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Why does a space heater (electric fan heater for home use) have a fuse?

Isn't the house itself connected via a circuit breaker? Wouldn't that stop potential safety issues?

PS: First post on here :)

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  • \$\begingroup\$ You should re-post this on the Home Improvement site. This questions has been answered there. \$\endgroup\$
    – SteveSh
    Commented Feb 17, 2022 at 0:24
  • \$\begingroup\$ @SteveSh Haven't found the question on there. Do you have a link? \$\endgroup\$
    – Hegus
    Commented Feb 17, 2022 at 0:34
  • \$\begingroup\$ "Wouldn't that stop potential safety issues?" If you start quantifying things you will see it doesn't. Just tripping is not enough because you can trip too slow or trip too late. A house breaker also doesn't stop a damaged heater from becoming an house-burning device the next time it is plugged in. \$\endgroup\$
    – DKNguyen
    Commented Feb 17, 2022 at 0:34
  • \$\begingroup\$ diy.stackexchange.com/questions/124618/… \$\endgroup\$
    – DKNguyen
    Commented Feb 17, 2022 at 0:35
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    \$\begingroup\$ @Hegus The breaker is sized to prevent the house wire from catching fire or melting, but things connected to it don't necessarily draw the full current it can support. If these device fail short, they can get hot enough to produce enough heat to cause fires long before the breaker trips. The breaker might not even trip in some cases because the current being drawn isn't enough to trip the breaker but is concentrating enough power in a small enough volume to get hot enough to start a fire. \$\endgroup\$
    – DKNguyen
    Commented Feb 17, 2022 at 1:39

3 Answers 3

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The breaker is sized to prevent the house wire from catching fire or melting, but things connected to it don't necessarily draw the full current it can support. If these device fail short, they can get hot enough to produce enough heat to cause fires long before the breaker trips. The breaker might not even trip in some cases because the current being drawn isn't enough to trip the breaker but is concentrating enough power in a small enough volume to get hot enough to start a fire.

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The breaker installed in the house protects the cable to the wall outlet. The maximum current of this breaker is selected depending on the length, the crossface and the ambient conditions of the cable. This breaker should act if there is a short circuit in the cable to the outlet or the outlet itself. It should also act if there is a short circuit in the cable between the wall outlet and the heater.

The heater itself should have another breaker for a lower current selected depending on the heater maximum current. There should not only be an excess current breaker but also an excess temperature breaker.

If there is a very long and thin cable used with the heater another breaker in the plug at the wall outlet should be used to protect the thin cable.

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Key piece is that the heater does not depend on the house breaker. You may have an outlet on a 15 or a 20 amp circuit. All the outlets on it total can draw that much. Heater itself must still protect itself without knowing how much the house breaker is and what other devices are on the circuit.

The wires in the heater can only take so much current. Too much and you might melt solder, burn insulation, and all manner of bad. Just monitoring temperature may not be enough since measuring one point does not guarantee that temperature in another. Current should be the same for everything in series on that circuit. Better than that, if it shorts, resistance drops and it trips the failsafe.

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