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#560865 - 08/04/09 09:25 PM Fuse blows when TV is plugged in
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
I own a circa-1924 apartment building with electrical wiring that runs the gamut from probably original to 80's vintage. Most of the wiring is old and is connected through a fusebox with screw-in fuses. There's also a circuit breaker panel near the electric meters, and I think both are active, but the fuses are the first in line to blow when there's a problem.

In one apartment (where I've never had any previous problem), the 20A fuse for that circuit blows whenever you plug the tenant's new LCD television into the nearby jack in the bedroom. By trial and error I narrowed the problem down and figured out that it was plugging in the TV that causes the fuse to blow. Other things (lights, etc.) plugged into the same jack do not cause a problem.

A 3-light electrical tester, when plugged into any of the sockets in the bedroom (all of which are on the same circuit) indicates HOT and NEUTRAL are reversed on all of the outlets in the bedroom. Other outlets on the same circuit (but outside the bedroom) indicate OPEN GROUND (which is undoubtedly correct, most of the wiring is old 2-wire stuff and there is no real ground.)

I know that electronic equipment is much more sensitive to incorrect wiring, so I'm not surprised that lamps might work while the TV instantly blows the circuit's fuse, if indeed the wiring is incorrect. Can I assume that the 3-light tester is correct, and that I just need to reverse the positions of the wires on all of the outlets in the bedroom, as they are probably just backwards? Or is this a bigger problem that requires an electrician? In the absence of a good ground, I'm not sure if the plug-in circuit tester is completely accurate or not.

I'm no expert, but I'm comfortable doing basic stuff. I have a multimeter which I can also use for diagnosis though I haven't pulled it out yet for this problem.

The TV, by the way, works fine when plugged into a jack in the other room (which is on the same circuit as the bedroom jacks) once I replace the blown fuse. So I'm thinking that all the bedroom jacks are just wired backwards, but I'd love to have someone more knowledgeable confirm that my thinking makes sense.

Thanks so much!

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#560866 - 08/04/09 10:39 PM Re: Fuse blows when TV is plugged in [Re: SF_Dave]
RCovell Offline
Carpal Tunnel

Registered: 09/26/03
Posts: 6011
Loc: Jayhawk Land
Welcome to the Forum.

Just because it's the right thing to do, I'd correct any reverse wiring in the house. But, be careful, you might run into phantom problems with open neutral readings.

I'd start at the panel breaker or fuse and verify the wire color for hot and neutral. Then, consider a continuity check to the first receptacle, verifying the same color wire is used for 'hot' as found at the fuse.

But, that said and the work done, a reverse hot/neutral shouldn't trip the breaker when the TV is plugged in. I'm puzzled even more by the fact that the TV plugged in to another receptacle on the same circuit does not cause the fuse to blow. Are you positive that it's the same circuit?

If so, I'd look at the suspect receptacle and check for tight connections, possibly even replacing the device itself. And, just for peace of mind, I'd do a load check at the fuse. Do you have an inductive ammeter?

Let us know what you find.

Bob.

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#560867 - 08/04/09 10:54 PM Re: Fuse blows when TV is plugged in [Re: SF_Dave]
MCA Offline
Carpal Tunnel

Registered: 03/09/06
Posts: 4943
Loc: Illinois
I'm confused too. If plugging it in instantly blows the fuse I'll say its a problem with the applaince, but it works on other circuits.
If just plugging something in a receptacle causes the fuse to blow instantly I'll say maybe a wire is pinched at the box and the movement causes it to touch, but other plugs applainces work. I agree that hot and neutral reversal won't blow a fuse when something is plugged in. I'll have to think about this some more, I'm confused too.
Does it blow instantly meaning within a second or so?
Or maybe just too much load. When it worked on another receptacle on that same circuit maybe it was a coincidance that a load that kicks on and off was off at the time. What else was on that circuit?

Not to complicate any further but the 20 amp fuse may possibly be too large. It may have to be a 15 amp fuse. If any wire on the circuit is #14 (thinner than #12) then the fuse must be 15 amp.


Edited by MCA (08/04/09 10:58 PM)

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#560868 - 08/04/09 11:03 PM Re: Fuse blows when TV is plugged in [Re: SF_Dave]
Gizmologist09 Offline
Handyman

Registered: 02/08/09
Posts: 606
Loc: Palmdale, California USA
There is another possibility. The coax that supplies cable TV (or MATV in many apartments) will be providing a ground path the to TV set via the antenna coax connection. If the outlet is wired backwards, there MAY be a hot to ground path through that coax connection. To test, be sure all input cables are disconnected from the set, then turn it on. If it does not blow a fuse, you are halfway there. Use a voltmeter set on 200volts AC and carefully check for voltage between the coax ground and the coax ground on the set. You should not see more than 10VAC. If you read a higher voltage, do NOT connect the set to the coax and get the outlet and set checked. The TV set must pass a test during manufacture called HiPot for high voltage potential. There should be approval stickers or a seal in the instruction/owner's manual that assures it passed.

If you read that higher voltage between the cable ground and the chassis ground of the set, check one more thing. Use the meter to check for voltage between the ground pin of the outlet and the larger slot of the outlet(neutral) you should not see more than a couple volts. Also check between the ground pin and the coax of the cable TV line. Again, no high voltage. If you read any high voltage, call the cable company and inform them what you found. There cable may be shorted to an AC line in the wall somewhere possibly from a nail.

In anyevent, this situation can be very dangerous so use caution.

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#560869 - 08/04/09 11:12 PM Re: Fuse blows when TV is plugged in [Re: MCA]
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
I was also very confused about why the fuse blows when you plug in the TV. It's a new TV, and so it's always "on" to some degree... and maybe there's something weird with it, but it works when I plug it into a socket in the other room (a socket on the same circuit, because the sockets in the other room go dead when that fuse blows.)

I don't think it's a socket issue, because it doesn't matter whether I plug the TV directly into the socket, or into a power strip plugged into the socket. Either way, POOF, instant fuse blow.

I don't think I'm overloading the circuit, since I have everything else on the circuit turned off (and that circuit is normally capable of powering a bunch of incandescent lighting, bathroom fan, etc. without a problem.

I'm pretty sure the 20A fuse is correct; these are the type of screw-in fuses where the bases/threads are different sizes depending on the amp rating of the fuse. So a 15A fuse won't really screw in there, I don't think. In any case, a 15A fuse would blow even more easily than the 20A fuse.

I was sure the problem was in the TV until I plugged it in elsewhere and it worked just fine. If I could blame the TV, that would be an easy solution. Hmm, here's another thought: the TV was connected to cable TV in the bedroom, and might have gotten some ground through the TV cable, right? When I plugged it into the other room, it wasn't connected to anything but the AC power. Do you think that could be a factor? I could try plugging it into the "problem" outlet without the TV cable connection in place and see if it still blows the fuse. Would that indicate a problem in the cable TV wiring?

I'm not sure I've got the know-how to trace all the wiring back from the fusebox and figure out in what order the outlets are wired. If I need to do that it may be time to contact a pro.

Thanks for all the help and any further ideas.

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#560870 - 08/04/09 11:16 PM Re: Fuse blows when TV is plugged in [Re: Gizmologist09]
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
How funny, while you were posting that reply, I was posting another reply where I suggested the cable TV wire might be involved. I am thinking this may be it.

Tomorrow, I will do the tests you suggest and call the cable company if I find some voltage there.

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#560871 - 08/05/09 12:32 PM Re: Fuse blows when TV is plugged in [Re: Gizmologist09]
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
OK, I confirmed that there is 120VAC between the ground of the cable wire and the TV ground (the TV is plugged in but not connected to the TV cable.) The TV doesn't blow the fuse when the cable isn't connected, so we've ruled out the TV as being a problem and there seems to be voltage flowing into the TV cable somewhere.

But there's ALSO 120VAC between the neutral and ground pins of the AC outlet. So does this mean that I have BOTH an electrical wiring issue and a cable TV wiring issue? Or does it just mean that I have an electrical wiring issue (hot & neutral reversed) and the TV cable is just serving as a path to ground (as it probably should.)

Can I solve the problem by reversing hot and neutral on all the outlets which show hot and neutral reversed on the plug-in outlet tester? Or do I need to call an electrician to trace things back from the fuse box?

THANKS so much for all the help!

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#560872 - 08/05/09 02:27 PM Re: Fuse blows when TV is plugged in [Re: SF_Dave]
Gizmologist09 Offline
Handyman

Registered: 02/08/09
Posts: 606
Loc: Palmdale, California USA
Okay, first the ground on the antenna cable is correct. That is grounded for lightning protection. The outlet is wired backwards and should be corrected. There should be nothing higher than 1-2 volts max between the ground pin and neutral.

There may also be a problem in the set as there is a switching power supply in the set that is NOT tied to the chassis ground. Older CRT TVs sometimes had one side of the chassis connected directly to the neutral side of the AC line. That was the purpose of polarized plugs.

Check the TV unplugged ac cord. Set the meter to OHMSx100 and measure between the antenna ground on the set and both sides of the AC cord to the set. You should not get a reading. If you do, the set has a dangerous power supply problem.
It should be checked by a certified trained repair facility. It is NOT a DIY repair.

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#560873 - 08/05/09 03:50 PM Re: Fuse blows when TV is plugged in [Re: Gizmologist09]
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
I tested between the antenna ground on the set and all three terminals on the AC cord with a continuity tester. There's continuity between the TV and the ground pin, but neither of the other two. So I think that means the TV is ok... ?

But here's the stumper: I reversed hot and neutral on the wall outlet. My little outlet tester now indicates that the outlet is correctly wired (before it indicated that hot and neutral were reversed.) But when I plug in the TV, with the TV cable connected... poof, the fuse blows instantly still.

So I think I need to call an electrician, unless you have some other idea.

Thanks again for the help, I appreciate it so much.

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#560874 - 08/05/09 06:53 PM Re: Fuse blows when TV is plugged in [Re: Gizmologist09]
SF_Dave Offline
newbie

Registered: 08/04/09
Posts: 6
Loc: San Francisco, CA
The problem is solved, though I can't tell you for sure what the solution was. After trying a zillion different things, and confirming that the TV cable was ok (no unusual voltage coming from it) we were still getting a blown fuse whenever the tv cable was connected, and the outlet tester was indicating reversed hot/neutral even though they were NOT reversed, as confirmed by the electrician. We traced the wires back to a junction box, which was also wired correctly, but we disconnected and then reconnected all the myriad of wires in the junction box just in case. After doing that, we were then getting a correct reading on the outlet tester for that outlet.

We then replaced the outlet which the TV plugs into for good measure, and now everything works. So even though we didn't change any of the connections in the junction box, the act of reconnecting them and replacing the twist-on wire connectors seems to have solved the problem.

All's well that ends well, and I thank you all again for your help!

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