Those darn fuses

[Gerald Newton]

My bother-in-law called and said his furnace was off again after he was billed $240 by the local repair shop. I called the repair shop but they wouldn't come again until the bill was paid. I said that the bill shouldn't be paid until the furnace works. So I took my tools and meter to town and began trouble shooting. A relay in the fire eye control for the oil burner chattered every time we tried to start the burner. Well, this was easy, I replaced the control. Turned it on and it chattered again. I checked the voltage at the switch, it read 108 volts. I checked the voltage at the panel - 108 volts from A to neutral and 120 volts from B to neutral, and 0 volts from A to B. What the hell, I thought. Something is weird here. The burner breaker was on A bus so I changed it to B bus and the burner took off. I next went to the service. And guess what I found? A fifty year old service panel with two

60 ampere fuses and the one on A phase was open. Apparently, the hot water heater that was on 240 volts was allowing B phase to back feed into the neutral through A phase loads setting up a voltage divider. When the burner was first turned on the relay got 108 volts and pulled in, but when the motoer and iginition transformer were added to the load the voltage dropped and the relay dropped out. This cycle repeated itself over and over causing the relay to chatter. I asked my brother-in-law if he had any hot water and he said not since the furnace went off. Hey, this is what I should have asked before I spent $80 for a new controller. I asked if some of the lights had been dimming at times. Oh yes, he said. Again, I should have asked that too. But mainly, I should have looked at the service. Who would think a house would have fuses this day and age? As for the service repair shop - well I asked my brother in law if the hot water went off after the repairman was there and he said yes, and that his brother who had been staying in the basement was using some electric heaters to get warm. Darn residential work, it will get you every time.

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2005 NEC Section 220.55 I just finished one. It took about 3 weeks to write in JavaScript and runs in a web page, and is free as are the other 24 electrical calculators at my site:

[Jeff Wisnia]

Well, my 21 year old home has a conventional breaker panel, but the HVAC equipment (2 heat pump systems) was installed with a total of four fused disconnect switches located near the compressor and air handler units to provide appropriate safety disconnecting means for service people.

Those disconnects were fitted with cartridge fuses of exactly the same amperage as the panel breakers which protected the dedicated lines feeding them, so the fuses inside them were redundant, 'eh?

The reason I'm mentioning this is that one of those fused disconnects, a

60 amp GE unit adjacent to our attic mounted air handler with auxillary strip heaters in it, has given me trouble with fuse failures about once every couple of years.

The fuses didn't "blow" in the conventional sense, what happened is that the closed switch contacts oxidized to the point where they developed enough resistance to generate significant heat when the 40 amp strip heaters were drawing current. After a while the end cap on one of the fuses adjacent to that switch contact became heated enough so that the fibre fuse body would char and the solder holding the fuse link to the end cap melted, "opening" the fuse, but leaving the entire fuse link still intact and "unblown".

I'd clean up the switch contacts and fuse clip and install a new fuse and things would be OK for another couple of years and then the same damn problem would occur. About ten years ago I bought and swapped in a whole new identical model GE fused disconnect only to find the same problem occured a couple of years later.

A fuse "charred out" again last week, so over the weekend I bought a Square-D 60 amp UNFUSED disconnect and replaced the fused one with it. I'm hoping the "plug and socket" contacts in the new disconnect will fare better than the switch contacts in the old disconnect. I put my old Simpson 260 meter across them with the strip heaters running and couldn't see any movement of the meter pointer on the 2.5 vac scale, so I know they're making good contact now, I just hope they stay that way.

Jeff

[professorpaul]

How about a light coat of silicon grease (any auto parts store). That will slow down the corrosion issue substantially. Light coat, wipe most of it off.

[Jeff Wisnia]

The first thing I tried, when the first fuse "fried" that way, was to brush a little Thomas & Betts "Koppershield" on the switch contact surfaces and on the cartridge fuse clips themselves.

That may have helped, but it didn't stop it from happening over and over again every couple of years.

I've mentioned the problem to several experienced electricians and they all said they've seen the same problems occur in GE fused disconnect switches.

I suspect that the first time the overheating happens the heat anneals the copper swich blades and or the springs which apply pressure so they are no longer stiff enough to maintain a low resistance contact.

Whatever, nuttin's perfect is it?

Jeff