# Thread: defrost heater fuses blowing

1. can you post a few pictures ?

what gauge wire ? copper wire ? aluminum wire ? clean ?

electricity is actually very predictable .. you must be overlooking something ...

2. It is Always the heat which opens the fuse. There is no other factor.

PHM
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Originally Posted by VTP99
Would overheating blow the fuse ? I've seen some really burned fuse ends from poor contact that were not blown.
I suppose it can be calculated by amps to watts to btu/h.

3. Are you using 250 volt fuses, when they should be 600 volt fuses.

4. Originally Posted by Poodle Head Mikey
It is Always the heat which opens the fuse. There is no other factor.

PHM
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I understand that, my question is how much resistance heat vs heat from current amps or is that conductance ?
How do you calculate that ?

5. I know it’s reaching but are the heaters the proper voltage as well. I know not your case but had one that had a crack in it would ohmn fine heat up and water would run into the crack and blow the fuses. ( just throwing darts here)

6. Originally Posted by VTP99
I do as a habit sand the fuse ends if I remove them.
I just wonder what the threshold is for the amount of heat.
It would have to be higher then the heat generated by the normal current.
While someone could make a calculation, it isn't necessary to me.

The total heat is the amount of current generated heat and external, "fuse holder" heat, along with ambient heat.

One of the factors in keeping a fuse holder or breaker cool is to have a sufficiently large conductor attached to the terminal. One reason for the recommended wire gauge size is the function of the conductor in the terminal to act as a heat sink.

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the 30 amp fuse r rated for 600v and the 60 amp r 480v, wire gauge on the 30 amp i think is #10 and the 60 amp is at lease #8 or #6 , i dont think it's the wire. i noticed the the fuse holder clip is attached to the lug portion of the fuse block with a rivet and it isn't tightly connected, there is some play. these are bussmann fuse blocks the cadilac of fuses. these are krack evaps sm24e-759-ckk there are 4 of them hooked up to a krack condensing unit cdd-0600-l4k-2230c two compressors on one unit, i did some searching here i found that these heaters burn out alot, and i have replaced about 8 heater on one evap that has 15 heaters on it, so 50% of heaters were bad, but still the fuses get hot and when one blows i replaced it and amp draw is fine so no heater blew. i'm going to replace one of the circuits of the fuse blocks with breakers, can anybody recommend a breaker that i can panel mount

8. Your call to change the design to a breaker.

I would just get new fuse holders, there cheap and you'll know is shot order if that was the issue!

9. Originally Posted by mrcool
...can anybody recommend a breaker that i can panel mount
You'll want DIN-rail mountable breakers such as these Eaton WMZS Series. Since defrost heaters are strictly a resistive load, you'll want ones with a B trip curve. Don't forget to buy the DIN rail to hang them on.

10. We had a Carrier roof top blowing it's 75 A breaker this past summer. We checked everything with the unit. Only drawing 38A a leg. We disabled individual components on the unit and the breaker would trip and it would take the same amount of run time to do it. Big old breaker panel. We found an unused breaker in the panel and swapped it. Same thing. Even hummed when on and warm to the touch. We have a big used electrical supply house near us and got another breaker. This cured the problem verified with an infrared camera we borrowed.

11. If you alter the overcurrent device type (from fuse holder to din rail breaker) you are:

1) invalidating the UL listing for the equipment, and

2) experimenting with liability should a negative event take place after the change.

I might do this with my own equipment, but not a customer's.

12. Originally Posted by timebuilder
If you alter the overcurrent device type (from fuse holder to din rail breaker) you are:

1) invalidating the UL listing for the equipment, and

2) experimenting with liability should a negative event take place after the change.

I might do this with my own equipment, but not a customer's.
Are the fuses integral to the evaporators or did a sparky install them at a panel ?

13. Originally Posted by VTP99
Are the fuses integral to the evaporators or did a sparky install them at a panel ?
From the description, I am going on the idea that the fuseholders are in the unit. I have seen a lot of equipment with fuseholders inside.

I can't imagine a contemporary electrician installing fuseholders in a service pane.

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