And here's how I almost killed myself today... - Page 1
Forum section: electronics / beginners

author: Stonent
author_rank: Super Contributor
timestamp: September 19, 2015, 06:20:11 am
content: And I put it in beginners because it was a dumb mistake.I was investigating a piece of production testing equipment that was called in as an IT issue because the software on the computer was failing everything that was being tested. During the various parts of the test, current snapshots are taken and compared against known good current ranges. The test operator has to activate certain functions in the device being tested based upon the prompts on the screen. (Turn this knob half way, and press this button etc) The software also has some ability to inject commands directly into the device's communication bus and during certain parts of the test, you will see the device being operated at a high speed with lights going off and on quickly as if a person was pressing buttons and flipping switches at 100 times their normal capability.The current consumption for the vast majority of the test was not wavering at all, even when it should at times increase by 2000%One of the suggestions by the support group for that device was make sure the power rails were good which is an absolutely reasonable thing to check. The previous day, one of the electrical maintenance guys said he tested the output voltages and they all seemed within spec. But I wasn't sure if it was tested under load or just free running.My mistake? On the back of the unit were several twist in fuses. Each fuse was labeled as to which power rail it corresponded to. I put my left hand on the painted metal cabinet and bent down to remove the fuse that said "12A 1A supply" with my right hand and as soon as it was being withdrawn out of the socket I suddenly felt a rush of excitement, 60 times a second.That's when I realized... Though it was for the 12V 1A supply, it was not fusing the output, it was fusing the input side. And confirmed when I saw 250V 5A stamped on the fuse.So yeah. Even when you think you're dealing with small voltages on an unknown device.... make sure you're not the path of least resistance.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: September 19, 2015, 08:00:37 am
content: I'm not familiar with the type of fuse, care to elaborate on how you made contact with the circuit?

author: Psi
author_rank: Super Contributor
timestamp: September 19, 2015, 08:05:22 am
content: QuoteSounds like you had a quite indirect 230V shock. Luckily.The full wack is considerably less exciting.

author: tron9000
author_rank: Frequent Contributor
timestamp: September 19, 2015, 08:26:46 am
content: Quote from: RobertHolcombe on September 19, 2015, 08:00:37 amThink he means the holders with twist off caps that pull the fuse out along with it. Some of them don't have a designated preference of where you connect the feed into the fuse, so if its connected to the contacts nearest the panel, then you could accidentally rest the far end of the fuse on that contact whilst touching the near end.lil' kiss from Miss Electra then?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 19, 2015, 08:46:24 am
content: Quote from: Psi on September 19, 2015, 08:05:22 amthis is personal taste but i'm not in agreement with you. the tingling or itchy itchy feeling all the way from the contact point throughout the path... shoulder, side of the back, thigh down to the calf and foot, is really something, but luckily the ELCB circuit breaker is keeping "him" alive, thats the only thing in the middle between "him" and the great cable on the pole...

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 19, 2015, 09:03:45 am
content: The convention is to connect the "hot" wire from the mains to the END of the fuse post, while the end towards the panel connects to the load. That way, when you take off the cap and pull out the fuse, there is no danger of coming in contact with the mains voltage.It sounds like the fuse post is wired BACKWARDS with the mains voltage source connected to the panel-end of the fuse post.And the fuse is put in series with the HOT side, and never in the NEUTRAL side of the mains power.

author: Psi
author_rank: Super Contributor
timestamp: September 19, 2015, 09:24:40 am
content: Quote from: Mechatrommer on September 19, 2015, 08:46:24 amYou think a full mains shockisexciting?Most people never experience a direct mains to earth shock. They only experience mains indirect to ground with resistance in the path.

author: RobertHolcombe
author_rank: Frequent Contributor
timestamp: September 19, 2015, 09:39:33 am
content: Quote from: tron9000 on September 19, 2015, 08:26:46 amI thought that may be the case but didn't want to assume anything, cause I google image searched and was presented with a few different styles - also good to know about the wiring convention in regards to safety, I hadn't thought about it before!

author: fubar.gr
author_rank: Unknown
timestamp: September 19, 2015, 10:26:07 am
content: Quote from: Richard Crowley on September 19, 2015, 09:03:45 amYep, this is the correct way to wire these fuseholders.However, even if it is wired correctly, there's still a potentially hazardous situation.Normally, when the screwcap is removed, the fuse will come out with it. But if for whatever reason the cap comes off and the fuse stays in the fuseholder, the fuse will be energized. Trying to remove it by hand will give you a shock.Maybe the fuseholder in the OP's case was wired correctly and he accidentally touched the tip of the fuse

author: Cliff Matthews
author_rank: Unknown
timestamp: September 19, 2015, 02:24:40 pm
content: Quote from: Richard Crowley on September 19, 2015, 09:03:45 amNo, it sounds like the wiring and fuse was good, but it stayed inside after taking the cap off and he grabbed it with his finger tips (just as though he were touching the inside line-side contact).The label "12V 1A supply" was somewhat misleading.

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 02:45:53 pm
content: So the fuses are for the supplies, not the output of the supplies. Might be good to have had a label there saying the fuse holders are at incoming mains potential ( 1125VAC 60Hz), and are not on the output side.So, what ( aside from the disco dance test) was actually wrong? Faulty power supply, faulty connections to the DUT, broken wire?Just fixed a SCR control board, it had a faulty power LED. You would think a power LED is something you can run without inside the case of a unit, but not this one.......

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 19, 2015, 03:07:37 pm
content: Quote from: SeanB on September 19, 2015, 02:45:53 pm

author: Cliff Matthews
author_rank: Unknown
timestamp: September 19, 2015, 03:14:46 pm
content: Quote from: PA0PBZ on September 19, 2015, 03:07:37 pmIt's a good thing the poster lives in the US.. He might be pushing up daisy's if he lived where 230v is the norm (granted, their fuse holders might be extra-special too).

author: SeanB
author_rank: Super Contributor
timestamp: September 19, 2015, 03:20:36 pm
content: Bloody Microsoft keyboard, I really must put a better one in than this one, it makes too many spelling errors. Must go back to using a PS/2 keyboard rather than the USB one.Non US fuse holders are nothing different to the US ones, they all are rated for 250VAC and up to 10A, as the fuses used in them are also rated for 250VAC use, and over 10A the brass required gets thicker.

author: Timur Born
author_rank: Regular Contributor
timestamp: September 19, 2015, 04:19:33 pm
content: As a kid I stuck a sewing needle into the interesting little holes of our living room's mains. Left quite a taste in the mouth. My guess is that the carpet below my feet helped in releasing the needle quickly and being quite alright after a short shaking experience. No idea if anyone ever pulled the needle out of the socket. One could call it the taste of the 80s.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 19, 2015, 04:43:11 pm
content: I guess unplugging from the mains before checking/changing fuses is rather old-fashioned. Still, it works for me....

author: IanB
author_rank: Super Contributor
timestamp: September 19, 2015, 04:54:16 pm
content: Quote from: Stonent on September 19, 2015, 06:20:11 amI don't think you have properly understood your mistake here. Your mistake was to work on the equipment while it was energized. You should have made sure it was isolated from the mains before touching it.

author: fivefish
author_rank: Frequent Contributor
timestamp: September 19, 2015, 05:00:57 pm
content: I dont get it.If you need to measure if the power rails are within spec, why are you removing the fuse?

author: tszaboo
author_rank: Super Contributor
timestamp: September 19, 2015, 05:10:53 pm
content: Quote from: alsetalokin4017 on September 19, 2015, 04:43:11 pmI usually put the end of the cord into my pocket, so others cannot kill me while I'm working on mains.And if it is an undocumented one-off test gear, I dont assume what a cable does.

author: sarepairman2
author_rank: Frequent Contributor
timestamp: September 19, 2015, 06:06:00 pm
content: I am extremely timid around anything that is connected to mains and only touch it with one hand ever.

author: C
author_rank: Super Contributor
timestamp: September 19, 2015, 09:51:50 pm
content: Sounds like the wrong fuse holder type was used]https://www.borderstates.com/UserFiles/MdmContent/images/3453LF3A_Littelfuse.jpg]The shrouded or recessed contact type cap should be used on mains if you want to be safe.C

author: Zeyneb
author_rank: Regular Contributor
timestamp: September 20, 2015, 10:55:49 am
content: Quote from: Timur Born on September 19, 2015, 04:19:33 pmYou're surely funny Timur!

author: calin
author_rank: Regular Contributor
timestamp: September 21, 2015, 04:42:36 am
content: He he .. same here I "probed" the 220V outlet with a nail when I was around 5 or years old. Luckily I did not got the phase before my mom saw me ...because probably I will not be here to write this post. I did got a very good parental spanking out of it ... good ol days when a good slap was still normal. Since then I don't remember sticking my fingers in an outlet anymore .. so it worked[size=78%]Quote from: Timur Born onYesterdayat 08:19:33 AM

author: MarkF
author_rank: Super Contributor
timestamp: September 21, 2015, 05:08:45 am
content: Shortly after getting out of school, I was working in a large rack with some military equipment and got against 120VAC @ 400Hz. Talk about reaching out and touching some one. The 400Hz has quit a little bite to it.

author: Monittosan
author_rank: Regular Contributor
timestamp: September 21, 2015, 07:19:24 am
content: Iv touched 320v dc. one hand on + the other on chassis earth on a stupid open frame switching PSU on a single board computer. That was a serious shock it throw me quite a bit!240vac 50hz a few other times but nothing too series. In all instances the RCBO did not trip and with what I have observed while testing RCBO's if you don't manually trip them frequently they are slower to react. Fresh out of the box they behave like they should.

author: fubar.gr
author_rank: Unknown
timestamp: September 21, 2015, 09:04:31 am
content: Quote from: alsetalokin4017 on September 19, 2015, 04:43:11 pmThere are many cases where you absolutely have to work on live equipment, either because turning it off is not possible, or because the troubleshooting procedure requires it to be live.

author: IanB
author_rank: Super Contributor
timestamp: September 21, 2015, 12:15:21 pm
content: Quote from: fubar.gr on September 21, 2015, 09:04:31 am"Working on live equipment" includes using insulated probes while observing proper safety precautions."Working on live equipment" does not include touching working parts with your bare hands.

author: tggzzz
author_rank: Super Contributor
timestamp: September 21, 2015, 01:24:57 pm
content: Quote from: alsetalokin4017 on September 19, 2015, 04:43:11 pmUnpluggingis the operative word. Switching off is not sufficient: I've come across mains sockets that were still live when the switch was in the off position

author: Stray Electron
author_rank: Super Contributor
timestamp: September 21, 2015, 02:14:46 pm
content: Quote from: Stonent on September 19, 2015, 06:20:11 amLive and Learn!FYI sometimes (frequently!) disconnecting a low voltage supply under power can still give you a real shock if there's enough inductance (usually a coil) in the circuit. Now I just turn things off before checking fuses.I learned this when tuning up a car that had points in it. The ignition was on but the motor wasn't running and in fact the rotor and distributor cap were removed. The points were closed so the primary of the coil was charged up. I opened the points to adjust the gap and it gave me a jolt that I'll never forget! I would have expected that from the secondary but not the primary side of the ignition coil!

author: Stonent
author_rank: Super Contributor
timestamp: September 21, 2015, 02:27:43 pm
content: Yes, that was the correct style of fuse. Funny thing was an electrical maintenance tech and his boss were right there when I did it. They just saw me suddenly jump back and say "Shit!" which I think just elicited a reaction of "Careful!" and a smile.The problem ended up being a bad pin in the cable that plugs into the device. I had one of the maintenance guys unlock the door to the Electrical Engineering research lab where I knew they had all the various connectors used in the products for prototyping with and we acquired a new one.

author: N8AUM
author_rank: Regular Contributor
timestamp: September 27, 2015, 07:57:55 pm
content: Keeping one hand in the pocket is always a good idea when working on live stuff. Most times I had to work on live 480v and turning off the power wasn't an option.oh ya, getting bit by 208v @ 400Hz hurts like a M#$^*% !!!

author: firewalker
author_rank: Super Contributor
timestamp: September 27, 2015, 08:12:00 pm
content: They were using a 5 amp fuse on the 120 volts for protecting a 12 volts 1 amp rail?Alexander.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 27, 2015, 08:42:09 pm
content: Quote from: Richard Crowley on September 19, 2015, 09:03:45 amConnections aside, this picture bugs me. Does anyone know why some ring-nuts like this have that pre-thread punch-out inside? Would it be a place for a big dose of glue or thread-lock?

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 27, 2015, 10:30:07 pm
content: The nut has a "notch" to allow it to pass over the parallel connection lug when you install the fuse post.Especially if you have soldered a wire to the parallel lug before mounting it to the panel.I can't see how the notch could affect the "strength" of the attachment in any meaningful way.

author: Cliff Matthews
author_rank: Unknown
timestamp: September 28, 2015, 11:50:10 am
content: Quote from: Richard Crowley on September 27, 2015, 10:30:07 pmEverything's got a reason (except bloody amnesia). Yes, I recall the thoughtful notch ~30 years ago. I left electronics for IT and got brain freeze..

author: katzohki
author_rank: Frequent Contributor
timestamp: September 30, 2015, 05:35:31 pm
content: Most I've seen is 6kV DC across one hand, 4kV DC hand-to-hand and numerous other lower voltages. Most everything I work with is less than 1 Watt, so I generally just shrug it off (after some swearing of course).But recently, working with a 15 Watt, 4000 Volt unit I went to a lot of extra caution. Plastic shield, Kapton tape everywhere, stuff secured and locked into place, HV relays for switching loads and the hand behind the back.Once in a while people get the full output directly into an oscilloscope channel around here...The most exciting failure I found was a unit that had its case energized to 1kV, kept that one as a souvenir.

author: bigdawg
author_rank: Regular Contributor
timestamp: September 30, 2015, 05:52:08 pm
content: Quote from: Cliff Matthews on September 19, 2015, 03:14:46 pmI thought our mains here are 230V too before we ground at the center creating the split phase of 110v. I mean my washer dryer, A/C etc all have 230V so wouldnt my main "fusebox" be at 230V too?

author: Richard Crowley
author_rank: Super Contributor
timestamp: September 30, 2015, 06:00:50 pm
content: Quote from: bigdawg on September 30, 2015, 05:52:08 pmConventional domestic power service in the US is 230V with center-tap ground/neutral.So a conventional branch circuit for lighting or outlets would be either "phase" to neutral.ground,at 115V.But large loads (water heater, clothes-dryer, stove/oven, HVAC) use "phase" to "phase" at 230V (floating).So, yes, there is 230V inside the breaker/distribution panel, but the most common exposure from branch outlets is only 115VAC.

author: MrWizerd
author_rank: Contributor
timestamp: October 01, 2015, 01:47:41 am
content: I touched a little tweaker screwdriver, the all metal jobs that have the top that spins commonly used for eye glasses and such to a hot 120v while in contact with a grounded work bench, the kind with fluorescent lights in it. That wracked my whole body stiffened every muscle lucky that caused my arms to swing out disconnecting the screw driver. At this point I had been working around 36-48 hours I don't remember which, the company I worked for didn't give a rat's arse about ocha regs. Needless to say I was well aware after that jolt, but do think it may have triggered some adrenaline or something. I had gotten slightly shocked by 120 before but always lightly and indirectly, just a tingle or something that time I took it all and... Yeah I don't know exactly how it pathed but my heart seemed fine after thank goodness... Later on the company got busted for not having proper safety circuits built in I believe gfci I doubt it would have hit me less but if my arm didn't swing out and just kept contact the lack of gfci probably would have sealed my fate.Sent from my SPH-L710 using Tapatalk

author: bigdawg
author_rank: Regular Contributor
timestamp: October 01, 2015, 11:19:19 am
content: Quote from: Richard Crowley on September 30, 2015, 06:00:50 pmThanks for the explanation; I always was a bit confused about at what point in a house does the VAC drop down from 230 to 110.

author: Cliff Matthews
author_rank: Unknown
timestamp: October 01, 2015, 11:59:28 am
content: Small adjustment - I read the average split phase is 234vac in the US and Canada.http://forum.allaboutcircuits.com/threads/what-is-the-actual-household-voltage-110-115-120-220-240.3320/andhttp://www.school-for-champions.com/science/ac_world_volt_freq.htm

author: nfmax
author_rank: Super Contributor
timestamp: October 01, 2015, 12:28:05 pm
content: A colleague of mine had an interesting incident the other day. He was working on a piezo-electric ceramic actuator element, soldering leads to the plated metal terminals. picked it up by the leads after the job was done - and received an electric shock! It turned out the pyro-electric effect was enough to generate 100's (?) of volts across the device from the heat of soldering, and he was using a non-grounded, ESD safe iron. No real harm done, but a big surprise!