Not sure if this is a common issue, or how to formulate a single question, so please feel free to edit. I'm just trying to understand the situation and don't know where else to turn.

I have an electrical outlet in my office that powers my computers. They work just fine, and I never had any issues.

I plugged in a tube guitar amplifier the other day and instantly burned a fuse on the input section of the amp's circuit. Swapped out a new fuse, same thing. Took the whole amp (again with a new fuse) and plugged it into a different office in my building. Worked like a charm (as it did before bringing it to the office).

I decided to plug in a surge protector power strip. It is the type that lights an LED when the outlet is grounded. I plug it in and it exploded! Well, it made a huge pop inside the power strip and stopped working followed by a stream of smoke and burnt plastic. There was nothing plugged into this power strip.

I pull the outlet out, and there are only two wires. There is no ground wire. The hot is on the correct side (provided that the black is indeed hot).

Does this sound like a normal string of events considering there is no ground? Or is there something SERIOUSLY wrong in this scenario?

I've played in an industry where we pop out the ground on amp cords on a regular basis to stop 60hz hum in some installations. Why would the absence of a ground here cause this problem?

Thanks for any info. If this seems off-topic, please help me re-formulate the question so that I can understand what's going on.

  • \$\begingroup\$ Whats the color of the other wire? Black and ...? Maybe its L-L (two different phases) instead of L-N which would result in much higher voltage (L-L is L-N multiplied by factor 1.73). But I think this is pretty unlikely. \$\endgroup\$
    – Rev
    Aug 3, 2014 at 15:29
  • \$\begingroup\$ The outlet should be grounded through its mounting screws, even without an actual ground wire. \$\endgroup\$ Aug 3, 2014 at 15:54
  • \$\begingroup\$ Color of the other wire is white. I don't have any of my tools here to check reverse polarity / grounding. However an electrician will check it out this week. I'll come back and re-post the results. Thanks for everyone's input. I suspect that the polarity is switched from what Spehro posted. It makes perfect sense, and the fuse probably just did it's job & saved me from getting shocked. \$\endgroup\$ Aug 3, 2014 at 21:41
  • 4
    \$\begingroup\$ Rev was right. The outlet was wired to 240v. There were two hots, and the outlet enclosure was grounded through the mounting screws. Computers worked just fine because the power supplies were international 120-240v. Everything else exploded on plug. Quite dangerous. Thanks for all the input on this weekend mystery! \$\endgroup\$ Aug 4, 2014 at 19:53

3 Answers 3


Have you checked the voltage at the outlet? Your guitar amp may be 120 volts AC only, while most computers use switch mode power supplies that can function with anywhere fron 100 volts AC to 240 volts AC at the input. If the outlet supplies significantly more then 125 volts AC the switch mode supplies in the computer gear may work fine but any device rated for 125 volts AC (or a bit less) will, at best, blow the AC line fuse.

  • 1
    \$\begingroup\$ Bingo... It took awhile, but you got it... Outlet was two hots 120/120 from two different phases... Neutral was connected where the ground should have been. The electrician was fired! \$\endgroup\$ Feb 17, 2015 at 18:18

There are some old tube guitar amplifiers that tie one side of the output, input and chassis directly to one side of the AC input. If you swap L and N, then the chassis is electrically "hot" and a connection between the chassis and earth will blow the fuse.

For example:

enter image description here

Similar design, but fitted with isolation transformer:


It could also electrocute you. Exercise extreme caution.

  • \$\begingroup\$ The schematic you posted dosen't seem to have this problem... \$\endgroup\$ Aug 3, 2014 at 16:16
  • \$\begingroup\$ @VladimirCravero Sorry, and thanks, I posted the "after" schematic (with isolation transformer installed) rather than the original. The original has the line going through a switch directly to the chassis. See edited version. \$\endgroup\$ Aug 3, 2014 at 16:56
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    \$\begingroup\$ Did they use to do that to...? Save money? \$\endgroup\$ Aug 3, 2014 at 17:34
  • \$\begingroup\$ @VladimirCravero Save money and weight on the transformer. Safety wasn't such a big thing back then. \$\endgroup\$ Aug 3, 2014 at 17:56
  • \$\begingroup\$ Yeah like if the laws of physics has changed since :D I guess being a guitarist "back in the days" was a dangerous job. \$\endgroup\$ Aug 3, 2014 at 18:25

I'd suggest you get one of those cheap line phase detectors, the ones you plug into the wall with 3 LEDs to show you how things are connected. My guess is that your line and neutral are reversed, and there is an inadvertent ground connection being made somewhere. I've seen this sort of problem (actually, a reversed ground-neutral) produce an arc on a solid-fuel rocket sitting on the pad, which definitely counts as a "bring me my brown pants" incident.

The OP does not make clear where the ground is missing - in the surge protector or in the wall socket. A mains checker applied to the wall socket should detect a missing ground. If this is so, the socket must be repaired and the ground connected. As part of this process the electrician should check for proper connection of the line and neutral, and doing it yourself after the repairs are complete can't hurt.

If the ground is missing in the surge protector, this suggests a pretty straightforward explanation for the pop and smoke incident - it's a really cheap piece of junk. As a secondary explanation, the blown part was defective. This can happen.

  • \$\begingroup\$ Unfortunately, the OP already said that there is no ground connection in the outlet. Without that, such a device cannot detect swapped hot/neutral. \$\endgroup\$
    – DoxyLover
    Aug 3, 2014 at 16:26
  • \$\begingroup\$ As I read it, it was unclear if the missing ground was on the wall outlet, or not connected in the surge protector, and I assumed (since the surge protector was opened up) that the missing ground connector was in the protector. Furthermore, if the wall socket is a two-prong outlet (no ground), a standard 3-prong surge protector will not plug into it. \$\endgroup\$ Aug 3, 2014 at 16:35
  • \$\begingroup\$ Quoting the OP: "I pull the outlet out, and there are only two wires. There is no ground wire. The hot is on the correct side (provided that the black is indeed hot)." Clearly he is talking about a grounded (3-pin) outlet with no ground connection. However, as Peter Bennett noted, the outlet could be self-grounded to the box. So using a tester as you describe could have value. However, if the tester shows an open ground, it will not be able to detect reversed hot/neutral. \$\endgroup\$
    – DoxyLover
    Aug 3, 2014 at 17:07
  • \$\begingroup\$ There is clearly something seriously wrong with that outlet. Perhaps the wires are swapped somewhere upstream, and black is NOT indeed hot. A qualified electrician needs to check the building wiring. \$\endgroup\$
    – Dave Tweed
    Aug 3, 2014 at 17:15
  • \$\begingroup\$ @DoxyLover - to continue my response to your first comment: a standard $10 outlet checker will detect a missing ground. This will (one hopes) cause an electrician to be called to fix the problem. A second application of the checker will then (if the electrician didn't check the lines while fixing the ground problem) detect a swapped hot/neutral. \$\endgroup\$ Aug 3, 2014 at 17:20

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