Thermal Fuse

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Little Bill

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Tennessee NEC:2017
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What would be the reason that two thermal fuses were wired in series?
Both fuses are the same rating.
Nothing between the fuses just wired in series, actually end to end.
 

templdl

Senior Member
Location
Wisconsin
Redundancy, to assure that one will respond as they both will see the same current as on will respond. I doubt if both will respond at the same time so I don't think it's a voltage thing.
 

Haji

Banned
Location
India
Yes. One thermal fuse is back up to the other in case of failure of either one to blow to prevent a fire in a hair dryer for example, by cutting off the power supply to the heater elements when the air flow is interrupted (e.g., the blower motor stops or the air intake becomes accidentally blocked)
 

Little Bill

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Location
Tennessee NEC:2017
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Semi-Retired Electrician
The fuses are actually in a coffee maker. It's the type that keeps hot water in the reservoir and when you pour in fresh water it forces the hot water out.
The fuses are in line with both the reservoir and the hot plate. Just couldn't understand why two fuses in series.
 

GoldDigger

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Placerville, CA, USA
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The fuses are actually in a coffee maker. It's the type that keeps hot water in the reservoir and when you pour in fresh water it forces the hot water out.
The fuses are in line with both the reservoir and the hot plate. Just couldn't understand why two fuses in series.
Possibly to detect overheating in two different locations?
I initially overlooked the significance of "thermal fuse" in the description.
 

Little Bill

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Staff member
Location
Tennessee NEC:2017
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Semi-Retired Electrician
Possibly to detect overheating in two different locations?
I initially overlooked the significance of "thermal fuse" in the description.

If they were separated some I could see that but these are not.
Picture a wire/conductor that is the line wire running to the lead on the fuse. The other lead from the fuse is crimped to the first lead on the second fuse but making close to a 180° turn, then the other lead from the second fuse is crimped to the wire that goes to the loads.
If you draw two lines about 1/4" apart and connect them at a point it looks like "^".

That's the way they are connected. So the two fuses are actually linked together with the other leads on each fuse connected to line/load wires.
 

Haji

Banned
Location
India
Even when it is coffee maker and not hair dryer, the same phenomenon i.e over heating due to loss of medium (air in case of hair dryer and water in case of coffee maker) happens and to afford double protection against it is the purpose of two fuses in series.
 

iwire

Moderator
Staff member
Location
Massachusetts
Yes. One thermal fuse is back up to the other in case of failure of either one to blow to prevent a fire in a hair dryer for example, by cutting off the power supply to the heater elements when the air flow is interrupted (e.g., the blower motor stops or the air intake becomes accidentally blocked)

Even when it is coffee maker and not hair dryer, the same phenomenon i.e over heating due to loss of medium (air in case of hair dryer and water in case of coffee maker) happens and to afford double protection against it is the purpose of two fuses in series.

Can you link to any hair dryer or coffee maker that uses two thermal fuses in series?

I don't think you can.

Every one that I have opened had a single bimetallic switch as the high temp cut out so that it would reset. Thermal fuses are one time items.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Can you link to any hair dryer or coffee maker that uses two thermal fuses in series?

I don't think you can.

Every one that I have opened had a single bimetallic switch as the high temp cut out so that it would reset. Thermal fuses are one time items.

I can.

Bunn coffee makers are set up that way. Here is a pic of the thermal fuse replacement kit:

mR9Q_oNMyoW-PC0ijEjen5A.jpg
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I also found some possible pertinent info:

http://alt.home.repair.narkive.com/GfbvFkLP/why-two-thermal-fuses#post7

They probably used 2 thermal fuses in series to avoid the problem GE
had with many of its drip coffee makers produced from 1976-1984
causing house fires when their single thermal fuse failed to melt:

http://www.cpsc.gov/en/Recalls/1991...rip-Coffeemakers-That-May-Pose-A-Fire-Hazard/


The company believes that a fire hazard may occur when a purchased thermal fuse in the coffeemaker fails to function as intended. About nine million drip coffeemakers bearing the GE or Universal brand name were manufactured between 1976 and 1984.
 
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