UV light + electric current = boom?

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Discussion Overview

The discussion revolves around an incident where a UV pen light was pointed at a light switch, resulting in a loud pop and sparks, leading to a tripped circuit breaker. Participants explore potential explanations for the phenomenon, including electrical behavior and the effects of UV light on materials and components involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that the UV light may have ionized the air between voltage terminals, creating a conductive path that led to an electrical arc and a short circuit.
  • Another participant proposes that the resistance of semiconductor components might change under UV light, potentially affecting the circuit.
  • Some participants inquire about the specifications of the UV pen light and the light switch, suggesting that further information could clarify the situation.
  • There are discussions about the possibility of mercury in the switch being ionized by UV light, contributing to the electrical event.
  • Questions arise regarding the conditions of the switch at the time of the incident, including whether it was in the ON position and if any probing with metal objects occurred.
  • One participant expresses skepticism about the likelihood of the proposed explanations, emphasizing the need for more controlled experimentation.
  • Another participant raises the possibility of thermionic emission or the photoelectric effect being involved in the arcing event.

Areas of Agreement / Disagreement

Participants express a range of hypotheses and uncertainties regarding the cause of the incident, with no consensus reached on a definitive explanation. Multiple competing views remain, and the discussion is ongoing.

Contextual Notes

Some participants note the importance of understanding the specific components involved, such as the type of switch and the characteristics of the UV pen light. There are also mentions of potential limitations in the original observations, such as visibility of soot or residue.

Who May Find This Useful

This discussion may be of interest to those exploring electrical phenomena, the effects of UV light on materials, and safety considerations in electrical circuits.

SW Dad
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Hi, so I was hoping to get an EE's explanation for what just happened. So my son was apparently over by the light switch pointing a UV pen light at the switch. Suddenly there was very loud, gunshot-like pop, sparks flew, and I had to go reset the breaker.

Maybe correlation doesn't imply causation, but this was freaking weird.

nzeg05.jpg

This is the switch. He had it pointing at the switches on the right.
1r9or6.jpg

And this is the pen.

Any explanations?
 

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SW Dad said:
Hi, so I was hoping to get an EE's explanation for what just happened. So my son was apparently over by the light switch pointing a UV pen light at the switch. Suddenly there was very loud, gunshot-like pop, sparks flew, and I had to go reset the breaker.

Maybe correlation doesn't imply causation, but this was freaking weird.

View attachment 216963
This is the switch. He had it pointing at the switches on the right.
View attachment 216964
And this is the pen.

Any explanations?
The UV light was probably able to partially ionize some of the air between the positive and negative voltage terminals that were originally open circuited. Once the air is ionized, it becomes more conductive, and the resistance goes down as an arc gets established in the air, and the current in the arc and wires increases, basically creating a short circuit until the circuit breaker blew.
 
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Hmm. That is odd. I suppose it would have to be repeated before you could rule out coincidence. But I wouldn't recommend the same scenario. Perhaps a more controlled environment. Like measuring the resistance of the device, outside a circuit, then applying the UV light.
It is possible that the semiconductor components could change its resistance as UV light might have an effect on it. Remember that UV photons have a higher energy than visible light.
 
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Thank you for both these responses. The first for a potential explanation of the phenomenon, the second so I could reinforce to my wife NO WE ARE NOT GOING TO TRY AND MAKE IT HAPPEN AGAIN.

She is why we can't have nice things
 
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LOL. Indeed a strange one. Welcome to the PF, BTW. :smile:

Can you post the model number of the UV penlight? A datasheet would be even better, but any info would be helpful so we can look up its specs. Do you know what its claimed output power is?

And do you know the model number and manufacturer of the light switch? Is that light circuit on a GFCI protected breaker by any chance? Or 2was it a regular 20A breaker that popped?
 
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SW Dad said:
Thank you for both these responses. The first for a potential explanation of the phenomenon, the second so I could reinforce to my wife NO WE ARE NOT GOING TO TRY AND MAKE IT HAPPEN AGAIN.

She is why we can't have nice things
One time when I was a youngster, I was plugging in an electrical plug and the prongs were still visible as I pushed the plug half-way in, and so I did an experiment of what happens when you put a metal safety pin across the prongs. The pin quickly heated up, and also blew the fuse. That was an experiment I never repeated. I was lucky I wasn't punished by my parents for my stupidity.
 
berkeman said:
LOL. Indeed a strange one. Welcome to the PF, BTW. :smile:

Can you post the model number of the UV penlight? A datasheet would be even better, but any info would be helpful so we can look up its specs. Do you know what its claimed output power is?

And do you know the model number and manufacturer of the light switch? Is that light circuit on a GFCI protected breaker by any chance? Or 2was it a regular 20A breaker that popped?

Yeah, it's manufactured by Scholastic with the model number 556156 06/17. I tried googling it with no luck.
 
Charles Link said:
One time when I was a youngster, I was plugging in an electrical plug and the prongs were still visible as I pushed the plug half-way in, and so I did an experiment of what happens when you put a metal safety pin across the prongs. The pin quickly heated up, and also blew the fuse. That was an experiment I never repeated. I was lucky I wasn't punished by my parents for my stupidity.

Okay, let's all resist the urge to tell our favorite "I blew the breaker being dumb" stories. Not that I have any of those, but I'm just sayin'

No need to give the young-uns who read the PF any more ideas than they already have...
 
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As for the switch, I'll have to have a look and get back to ya.
 
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  • #10
SW Dad said:
As for the switch, I'll have to have a look and get back to ya.
It's my guess that the switch is very mechanical, rather than semiconductor. I believe some switches also contain liquid mercury as the conductive element (I remember reading that somewhere). If that is the case, it is certainly also possible that the conductive path was assisted by mercury atoms in the vapor phase being ionized inside the switch by the incident UV. ## \\ ## Editing: A google of this shows these (mercury) switches were manufactured before 1991, and do not make a click when you turn them off and on, because there is no solid mechanical contact that gets put in place, but rather they function on the change in position of the electrically conductive mercury liquid. ## \\ ## Additional editing: Here is one UV penlight I was able to find in a google. The UV wavelengths of the one that you have could be the same, or they could be different: https://www.amazon.com/dp/B00NLREN8Y/?tag=pfamazon01-20
 
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  • #11
Wait a minute. Is it the premise that the UV penetrated the plastic face plate? Or came in through those plug slots?
 
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  • #12
anorlunda said:
Wait a minute. Is it the premise that the UV penetrated the plastic face plate? Or came in through those plug slots?
This idea had occurred to me as well.
 
  • #13
SW Dad said:
So my son was apparently over by the light switch pointing a UV pen light at the switch. Suddenly there was very loud, gunshot-like pop, sparks flew, and I had to go reset the breaker.
It is an interesting occurrence. Was the ON-OFF switch in the ON position at the time? Can we take it that the sparks emerged from one particular slot under the Fan Light controls, is that definite? Was there soot or residue on the switch cover after the blast, but it is not visible in the photo because it has been cleaned away? Is your son of an age where he may have been probing with a short length of metal, such as a paper clip, and not wish to disclose this to you?
 
  • #14
NascentOxygen said:
It is an interesting occurrence. Was the ON-OFF switch in the ON position at the time? Was there soot or residue on the switch cover after the blast, but it is not visible in the photo because it has been cleaned away? Is your son of an age where he may have been probing with a short length of metal, such as a paper clip, and not wish to disclose this to you?
This one is interesting enough, that we could really make a laboratory experiment out of it, or perhaps even the manufacturers of this UV penlight need to do that kind of testing. I am lead to believe that an electric arc was established that was aided by the UV, perhaps in the outlet as @anorlunda has suggested, or somewhere in the switch. Another possibility, although I think somewhat unlikely is that the UV heated up the surface of one of the metal electrodes and aided in thermionic emission. Alternatively, did the UV photons have sufficient energy to free the electrons=photoelectric effect?, and thereby made the arcing occur quite readily at the 115 volts ac?
 
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  • #15
The speculations offered so far are very unlikely.

The image shown in the first post doesn't show an outlet--it shows switches which operate a fan and light. If a closure of those circuits occurs there wouldn't be a short that would trip a breaker; there would simply be a fan or light turned on. If the devices in the box are wired in accordance with the NEC, the hot and neutral wouldn't be anywhere near close enough for a short across them without probing through an opening with a metal probe even more capable than the one that Underwriter Labs uses to test these things.

Also, the long wave ultraviolet produced by the near visible led in that pen can't ionize air anyway, and that pen probably emits a few tens of milliwatts of UV, hardly enough to heat metal significantly.

It's hard to see how low-power long wave UV light could cause anything like what is described. The explanation must lie elsewhere.
 
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  • #16
SW Dad said:
Hi, so I was hoping to get an EE's explanation for what just happened. So my son was apparently over by the light switch pointing a UV pen light at the switch. Suddenly there was very loud, gunshot-like pop, sparks flew, and I had to go reset the breaker.

Maybe correlation doesn't imply causation, but this was freaking weird.

Your use of the word "apparently" can be interpreted as suggesting that you might be doubtful about your sons account of what happened.
 
  • #17
The Electrician said:
It's hard to see how low-power long wave UV light could cause anything like what is described. The explanation must lie elsewhere.
i agree. There's something fishy in Denmark -
Sparks indicate a connection between 'hot' and either 'neutral' or 'ground'.
Hot is black , neutral is white, green or bare is ground. If it's a metal box that too is grounded.
..........................

In photo, top cover plate mounting screw is loose and right dimmer handle is missing. Suggests amateur installation.
I always align the mounting screw slots vertical to give a neat appearance, yours are helter-skelter.

I suspect wires behind the switch are in too close proximity .
My advice is
Open the breaker , remove cover plate (carefully) and look. Should be smoke residue wherever was the origin of your sparks.

My suspicion is you'll find the dimmer mounting screws are loose and it can be moved enough to make some wires back there touch one another.

Get a handyman neighbor to help if you're uncomfortable .

Repeat - Open the breaker first.

old jim
 
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  • #18
Charles Link said:
Alternatively, did the UV photons have sufficient energy to free the electrons=photoelectric effect?
I find photoelectric effect is more likely than air ionization as the cause of the spark. The photon energy in the high frequency end of visible spectrum is about 3.25 eV compared to the ionization potentials of air constituents which are at least ~10 eV. Unless the UV light is focused, you will only have trace level of ions from UV-ionized molecules. On the other hand, the work function of most metals is below 5 eV.
 
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  • #19
I've heard of photoflash strobe lights causing plastic case transistors to mis-operate . But that was back in 1960's - surely manufacturers have switched to UV-proof plastic by now ?
But as @The Electrician pointed out that should only turn on the fan or light, and briefly.
I suppose if it fed DC to the fan motor you might get overcurrent. Do the fan and its speed controller still work okay ?

old jim
 
  • #20
Did your son push against the switch plate with the UV light or was he definitely at at distance?
If there was contact, than I would say that Jim's comment about poor installation is likely the answer, and you should get an electrician to inspect it right away.

I experienced fuses blowing randomly that took me a couple years to determine root cause. Whoever installed the garbage disposal had access to both phases under the sink and decided to wire nut them together. Turns out that the wire nut was poorly done, so the phase-to-phase short only happened if a certain spot on the wall was bumped.
 
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  • #21
Charles Link said:
This idea had occurred to me as well.
I believe it came in through the exposed gaps in the switches, rather than passing through the plastic.
 
  • #22
SW Dad said:
I believe it came in through the exposed gaps in the switches, rather than passing through the plastic.
This kind of puzzle can be a fun adventure for the experimentalist in the laboratory. Oftentimes, it turns out to be resolved only after a series of experiments has ruled out some of the possibilities that get proposed for being the cause or mechanism responsible, and then, in some cases, what seems to be a somewhat unlikely cause when it is first proposed, turns out in fact to be correct. Unless the bunch of us have overlooked the obvious and someone can come up with that new answer, this one doesn't look like it can be too easily resolved without some additional experimentation of some kind. The correct answer has perhaps already been proposed, but further experimentation would be necessary to resolve the puzzle. @jim hardy had a good suggestion of having an experienced electrician look over the wiring connections to see if there is something that isn't installed properly.
 
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  • #23
I don't see any comment that the "switches" on the right side are variable controls and not off-on switches.
 
  • #24
DarioC said:
I don't see any comment that the "switches" on the right side are variable controls and not off-on switches.

Post #17 by Jim Hardy: "In photo, top cover plate mounting screw is loose and right dimmer handle is missing."
 
  • #25
The Electrician said:
Post #17 by Jim Hardy: "In photo, top cover plate mounting screw is loose and right dimmer handle is missing."

So what? Those dimmer sliders are normally fitted with a piece of cardboard behind the plate. I still don't see a path for UV light to shine on the electrical contacts. If there was such a path, the contacts should be visible in the picture.
 
  • #26
The Electrician said:
Post #17 by Jim Hardy: "In photo, top cover plate mounting screw is loose and right dimmer handle is missing."
They're dimmers. It might look it in the photo, but the screw isn't lose. And yeah, that handle kept popping off and ended up getting lost at some point. It's just a plastic cap, the switch still works.
MVIMG_20171219_072838.jpg

Here it is from a different angle.
 

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  • #27
the_emi_guy said:
Did your son push against the switch plate with the UV light or was he definitely at at distance?
If there was contact, than I would say that Jim's comment about poor installation is likely the answer, and you should get an electrician to inspect it right away.

I experienced fuses blowing randomly that took me a couple years to determine root cause. Whoever installed the garbage disposal had access to both phases under the sink and decided to wire nut them together. Turns out that the wire nut was poorly done, so the phase-to-phase short only happened if a certain spot on the wall was bumped.
No, he's positive he didn't come into contact with the plate. He was just shining the light in there.
 
  • #28
NascentOxygen said:
It is an interesting occurrence. Was the ON-OFF switch in the ON position at the time? Can we take it that the sparks emerged from one particular slot under the Fan Light controls, is that definite? Was there soot or residue on the switch cover after the blast, but it is not visible in the photo because it has been cleaned away? Is your son of an age where he may have been probing with a short length of metal, such as a paper clip, and not wish to disclose this to you?
No, I'm positive he wasn't poking at it. I was in the room when it happened. The switches were in the exact positions shown in the first photo. For the slider switches, you slide them up for increased intensity.
 
  • #29
Also, the slider switches are different. The fan one mechanically clicks at four separate stops, the light one does not.
 
  • #30
I'd remove the plate and have a look inside, but I'm still recovering from hip surgery at the moment and don't want to mess with electronics right now :wink:
 

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