch with suitable cable and clamps. Think this is bad? Try measuring some of the 12 VDC to 120 VAC inverters. That is why many manufacturers don't recommend tying these "systems" into the house circuit.
As long as you know what you are doing (and you seem to) and are aware of the limitations, you will be safe.
Problem is many people haven't a clue, the regulations are written in jargon, the specifications were translated from the same language Latka on Taxi spoke, and the instructions have been attorneyized.

 

All good points, so should the ground wire in the cables running into the house from the 240V connector on the generator be tied to the ground in the main power box? This would then provide a "ground" back to the generator for the 240V plug, right?

 

All good points, so should the ground wire in the cables running into the house from the 240V connector on the generator be tied to the ground in the main power box? This would then provide a "ground" back to the generator for the 240V plug, right?

The generator is grounded externally, now as you said, uses the building ground.
There is no electric shock hazard as long as the generator body, ground and the user are at the same potential even if the 240 volts supply is NOT grounded. You get shortcircuit protection once the neutral is also grounded.

regards,
Maanga

 

This where we can get attorneyized. Start with the generators owners manual as to whether or not they want it grounded. If no, (and I don't know why they would say that) then the building ground will have to suffice. It is highly suggested (on the order of it is suggested you pay income tax) to follow your state and local codes as to the grounding. Most places specify a (single point ground) this would be the where your service box or meter box is grounded. As grid service is not present at this time (the transfer switch is in the Aux position) a separate ground point may in some cases be permitted.

 

In order to provide 240 volts, most portable generators utilize alternators that have two 120 volt windings, and connect those two windings in series to provide 240 volts. It sounds to me like one of the windings is bad. I don't think the floating ground has anything to do with the problem.