Issue with fuse

Status
Not open for further replies.

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
this fuse is for hvac system, but it keeps blowing every 4-6 months. but, trying to understand whats going on. the view window (plastic) seems to get hot and fall in. the fuse is screwed in horizontally. wondering if the collapse of the view window happens after the fuse gets hot and opens, or is the plastic window falling in and touching the fuse element causing the fuse to open?

fuse.jpg
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
My money would rest on the fuse getting hot, most likely due to pitted and/or poor contact area where it touches the buss and that heat eventually causing your failure.
 

big john

Senior Member
Location
Portland, ME
My money would rest on the fuse getting hot, most likely due to pitted and/or poor contact area where it touches the buss and that heat eventually causing your failure.
Agreed. If that same fuse keeps failing due to severe heat, and there are no other indications of a fault, then likely there is much poorer contact-resistance on that fuse holder.

A simple check might be to put this thing under full load and look at the results of a AC millivolt-drop test across each fuse holder. If the suspect fuse has a significantly higher voltage drop, then there's your problem.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
the only thing that runs in the hvac is a small evac fan that runs before the gas heater ignites, and then the main blower motor. from the descriptions i could find online about this York unit the main blower motor is 1/3hp (~2A at 120vac), i did look at tag some time ago, just forget what it said. now, about 3 years ago the control board in this hvac had gone up in smoke, had to replace that, but when i asked hvac guy why it burned up he said the blower motor is probably going. the whole thing (less control board) is just 7yrs old. the fuse seems to blow when the fan blower has been running for some time (it can ruun in excess of 30min depending on how far the temp is swinging, usually less for heating and more for AC.

so i am wondering if i am fighting a blower motor that is starting to seize up and because of that it itself is getting hot, dragging, and then blowing the fuse. or is the socket simply getting too hot from a ~2A continuous current? replacing the blower motor is a pita because its buried inside the hvac unit. with some doing, i could measure the actual current with my Fluke.

the next item is the fuse socket/outlet. the hvac is a plug. the bottom of the fuse socket looks fairly clean to me, and i do make the fuse tight, but in reality the bottom of the fuse is a small nipple and there's not a big footprint between fuse bottom and socket, but 2A should not be causing much heat to have the thing get too hot.

so, if i were to replace the socket/outlet with a better type of fuse or breaker, what type could i use?
 

growler

Senior Member
Location
Atlanta,GA
the only thing that runs in the hvac is a small evac fan that runs before the gas heater ignites, and then the main blower motor. from the descriptions i could find online about this York unit the main blower motor is 1/3hp (~2A at 120vac), i did look at tag some time ago, just forget what it said. now, about 3 years ago the control board in this hvac had gone up in smoke, had to replace that, but when i asked hvac guy why it burned up he said the blower motor is probably going. the whole thing (less control board) is just 7yrs old. the fuse seems to blow when the fan blower has been running for some time (it can ruun in excess of 30min depending on how far the temp is swinging, usually less for heating and more for AC.

so i am wondering if i am fighting a blower motor that is starting to seize up and because of that it itself is getting hot, dragging, and then blowing the fuse. or is the socket simply getting too hot from a ~2A continuous current? replacing the blower motor is a pita because its buried inside the hvac unit. with some doing, i could measure the actual current with my Fluke.

the next item is the fuse socket/outlet. the HVAC is a plug. the bottom of the fuse socket looks fairly clean to me, and i do make the fuse tight, but in reality the bottom of the fuse is a small nipple and there's not a big footprint between fuse bottom and socket, but 2A should not be causing much heat to have the thing get too hot.

so, if i were to replace the socket/outlet with a better type of fuse or breaker, what type could i use?

You really need to measure the current draw of this unit. You have figured out how much it should draw but not what it's really drawing. You need an AC ammeter.

It could be almost anything, even an intermittent short of some type ( maybe moisture). It could be a bad blower motor. You can get a new motor right off the shelf that's not good so the age of parts is not always a good indication of how reliable they are.

Where you say the HVAC is a plug I would hard wire with a fused disconnect. or 15 amp breaker.
 

John120/240

Senior Member
Location
Olathe, Kansas
Move the HVAC circuit to a different fuse holder. If the problem moves too, the problem must be in the HVAC equipment. Conversely if you quit experiencing problems at the new fuse holder, check the old fuse holder.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Is the fuse opening from an overload or from a short circuit? If the solder that holds the "spring" part of the element has melted letting the spring pull away from the base, the fuse opened because of an overload or because of high temperatures that are a result of a poor connection. If parts of the the fuse element have melted, a high current was responsible for the fuse opening.
 

Haji

Banned
Location
India
It looks the hvac is otherwise healthy except blowing of the fuse every 4-6 months. In that case, cost saving simply by replacing the fuse by a breaker in consultation with the hvac manufacturer.
 

kwired

Electron manager
Location
NE Nebraska
Is the fuse opening from an overload or from a short circuit? If the solder that holds the "spring" part of the element has melted letting the spring pull away from the base, the fuse opened because of an overload or because of high temperatures that are a result of a poor connection. If parts of the the fuse element have melted, a high current was responsible for the fuse opening.


You can see in the picture in post #1 that the spring has no tension on it. This indicates the element has melted and spring pulled it open, could have been from too high of load or from poor connection in the fuseholder/conductor connection to the fuseholder. I would think if we had a poor connection there may be melted conductor insulation, melted plastic components of the fuseholder or discoloring of the metallic components in the fuseholder. If none of those are present next thing is to check out the motor - though if it is only a 2 amp motor it should trip motor overload protection long before this 15 amp fuse.

Maybe run the fan for at least 15 minutes and see how hot the fuseholder gets, if much significant heat at all replace it, especially if load was only around 2 amps.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
some follow-on info. yes, the fuse element melted and spring pulled back. but i am wondering if the plastic window may have sunk in over time/use and touched the spring and while element was hot/soft the plastic window helped the element break (thus maybe not an overload at all, etc). this might explain why it happens every 4-6 months ? the fuse holder parts show no signs of melting, bad color, or odor.

there's just a one gang box there where the hvac unit plugs into, fuse on top with flip-up cover, and a black plastic snap-in 3-prong outlet below the fuse, etc. its a cheap setup from what i can see.

given the motor should not be near 15A i need to do some testing, which i need to do anyways to determine if i really need to open up the unit to replace motor.

does any maker have a lever type breaker that is equivalent to these type "T" dual element screw in fuses? and given i currently dont have a "panel" there how could i mount just one lever type breaker? i am thinking it might be better to hard wire the unit vs using the 3-prong plug that is there now.
 

kwired

Electron manager
Location
NE Nebraska
some follow-on info. yes, the fuse element melted and spring pulled back. but i am wondering if the plastic window may have sunk in over time/use and touched the spring and while element was hot/soft the plastic window helped the element break (thus maybe not an overload at all, etc). this might explain why it happens every 4-6 months ? the fuse holder parts show no signs of melting, bad color, or odor.

there's just a one gang box there where the hvac unit plugs into, fuse on top with flip-up cover, and a black plastic snap-in 3-prong outlet below the fuse, etc. its a cheap setup from what i can see.

given the motor should not be near 15A i need to do some testing, which i need to do anyways to determine if i really need to open up the unit to replace motor.

does any maker have a lever type breaker that is equivalent to these type "T" dual element screw in fuses? and given i currently dont have a "panel" there how could i mount just one lever type breaker? i am thinking it might be better to hard wire the unit vs using the 3-prong plug that is there now.

If your load is a ~2 amp motor a 5 or 6 amp type T fuse should not be giving you any trouble unless something is wrong somewhere.

The spring is not attached to the "window". Any melting of the window is likely due to heat from fuse element and is result of overcurrent - and fuse is blowing because of this same overcurrent.

I still have a hard time seeing a 15 amp fuse ever being loaded enough to trip if a 2 amp motor is the load, that motor is probably a PCS motor and maybe only draws 10-12 amps max if locked rotor conditions occur. Thermal overload in motor should open circuit before that 15 amp fuse ever blows. Is there other items supplied by the fuse?
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
i never heard any issue from the motor, unless some weird short going on for brief m-sec or something. i suspect the bearings are going and the motor is dragging.

i will do some current testing to see where the motor runs.
 

kwired

Electron manager
Location
NE Nebraska
i never heard any issue from the motor, unless some weird short going on for brief m-sec or something. i suspect the bearings are going and the motor is dragging.

i will do some current testing to see where the motor runs.

But if motor is dragging it likely doesn't come close to enough current to blow 15 amp fuse, and maybe doesn't even draw enough if locked rotor to blow the fuse. Short circuit/ground fault should be putting you into the instantaneous setting of the fuse and will likely have a blackened window and vaporized segment of the fuse link instead of just a melted segment of the fuse link.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
its a dual fuse type. it was the lower glob that split in half. it looks like a cast of tin/lead, brittle stuff. there was no black color anywhere. i suspect just an overheat of that glob which made it soft enough for it to split apart. there were two small fragments from that glob inside the fuse, etc. i ordered a General Tools CM660 clamp meter (has inrush feature for motor testing) and when it arrives i will go do some sleuthing to get a better understanding of why this fuse keeps blowing every 4-6 months. in the meantime, i will take my IR heat gun to see if the fuse runs hot while blower is running, i'll grab a pic of the 1gang fuse/outlet.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
some more info.
the small evac blower (casing is approx 6" diameter) is rated 2.3A and it runs the whole time when a call for heat is there.

the orange romex says on it "10 AWG C2 Ground" with that spacing, didnt measure the wire diameter yet, i will assume its 10ga romex for now. ambient temp of everything was 73F, after a call for heat and about 5min running, the shell of evac motor was 140F and the metal on the fuse/outlet was 120F. the unit label rates the amps (i assume max amps) as 14A w/ 20A(max) OCD. the hvac plug wire is approx 7ft long and 14awg.

10ga romex, 14A rated unit, 15A fuse, 14ga plug wire. these items together dont make sense to me. your thoughts?

DSC02535.jpg


DSC02534.jpg


DSC02536.jpg
 

kwired

Electron manager
Location
NE Nebraska
Apparently we are drawing more then the earlier mentioned ~2 amps.

I myself would use a 20 amp fuse based on what that nameplate says. Running current is not likley high enough to be a problem (assuming no excess heating from bad fuseholder or connection to it), but maybe it does get to where it runs for a long period, maybe has a short off cycle and then can't hold for the next startup.

BTW a 1/3 120 volt motor draws 7.2 amps according to NEC tables, that is low enough that a 15 amp fuse isn't likely blowing during starting if motor/bearings/capacitors/etc. are in good condition, but is just high enough that some of those issues mentioned may cause starting duration to be long enough to be a problem with a 15 amp fuse - plus it was mentioned there is a draft inducer motor which is probably where the ~2 amp figure earlier mentioned comes from.
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
i forgot to use the right "type" in the online converter ("electrical power")
1hp = 736watt (electrical)
2/3hp = 497watt = 4.14amp x 120vac(rms)

but, from another online motor calc tool, with efficiency 90% and PF 85% (one phase 120vac), 1hp = 8amp, thus 2/3hp = 5.3amp

when this hvac is running heat i should expect to see ~7amps on the wire, will find out as soon as my new clamp meter arrives.
 
Last edited:

Little Bill

Moderator
Staff member
Location
Tennessee NEC:2017
Occupation
Semi-Retired Electrician
Never seen an HVAC unit with both heat & cooling that plugs in. Well maybe units like in a hotel room or window/wall units. Never saw one with a fuse in with the receptacle either.

Is this an outdoor unit, or one of the type I mentioned?
Is this a dwelling or some type commercial building?
 

FionaZuppa

Senior Member
Location
AZ
Occupation
Part Time Electrician (semi retired, old) - EE retired.
the air handler is in attic (integrated with gas heater), AC condenser & compressor is outside.
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
the air handler is in attic (integrated with gas heater), AC condenser & compressor is outside.
So if this is just a blower motor on an air handler and you say is 2/3HP, 5.3A, what is that nameplate for? It clearly says 14A. You can't run 14A continuously on 14ga wire, so something is missing or the info is inconsistent in this puzzle.
 
Status
Not open for further replies.
Top