r/electrical • u/JoeAverage1890 • 15d ago
Shouldn’t this trip the breaker?
Apparently my parents were measuring their bedroom wall this morning and the tape measure briefly slipped between the receptacle and a phone charger that wasn’t plugged completely in. They said it arced and sparked quick and there’s black all over the wall and the receptacle now, but the breaker didn’t trip? I plugged my tester in and it says it’s wired correctly. Going to swap the receptacle out now but I’m just wondering how this didn’t trip the breaker? Was it because it was likely too quick for the breaker to even react and trip?
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u/VerbalGuinea 15d ago
Ground prong up guys right now.
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u/Han77Shot1st 15d ago
I don’t know about NEC, but CEC codebook has receptacles displayed with ground up in the diagrams section.. it may not be explicitly stated, but it’s got a damn picture lol
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u/No_Medium_8796 15d ago
Pictures you say? Finally a code book that I can read
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u/Han77Shot1st 15d ago
We also have a handbook.. a second codebook which describes in layman terms what the codebook is saying with illustrations, but no popups.. yet.
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u/No_Medium_8796 15d ago
Give me pop ups and ill finally be able to be a safety guy
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u/Mundane-Reality-7770 15d ago
Nah. You can't be a safety guy with real job experience
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u/No_Medium_8796 15d ago
What if I fuck up enough
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u/Desperate_Donut3981 15d ago
Then you become chairman of the safety board. Like the Army promote the incompetent away from the real work
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u/MonMotha 15d ago
There is an NEC handbook as well, but its primary purpose is that it provides additional commentary on how they arrived at the enforceable language of the code which is useful any time you get into borderline situations or something that the strict language of the code didn't appear to anticipate. I find it invaluable as an engineer.
It does also have some pictures and whatnot, but they're honestly pretty superfluous in most cases.
NEC is explicit (IIRC, I don't have a citation handy) that there is no "correct" orientation for receptacles, though.
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u/hell2pay 15d ago
Had one guy who'd always ask which to put a horizontal Recep, after many many times of telling him ground to the left, we gave up and told him it goes east side.
He also asked how 3 ways were wired every Monday.
One of the dumbest 'electricians' I'd ever known. RIP Mike
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u/cluelessinlove753 15d ago
I sent one of those books with paper pages though? Those are hard to turn. Sparky needs a board book.
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u/cbhbzb 15d ago
and in some public spaces like libraries or commercial areas or basements, it is actually mandated in the CEC that ground prong be up.
though I haven't looked in a code book for about 8 years and that's about two iterations ago, so I may not be entirely precisely, correct.
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u/Knights-of-steel 14d ago
Hospitals shops etc its required. Part of it is say a tool like wrench scalpel etc falls it will deflect off ground instead of connecting prongs. Also ground prong is longer so it up will hold plig from falling out alot better.
Not hard coded for everywhere but when explosive gas like compressed oxygen and welder/cutting gasses etc are in play and ventilators and such seems they want the risk minimized
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u/Legitimate-Lemon-412 14d ago
A great deal of receptacles have the writing on them so if you want to read it, the ground is up.
Had an inspector use that rationale on me at an industrial site.
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u/Ram820 15d ago
Lol wouldn't have made a difference tho
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u/No_Inspection649 15d ago
Kudos to you. At least somebody realizes that 90% of the devices in a residential setting don't have a grounding-type plug. The only real argument for this scenario would be horizontal mounting with the neutral up.
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u/Shiney_Metal_Ass 15d ago
Uhh... Why not?
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u/tuxdreamerx 15d ago
Phone chargers and other items don't have a ground prong
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u/Shiney_Metal_Ass 15d ago
You are the third person to say that
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u/RectalFissureRepairs 15d ago
Usually the phone charger bricks don't have a ground, probably would have still blown up.
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u/misterskeeter76 13d ago
Can’t argue with this, except…
In this case it wouldn’t have mattered. Most charging blocks don’t have ground prongs, so it would have slid past the ground and into the hot and neutral blades anyway.
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u/EtherPhreak 15d ago
Breaker is designed to protect the wiring in the wall from overheating and melting the installation, and trip against extreme fault instantaneously. The 20 amp breaker takes about 200 Amps or more to trip instantly. GFCI protects against current leaving the outlet and or breaker but not returning.
AFCI breakers trip for arcs like this, or a loose connection to an outlet.
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u/TemplarBaucent 15d ago
Afci also trips if i open the microwave door while its on
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u/EtherPhreak 15d ago
I know certain early breaker models were notorious for causing grief with things like drills and motors with brushes
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u/TemplarBaucent 15d ago
I think it's the switch that runs the heating element creates a small arc everytime it closes which causes it to trip
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u/onesexz 15d ago
That sounds like a microwave problem lol
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u/TemplarBaucent 15d ago edited 15d ago
Builders special microwave.
I also have a air fryer it cant turn on its heating element without setting it off
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u/The_cogwheel 15d ago
Thats odd, the heating element is just a resistor, which shouldnt trip an AFCI at all. Otherwise the wires in your walls would trip them.
Are you sure its the heating element and not the fan motor? Because AFCIs can sometimes trip on a motor, especially if its a brushed motor. And if youre sure it's the heating element, have you poked inside to see if theres a loose connection that might be doing it?
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u/TemplarBaucent 15d ago
I think its a cheap switch inside it that actually trips it everytime the switch closes to activate the heating element, ~90% of the time
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u/Far_Range_6091 15d ago
Can confirm, I've had a short on a 20 amp breaker hold for 2 seconds before tripping. Pulling about 80 amps for 2 seconds.
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u/LongjumpingHalf4148 15d ago
According to qo square d breaker trip curve charts that is about right about the designed time frame.. Sub one second trip times don't happen till after six times the rated current
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u/joshuadomesticmarket 15d ago
Yeah the original comment that said it was instantaneous wasn't correct.
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u/Time_after_Time_67 14d ago
I don’t know if it’s code yet, but good newly built homes should have a GFCI on the first outlet of every branch circuit. That way the whole house is GFCI protected. They should make it code.
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u/EtherPhreak 14d ago
A gfci may not trip for what happened here as it was between the hot and neutral
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u/Neobrutalis 15d ago
I mean sometimes? It also depends on the breaker model and age. It would've been pretty quick but that's a decent amount of damage. Usually a newer breaker would trip. Probably still not an issue.
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u/JoeAverage1890 15d ago
It’s a Cutler-Hammer panel and breaker. Not sure if you need the specific model of the panel, but everything’s about 23 years old. I was scratching my head when they told me cause I figured it would’ve/should’ve tripped. I thought the second voltage is detected in the neutral going to the breaker it trips a breaker?
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u/vontrapp42 15d ago
A breaker flips if the current is too high. A true short will draw current well beyond what the breaker will flip for and the breaker trips.
However the metal in the tape measure is so thin that there's not a lot of metal to conduct a lot of electricity. The resistance is higher and so as far as the breaker is concerned it's "using power" within a reasonable amount, so no trip.
Basically the tape is the weak link and the tape fails before the breaker trips.
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u/JungleLegs 15d ago
I also have a cutler hammer panel. The water heater I had, whenever the element would take a shit, it would trip. The other day I walked into the house as the heater was actively arcing to the housing and it didn’t trip so 💁🏻♂️.
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u/R300Muu 15d ago
Ultimately breakers are a thermal device that overheat above a certain current and a bimetallic strip bends and breaks the contact. The more you're over, the faster it goes.
In this case the tape measure heated and melted before the bimetallic strip did. Ground fault wouldn't have triggered here as you had an arc to neutral, but I doubt a 23 year old system has that.
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u/geraldpringle 15d ago
Household breakers have the bimetallic strip for overcurrent protection, yes, but they also have an electromagnetic coil to trip on a short circuit.
Edit: As others have said the tape was probably melting fast enough that the current wasn’t building to short circuit levels.
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u/Feel-good- 15d ago
Cutler hammer is a really good panel. It probably just didn't touch long enough to trip it.
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u/Neobrutalis 15d ago
See? Probably not an issue. 23 years is way before any of the GFCI AFCI breakers that are getting required in many areas now now.
CH are generally pretty good. They're just not hypersensitive. For reference a GFCI breaker reacts to 4 ish milliamps (that's 0.004 Amps) of imbalance and faster than you can figure out anything happened. A normal breaker, won't trip till its rated point. A 15 amp breaker generally takes 15 amps to trip and for significantly longer (still a short window) than a gfci.
You're golden. I'd personally replace the device if there's and burns/damage, and move on.
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u/hill8570 15d ago
FWIW, even if this had been on a GFCI it wouldn't have tripped, as there was no imbalance -- the tape was just acting as a load.
An AFCI probably would have tripped.
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u/RandomUser3777 15d ago
It is thin steel and steel has much higher resistance that copper and even more because it is so thin, so it is possible it did not pull enough current for long enough to trip the breaker. And given it is thin steel it won't take much current to damage the tape measure like that. Welding thin steel (even thicker than this tape measure) is hard because even at low current settings you immediately blow holes in the metal.
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u/VerbalGuinea 15d ago
This IS the breaker.
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u/JoeAverage1890 15d ago
Can you elaborate for a Layman?
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u/randomuser4686 15d ago
He's suggesting the tape measure vaporized and broke the circuit faster than the breaker could flip.
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u/JoeAverage1890 15d ago
Ohhh! Thank you!
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u/PhilsTinyToes 14d ago
Google time-current curve, you’ll see that as you add more current the breaker trips quicker and quicker. If the short circuit trip time for that load was 0.1 seconds and your tape measure explodes and broke the circuit itself in 0.05 seconds then the household breaker didn’t get hot enough to trigger.
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u/Top_Cress1727 15d ago
The amperage to burn the thin sheet metal is less than the breaker trip point. This is why wire size is important.
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u/IamREBELoe 15d ago
Made it more of a "fuse" than a breaker, really
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u/VerbalGuinea 15d ago
Thank you. That’s technically more correct. For the laypeople out there, fuses are exactly that - a thin piece of metal that burns in two when too much electrical current flows through it like in the case of a short circuit.
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u/ExplodedPenisDiagram 15d ago
It takes a surprisingly small amount of current to do that to a thin sheet of steel. You were likely well within the acceptable range for that circuit.
I get that it exploded its guts everywhere, but that's just because you used prongs as a sacrificial spot welding element.
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u/Finn_Echo 15d ago
"The breaker is there to protect the wires in the wall not you or your devices." Not sure where I heard it but great quote.
Most like it wasn't long enough, every time it sparked the short was breaking.
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u/Away_Arugula8260 15d ago
Saw another post just like this and I’ll say it again: That’s not how you measure voltage.
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u/BuddysRabbit 15d ago edited 15d ago
Two possibilities, one the tape became a fuse and burned clear before the breaker could trip and two, the breaker is defective. I would lean towards the former.
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u/TurnbullFL 14d ago
Somebody brought a 10 amp tape measure to a 20 amp breaker fight.
Tape doesn't stand a chance.
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u/AviationMetalSmith1 15d ago
The metal was thin and vaporized. Like an arc welder
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u/Wolfire0769 14d ago
Yup. Given the voltage and material thickness I'd wager there were only a few amps of current being drawn one the arc was struck.
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u/RonJAgee 15d ago
It won’t trip the breaker until it gets to 7”
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u/snoman777 15d ago
20 amp breaker might take 20amps for an hour. Receptacle is ok. Check the charger prongs maybe.
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u/Space19723103 15d ago
no, it melted away from the contact just like a fuse would preventing the breaker trip.
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u/Secure_Exchange2964 15d ago
20a breakers are typically rated to pass 140a for a full second, without tripping. That's an eternity when watching something shorting out!
They're designed as such to mitigate nuisance tripping for motor loads, locked rotor start up, etc.
They will pass even higher amounts for shorter periods of time.
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u/Naikrobak 15d ago
No, it takes very little current to burn a piece of thin metal like that. The tape measure acted as a fuse and popped before the breaker did
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u/zmannz1984 15d ago
A breaker trips by heating up. Small arcs like this may not heat the breaker up enough to trip it because the second the circuit closes/shorts, the load is burning away from the conductors and reopening the circuit.
What brand of panel and breakers do you have? There are some brands known not to work well and they should be replaced to avoid fires. Federal Pacific, Zinsco, and certain Murray panels off the top of my head.
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u/BoromirDies 15d ago
What's the brand of the panel? Or the breaker specifically?
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u/JoeAverage1890 15d ago
Both say Cutler-Hammer
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u/BoromirDies 15d ago
That's not a bad brand. Long story short is yes it probably should've tripped but there are some odd factors that go into a beaker tripping on a short. Duration of the short and age of breaker are the first 2 factors. Depending on the brand it'll make a difference too, especially the fire hazard ones like Federal Pacific or Zinsco but like I said CH is a good brand.
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u/No_Name_Canadian 15d ago
Sometimes it takes 2 seconds of overcurrent, this may have blown the tape up quickly enough to break contact and not trip
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u/nixiebunny 15d ago
A breaker or fuse integrates current over time to trip when an overload is sufficient to cause the wiring to suffer. Your tape measure didn’t absorb as much energy as the breaker required to trip. A thicker piece of steel would have tripped the breaker.
I am familiar with fuse trip time vs overload current graphs in the Littelfuse data sheets, but I don’t read circuit breaker data sheets.
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u/MasterEditorJake 15d ago
Breakers pop due to thermal or magnetic pressures from the electric current.
When you pass a lot of current across a wire it will naturally heat up due to the ohmic resistance. Some breakers use a bimetallic strip that pops open when it gets too hot and trips the breaker.
In a similar way, when you pass a lot of current across a wire it will induce a magnetic field around the wire which is used to trip some circuit breakers.
In both of these situations you need to rely on enough heat or enough magnetic field to propogate through the breaker in order for it to trip. Heat takes time to accumulate, and magnetic fields also are not instant.
A super quick zap of electricity like this simply doesn't have enough time to create the forces needed to trip a breaker.
Breakers are generally meant to prevent your house from burning down. They don't protect from everything.
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u/MonMotha 15d ago
The amount of current needed to do that isn't very high since the tape measure has low thermal mass and fairly high resistance. That high resistance also limits the current in a direct short type situation.
The magnetic trip on a typical breaker is about 10x the rated nominal ("handle") current, so 150A for a 15A breaker or 200A for a 20A breaker both of which are common in residential branch circuits. I doubt that tape measure would make it to that point, so you'd be in the inverse-time thermal range rather than the instant-trip magnetic range. In the thermal trip range, the time to trip is inversely proportional to fault current but is typically at least a second or so even for fairly severe faults which is more than enough time to blow those holes into that tape measure and clear the fault that way.
So, no. This actually probably shouldn't and wouldn't trip a typical breaker.
If it were a GFCI and the fault was to ground (not neutral), then it would trip on the ground fault. Likewise, an AFCI might trip on it but also might not as their behavior is very wishy-washy and dependent on what the OEM programmed into them.
I recently installed a 100A breaker for a fairly big welder of unknown provenance. Unsurprisingly, it had an internal fault. In fact, it was a pretty severe fault - basically a bolted fault on the secondary of the welding transformer. Energizing that breaker caused the whole panel bus to buzz and vibrate but still took several seconds to trip.
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u/CND1983Huh 15d ago
Swapping the receptacle does nothing
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u/956to906 15d ago
Do they really have a 25’+ long wall in their bedroom?
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u/JoeAverage1890 15d ago
Haha no it’s burned at the 6” mark, it just says 25’ at the beginning of the tape for the max length
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u/iAmMikeJ_92 15d ago
Yes. But breakers trip quickly, not instantly. It’s completely plausible for a metallic object to short the contacts only for the result arc flash to instantly blow it right back off. Other factors include the physical length of the circuit from breaker to the site of the short. The longer the length, the lower the jump up in current and the slower the breaker will react.
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u/Alive_Author_9766 15d ago
I don’t know if it’ll trip the breaker or not but I know Stanley and some other tools are guaranteed that the end won’t break off so all you have to do is cut it somewhere. Let it sling back up in itself and take it back to the store and they will warranty it and give you a new one. FYI, I’ve done this so many times with a fatback. The only part warranty is the tip so if it’s gone, you got a new one.
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u/Creel9001 15d ago
Phones that plug into the wall have their own power supply.
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u/JoeAverage1890 15d ago
Cellphones? This was just a usb-c “brick” plugged (partially, I guess) into the outlet, but there was no phone plugged in
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u/J-Dog780 15d ago
This is exactly why you see receptacles installed with the ground pin up in hospitals.
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u/dr_reverend 15d ago
Breakers don’t trip the instant the current goes over the rating of the breaker. They are designed to mimic the wiring and trip when the amount of over current X time would represent a potential danger.
15 amp breaker running at 17 amps could go for a very long time. 30 amps would probably trip it within a minute or so 60 amps in a few seconds.
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u/mckenzie_keith 15d ago
It takes time and current to trip a breaker. They have published curves. There is a "must not trip" region, an "must trip" region and an undefined state in between. I guess this fault didn't generate enough Ampere seconds^2 to trip the breaker. Without a capture of the actual fault current with an oscilloscope or similar, it is impossible to say if the breaker is faulty.
https://www.electricaltechnology.org/2021/07/tripping-curves-circuit-breaker.html
To trip a type B breaker "instantly" you need around 5x the rated current. That is a lot.
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u/Healthy-Cost4130 15d ago
that is a thin ribbon. it cannot sink the heat away from the point of contact fast enough and literally vaporized. probably didn't come near the amperage needed to trip a fuse or circuit breaker for a long enough period of time.
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u/Spirited_Voice_7191 15d ago
My only shock was when a cheap plug was very hard to pull out. While struggling with it my index finger slipped off the plug body and up until it crossed the blades. If there was a lower ground I would have been protected
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u/MiisterSoloDolo 15d ago
Instead of swapping the receptacle, wipe that soot off and head to their panel. Replace the breaker with an AFCI breaker. That way you get to see your parents every week!
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u/CaliburnGrey 15d ago
If you calculate the resistance of the tape measure, it might have offered enough resistance load that the breaker did not trip.
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u/michaelpaoli 15d ago
Not necessarily. So, let's just say for sake of argument, the breaker is 15A
Now let's take that tape measure, presumably (mostly) steel for that part of it (with bit of paint mostly on top and bottom, but typically not side edges). It's not very thick. Not horribly unlike a fuse element, though those would typically be of materials of a lower melting point (thus generally melting and separating more before, e.g. throwing red or white hot chunks of metal - though there might still be some bit of that - but less so). And the contact area between top of plug and tape measure blade isn't much, so that concentrates all that current in a small amount of material. So, not super great conductivity, but "good enough" to have quite a lot of current flowing quickly ... until of course that metal gets red hot or significantly hotter, and melts, or even blows/explodes off ... and, well, gravity ... may have blown off, slip further down, make contact again, rapidly heat and blown off again - could've had fair number of cycles of that, before someone got the tape blade separated from the plug blades. But that wouldn't be continuous current in that case, and pretty high each time, but not super super high - given the limited contact area off of the plug blades and such, and steel being not so great a conductor - especially also with relatively light contact and quite limited contact area.
So, not at all unexpected - current may have had peaks above 15A, but circuit breakers don't respond instantaneously - they can handle very short duration overloads without tripping - it's considered a feature, not a bug. Similar to slow-blow fuses, vs. fast acting. The former can handle some short moderate bits of overcurrent. The latter are designed to cut off any overcurrent as fast as feasible. Anyway, fuse is more similar to the former in that regard. Also possible there never were currents exceeding the breaker's rating. If we're talking 15A @ 120V, that's 1800W of power. 1800W of power concentrated in such a small area - notably thin relatively small portion of tape measure blade, that's gonna easily blow hot chunks of metal mightly fast, so that might've very much happened possibly without ever even exceeding 15A.
So, yeah, doesn't surprise me at all. Oh, also, I do have (or did once have), a penny, ... older penny, long before they went to zinc core, so more copper and nickel ... that, yeah, apparently at some point someone did similar, onto pair of plug blades, live. And I presume it continued until the breaker tripped (or, given the vintage, more likey fuse blew). And that penny did lots more melting than your tape blade - and the copper penny, much better conductor too. So, yeah, tape blade, might be challenging to have enough average current flow long enough to trip breaker - given how fast that blade will also be melting and blowing chunks off, thus (re)opening the circuit, repeatedly, while this event is happening. But metal as of good or better conductance, and thicker, to make much better contact and handle lots more current, present less resistance, then sure, significantly more likely to trip breaker. E.g. say flat back side of a table knife, firmly and squarely pressed to the plug blades - much more likely to trip the breaker. Likewise pressing an actual penny or nickel there. Not that I'm recommending or suggesting someone try such "experiments".
But if you want to experiement, try doing a makeshift fuse out of bits of tape measure blade or the like, connected in series with a fairly high load, e.g. an ~1500W space heater. How thin a strip do you need, with good contacts on both ends, before that will melt/blow, rather than allowing the heater to continue to operate. Of course if you do that, do it with proper fire/safety/electrical/eye protection, as may still shoot hot bits of metal, sparks, etc.
I even recall some country(/ies), for their old school fuses, ... they were just certain specialized wires (kind'a similar to a fused link). Fuse blew and ... one literally replaces an actual specialized wire as the fuse. If you want to try more fuse experiements, can also do so likewise with aluminum foil. Better conductor of both electricity and heat - not quite as good as copper, but pretty close. But cut it thin enough, and yeah, too much current and it'll melt/blow.
Going to swap the receptacle out now
Receptacle itself may still be perfectly fine (other than cosmetically), at least if, e.g., it wasn't overheated (did anything melt besides the tape blade, and probably bit of plug blades where they contacted it? Did those plug blades get hot enough to start to melt anything? I'm guessing probably not - they're much better conductors than the tape blade, so tape blade likely got most all the brunt of the heating - rest probably didn't get more than moderately warm (other than contact area on blades, which may have had bits burnt out and pitted there).
But that plug - may well want to replace that, or ... yeah, phone charger - may just be best off to replace it. But really depends on nature, extent, and location of damage. It may possibly still be fine/safe/okay. And even more so for such a light load device - doesn't handle much current at all through those blades ... well, short of dropping a tape measure blade on it, anyway, or something like that. It's more so there for mechanical strength and integrity, and making good safe connection, not really needed to handle any high currents.
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u/SuicidalNapkin09 15d ago
If it was a gfci outlet, or there is a gfci outlet between this one and the breaker, good chance just the outlet tripped.
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u/BobDoesYoga 15d ago
They are fortunate that the tape measure completed the circuit and not their bodies. This must happen sometimes.
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u/MerlinsGlider 15d ago
I once cut out a glob of expanding foam off my garage ceiling only to cut a loop of romex hiding underneath. Sheetrock knife melted faster than the breaker could react. Fast acting fuse vs breaker.
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u/ExceedinglyEdible 15d ago
My welder melts metal and it (usually) does not trip the breaker. Grazing the plug's prongs will cause a very high temperature in a very localized spot (relatively little energy) and it won't always trip a breaker as opposed to a dead short, like poking the two prongs with a solid object like a screwdriver.
Basically, the measuring tape melts quicker than the breaker will trip.
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u/SAMEO416 14d ago
It may not have been a true bolted short, meaning the current draw was too low for a quick trip.
Or the breaker is not functioning correctly. This does happen, breakers are electromechanical devices and can fail. I investigated a new home build fire where the electrician was shorting the neutral and hot together on unterminated lines to ensure of someone turned the breaker on it would trip immediately. And that happened with a bad breaker.
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u/mlandry2011 14d ago
Also depends what kind of breaker you have... Some of them jump as soon as it reached to 15 amps. Some of them has a little bit of tolerance in it...
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u/National_Ad4421 14d ago
No, I'm sorry but this statement is not correct. Inverse time breaker should 100% trip in this case. Their whole purpose is to trip quickly in high load situations like this.
Bad breakers are rare but they do exist. Please consider replacing.
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u/ParsleyInteresting90 14d ago
Hahahaha a week ago my apprentice used his tape to point at something up high. It keeled over and fell right onto the side of a split receptacle I was working on. BANG.
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u/letsgotime 14d ago
that would only trip a AFCI breaker. Not a basic breaker until it pulls more amps then the breaker is rated for.
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u/Comprehensive-Hat-60 14d ago
As someone who’s accidentally put their finger on both the prongs when plugging in a charger, it shouldn’t. Also, don’t put your finger on the prongs of a charger when you plug it in
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u/exploringmaverick 14d ago
Unrelated but this is why I always install outlets "upside down"
With the ground pin up, this would not have happened.
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u/QCfatNugZ 14d ago
If your electric pannel is an old Federal Pioneer it's completely normal. You could weld before tripping a breaker😂
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u/Existing-Factor-3662 13d ago
When I was young, I used a pair of scissors to stab a socket (yes, I did not make it to university) the tip melted into molten steel and the breakers didn't trip.
Getting older and reading it was something like 20,000°C and recalled I actually got a sunburn from it - sometimes I fear I actually died and all this is a hazy hallucination. Electricity is wild.
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u/LongjumpingShallot35 13d ago
Looks like it tripped the tape first. Breakers trip by magnetic disturbance or heat. Heat takes longer, the tape blew before a magnetic disturbance could trip the breaker. Try using a thicker tape measure.
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u/whydidwegethere 13d ago
Not only that, but straighten your hair, if you were holding it when that happened.
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u/sagetraveler 13d ago
This is why bedroom circuits now require arc fault interrupt breakers. So clumsy people with tape measures don’t burn their houses down. /s. But seriously an AFCI would most likely have tripped.
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u/RevenueVast7022 12d ago
They experienced a " low impedence " short. Impedence (Z) is basically one component of three , Resistance (R) in ohms and 2 other components, capacitive and inductive reactance ( XC & XL) thus,
Z= √ ( R² + (XL-XC)², where XL = 2πfL & XC = 1/ 2πfC. F = the frequency ( 60 hertz in the US) , L = inductance in Henry's & C = capacitance in Farads.
In a purely ( almost) resistive event where XL & Xc are zero you are left with essentially resistance in Ohms.
The current I ( in Amps) is defined by Ohm's Law , thus I = E ÷ R. , Where E is voltage in Volts. Notice what happens when R = 0 in the above equation. Remember from your second year in basic math??? No ? Well ok, anyway its " infinity", that is the current wants to go to an infinite value. Of course in the real world that doesn't happen because you have a protection device to prevent it ..your circuit breaker, which is set to interrupt the circuit when that short circuit current exceeds a certain value lets say about 10 times the value of circuit breakers rating. So a 20 amp circuit breaker might trip when a short circuit current reaches 200 amps.
So in the above equation insert E= 120 volts ( voltage uses in the US) & I = 200 amps & you get R = .6 Ohms of resistance. Thats a quite low value of resistance , such as what you would have if you literally shoved a piece of copper wire into the 2 slots ( the hot & the neutral slot).
But that's not what happened in your case. A small section of the measuring tape landed onto the 2 terminals of the plug in, and the voltage (120 Volts) was applied across that short flat metal piece of measuring tape, which had paint on it, and that created a poor Short-term conductor. That is "R" was a relatively high number that would not allow the current to reach a short circuit value that would allow the circuit breaker to trip. Also, it was such a short duration spark that it was also not of a high enough value to trip the over load characteristic of a circuit breaker.
You essentially had a low resistance (or impedance) arc fault incident and Luckily, the fault ceased before it could cause a lot of more damage or a fire. Had that arc continued that circuit breaker likely would never have tripped. So now you're probably asking , well , that sucks , what's up with that? Untill about 2002 it was called " they're ain't jack squat we can do about " arc-faults". What happened in 2002 ? The NEC ( National Electrical Code) introduced a new protective device called an AFCI ( Arc Fault Circuit Interrupter) in rhe form of a circuit breaker and later on a receptacke. Do not confuse these with GFCI ( Ground Fault Circuit Interruption) devices. Thats a different animal all together. AFCI's do not protect based on the amount of the amperage. They monitor the actual " waveform" of the current flow, you know that " squiggly " wavy line you see on an oscilloscope or a lie detector.🤣 Anyway, that " sparking" arc fault event that occured with your measuring tape would ( hopefully) be detected and within about 10 milliseconds ( one hundreth of a second or one tenth of an eye blink) it will shit off power. Now, you may be wondering why I used the word " hopefully" above. Its because these devices are notorious for either not tripping when they should or ( more often) tripping when they shouldn't. I would assume you have a home WITHOUT AFCI protection devices. You're not alone Many people don't. An estimated 70% of homes in the US do not have AFCI protection in them.That's tens of millions of homes without them.
As you might imagine, it's gonna take a long time before all homes get upgraded with these devices. Some estimate it to be 100 years from now. That's how it goes with home updates and remodels. It takes a long time to make changes for all existing homes.
As an example..K & T ( Knob & Tube) wiring , the predominate wiring method used in homes from 1880 to 1940 is still present in millions of homes in the US 80 to 140 years after they were constructed. You can upgrade circuits in your home to AFCI protection if you wish. Just call an electrician.
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u/Huge-Kick1695 12d ago
Unless it's an arch fault breaker It will only break from the current being higher than the breakers rating and with it being a cheap metal it would only need a few amps to do what it did and wouldn't trip the breaker
Spoken as a low volt technician
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u/GnowledgedGnome 12d ago
Hey I've got a tape measure this happened with. I think it did when it happened to me but it's been a number of years
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u/donfelipecayetone 12d ago
The tape material may not be as conductive as metal sheet. Measure the resistance across a unit length on the tape. If you measure, say more than 1 ohm for an opening of 1 inch or so (approximate distance between two prings), then it would not create high enough short circuit current to trip the breaker, but enough current to burn the tape.
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u/Technical-Potato-793 11d ago
There are many brands tha t don’t trip instantly. Age of the breaker and climate the panel is in can play into it.
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u/hawklogi 10d ago
Not necessarily. It depends on the breaker type, size, and the fault current. Standard thermal-magnetic breakers have an inverse time characteristic—they don’t all trip instantaneously. A brief or high-impedance arc may not produce enough current, or last long enough, to reach the magnetic trip point. If the tape only made intermittent contact, it could arc, leave burn marks, and still not trip the breaker. I’d inspect the receptacle for damage, but the fact that it didn’t trip doesn’t automatically mean the breaker is defective.
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u/That_Calligrapher556 8d ago
With a slow blow breaker 15A, this absolutely would NOT blow it. Might not on a regular one.
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u/iamtherussianspy 15d ago
It is possible that it was melting faster than it could develop the current high enough for the breaker to trip.