3 Amp Fuse Blows Sporadically

109A_5

In an effort to NOT satisfy @EdTheHeaterMan 'amusing' statement "Or we can just tell @Blower55 to look for bad wiring again."
And also to help eliminate stuff that is apparently not relevant.

Blower55 said:

Blower55 said:
I looked back at my notes from the heating season and I had jumped R & G on the control board to isolate the blower with all other tsat wires disconnected from the board. The blower ran, but blew the fuse after a few minutes. It did not always blow immediately however.

Which I apparently forgot, overlooked or was distracted. If this above quote is true it eliminates all the external wiring, the thermostat and the compressor / condenser unit. The control board was changed, not much left that would blow that fuse!!!

Again my best guess so far is the safety device 11C Link, Fusible (over temperature) which may be a special heat sensitive wire that may surround the heat exchanger assembly has rubbed through. Or it has an integral grounded conductor (not shown on the wiring diagram) so when overheated it intentionally causes a short circuit blowing the 3 Amp fuse, shutting down the furnace. Age and vibration may have caused its dilapidation or there was a mild overheating event due to a restricted filter or evaporator core making this device intermittent.

Blower55

Good morning. I do not have the old board, so I cannot show pictures of it. I have included the instructions that came with the new board. The AC worked throughout the night and is still working as I write this at 9:30am EST. Any chance this might be a transformer that is going bad and sending inconsistent voltage that would cause the fuse popper to trip? When I tested it at the board, it was reading 26.5, but I am wondering if this type of component simply fails outright or can go bad gradually, resulting in the kind of unpredictable operation I am experiencing.

mattmia2

109A_5 said:

Connecting an ammeter, to me, (as much as I do like ammeters) probably will only reveal three things two of which we already know exist. (1) normal current, (2) fault current (if it will fail while you are watching, don't blink), (3) an abnormally high running current (just over 3 Amps), like if the blower control relay coil is partially shorted. However I think that relay was changed when the board was changed.

It could also reveal something like a thermostat with a bad charging circuit or battery or other component that is causing it to draw far more than its rated current and put the current through the fuse at a little over the fuse's rating causing it to blow/trip over time.

The breaker may not be a very consistent device either, it may vary considerably when it trips and if it tripped on a thermal mechanism you need to let it cool before it will handle its full rating without tripping again.

Did it trip with the only thing connected to the board a jumper between r and w or y g and r? I might try the jumper test gain if it trips with the jumper and the breaker with the jumper and a real fuse.

As far as inspecting the wiring in the furnace itself, if there are any harnesses with loom around them, you have to look inside the loom, it isn't uncommon for cables to melt or chafe inside loom. Also look very closely at terminals to see if there is a stray strand of wire poking at something and see what happens to the wire when the covers are on, if something gets pushed over or is touching the cover itself.

JUGHNE

One intermittent short I found in an older furnace was the high limit cut out.

It was inserted into the heat exchanger mounted on 2 buss bars. The thermal disk was mounted on the end of the busses.

Foil covered insulation was inside the furnace air plenum.

When the blower ran the foil would flop around and cause a short to ground....sometimes.
It finally showed out to be a solid dead short and was detected.

Removing the foil in that area fixed the problem.

Just another long shot to consider.

pedmec

you didn't state whether the blowing of the fuse was still happening in heat or a/c.

as stated before the reason it doesn't show the fuse in the wiring diagram is because the old board did not have a 3 amp fuse. diagram shown is what comes with the furnace on the inside of the door.

EdTheHeaterMan

109A_5 said:

@EdTheHeaterMan, I love the colored annotations. It may help some folks. Using your annotations I doubt it is #2 the (orange circuit) since the 3 Amp over-current device trips out in A/C mode too. 

I was not suggesting that those were the only three possibilities. I was trying to make a point that:

@Blower55 was not helping his cause with offering very limited information on his issue.  As you know, there is no possible way to help him without going in person.  We can not send our many years of experience and understanding of electrical circuits in a single post.  

But your attempt is admirable 

EdTheHeaterMan

JUGHNE said:

One intermittent short I found in an older furnace was the high limit cut out. It was inserted into the heat exchanger mounted on 2 buss bars. The thermal disk was mounted on the end of the busses. Foil covered insulation was inside the furnace air plenum. When the blower ran the foil would flop around and cause a short to ground....sometimes. It finally showed out to be a solid dead short and was detected. Removing the foil in that area fixed the problem. Just another long shot to consider.

Great observation!

I remember the first time that happened to me. It took forever to find that problem. That might be a good place to look

To test this theory, remove both wires from 7H1 limit switch. Leave them disconnected for the summer. If the limit switch is the issue, the fuse will no longer blow because the red wire is no longer connected to the limit switch and will no longer be able to short out as a result of the foil backing On the insulation to create the dead short that could blow a fuse or trip a circuit breaker

EdTheHeaterMan

To test this theory, remove both wires from 7H1 limit switch. (As shown on the original diagram) Leave them disconnected for the summer. If the limit switch is the issue, the fuse will no longer blow because the red wire is no longer connected to the limit switch and will no longer be able to short out as a result of the foil backing on the insulation to create the dead short that could blow a fuse or trip a circuit breaker

come heating season you will need to solve the problem by repairing the insulation so the foil does not touch the buss bars of that limit 



This would be the location of the short circuit on the diagram I posted earlier.  The RED line on the diagram is always powered with 24V from the Hot side of the transformer.  If the common side of the transformer is grounded. This would be a dead short that can cause a blown fuse or tripped breaker

Blower55

Folks, I am soaking all of this in and will definitely check that limit switch. I am trying to provide as many details as I possibly can, based on my observations and any testing I have done to this point. In terms of what is happening today, the system has worked flawlessly, so that is nearly an entire day now without any issues. This unpredictable operation makes it very hard to perform tests at the moment things fail. The annotated diagrams are very helpful, so thank you @EdTheHeaterMan for taking the time to do that.

Jamie Hall

In my experience the sort of very random failure which you are seeing is almost always a chafed or chewed wire or, as has been suggested, a random piece of metal occasionally touching something. It's almost never a component failure (I said ALMOST never...). The problem is, of course, that even if you disconnect all the wires which are in the circuit -- and the fuse doesn't blow -- you haven't proved anything. Worse, when you reconnect them a circuit at a time and, eventually, the fuse does blow... it isn't necessarily the last circuit you connected (it's tempting to think so, but remember it was random). Nor can you find it with a multimeter. The best approach is a little tedious, but you really have to go the length of each circuit and verify that the wire insulation is good. Replace anything suspicious.

jringel

I know a lot off information has gone back and forth in this thread but I didnt see if it was mentioned whether or not this unit was tied into a heat pump as it sounds a lot like the ac contactor or wiring to the outdoor unit. If a heat pump were installed that would explain the issue happening in the summer and winter.

109A_5

@EdTheHeaterMan, Thank You.

If you highlighted every possible scenario on this wiring diagram it may have become unreadable. I just wanted to logically remove paths that seemed not valid to this situation with the previously given information. I was hoping it would build some clarity.

Troubleshooting is a process of elimination. I understand that @Blower55 and others posting here may not have the technical savvy, skills, tools and equipment to maybe do all the needed tests, devise new tests and build on their outcome to find this type of intermittent defect.

I am hoping that even if it comes down to @Blower55 replacing the Furnace to fix the problem (I hope it does not come to that, seems that would be a waste of money) someone else may get some value out of these postings in the future. I think @Blower55 is doing especially well with the nature of an intermittent problem where patience, stamina and perseverance may be the best tools.

It seems to me (and I am guilty of it too) there is a lack of reading comprehension, lack of memory, lack of reading some posts, etc. here (not sure what or why, and it may be different in each case). Items that have been functionally eliminated with a valid troubleshooting strategy keep coming up to be retested, revisited and reanalyzed. In my experience this tactic is a total waste of time. I believe I have corrected my oversights in my recent posts.

Example; why are folks wanting to current test anything outside of the furnace cabinet? @Blower55 eliminated those loads in a previous post and the over-current device still tripped. And didn't the blowing of the 3 Amp fuse start this whole thing with the original controller board ?

I still believe that the safety device 11C Link, Fusible (over temperature) and / or the 7H1 limit switch are the most likely culprits in this case. However I can't force @Blower55 to find and do a through inspection, test them for intermittent behavior or bypass test (for the non-heating season) of these devices, and yes, I probably can't be there to do it for him.

I would not have discarded the old board. Was there a significant core charge? It could have been a spare since there was probably nothing wrong with it.

To address @Blower55 transformer question, I highly doubt (although probably not impossible) the transformer has a intermittent primary side partial short that would increase the secondary's voltage enough to trip an over-current device (intermittent transformer turns ratio change). If the case of the transformers output voltage decreasing or 'browning out' it probably would not trip an over-current device.

HVACNUT

Agree with @Jamie Hall. SOMEWHERE 
there is 24v wire intermittently shorting to Common or another ground. 
Since you mentioned that the condenser would stop, then start again a few minutes later, I would start there. 
Turn off the thermostat.
Shut off power to the condenser and remove the electrical access cover. Could be webs, dead mice, acorns, etc. in there. Webs do conduct electricity. Clean it out. I use a 3" diameter soft nylon brush. Inspect the low volt wires. Spade connections at the contactor. Are the connections rusty?Splices at the field to factory connections. To and from pressure switches if any. 
Look for any spot where the outer jacket of the wires might be torn or broken. 
Follow it back to the furnace inspecting every inch.

I had a callback today. (I'm very ashamed.) I did the A/C maintenance on Friday. First time the systems ran this season. 
No cooling, Master Bedroom. So I get there and its actually no cooling on all 4 zones. (Damper system, one air handler). 
At the air handler I measure 0v across Y and C, and 24v across R and Y. I go to the zone panel and on the terminals supplying the air handler I get 0v across R and Y, so the circuit is closed there. As I pull my hand away I moved the low volt wires and I heard a click. It turns out the wire at the Y terminal was broken and only being held by the insulation. R and Y were closed at the terminals, but never made it past that. On Friday I cycled it off while checking the air handler, and again while checking the condenser. Started right up no problem and continued to run while I was servicing other systems. 
This was a fairly simple find, but it can certainly be a PITA sometimes. 

JUGHNE

If you go for the test of the high limit, reference the picture that Ed posted, that is the type of limit I had problems with.

Some boards are smart enough to see the limit open on high furnace temp and will run the blower at high speed until corrected (furnace cools down).

If this is the case with your board then remove and jumper both the wires as if the limit switch was still closed, be sure the wires are isolated in free air and not touching anything.......this is a TEMPORARY test......do NOT leave the wires jumped as this creates a dangerous condition should you go to the heating mode.

Best to even shut off the gas supply valve as a reminder to correct the jumped switch.

You could remove the limit switch and show us what it looks like, please.

109A_5

@Blower55, Sorry I did not get back to you sooner on this question, I've been wading through many irrelevant but Good-Hearted posts to your problem.

Blower55 said:

The dim bulb option sounds interesting, but let's say I try that, and wiggling wires does not produce clues. The brightened bulb tells me a problem is occurring, but I think I would be right back to the mystery of root cause. As one person commented, maybe the new board I purchased is bad. It is an OEM part.

You have commented that the fuse and the 'lil popper' has tripped numerous times, I have lost count and also there has been many hours of normal run time also in-between trips. The Dim Bulb tester basically does two things when set up properly. Current limiting, so if it is used to replace the fuse or in series with the fuse, the fuse no longer blows and current is limited to a safe level just below the rating of the normal fuse's value. You can tell when a Tech has had a bad day when you see blown fuses littered around at a job site. Secondly if an over-current event happens the lamp(s) light up brightly if the event goes away they dim, nothing to reset to try again, enhances fast focused stimulation. Instant repeatable feedback.

Part of the Art of troubleshooting intermittent problems like this is imagination, what can I do to strategically, repeatably, to stimulate what ever is causing the intermittent situation. Including but not limited to Heating , Cooling, Vibration, gently moving wires, flexing housings, operating the equipment in different modes, what ever it takes, be creative. Something will stimulate it, I doubt it is Gamma Rays. However you have to find the correct place to administer the stimulation. If you knew what was causing the problem from the start you would have fixed it. It literally is a treasure hunt.

If you get the bulb to light up brightly one at a time disconnect all Fused 24 VAC paths that leave the board. When the bulb dims reconnect that circuit, load or sensor and it should get bright again, chase that path. Focused agitation and Strategic load shedding is the key. If you get to the point that all is disconnected from the board (except the power transformer) and the bulb(s) is still bright the problem is the board or something is touching under the board.

One of your previous posts, I missed the fact that you disconnected all the thermostat wires and jumpered the blower on continuously and the over-current device still tripped. This, to me, defines that the problem is within the Furnace housing. If you suspect a specific sensor (like I do) the Dim Bulb tester should be inserted between the Fused 24 VAC from the board and that sensor. Good quality Alligator clip leads help a lot. The 'lil popper' can be used for general protection at the 3 Amp fuse location.

Hope this helps and makes sense to you.

mattmia2

109A_5 said:

Example; why are folks wanting to current test anything outside of the furnace cabinet? @Blower55 eliminated those loads in a previous post and the over-current device still tripped. And didn't the blowing of the 3 Amp fuse start this whole thing with the original controller board ?

It is not clear the way it is worded if everything was disconnected including the condenser and a jumper was placed in place of the thermostat or if only 2 wires from the thermostat for the desired function were connected and/or the contactor in the condenser was still connected.

109A_5

mattmia2 said:

109A_5 said:

Example; why are folks wanting to current test anything outside of the furnace cabinet? @Blower55 eliminated those loads in a previous post and the over-current device still tripped. And didn't the blowing of the 3 Amp fuse start this whole thing with the original controller board ?

It is not clear the way it is worded if everything was disconnected including the condenser and a jumper was placed in place of the thermostat or if only 2 wires from the thermostat for the desired function were connected and/or the contactor in the condenser was still connected.

Blower55 said:
I looked back at my notes from the heating season and I had jumped R & G on the control board to isolate the blower with all other tsat wires disconnected from the board. The blower ran, but blew the fuse after a few minutes. It did not always blow immediately however.

I was not there to supervise, so I have to take @Blower55 at his word that he did what he said he did. Seem quite clear to me "all other tsat wires disconnected from the board" I'm thinking "all" means every thermostat wire. If you look at @EdTheHeaterMan's annotations of the wiring diagram where he added the thermostat and the compressor / condenser unit (looks fairly typical to me) even if you just remove the "R" wire you have removed the only non-grounded (probably fused) 24VAC power source from all the external wiring.

Troubleshooting is about systematically eliminating what is NOT the problem, eventually you get to the problem. If done correctly with the correct tools and strategies defects like this are found amazingly fast.

Why make this harder than it needs to be ???

mattmia2

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

109A_5

mattmia2 said:

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

I should not even respond, Sorry folks, this is beyond silly and probably not helping @Blower55 except for entertainment value. Do you have any clue as to how the thermostat controls the compressor / condenser unit ? My guess by what you have written is NO.

HVACNUT

mattmia2 said:

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

Typically 18 ga. thermostat wire, yes. And for straight up single stage A/C it's just 2 wires. Common (Ground) and 24v hot from Y. 
The contactor is not a switch like a thermostat. It uses voltage, in this case 24 volts, to energize a coil and magnetize it. That action will close contacts for another voltage source, in this case sending 240 volts to the compressor and condenser fan motor. 

109A_5

HVACNUT said:

mattmia2 said:

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

Typically 18 ga. thermostat wire, yes. And for straight up single stage A/C it's just 2 wires. Common (Ground) and 24v hot from Y. 
The contactor is not a switch like a thermostat. It uses voltage, in this case 24 volts, to energize a coil and magnetize it. That action will close contacts for another voltage source, in this case sending 240 volts to the compressor and condenser fan motor. 

@HVACNUT maybe you should also explain for the benefit of all, using the original wiring diagram above provided by @Blower55 and the annotated version by @EdTheHeaterMan that includes the thermostat and the compressor / condenser unit, where the 24VAC Hot that energizes the "Y" terminal comes from during an A/C call in this case. I'm sure @mattmia2 won't believe me.

EdTheHeaterMan

mattmia2 said:

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

This might be off topic

This is an interesting question. If you call a refrigerator a "Frigidaire", or a bowl of flavored gelatin, "Jello" or a utility knife a "Stanley", does that mean we can't use it? Calling a roll of 18 gauge solid core two conductor cable, "Thermostat Wire" does not make it unusable for connecting the contactor of a condensing unit to the furnace.

This is like calling a closed system circulator a PUMP. or saying that every fuel oil pump system needs a Tiger Loop, or choosing 26 gauge or 24 gauge galvanized smoke pipe. When we all should be more accurate by saying things like "Circulator pump", or Fuel "Deareator" or "vent connector" pipe

But I don't think we should split hairs when all that is going to do is confuse the issue. Just try to be clear in your original post, then we will all be better understood.

Jamie Hall

May I, with all respect to the folks above who have responded, go right back to the first two posts in this thread -- the OPs, and @EdTheHeaterMan 's? And to one of mine up there.

You are NOT, repeat NOT going to find this sort of intermittent problem with a measuring device. Nor, unless you get incredibly lucky, with swapping out parts, nor again -- unless you are incredibly lucky, by disconnecting something and having it not happen. Rather, you need to identify every single wire or board which is supplied power by that fuse, and then inspect every inch of it. Cut insulation. Chafes. A connector with a whisker of wire sticking out. A loose bit of metal occasionally touching something. If a relay is involved, try a more or less gentle tap to see if something is loose inside. Another rare, but possible problem, is a solder bridge on a printed circuit board. Once you have done that, and verified that everything is in top notch condition (or fixed whatever isn't) then perhaps you can start fussing about parts.

HVACNUT

109A_5 said:

mattmia2 said:

Is a condenser contactor control cable a thermostat wire? It uses thermostat cable but it isn't a thermostat...

Typically 18 ga. thermostat wire, yes. And for straight up single stage A/C it's just 2 wires. Common (Ground) and 24v hot from Y. 
The contactor is not a switch like a thermostat. It uses voltage, in this case 24 volts, to energize a coil and magnetize it. That action will close contacts for another voltage source, in this case sending 240 volts to the compressor and condenser fan motor. 

@HVACNUT maybe you should also explain for the benefit of all, using the original wiring diagram above provided by @Blower55 and the annotated version by @EdTheHeaterMan that includes the thermostat and the compressor / condenser unit, where the 24VAC Hot that energizes the "Y" terminal comes from during an A/C call in this case. I'm sure @mattmia2 won't believe me.

It looks like the Y terminal on the circuit board is just a screw. It might as well be a wire nut. It's just a splice point. There's no circuitry in that board connected to Y.
That's on this particular board. 
I don't know what's not to believe. 

ChrisJ

@Jamie Hall you can say that but if this was my problem the very first thing I'd do is wire in an ac ammeter so I could watch it as well as get a feel for how things behave.



For example we don't know how much current it normally draws and we don't know how much it's drawing when the fuse or breaker trips.  We don't know if it's slightly over for a period if time or if it's a dead short.  



Regardless, I don't like troubleshooting blind.

pedmec

this is my last suggestion and i know you might have answered this but its a long thread. i was going to make this suggestion before i saw the new board. i assumed the new board had pin type molex connections but now that i see you still have the old style slide on type. i'm going to offer this.

years ago carrier and bryant used these types of molex connectors but they change the connection type due to the fact that the fingers inside the molex connectors break off. its just flat piece of metal folded over 180 degrees, not a female insert picking up a pin on the board. eventually that crease from folding over causes the metal to break off and it might float around in the molex connector. if you reused the old connectors then it could be from that connector. you can tell by removing the connector and looking straight down into it.

i apologize if this has been addressed already.

mattmia2

pedmec said:

years ago carrier and bryant used these types of molex connectors but they change the connection type due to the fact that the fingers inside the molex connectors break off. its just flat piece of metal folded over 180 degrees, not a female insert picking up a pin on the board. eventually that crease from folding over causes the metal to break off and it might float around in the molex connector. if you reused the old connectors then it could be from that connector. you can tell by removing the connector and looking straight down into it.

It doesn't look like they have tis style of connector, but I haven't had trouble with them just connecting and disconnecting them. Where i have problems is if you jam a meter probe in there, then that bends the contact so it breaks.

pedmec

@mattmia2 they do. it connects on top of the control board. 7 pin edge connector. its a common problem. thats why the don't use them anymore. had 28 unit apartment building that had the old carrier atmospheric furnaces. had enough break

mattmia2

oh it is a board edge connector. those suck for a lot of reasons. the edge of the board corrodes and causes problems a lot too. they use them a lot in cheaper car dashboards

109A_5

pedmec said:

years ago carrier and bryant used these types of molex connectors but they change the connection type due to the fact that the fingers inside the molex connectors break off. its just flat piece of metal folded over 180 degrees, not a female insert picking up a pin on the board. eventually that crease from folding over causes the metal to break off and it might float around in the molex connector. if you reused the old connectors then it could be from that connector. you can tell by removing the connector and looking straight down into it.

pedmec said:

it connects on top of the control board. 7 pin edge connector. its a common problem. thats why the don't use them anymore. had 28 unit apartment building that had the old carrier atmospheric furnaces. had enough break

Although @pedmec did not say what the symptoms were (blown fuse or other) it is easy enough to inspect. If there is a piece of metal floating around between

Pin 1 RED Wire (10B1-1) (24 VAC Hot)
and
Pin 2 BLU Wire (10B1-2) (Ground)

of the Control Board's 7 Pin Edge connector (10B1) body or housing it will probably blow the 3 Amp fuse. With the vibration of the blower it could be intermittent. Inspect the connector closely.

Blower55

Hello all - I have not responded for a day or so, as the AC has been working perfectly over the last three days without a hiccup. This makes it very difficult to try to identify the source of the problem. That said, I see some additional things in the posts that I can double check. I appreciate all of your suggestions on this!

EdTheHeaterMan

Blower55 said:

Hello all - I have not responded for a day or so, as the AC has been working perfectly over the last three days without a hiccup. This makes it very difficult to try to identify the source of the problem. That said, I see some additional things in the posts that I can double check. I appreciate all of your suggestions on this!

Did you check the limit switch (7H1 on the diagram) that I pictured in one of my previous posts? The actual part is usually located on the front wall of the furnace (inside the removable front door) about mid point between the burner compartment and the top of the appliance (on an upflow furnace). You will see two red wires connected to the spade terminals.

JUGHNE

Yes, if you would disconnect the limit now while not in heating season and then if fuse opens you could eliminate that as the shorting device.

The limit itself may be held in place with only 1-2 screws.
Put your finger in the hole to check for the foil insulation.

Shut off the power while you do this.

As I stated above, disconnect the wires and isolate them from touching anything.
If that action then causes the blower and inducer fan to run constantly, jumper the wires together and shut off the gas......this is only a temporary test....the limit MUST be place for the heating mode.

EdTheHeaterMan

JUGHNE said:

Yes, if you would disconnect the limit now while not in heating season and then if fuse opens you could eliminate that as the shorting device.

The limit itself may be held in place with only 1-2 screws.
Put your finger in the hole to check for the foil insulation.

Shut off the power while you do this.

As I stated above, disconnect the wires and isolate them from touching anything.
If that action then causes the blower and inducer fan to run constantly, jumper the wires together and shut off the gas......this is only a temporary test....the limit MUST be place for the heating mode.

I was thinking that @Blower55 could leave the wires disconnected therefor when heating season arrives, the heating circuit would be inoperative. You however pointed out that this often energizes the blower circuit because an open limit logically should make the blower operate. Shutting the gas valve OFF will also accomplish this, as long as there is some notation on the gas valve handle to remind someone to fix the problem and reconnect the limit before operating the heating system.

My money is on your observation @JUGHNE!

Blower55

Hello again - after nearly a week of perfect operation, the popper tripped this evening. I removed the high limit switch as suggested and have attached pictures of the switch and piece of foil I found loose under the insulated portion of the switch. When I turned the main power back on, the blower did run continuously, so I jumped the two limit switch wires, put the power back on and set the tstat to call for cooling. Unit started up and has been running properly for about 15 minutes. Will obviously keep an eye on it and update you over the next few days. By the way, why is the foil there in the first place and when I reinstall the switch, should I replace the foil?

EdTheHeaterMan

@JUGHNE NAILED IT!

The insulation used by equipment manufacturers to keep the heat from the heat exchanger from overheating the exterior of the furnace is a standard 1/2" or 3/4" foil faces insulation very similar to this
After years of heating and cooling off and blower forcing air at high speed, that foil face can break down and peal off. Take an inspection mirror and flash light, look inside the furnace thru the limit control opening to see if there is any other loose pieces. If there are no other loose pieces of foil, then just replace the limit and connect the wires. That piece of foil can be discarded. It no longed serves any purpose.

109A_5

Cool, see perseverance pays off. While you are in there I would inspect that board edge connector for the defect @pedmec spoke of.

JUGHNE

There is another foil insulation story but this involved an electric furnace.

Homeowner would feel a "tickle" between the kitchen sink and the microwave/hood.
Even with the MW unplugged it would still be there.
IIRC, the MW was on a 2 wire outlet.

50's house with wall stack supplies on interior wall.

Hood mounting screws were penetrating the wall stack.

Electric furnace had foil insulation come loose and lay on elements.
Only one leg of elements are switched so the element is always hot, 120 volts to ground.

The grounding wire was hanging loose in the furnace. There was a lug on the end but no hole to mount it. Installed in the 70's when battery drill was not available and no easy outlet at furnace......I am sure he meant to drill a hole later and secure the grounding lug....but stuff happens.

Removing the insulation solved the problem and grounding the furnace was done at this time.

The foil connection to the sheet metal was a high resistance path.
This amounted to stray voltage.

A meter would read 120 volts but there was fortunately not enough current flow to light up a light bulb let alone a person.

So today there are factory installed grounding lugs in all equipment.
The NEC does state that all duct work, piping etc must be bonded to ground.
This is usually over looked, assuming the ductwork is continuous...but not with canvas duct connectors...not in use in this case BTY.

And if there was a 3 wire outlet for the MV, that would have bled the current off and in effect grounded the furnace.
However if there was a major fault current in that furnace, the grounding wire in the MV cord is only perhaps only 16 AWG and would have cooked itself and become a potential source of ignition. Furnace may never be blamed for the fire.

mattmia2

Since the heating element is a resistor the current and voltage available in the middle is less than connecting directly to the mains.

The 16awg wouldn't carry the fault current of the furnace continuously but a dead short across it would trip a breaker or blow a fuse before it melted the cord.

JUGHNE

If the element burned thru and the hanging coil came in contact with the SM then the SM would be energized up to the MW case.

There were plenty of 20 KW electric furnaces, a few 25 even.
All were replacements that never had enough ductwork to provide adequate air flow.
Today 15 KW is about the max, mostly 10 KW.

The elements are/were fused at 60 amp.
True, a dead short leg to leg would trip CB or fuse.

Another situation I recall is when a house lost the neutral to it's service.
No ground rod or water pipe grounding connected.
The water system was metallic and so the only ground connection was the 1/4" OD copper water line to the ice maker.
So the neutral current was flowing thru the fridge cord ground and the 1/4".

Fortunately it was night, with little 120 to neutral load.

Both got hot enough to smoke up the place.