T. Rex wrote:

In article ,
says...

Doug Miller wrote:

In article , HerHusband wrote:

Whether resistive or inductive, how can you have current flow unless the
circuit is completed?

Back-EMF can flow through the neutral wire because the neutral is connected to
ground at the breaker box.


Open the hot wire and current shouldn't flow through
the neutral either, unless there really was a fault in the wiring
somewhere?

Back-EMF from the collapsing magnetic field in the motor windings when it's
switched off.


You basically have a 2 wire inductor. You are saying that when the
circuit is broken, magnetic field collapse produces a current flow in
one wire that is not equal to the current flow in the other wire. You
may be in line for a Nobel Prize. Or maybe not.



Pay attention this time. The currents _at_the_GFCI_ are unequal, because
one of the two wires is not connected to the GFCI.

.... or anything else, for that matter (except one side of the switch).

--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

On Dec 30, 10:12 am, M Q wrote:
T. Rex wrote:
In article ,
says...

Doug Miller wrote:

In article , HerHusband wrote:

Whether resistive or inductive, how can you have current flow unless the
circuit is completed?

Back-EMF can flow through the neutral wire because the neutral is connected to
ground at the breaker box.

Open the hot wire and current shouldn't flow through
the neutral either, unless there really was a fault in the wiring
somewhere?

Back-EMF from the collapsing magnetic field in the motor windings when it's
switched off.

You basically have a 2 wire inductor. You are saying that when the
circuit is broken, magnetic field collapse produces a current flow in
one wire that is not equal to the current flow in the other wire. You
may be in line for a Nobel Prize. Or maybe not.

Pay attention this time. The currents _at_the_GFCI_ are unequal, because
one of the two wires is not connected to the GFCI.

I think that YOU need to pay attention:
In the wire that is not connected we can agree that the current is zero?
In the wire that is connected, what do you think the current is?
If it is zero, they are equal and the GFCI should not trip.
If it is not zero, where do you think that current is going?
Those electrons have to go (come from) somewhere. They are not
getting stored in the fan. They could be going to ground, but
only if you have a ground fault.

MQ-

Sorry for the potentially redundant post but......

Since the GFI is tripping we know that somehow the currents compared
by the GFI's circuit are different; the hot is zero since the switch
is open, thus the current at on the neutral is non-zero.

If it is not zero, where do you think that current is going?

It's going from the fan collapsing field thru the neutral wire back to
the neutral / ground bus at the service panel.


Those electrons have to go (come from) somewhere.

They are not getting stored in the fan.

Actually, they're kinda stored "stored in the fan"......in the
magnetic field, while the fan is running.

They could be going to ground, but only if you have a ground fault.

They are going to ground, but not thru a fault....but thru the neutral
wire.


MQ, this is just my understanding of how motors & GFI can
interact......... I could be wrong but I believe this is a reasonable
theory.

cheers
Bob

On Dec 30, 9:02*pm, BobK207 wrote:
On Dec 30, 10:12 am, M Q wrote:





T. Rex wrote:
In article ,
says...

Doug Miller wrote:

In article , HerHusband wrote:

Whether resistive or inductive, how can you have current flow unless the
circuit is completed?

Back-EMF can flow through the neutral wire because the neutral is connected to
ground at the breaker box.

Open the hot wire and current shouldn't flow through
the neutral either, unless there really was a fault in the wiring
somewhere?

Back-EMF from the collapsing magnetic field in the motor windings when it's
switched off.

You basically have a 2 wire inductor. You are saying that when the
circuit is broken, magnetic field collapse produces a current flow in
one wire that is not equal to the current flow in the other wire. You
may be in line for a Nobel Prize. Or maybe not.

Pay attention this time. The currents _at_the_GFCI_ are unequal, because
one of the two wires is not connected to the GFCI.

I think that YOU need to pay attention:
In the wire that is not connected we can agree that the current is zero?
In the wire that is connected, what do you think the current is?
If it is zero, they are equal and the GFCI should not trip.
If it is not zero, where do you think that current is going?
Those electrons have to go (come from) somewhere. *They are not
getting stored in the fan. *They could be going to ground, but
only if you have a ground fault.

MQ-

Sorry for the potentially redundant post but......

Since the GFI is tripping we know that somehow the currents compared
by the GFI's circuit are different; the hot is zero since the switch
is open, thus the current at on the neutral is non-zero.

If it is not zero, where do you think that current is going?

It's going from the fan collapsing field thru the neutral wire back to
the neutral / ground bus at the service panel.

Those electrons have to go (come from) somewhere. *
They are not getting stored in the fan.

Actually, they're kinda stored "stored in the fan"......in the
magnetic field, while the fan is running.

They could be going to ground, but only if you have a ground fault.

They are going to ground, but not thru a fault....but thru the neutral
wire.

MQ, this is just my understanding of how motors & GFI can
interact......... I could be wrong but I believe this is a reasonable
theory.

cheers
Bob- Hide quoted text -

- Show quoted text -

Any current produced in this way will be way too small to trip the
GFCI. The capacitance to ground of the now open hot wire bewteen the
motor and the switch is way too small to support the required current.
If this theory was correct then the GFCI could be made to trip by
applying line voltage across the motor from a separate branch circuit,
one on the same leg, while the switch is open. Back emf is voltage,
not current.

Though I applaud the effort.

How about static discharge from their finger to the switch when they
go to turn it off?

How about static discharge from their finger to the switch
when they go to turn it off?

I considered that, but the GFCI trips even if I just touch the switch
handle (no metal conductive parts).

It will sometimes also trip three or four times in a row. Trip, reset,
trip, reset, etc. Then it'll go several weeks without tripping again.

The GFCI, switch, wiring, and fan are all new. I've already replaced the
GFCI and switch to rule those out as faulty. Everything is properly wired
and grounded.

My in-laws live out of town so I don't get up that way very often, but I'm
going to try the double-pole switch on our next visit. If that fails, it
has to be a faulty fan, or a problem with the wiring.

Anthony

On Dec 31, 11:24*am, HerHusband wrote:
How about static discharge from their finger to the switch
when they go to turn it off?

I considered that, but the GFCI trips even if I just touch the switch
handle (no metal conductive parts).

It will sometimes also trip three or four times in a row. Trip, reset,
trip, reset, etc. Then it'll go several weeks without tripping again.

The GFCI, switch, wiring, and fan are all new. I've already replaced the
GFCI and switch to rule those out as faulty. Everything is properly wired
and grounded.

My in-laws live out of town so I don't get up that way very often, but I'm
going to try the double-pole switch on our next visit. If that fails, it
has to be a faulty fan, or a problem with the wiring.

Anthony

Actually it sounds like a wiring issue. Conductors have an inductive/
capacitive link between them. If you have a stray conductor running a
different load that is close to and parallel to the wires feeding the
fan motor, then you'll get a current imbalance through the GFCI.

In article , hvacrmedic wrote:

Actually it sounds like a wiring issue. Conductors have an inductive/
capacitive link between them. If you have a stray conductor running a
different load that is close to and parallel to the wires feeding the
fan motor, then you'll get a current imbalance through the GFCI.

That doesn't explain why the GFCI trips _only_ when the fan is switched off.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

Actually it sounds like a wiring issue. Conductors have an inductive/
capacitive link between them. If you have a stray conductor running a
different load that is close to and parallel to the wires feeding the
fan motor, then you'll get a current imbalance through the GFCI.

Nope, other than where the cable exits the switch box, there are no other
cables anywhere near the cable running to the ceiling fan.

Also, the fan NEVER trips while running, only when it is turned OFF. And
even then, it's only once every few weeks or so.

Anthony

On Jan 1, 10:32*am, (Doug Miller) wrote:
In article , hvacrmedic wrote:

Actually it sounds like a wiring issue. Conductors have an inductive/
capacitive link between them. If you have a stray conductor running a
different load that is close to and parallel to the wires feeding the
fan motor, then you'll get a current imbalance through the GFCI.

That doesn't explain why the GFCI trips _only_ when the fan is switched off.

Maybe the two effects (motor generator action, and capactive bleeding
to ground) are combining thier efforts to trip the GFCI. I don't
know. But I do know there are millions of GFCI's out there that don't
trip when a motor connected to them is turned off. I also know that
if you run more than about 250 ft of extension cord that the
capacitive coupling between the hot wire and the ground wire will
cause the GFCI receptacle that you have it plugged into to trip.

Capacitive/inductive coupling can lower the GFCI's tolerance by
providing a continuous background leakage. If the GFCI is rated to
trip at .X amps, then capacitive leakage to ground can reduce that
trip current to a small fraction of .X. Any spurious signal is likely
to cause a trip at that point, even the neighbors ham radio.

It's possible that this particular GFCI is a little more sensitive
than it should be, and swapping it with a different one may cure the
problem.

Switching both the hot and the neutral through a double-pole switch is
almost certain to fix it.

Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
days ago.

As always, it trips when the fan switch is turned "OFF", not while it is
running.

I've tried two different GFCI's, and three different switches, and they all
have the same results.

I'm baffled. The wiring is new and in good condition. The only two things I
can think are the fan itself is bad, or condensation is draining back into
the fan and causing problems.

Still, it seems like the DPST switch would completely isolate the fan from
the GFCI. There shouldn't be any current flow even if there was a problem
with the fan. And there have been absolutely no problems while it is
running. Only when it is turned off.

Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?

Anthony

In article , HerHusband wrote:
It's possible that this particular GFCI is a little more sensitive
than it should be, and swapping it with a different one may cure the
problem.

Switching both the hot and the neutral through a double-pole switch is
almost certain to fix it.

Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
days ago.

Please describe how you wired the DPST switch; post a diagram if possible.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On Mon, 21 Jan 2008 13:15:26 -0600, HerHusband
wrote:

It's possible that this particular GFCI is a little more sensitive
than it should be, and swapping it with a different one may cure the
problem.

Switching both the hot and the neutral through a double-pole switch is
almost certain to fix it.

Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
days ago.

As always, it trips when the fan switch is turned "OFF", not while it is
running.

I've tried two different GFCI's, and three different switches, and they all
have the same results.

I'm baffled. The wiring is new and in good condition. The only two things I
can think are the fan itself is bad, or condensation is draining back into
the fan and causing problems.

Still, it seems like the DPST switch would completely isolate the fan from
the GFCI. There shouldn't be any current flow even if there was a problem
with the fan. And there have been absolutely no problems while it is
running. Only when it is turned off.

Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?

Anthony



I have a similar problem. I noted that when the light bulb on the fan
fixture is illuminated the problem does not occur. Perhaps a
resistive load across the fan absorbs any power line spikes which
trip the GFCI. Just an idea.

Mike


Note: my return address contains no numeric characters.

Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a
couple of days ago.

Please describe how you wired the DPST switch; post a diagram if
possible.

As you would expect. The incoming hot wire goes to one pole of the switch,
the incoming neutral wire goes to the other pole. Then the switched hot and
neutral go directly to the fan.

The ground wire is directly tied to all other grounds.

The switch clearly indicates which two terminals are the "line" and which
two terminals are the "load". Outgoing hot wire is on the same pole as the
incoming hot wire, and the outgoing neutral is on the same pole as the
incoming neutral.

I don't see any other way you could wire the switch?

Anthony

I have a similar problem. I noted that when the light bulb on the fan
fixture is illuminated the problem does not occur. Perhaps a
resistive load across the fan absorbs any power line spikes which
trip the GFCI. Just an idea.

In my case, the fan is just a fan. No light or heater in it.

Wiring is about as simple as it gets. A single incoming 12/2 cable coming
from the switch. Fan housing is grounded, hot and neutral are connected
according to the fan instructions (black to black, white to white. No
brainer).

Anthony

In article , HerHusband wrote:
Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a
couple of days ago.

Please describe how you wired the DPST switch; post a diagram if
possible.

As you would expect. The incoming hot wire goes to one pole of the switch,
the incoming neutral wire goes to the other pole. Then the switched hot and
neutral go directly to the fan.

The ground wire is directly tied to all other grounds.

If it's *still* causing the GFCI to trip, then I see only two possibilities:
1) the original GFCI *and* the replacement are defective
2) there is a ground fault in the fan itself.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On 2008-01-21, HerHusband wrote:

As always, it trips when the fan switch is turned "OFF", not while it is
running.

How quickly after you turn off the swich does the GFCI trip, and how
long does it take for the fan to fully spin down?

Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?

I can't really think of any. Sorry.

Cheers, Wayne

As always, it trips when the fan switch is turned "OFF", not while it
is running.

How quickly after you turn off the swich does the GFCI trip

Instantly as far as I can tell. Flip the switch off and the GFCI trips at
the same time.

how long does it take for the fan to fully spin down?

I don't know. Probably a couple of seconds at least.

Anthony

Doug,

Well, it was worth a try, but the DPST switch did not fix the
problem either. It worked fine for about two weeks, then tripped
again a couple of days ago.

If it's *still* causing the GFCI to trip, then I see only two
possibilities:

1) the original GFCI *and* the replacement are defective

Seems unlikely. I've never had a GFCI fail before, so I can't imagine two
being bad, especially since they are two different brands.

2) there is a ground fault in the fan itself.

That seems to be the only thing remaining, though I don't understand how it
could trip the GFCI if the DPST switch cuts it out of the circuit
completely?

I wired the two bathrooms in our house the exact same way and we've never
had a problem with the fans tripping the GFCI's.

Anthony

In article , HerHusband wrote:
Doug,

Well, it was worth a try, but the DPST switch did not fix the
problem either. It worked fine for about two weeks, then tripped
again a couple of days ago.

If it's *still* causing the GFCI to trip, then I see only two
possibilities:

1) the original GFCI *and* the replacement are defective

Seems unlikely. I've never had a GFCI fail before, so I can't imagine two
being bad, especially since they are two different brands.

Oh, I agree, it's very unlikely. Still possible, though...

2) there is a ground fault in the fan itself.

That seems to be the only thing remaining, though I don't understand how it
could trip the GFCI if the DPST switch cuts it out of the circuit
completely?

Ground and neutral wired together at some point in the circuit?

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.

On Dec 24 2007, 3:19*pm, "RBM" wrote:
This is not an uncommon anomaly. As others have said, the NEC doesn't
require GFCI protection for the fan unless it's located over the tub, and
the manufacturer requires it

"HerHusband" wrote in message

...



We recently remodeled my in-laws bathroom and, as per code, the entire 20
amp bathroom circuit is protected by a 20A GFCI outlet. Everything was
inspected and functions as it should.

Unfortunately, the GFCI is tripping occasionally when the bathroom vent
fan
is turned "off". It never trips when turning on the fan, only when turning
it off. And, it may go a few weeks before it trips, and then suddenly trip
three times in a row. Then it'll work fine again for a few weeks.

I suspected a bad GFCI, so I replaced it with a new one, and also replaced
the switch that controls the fan. It doesn't seem to trip as often now,
but
it tripped for me again last night when we were visiting.

The only remaining item seems to be the fan itself, but I'm curious if
there's some other possible cause I might be overlooking? The fan has been
installed for a few months now, so I probably can't return it at this
point.

I thought maybe condensation from moisture draining back through the vent
pipe (it had to be routed up over a beam to get out through the side
wall).
But, the tripping doesn't really seem to be related to when the shower was
used recently.

Thoughts?

Anthony- Hide quoted text -

- Show quoted text -

the NEC doesn't require GFCI protection for the fan unless it's
located over the tub, and the manufacturer requires it

I'm not pushing back...just trying to understand the wording above.

The "and" in that sentence seems to indicate that the manufacturer has
the final say. In other words, the NEC requirement is based on 2
criteria: location and the manufacturer's requirement.

Are there fans that can be installed above a tub but not require a
GFCI protection because the manufacturer doesn't require one?

Thanks

On 2008-01-23, DerbyDad03 wrote:

the NEC doesn't require GFCI protection for the fan unless it's
located over the tub, and the manufacturer requires it

I'm not pushing back...just trying to understand the wording above.

The "and" in that sentence seems to indicate that the manufacturer has
the final say. In other words, the NEC requirement is based on 2
criteria: location and the manufacturer's requirement.

The NEC requirement is just the general one that equipment be
installed according to manufacturer's requirements. Manufacturers
typically say that their fans may be installed over showers if they
are installed on a GFCI. I don't know if there are any that allow
installation over a shower without GFCI protection.

Cheers, Wayne

Mike Hennessey wrote:
On Mon, 21 Jan 2008 13:15:26 -0600, HerHusband
wrote:

It's possible that this particular GFCI is a little more sensitive
than it should be, and swapping it with a different one may cure the
problem.
Switching both the hot and the neutral through a double-pole switch is
almost certain to fix it.
Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
days ago.

As always, it trips when the fan switch is turned "OFF", not while it is
running.

I've tried two different GFCI's, and three different switches, and they all
have the same results.

I'm baffled. The wiring is new and in good condition. The only two things I
can think are the fan itself is bad, or condensation is draining back into
the fan and causing problems.

Still, it seems like the DPST switch would completely isolate the fan from
the GFCI. There shouldn't be any current flow even if there was a problem
with the fan. And there have been absolutely no problems while it is
running. Only when it is turned off.

Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?

Anthony



I have a similar problem. I noted that when the light bulb on the fan
fixture is illuminated the problem does not occur. Perhaps a
resistive load across the fan absorbs any power line spikes which
trip the GFCI. Just an idea.


I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of the
spike depends on where in the sine wave the fan is turned off - random
effect. Fans intended to be used on GFCI circuits could be built with
more winding isolation. I haven't seen aanswer yet that better fits what
happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike from
the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".

--
bud--

On Thu, 24 Jan 2008 11:13:42 -0600, bud--
wrote:

Mike Hennessey wrote:
On Mon, 21 Jan 2008 13:15:26 -0600, HerHusband
wrote:

It's possible that this particular GFCI is a little more sensitive
than it should be, and swapping it with a different one may cure the
problem.
Switching both the hot and the neutral through a double-pole switch is
almost certain to fix it.
Well, it was worth a try, but the DPST switch did not fix the problem
either. It worked fine for about two weeks, then tripped again a couple of
days ago.

As always, it trips when the fan switch is turned "OFF", not while it is
running.

I've tried two different GFCI's, and three different switches, and they all
have the same results.

I'm baffled. The wiring is new and in good condition. The only two things I
can think are the fan itself is bad, or condensation is draining back into
the fan and causing problems.

Still, it seems like the DPST switch would completely isolate the fan from
the GFCI. There shouldn't be any current flow even if there was a problem
with the fan. And there have been absolutely no problems while it is
running. Only when it is turned off.

Other than rewiring to take the fan off the GFCI, or replacing the fan, are
there any other things I could try?

Anthony



I have a similar problem. I noted that when the light bulb on the fan
fixture is illuminated the problem does not occur. Perhaps a
resistive load across the fan absorbs any power line spikes which
trip the GFCI. Just an idea.


I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of the
spike depends on where in the sine wave the fan is turned off - random
effect. Fans intended to be used on GFCI circuits could be built with
more winding isolation. I haven't seen aanswer yet that better fits what
happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike from
the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".

If this is true then a bleed off resistor could serve as an
incandescent lamp.

This is a total wild ass guess on my part. I would be very interested
in knowing if this could work.

Wayne Whitney wrote:
On 2008-01-23, DerbyDad03 wrote:

the NEC doesn't require GFCI protection for the fan unless it's
located over the tub, and the manufacturer requires it

I'm not pushing back...just trying to understand the wording above.

The "and" in that sentence seems to indicate that the manufacturer has
the final say. In other words, the NEC requirement is based on 2
criteria: location and the manufacturer's requirement.

The NEC requirement is just the general one that equipment be
installed according to manufacturer's requirements. Manufacturers
typically say that their fans may be installed over showers if they
are installed on a GFCI. I don't know if there are any that allow
installation over a shower without GFCI protection.


A good description of manufacturer's requirements being mandatory, not
optional.

There are also requirements from the agency approving the device. That
is usually UL. Those requirements are also mandatory.

Bath fans and GFCIs came up about a month ago:
From the UL "White Book"
"Fans intended to be mounted over tubs or showers have been evaluated
for such purposes and are marked 'Acceptable for use over a bathtub or
shower when installed in a GFCI protected branch circuit.’"

The manufacturer will always require GFCI protection for a fan over a
tub. If the manufacturer misses it, it is required by UL. [Old fans may
have been made when the UL standard was different.]

(The UL "White book" is available at:
http://www.ul.com/regulators/2006WhiteBook.pdf
5.5M and not real easy to use)

The "final say" may be NEC, manufacturer, or approval agency.

--
bud--

bud-- wrote:

Mike Hennessey wrote:
....
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of the
spike depends on where in the sine wave the fan is turned off - random
effect. Fans intended to be used on GFCI circuits could be built with
more winding isolation. I haven't seen aanswer yet that better fits what
happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike from
the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".


If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.

Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.

M Q wrote:


bud-- wrote:

Mike Hennessey wrote:
...
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of
the spike depends on where in the sine wave the fan is turned off -
random effect. Fans intended to be used on GFCI circuits could be
built with more winding isolation. I haven't seen answer yet that
better fits what happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike
from the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".


If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.


Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.


If a supply wire was near an end of the armature - could be.

--
bud--

bud-- wrote:

....

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.


Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.


If a supply wire was near an end of the armature - could be.


Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.

The point is: don't ignore a ground fault on the neutral side.

On Thu, 24 Jan 2008 12:56:52 -0600, bud--
wrote:

M Q wrote:


bud-- wrote:

Mike Hennessey wrote:
...
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of
the spike depends on where in the sine wave the fan is turned off -
random effect. Fans intended to be used on GFCI circuits could be
built with more winding isolation. I haven't seen answer yet that
better fits what happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike
from the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".


If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.


Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.


If a supply wire was near an end of the armature - could be.

Reverse the motor leads in single phase? Doesn't this happen 120
times per second?

Wouldn't an incandescent lamp be a purely resistive load?

Terry wrote:
On Thu, 24 Jan 2008 12:56:52 -0600, bud--
wrote:

M Q wrote:

bud-- wrote:

Mike Hennessey wrote:
...
I suggested earlier the high voltage spike that can be produced in the
fan winding when the switch is turned off could produce capacitive
currents from the fan winding the motor poles (ground). The size of
the spike depends on where in the sine wave the fan is turned off -
random effect. Fans intended to be used on GFCI circuits could be
built with more winding isolation. I haven't seen answer yet that
better fits what happens.

In Mike's description above, if the light is connected across the fan
when the fan is turned off the light could absorb part of the spike
from the motor winding.

I don't think there is a fix other than "take the fan off the GFCI, or
replacing the fan".

If, as you suggest, there is a voltage spike when the fan is shut off,
and there is more capacitive coupling from one of the fan terminals to
ground than the other, then adding a capacitor across the fan leads
might reduce that.

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV might
work but I wouldn't connect one L-N without protection. My guess is a
shunt resistance would probably have to be too low value so that it
dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.

Another possibility: I have often seen the armature of motors, when they
turn on or off, shift axially. If so, there could be a transient short.

If a supply wire was near an end of the armature - could be.

Reverse the motor leads in single phase? Doesn't this happen 120
times per second?

If the trip is caused by capacitance to ground and spike, and if the
capacitance to motor frame (ground)is lower from the neutral end of the
winding than the hot end, reversing the leads may help. I wouldn't bet
on it.

Probably not stated previously, a spike is not only high voltage, but
because it is short duration it is high frequency. High freq increases
capacitive currents.

Another possibility - capacitance from hot wire to ground wire in Romex
between fan and GFCI.


Wouldn't an incandescent lamp be a purely resistive load?

Sure. If you don't mind a light turned on with the fan, it works for
Mike. May work for the OP.

--
bud--

M Q wrote:


bud-- wrote:

...

Assume capacitance is equal from both hot and neutral to pole. Leakage
also depends on voltage. The neutral end is connected to pole (at the
service panel). The voltage from N-to-pole will be minimal. I suspect
the winding is constructed so the neutral end is most toward the pole
but don't know. Could try reversing the motor H-N leads.

Capacitor across H-N - maybe. The capacitor does not dissipate the
energy but may spread it over time and lower the peak voltage. MOV
might work but I wouldn't connect one L-N without protection. My guess
is a shunt resistance would probably have to be too low value so that
it dissipates significant energy.

I wouldn't guarantee trip is from capacitance-spike but it is the best
guess I can come up with.


Another possibility: I have often seen the armature of motors, when
they
turn on or off, shift axially. If so, there could be a transient short.


If a supply wire was near an end of the armature - could be.


Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher
frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.

The point is: don't ignore a ground fault on the neutral side.


What I remembered was inducing a current into just the neutral. A
manufacturer's datasheet shows common mode induction into both H & N
like you said. That would also give "neutral"-ground fault detection if
the hot and neutral wires were reversed. The data sheet shows a fullwave
rectified 60Hz source, which would be 120Hz with harmonics.

When there is load on a circuit with a GFCI, resistance will cause a
voltage from neutral to ground, and there will be current in a N-G fault
that will trip the GFCI. With the added circuitry a GFCI will trip on a
N-G fault with no load.

(datasheet is at
http://cache.national.com/ds/LM/LM1851.pdf

--
bud--

According to M Q :

Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.

Detecting current difference between hot and neutral is a lot
simpler than that.

If the neutral is shorted to the ground, the ground wire acts
as a parallel conductor to the neutral which reduces the neutral's
current, hence, the hot and neutral won't match.

GFCI's work by running the neutral and ground around a magnetic core,
measuring the net magnetic flux, and triggering if it exceeds a certain
threshold. If the neutral and hot currents are the same, the magnetic
flux cancels out.

http://www.codecheck.com/gfci_principal.htm
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.

Chris Lewis wrote:

According to M Q :


Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.


Detecting current difference between hot and neutral is a lot
simpler than that.

If the neutral is shorted to the ground, the ground wire acts
as a parallel conductor to the neutral which reduces the neutral's
current, hence, the hot and neutral won't match.

GFCI's work by running the neutral and ground around a magnetic core,
measuring the net magnetic flux, and triggering if it exceeds a certain
threshold. If the neutral and hot currents are the same, the magnetic
flux cancels out.

http://www.codecheck.com/gfci_principal.htm

I see that you didn't even read the link that you provided.
"To detect a Neutral to Ground fault there is a second transformer placed
upstream of the H-G sense transformer. A small drive signal is injected ...
which induces equal voltages on the H and N wires passing through its core."

According to bud-- :
M Q wrote:
Chris Lewis wrote:

According to M Q :


Note that GFCIs detect a fault from neutral to ground also. From the
schematics I have seen, I believe they do this by inducing a higher
frequency
common voltage on the the hot and neutral. If there is neutral
to ground fault there will be a common mode current of this higher
frequency signal which will be detected by the same circuitry and trip.


Detecting current difference between hot and neutral is a lot
simpler than that.

If the neutral is shorted to the ground, the ground wire acts
as a parallel conductor to the neutral which reduces the neutral's
current, hence, the hot and neutral won't match.

GFCI's work by running the neutral and ground around a magnetic core,

A typo - neutral and hot instead of neutral and ground.

Whoops yup.

measuring the net magnetic flux, and triggering if it exceeds a certain
threshold. If the neutral and hot currents are the same, the magnetic
flux cancels out.

http://www.codecheck.com/gfci_principal.htm

I see that you didn't even read the link that you provided.
"To detect a Neutral to Ground fault there is a second transformer
placed upstream of the H-G sense transformer. A small drive signal is
injected ... which induces equal voltages on the H and N wires passing
through its core."

What is detected by adding the CT in MQ's post is a N-G fault with no
load. Without the added CT, the simple GFCI in Chris' post won't detect
a N-G fault until there is a load on the circuit. Other than that a GFCI
operates as Chris describes.

It seems rather strange even to do that. The slightest bit of load
on the circuit, either before or after the GFCI, will mean
that the neutral is at a different potential than the ground. If
the neutral shorts to the ground after the GFCI, then there will be
current flow thru the neutral at the GFCI to the ground without the hot
at the GFCI seeing anything.

I was surprised to learn this function was built into GFCIs. But it only
adds 2 parts.

Me too. I shoulda read a bit farther ;-)
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.