Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
With the common fuse melting, I've decided to do some testing of how and why the fuses melt the plastic and still keep working.
My first testing was done with my fan fail unit with a thermocouple RTV'ed on the front side to measure the heat. The fuse heating is transferred down into the fan fail unit via the fuse terminals and fuse socket. The heat produced in this test also is some heating components other than fuses. I'm using this test because I have some PPTC fuses ordered for testing.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
Got my PPTCs (resettable fuses). Testing looks good so far. Runs my fan fail unit about 25 degrees cooler. But that may because the PPTC is much longer than the regular fuse so the heat is staying inside the PPTC more than the fuse. These PPTC are to long to fit installed in my fan fail unit in the car.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
Checking voltage drops which relates to heat. To get the heat in watts, multiply the voltage drop times the current in that circuit.
These drops at 10 amps current cold temps:
Poly fuse 15 amp = 0.063 volts
Poly fuse 20 amp = 0.042 volts
Fuse 15 amp = 0.058 volts
Fuse 20 amp = 0.051 volts
Tested the 40 amp breaker with 20 amps current and it drops 0.070 volts but it varies a little up and down a bit. Must be the mechanical contacts inside changing. This is why I recommend bypassing the fan circuit breaker if you have a fused fan fail. It's just another producer of heat and since it is redundant not needed.
Tested the voltage drop over my fan fail unit with the 15 poly fuse to be 0.093 volts at 10 amps. So it drops 0.030 volts not counting the fuse. That drop is due to the current sensor and the circuit board copper layout. I was just wondering if going to 4 oz. copper would help but it looks like it would not help much to reduce heat.
I'm next going to test the poly fuse for voltage drop after it has been tripped a few times. I'm checking if I could design the poly fuses into my fan fail to replace the fuses and their sockets. It would need to standoff the poly fuses above the fan fail unit to isolate the heat produced by the fuses.
Last edited by Bitsyncmaster; 08-06-2015 at 08:04 AM.
Location: Farmington Hills, MI
Posts: 180
My VIN: 16619
Interesting, oddly enough the temps seem pretty typical to what I have seen on modern vehicles. Have seen relays heat up to 150F with no issues and within spec. Most likely the plastic is just cheap plastic not capable of that kind of heat. Would love to have an extra one to put it in a temp chamber and see if it meets typical modern temp standards.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
It's not that my fan fail units are failing, I just hate producing heat in the relay compartment. A lot of people are still running OEM fans and it runs only warm with low power fans. My components are mostly rated for 80 deg. C or higher But all the fuses running any amount of current are real heat producers. I'm beginning to think that fuses are only good for short circuit protection and not protecting for marginal overloads.Originally Posted by Exolis
My testing has been at room temps so running in the enclosed relay compartment will raise those temp readings I recorded.
I'm going to do more fuse testing but am considering incorporating electronic "fuse" circuitry in a new design. But not sure the added cost will make it feasible.
Last edited by Bitsyncmaster; 08-06-2015 at 03:27 PM.
Location: Farmington Hills, MI
Posts: 180
My VIN: 16619
Yes, I can tell you for a fact fuses are not for marginal over loaded. Depending on who makes the fuses you are using, they will have different characteristics, so you might be able to find something searching for Fuse blow curve. The only goal for a fuse is to protect the wires from catching on fire/being damaged.
I will double check some of the curves we have (granted we mostly use Mini-Fuse low profiles for 5-20 Amps and Fusible Links for 30+), but running 30 Amps on a 20 Amp fuse seems bad. I would almost expect that to blow with 1-10 seconds.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
Tested the poly 15 amp fuses with 10 amp fan loads.
Also recorded voltage drop over one poly fuse since I want to see if that drop changes after a few over current trips of the fuse. That is a problem that occurred with poly fuses and want to see if that problem has been corrected.
7:09 76.8 0.063 volts
7:14 87.8 0.068 volts
7:19 96.4 0.070 volts
7:24 100.2 0.071 volts
7:29 102.2 0.071 volts
7:34 103.1 0.072 volts
7:39 103.8 0.072 volts
Again these poly fuses keep my fan fail unit cooler since their taller and that moves the fuse heat further from the fuse pins. Also note these temps were more consistent since I tested it early in the morning before my room AC was tuning on and off.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
I set up one 15 amp Poly fuse with a 20 amp load and it took 2 min. and 8 seconds to trip. I then tested the voltage drop after it cooled off. It was 0.063 volts new, it now is 0.086 volts after the first trip.
I did another 20 amp trip time and it was shorter, 1 min. 10 seconds as I would have expected since it's resistance is now higher and it gets hotter quicker. I waited longer for cool down but it still has a 0.083 volt drop. So I think it will probably stay there but will check it again in a few days.
Now I'm going to test a standard size fuse we use in the #7 position (for fuel pump). I want to find what current flowing through that 20 amp fuse will cause it to melt the plastic and maybe start a fire in the rubber fuse holder.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
Here is a table of temps of the plastic side of the fuse at increasing currents.
The end result was the fuse started to glow orange the plastic on one side of the fuse melted a hole in the plastic had I not shut off the current, the rubber fuse holder would have caught fire since I saw 500 deg. F on the plastic side that did not melt.
Now this testing was done at room temperature so I'm not sure what effect running in our hot relay compartment would have done to these readings. I wonder what the relay compartment temps run. My guess would be 110 to 120 deg. F in the summer. I would measure mine but it would be cooler than most since I have all solid state relays.
Now the point of this testing was to determine why my friends DeLorean fused fan fail jumpers caught fire. He opened the relay compartment and said the fuses had flames 6 to 8 inches high. That was from the rubber fuse holder sockets. My testing proved it could happen but you would need to be running right near the fuse trip current of 120% of the rating. Anything higher would blow the fuse before quicker.
Location: Leonardtown, MD
Posts: 9,008
My VIN: 03572
Test document in PDF
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