Help me fix this (air handler with heat strip) - blows fuses

[handyandy2]

I've spent the entire past 3 days trying to diagnose the root cause of blowing slow-blow fuses. Follow along:

Fuses are blowing NOT due to amperage overload, but due to heat (suspected, as there is no definitive way to discern). There is no amp spike.....EVER.

What is working, and in spec are:

• 10kw heat strip blows hot, but will run for perhaps 20 mins, then blow a fuse
• Critical to know is the slow blow fuse box in the attic heats up the terminals (1 in particular)
• Since I have been watching the temp with a thermal gun, I shut the system down at 115d.
• It takes perhaps 10 mins to gradually reach 115d.
• Heat strip specs out with both ohm and volts (21amp, 240 volt, 11 ohm) - there are two
• Thermal limiters on heat strip are new (and test out ok)
• Total amp draw on circuit is 43 amps (60 amp fuses/box) 2 amps are the fan.
• No amp spike during operation, either during warmup or at full heat
• When one heat strip is disconnected, the amps drop accordingly, BUT the fuse box terminal continues to heat up.
• Sequencer is new, and checks out, and does allow one side to kick in first, then the other.
• Fan motor and cap is not an issue

What perplexes me is the normal amps (43) but the excessive heat build at the fuse terminal. Yes, I have removed the fuse box, ensured tight connections, wiring gauge (at 6g) is in line with whats needed (though it is alum to the fuse box). There are no extra splices in circuits, no corrosion, no crispy wires, no burnt smell. I've owned and operated this unit for 25 years.

What say ye?

 

[Stuff]

Time to replace fuse box with breaker box.

 

[Sponsor]

 

[handyandy2]

I do appreciate the reply, but don't see how that resolves the problem. Please help me understand your solution?

 

[Stuff]

Fuse box is overheating and blows fuses with normal load less than max rating. Should be able to run indefinitely if at or less than fuse value. The fuse contacts probably are bent, corroded, or contaminated so providing resistance to the current.

 

[handyandy2]

Thanks. I fully understand the potential problem with the terminals. While I had noted in the above post the box was pulled, and terminal ends (both the box and fuse holder) are pristine, fully snug, not bent, and test with zero resistance. I have a spare pullout and have swapped that (testing zero ohms) but still producing heat. The pullout (with fuses installed) test with zero resistance as well.

Agree it should run indefinitely.

 

[Stuff]

It is heating up. It shouldn't. Assume it is a fused disconnect box which is cheap to replace.

Were you able to test with a megger? It uses high voltage to find issues that a multi-meter using a few volts DC won't find.

 

[WorthFlorida]

At 23 amps per leg, the fuse or circuit breaker needs to be at least 30 amp with 10 gauge wire. Is your fuse cartridge type?

From a google search
What causes a fuse to melt?
Heating in an electrical circuit is always caused by current flowing through a resistance. If there is significant resistance in the contact between fuse and fuse holder, current well below the fuse rating can cause enough heating to melt the fuse or fuse holder.

 

[handyandy2]

Its been root-caused. @Stuff, you nailed, tho my tests of volts/amps/ohms ruled out the box, but the box it was. I don't have a megger. My sincere thanks to you. There was just nothing about either the box, or the plugin portion that led me to believe anything was amiss. Plenty of spring tension on the fuses, and clamp tension on the flat blade plug-in.

Some 6 gauge wire nuts bypassed the box in order for me to recreate the heat buildup test, which it passed. After running for 20 mins, the ambient temp was 75 on the wires, the wires rose to 90F (vs 115F plus before I shut it down). Total amps increased from (with box 43) to (wire nuts 45). I'm chalking that up to efficiency of passing more current to the unit.

@FL, yes its a cartridge slow blow type.

I'm off to shop for a new breaker box to ensure a shutoff at the unit.

 

[Stuff]

Worth - OP has two heat strips @ 23 amps each. Assume in parallel.

Going from 43 to 45 amps figures out to be only 0.25 ohms but also means 460 watts. Guess is that numbers are rounded so ohms is a lot less. Even .01 ohms at 43 amps would mean 18 watts of heat would be generated in the fuse box; enough to heat things up.