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Today, I was working on a carrier Heat Pump. The unit was doing nothing and I suspected a fuse. I pulled the fuse and it was so hot I couldn't touch it but it tested good.

What I found was a neutral wire that had come undone. On these units, the neutral wire connects directly to one terminal of a combination fan/compressor start capacitor.

It seems that if the neutral was broken, and the ground started to carry the load, it would still not heat up the fuse.

20A fuse and the unit once fixed only drew 13A. What happened here?
I haven't even seen a heat pump that takes a neutral. Is this a split system?
(Is the fuseholder tight?)

[This message has been edited by electure (edited 01-16-2002).]
Actually, this is a PTAC unit. 277V.

All controls (t-stats, switches, etc.) switch the neutral side of all loads (compressor, fan motor, resistance heating element)so if the neutral breaks, all parts are still hot.
Look for a U/L or another Nationally Recognized Testing Lab sticker.
Switching a neut by itself has been forbidden forever. Some of the Japanese equipt. I've run into has asked to switch both hot & neut.

Look out before you guys kill somebody!! [Linked Image]
Of course switching the neutral to any appliance is not alowed, and perhaps it's not allowed by UL inside appliances either but it's something I've seen inside appliances before.

I've seen it in AC's, Commercial washers and dryers, and I don't know if I've ever seen it anywhere else or not. It's unusual but not unheard of to those of us who work inside these things alot.

Of course if you've got the power off to any of these units, you're safe. You just can't turn off the power switch on the appliance and start sticking your screwdriver inside.
As a maintenance electrician I've seen switched neutrals within UL approved appliances many times. It makes you wonder but there isn't a dern thing you can do about it. If you rewire the machine you've voided the UL.
To answer the question ... I only have a "what if": If the current was arching or pulsing to ground at some point near the fuse you may produce heat that is transmitted to the fuse but not necessarily through the fuse. Any signs of arcing or sparking?
No sign of arcing, contact surfaces clean. The paper wrapper on the fuse was browned alittle from the heat but that was the only sign.

Once I got the machine running again, I checked the fuse a few minutes later and it had cooled off considerably.

Something was obviously not the way it was supposed to be but I was just hoping someone would say "I've see this before and..."

Oh well, some things we'll never know.
I've seen this happen a few times at 600v.The fuse element will drop into the silica and turn into a resistor.When you pull out the fuse you shake it and it becomes open.I think the last time it was a CJ 400 amp fuse.
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