Whirlpool RF0100 electric stove trips breaker.

[David Farber]

This Whirlpool electric stove, model number RF0100, appears to be an older
model. If you were wondering what blasted the ground lead of my Kill-A-Watt
meter which I posted about two days ago in the thread about the Phillips
security screw, this is the culprit. The schematic is here:
http://members.dslextreme.com/users/farberbear/Repair/Electric-stove.jpg as
I photographed it from the back of the stove.

I used to have the stove hooked up to a 240 volt circuit as indicated in the
schematic and it worked ok. But, now I don't have a 240V line. Or at least
not yet. I was trying to at least get it to work on 120V because only the
oven will be used and at very low heat, probably around 200 degrees to cook
some ceramics. The stovetop and broiler wires have been disconnected because
they are not needed. This particular model does not have a light or a clock
as shown in the schematic. I wired it so that L1 went to hot (black), L2
went to N (white), and N to ground (green). The load across the electric
plug, hot to neutral, with the oven switch on, is 17 ohms. That makes sense
to me. However the impedance reading from neutral to ground makes no sense
to me. I've tried two different dvm's and they both give bizarre results.
When starting at the 2M ohm range, the meter starts at about 700k and counts
down to about 300k and holds there. When I move the switch over to the 20M
ohm range, the meter starts at about 2.3M ohm and counts up. The other meter
which is a portable meter counts backward too and then the minus sign
appears on the meter as if current is coming into the meter. From what I can
see, there aren't any capacitors or semiconductors interfering. The oven
thermostat has a thermocouple wired to it which for some reason is not shown
in the schematic. I opened the thermostat and found an interesting
mechanical coupling that moves a slider bar that depresses either the oven
element switch, the broil element switch, or both. Then there is the
thermocouple wire which is wrapped(?) internally around the rotating control
that selects the oven temperature. I don't usually open oven thermostats so
I'm not sure what exactly is going on inside because once you open the
thermostat,
there is a spring inside the device which displaces everything. In any case,
at room
temperature, there is a connection from L1 to the bake and broil terminals
so I know
it's sending power through to those elements.

Not trusting anything about these electrical readings, I first plugged the
oven into my Sencore PR57 isolation transformer. The transformer can only
supply about 300 watts but I was more interested in measuring leakage
current. No surprise when the low side leakage to the metal frame measured
full deflection even with the power off to the oven. Not exactly the best
situation. The high side leakage was zero. I turned up the voltage slowly
and the current seemed to rise proportionately to the 17 ohm load until I
stopped at about 300 watts. I should mention that the heating element did
not seem to get warm. Also, the heating element impedance matched the load
across the power plug so that seemed like a good sign.

Then I plugged it directly into the wall outlet via the Kill-A-Watt meter. I
turned on the oven switch and an instant trip of the circuit breaker at the
sub panel occurred. So where is all this extra current coming from to cook
the ground plug on my Kill-A-Watt and trip the breaker? I must be missing
something very obvious.

Thanks for your reply.

 

[David Farber]

I'm not really sure which heating element is still connected. Most of
the elements' wattages are listed on the wiring diagram, but the two
on the left side (bake, broil) of the diagram aren't (that I could
see). It could be, that the wattage of the element being used exceeds the
total wattage of the circuit breaker.
I would check to see if there are any other loads applied to the same
AC supply circuit.

Those temperature control symbols may represent the type that have
internal heated bi-metal type switches in them (dunno what they're
called in the trade, but commonly used in cooking appliances with
heating elements). In some Corning laboratory type hot plates that I have,
these types of
switches make a fairly distinct sound.. a sort-of "tink" when the
when the switch's internal heated bi-metal strip opens the switch.
I believe a cam closes the switch contacts, then the heated bi-metal
strip opens the switch (a calculated/designed time delay, I guess).

The small bi-metal heater wire (inside the switch(s) will naturally
have a DC resistance. I wouldn't think that internal heater wire
resistance would change from just a DMM test lead voltage, but maybe
they do. If I could get to those lab hot plates, I'd do some checking
instead of just speculating.

Hi WB,

There's an upper and a lower element in the stove section. The top one is
called the broil and bottom one is the bake. When you turn the stove knob
from off to on, both broil and bake elements get energized. I disconnected
the broil element because I will not be doing any broiling. So what's left
is the bake element at the bottom. That presents about a 17 ohm load which
translates into approximately 850 watts. If the supply voltage were 240V,
that would increase the power to 3,400 watts. I'm not even sure 850 watts
can raise the temperature to 200°F (assuming I can get it working) in a
reasonable amount of time. Regarding the circuit breaker, there was really
nothing else on that circuit at the time it tripped. Even so, why would the
ground terminal be the one to take the hit? There shouldn't be any current
flowing through there, right? The way the stove is wired now is that the
neutral wire of the stove is connected to where L2 was before so the circuit
path is from L1, through the oven switch, through the heating element,
through L2 which is now wired to neutral.

Thanks for your reply.

 

[David Farber]

Regarding the circuit breaker, there was really

David,

Is the breaker a GFCI type? That would explain the tripping, as the
way you have the stove wired, there is a large potential between
neutral and ground (in terms of a GFCI).

Not a GFCI. The circuit breaker is 20 amps and located in the sub panel.

How do you account for the large potential between neutral and ground? Since
there is no oven light or clock, the neutral wire in the schematic doesn't
really connect to anything except the chassis ground. Though I do agree with
you that in order for that circuit to trip and burn out the ground pin,
there is some major current flowing. I'd just like to be able to follow it
from beginning to end.

Thanks for your reply.

 

[David Farber]

There is a comment included in the text on the wiring diagram label,
wrt a bonding strap.

I think you already know, but the earth ground should only have a
path to the exposed metal cabinet parts, and all other parts need to
be electrically isolated from earth ground (the insulating quality of
the isolation needs to exceed the applied/working voltage, hi-pot
being the normal convention).
I believe an 800W element will create a lot of heat inside an
enclosure. I have a 500W 120VAC heater intended to heat engine block
oil pans in winter, and in low light in open air, the deep red color
of the element armor can be seen (not nichrome or resistance wire).

Here's the way I see it. Starting from L1, current flows through the limit
switch then to the thermostat. If the thermostat switch is turned on by the
thermocouple, then the current continues on to the bake element and then the
circuit is completed when it connects to L2, which is where the neutral line
is. I don't see any connection to that and the metal cabinet parts. So
either my analysis is wrong or there is current leaking to ground.

Just so there is no confusion, from left to right in the schematic, the
original 240V wiring of L1, N, L2 has been changed to 120V wiring of, Line,
Ground, and N. I can remove the bonding strap which connects the chassis to
N in the 240V schematic or Ground in the 120V version that I came up with.
I'd just like to understand what's going on here.

Thanks for your reply.

 

[David Farber]

Here's the way I see it. Starting from L1, current flows through the
limit switch then to the thermostat. If the thermostat switch is
turned on by the thermocouple, then the current continues on to the
bake element and then the circuit is completed when it connects to
L2, which is where the neutral line is. I don't see any connection to
that and the metal cabinet parts. So either my analysis is wrong or
there is current leaking to ground.
Just so there is no confusion, from left to right in the schematic,
the original 240V wiring of L1, N, L2 has been changed to 120V wiring
of, Line, Ground, and N. I can remove the bonding strap which
connects the chassis to N in the 240V schematic or Ground in the 120V
version that I came up with. I'd just like to understand what's going
on here.
Thanks for your reply.

Ok so I removed the bake and broil heating elements. I measured each
element's resistance from one end of the element to its metal supporting
plate. And that's where the funky resistance measurements are happening.
There must be some partial internal short from the hot wire inside the
element to the outside supporting tubing. I can't imagine that the
resistance between the two would be less than 1M ohm. The readings were
counting down to as low as 20K ohms. On the other hand, it sure seems
unlikely that both elements would have exactly the same fault. Anyone have a
heating element that they'd like to check the insulating resistance for
comparison?

Thanks for your reply.

 

[Jamie]

Ok so I removed the bake and broil heating elements. I measured each
element's resistance from one end of the element to its metal supporting
plate. And that's where the funky resistance measurements are happening.
There must be some partial internal short from the hot wire inside the
element to the outside supporting tubing. I can't imagine that the
resistance between the two would be less than 1M ohm. The readings were
counting down to as low as 20K ohms. On the other hand, it sure seems
unlikely that both elements would have exactly the same fault. Anyone have a
heating element that they'd like to check the insulating resistance for
comparison?

Thanks for your reply.

You have bad heaters... they shouldn't be below 1M of IR..

If they are connected to together in some way, it's possible one
shorted, causing the other to get over voltage...

Just a theory.. but sounds plausible...

Some food may have splashed on the heaters and cause an internal
thermo warp.. Like something cold hitting them..

 

[David Farber]

You have bad heaters... they shouldn't be below 1M of IR..

If they are connected to together in some way, it's possible one
shorted, causing the other to get over voltage...

Just a theory.. but sounds plausible...

Some food may have splashed on the heaters and cause an internal
thermo warp.. Like something cold hitting them..

Are these heating elements pretty much standard or do I have to get an exact
replacement?

Thanks for your reply.

 

[David Nebenzahl]

Are these heating elements pretty much standard or do I have to get an exact
replacement?

My go-to place for such appliance parts is Reliable Parts:
http://reliableparts.com, (877) 733-9241. They're well-stocked and the
people there are knowledgable. I'm going to guess that they'll have
replacements available.