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#171988 12/10/07 11:10 AM
Joined: Dec 2007
Posts: 35
adroga Offline OP
Hi guys,

I am living in a bungalow built in 1963 that has a 100 amp fused panel with copper wiring.

1 of my main 100 amp cartridge fuse blew last night and i woke up to a cold house (heatpump and oil furnace heating).

What could cause the main fuse to blow?

All i had on was the heat, a blower fan for my fireplace alarm clock and coffee maker set to turn on at 6:30.

My heat pump is old and has a tendency to need resetting on cold days to work properly. It was -12 celcius last night so it should not have been working anyways.

I tightened some neutral screws in the panel, the ground screws and tested all fuses.. none were blown.

I dont know where to look next. Could the clips that hold the cartridges be loose? I didnt see any evidence of arcing anywhere. Dont see any water infiltration anywhere.

I have an electrical background, took the construction electricity course (1350 hours) but havent started working yet as an apprentice.

I appreciate any and all feedback.


Joined: Jul 2007
Posts: 1,335
I see this is your first post so welcome to the board. There are several things that will cause the fuse to blow. A fuse protects against two things, overloads and short circuiting. Before you even think about replacing the fuse, you have to identify why it did what it did. There are too many things that it can be and since your bio states that your are a future apprentice, you are better off and safer to call in an electrician. For whatever reason the main blew before anything else typically indicates there is a other, possibly serious issue with your system. Even if it is just a problem with your heat pump, your over current protection for you heat pump did not kick out before the main therfore you have other issues as well especially sense you service is almost 40 years old.

Are you certain the fuse blew? The is the possibility that the you lost a leg from your service. This requires working around energized wires. Another reason to call an electrician.

This may not be the answer you are looking for, but without additional info and putting you at risk for getting additional info, again you are better off calling an electrician. At least you will still be around to post your second post.

"Live Awesome!" - Kevin Carosa
Joined: Nov 2007
Posts: 763
Hi adroga,
I think there are usually way too many unknown variables to safely diagnose something like this over the Internet.
I’m fairly new to this forum myself, so not really sure what the policy is on providing information for DIY’ers.

However, assuming you do not have a known rodent problem, and all wiring is in good condition, I would probably start by investigating the heat pump. Are you sure it was not running? Does it also have electric heater strips?, I don’t know why they would be needed with an oil-fired furnace.
If it’s that old and routinely needs to be “reset” as you say, then there is already one defined problem that is going to get worse as time goes by.
With fuses, a locked compressor, bad start capacitor or potential relay could cause a high inrush/LRA which could possibly cause a main fuse to open before or at the same time as the time delay fuse clears in the heat pump disconnect, if there even is one, or the branch circuit. Maybe you should call an HVAC pro to have the unit checked out. It’s also possible that the problem had entirely nothing to do with the heat pump.

Quirky things do happen.
I’ve seen a problem with a single 20 Amp branch circuit trip a 200 Amp SQD QO main breaker before.
Sometimes in older homes, a problem with an electric range would blow one of the 60 or 100 Amp main fuses with or without blowing the 40 Amp branch circuit fuses.
That is why I say, there are usually too many variables to safely and accurately diagnose something like this over the Internet.

Maybe someone else here will have some other ideas. Good luck with it.


Joined: Oct 2006
Posts: 745
Sounds like the main fuse(s) could be original from the 1960's, or could be one-time fuses instead of time-delay (FRN/S) type. Fuses don't last forever, sometimes they just blow due to age. I still wouldn't go poking around in that panel though. If the fuse(s) blew due to excessive heat from a loose clip, it could have caused the insulating plastic (bakelite) parts to weaken. Any stress placed upon them could result in breakdown and major arc flash exposure.

I tend to think that my life is worth more than the cost of a service call. I hope that you'll consider biting the bullet and paying for one instead of the possibility of biting a feeding tube for the rest of your life (if you are lucky).

Sorry to be so blunt, but this could be a truly dirty job and if there's any doubt, you should consult a pro. I do have one question though: If you've been in this training, haven't you met anyone who might be able to connect you with a licensed pro who might be able to help you out? If you don't have the money, maybe they will help you out and possibly hire you at the same time. Hey, anything is worth a try. Seriously, please don't mess with this on your own.


"But the guy at Home Depot said it would work."
Joined: Dec 2007
Posts: 35
adroga Offline OP
Thanks for the responses, and I understand your concerns trying to get an electrician to come in and diagnose a problem like this would take forever as I know people who have asked me to do electrical work in their basement as the waiting time for an electrician for a decent size job is 6 months.

I have nothing to lose by trying to diagnose the problem my self and if it blows again without any obvious signs I will try and get someone to come out and take a look.

The fuse was ancient looking and possibly blew due to age in my opinion. The sales rep at home depot said it shouldn't make a difference but intuitively speaking a 40 year old fuse should be replaced.

I was taught basic safety methods in school(lasted 1 year and 3 months) and I have a basic understanding of electricity but not much diagnosing experience.

I will keep an eye out and try to read as many threads as I can find of other problems to see if I can find any similar characteristics.


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