On Friday, March 10, 2017 at 3:22:28 PM UTC-5, wrote:
On Fri, 10 Mar 2017 11:12:27 -0800 (PST), DerbyDad03
wrote:
On Friday, March 10, 2017 at 1:55:04 PM UTC-5, wrote:
On Fri, 10 Mar 2017 08:16:03 -0800 (PST), DerbyDad03
wrote:
Oh boy, another ground question. These discussions are always lively. :-)
I lost power from the wind storm that hit the Great Lakes area on Wednesday. Last night I
borrowed a generator from a friend to get the furnace going. I simply unhooked the wires
from the breaker to the switch for the furnace and used an extension cord (with the receptacle
end cut off) and wire nutted the hot to hot, neutral to neutral, and ground to ground from
the generator to the wire that goes from the switch to the furnace. Everything worked fine,
other than the fact that the power came back on less than 3 hours later. It figures.
Since then, I've done some reading and found these 2 "facts":
1 - I read a generator related post in an HVAC forum that says that the control board in many
HE furnaces require a really good ground in order to operate properly.
2 - The manual for the generator says:
"Grounding
The generator system ground connects the frame to the ground terminals on the power panel.
- The generator (stator winding) is isolated from the frame and from the AC receptacle
ground pin.
- Electrical devices that require a grounded receptacle pin connection will not function
if the ground pin is not functional. "
The manual also says that the ground terminal on the power panel can be remotely
grounded by using a ground rod.
So, this indicates to me that I was running my furnace without a ground, which sort
of (sort of) goes against claim #1 that the control boards need a good ground to
operate properly. Of course, maybe my furnace *seemed* to have been operating
properly with the generator but really wasn't, or maybe my control board doesn't
even need a ground.
Comments?
The ground is for surge/transient protection.
If you did not remove the grounding connection in the switch box, you
are still grounded.
I did in fact remove the ground screw from the switch and attached the generator
only to the ground wire going to the furnace. Therefore I was not grounded.
That leads to the next question, just as learning experience...
Let's say I don't want the add the "inlet" mentioned below (more on that later) and just
use the extension cord like I did last night. If I leave the ground from the furnace to the
panel connected in that switch box, should I also attach the ground wire from the
generator to those ground wires? Wouldn't that ground the generator frame to the
panel ground as well as provide a ground for the other receptacles on the generator?
A quasi legal way to do this is to put in "inlet" in a box next to the
switch (or use a 4x4 box) then replace the switch with a 3 way.
The black (common) goes to the furnace and the other two switch from
line power to the inlet. Grounding and neutral are not affected and
since the neutral is not bonded in the generator, it is not a
separately derived system so this is legal.
That meets the intent of the "transfer equipment" rules even if it is
not part of the manufacturer's listing and labeling (110.3 (B)
violation)
I watched a video in which a "15 year HVAC tech" added a 3 prong appliance cord
to the furnace and installed a dedicated receptacle after the furnace switch. The purpose
was so that the furnace could just be unplugged from the receptacle and plugged into a
standard extension cord from the generator. Based on what you just said, that not only
doesn't sound legal, but it also ungrounds the furnace unless the generator is grounded
in some other manner, correct?
Short answer is there is never a reason to remove the ground from the
furnace, leave it connected to the house wiring. Just switch over the
power and if you use the DPST (3 way) solution you are really
switching the hot and leaving the neutral connected to both.
If you did go with a cord and plug solution, leave the ground
connected and connect the generator ground to the box.
That all makes sense but it eliminates the "plug and play" aspect of
the solution, making the "15 year HVAC tech's" method nonviable.
Here it is. You don't have to watch the whole thing...the plug and receptacle
can be seen at 0:24
https://www.youtube.com/watch?v=a2FDO3SVnVE
Once he unplugs the cord from the receptacle and plugs it into the extension
cord from the generator (which is electrically the same thing that I did at
the switch) the furnace is no longer grounded, unless the generator was
grounded in some other manner.
Assuming the cord and plug solution is legal in the first place, in order to maintain the ground, he would need to run a ground wire from the receptacle
box to the furnace chassis or mount the receptacle box *on* the chassis, correct? That way even though the ground wire from the generator isn't doing
anything, the furnace would still be grounded.
If the receptacle was mounted on a "stud" like in the video, would an
inspector take issue with a ground wire running from the receptacle box
(or from the panel) to the furnace chassis as a permanent installation?
In reality, the
box probably stayed grounded anyway, even if you moved the wire from
the switch. It is usually connected to the back of the box first and
then that continued on to the furnace. Back when I had a furnace, the
box was actually mounted on the furnace and that would be a
connection.
That's a good point. I just put a meter on my switch box, which is mounted
on the return duct work. Zero ohms to the furnace chassis and zero ohms to
the ground pin of a nearby receptacle which is on a different circuit. However,
it is grounded only when the switch is attached to the box because the
ground wires go directly to the switch, not to the box first.