[AT] Why Ground a Generator

Dudley Rupert drupert at premier1.net
Sat Dec 15 00:57:08 PST 2007


Maybe we can get a good argument going on generator grounding (but treat
this as a wind chill free zone) -

Dudley
Snohomish, Washington

I would think that grounding the frame of a generator that has a drop cord
or two plugged into it, as well as grounding one side of its' output, would
ensure that IF a short ever developed between either of the output terminals
and the frame of the generator then a potential difference would/could not
be created between the generator frame and the ground/concrete/grass/etc
that it's sitting on.  In other words, the innocent bystander would not get
shocked if he touched the generator and the chances of arcing between the
generator and the ground would be lessened.  Note:  I wonder, however, how
often the generator is ever grounded in this case.

BTW - This is the same grounding design as in the old steel power tools that
had the 3-prong plug.  The third prong, the grounding prong, connected the
steel case of the power tool back through the ground wire to the ground bar
in the service panel.  The neutral wire (white wire) was also connected to
the ground bar.  The ground bar, of course, is connected to a ground rod and
probably the copper/steel water pipes as well.  I think good grounding of a
generator with drop cords would/should. emulate the typical/good service
panel ground design.
When it comes to a generator that is connected to the service panel I don't
know.  I would think, however, that if the generator neutral is connected to
the frame of the generator (I have no idea on this ... would like to hear
from someone who does) then I think one could argue that the service panel
ground would ensure that the frame of the generator is kept at ground
potential and thus there would be no need to have a separate generator
ground.  However, I don't know what having one could hurt.  If the generator
neutral were not connected to the frame of the generator then it would seem
that providing a ground for the generator would give the safety benefits
described above.
WOILF (Way Out In Left Field) - Another case could be made for good
grounding of the power source but I am not sure how really applicable it is
to the generator in the home environment (I don't think any at all).  But -
not that any old iron lover should care - in the avionics world it is big.
A typical avionics unit, say a Flight Control Computer (Autopilot),
communicates (both inputting and outputting data) with literally dozens of
other units.  Protocols (i.e., ARINC specs) have been established to govern
the precise transmission and reception of this data.  For example, say the
Flaps Control Unit uses a logic discrete to output to other units if the
flaps are properly configured for takeoff.  So, let's say the Flaps Control
Unit does determine that the state of this discrete is True.  In this case
the protocol says it should output a DC voltage of 3.5 or less.  The
protocol also says that any unit inputting this discrete should consider it
True if it sees a DC voltage of 4.2 or less; otherwise consider it False.
This communication requires not only good grounding of each unit involved
but good grounding to a common source; that is, to the frame of the
airplane.  Otherwise the design used in many units permits the ground to
"float" to an undetermined state.  Say, for example, the sending unit is not
grounded, or not grounded well, to the frame of the airplane but that the
receiving units are.  Then it is a real possibility that the sending unit
could be outputting, for example, say 2 volts with respect to its' ground
(to indicate a True) but, if its' ground is at say +5 volts with respect to
the frame of the airplane, then the receiving units will see this as a +7
volt input and interrupt it as False.  OK, way off the subject of generator
grounding -





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