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It’s not just that I’ve been told by some people to never work on a live circuit as an amateur (I know that electricians often do work on live circuits)... but the package itself containing the insulated screwdrivers says to never work on a live circuit.

One might say that it adds an extra level of precaution in case the circuit you’re working on is live unintentionally... but it seems to me that this extra “safeguard” would just tempt people to work on live circuits.

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    Kris, that's the key point. Your information below, while informative, doesn't really address the question. – isherwood May 4 '18 at 13:39
  • Yep, @kris is the only one that answered this question. – Navin May 4 '18 at 22:20
  • In case you end up working on a live circuit by accident... – dalearn May 5 '18 at 1:37
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    What are air bags for if you're not supposed to drive your car into a lamppost? – Dawood ibn Kareem May 5 '18 at 12:06
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    @DawoodibnKareem Oh so that's what I've been doing wrong this whole time! I should write this down...I believe there is a pen in the glove compartment, it sure is hard to reach with one hand on the wheel! – n_b May 7 '18 at 9:34

11 Answers 11

89

The idea is to not rely on any single point of failure. For you to be planted six feet under, you want at least three things to have gone badly wrong at the same time.

  • Your lockout padlock fell off.
  • Someone didn't realise you were working on the circuit and switched the breaker back on.
  • You forgot to turn off the isolator switch.
  • You didn't notice the puddle of water you were standing in.
  • You were not using an insulated screwdriver.
  • Your dog barked at a squirrel and startled you.

Its kinda why you bought a car with seatbelts, airbag and ABS - Does having those items tempt you to jump red-lights in front of large trucks or freight trains?

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    DC-9 aircraft had a backup power supply behind one of the walls in a cargo bay. It also had a bleeder resistor and a placard that said "never trust a bleeder". It meant don't trust the resistor that was supposed to slowly drain the reservoir when the aircraft was in for maintenance and powered down. – jww May 6 '18 at 10:27
  • I do seem to recall reading something that said that people do actually drive more dangerously when their car has more safety features, but I think it wasn't dangerously enough to outweigh the safety features. (Not sure) – immibis May 7 '18 at 1:45
  • @immibis, you can think of insurance as a safety feature. Insurance shifts risk, allowing us to do more risky things. – mikeazo May 7 '18 at 16:17
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    @immibis: It's true. I have long advocated that other people's cars should have an iron spike right in the center of the steering wheel. I have been rear-ended three times while stopped for pedestrians crossing the street. A big spike in the middle of everyone else's steering wheel would make other people drive much more carefully around pedestrian crosswalks, and would probably lower accident rates overall, yet no one will adopt my entirely sensible idea. – Eric Lippert May 9 '18 at 14:13
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    @immibis: Due to inertia, the bumpee gets thrown backwards into their seat; the bumper gets thrown forwards towards the steering wheel. But it doesn't really matter since my car wouldn't have the spike. The plan is for other people's cars to have it. – Eric Lippert May 10 '18 at 13:05
63

A de-energized circuit is like an unloaded gun

Once I worked on a circuit. I shut off the breaker (I knew the circuit well, since it powered the lighting in the electrical parts crib) and double checked power was off. As a a third check, I brushed the now-dead hot wire against EMT ground. Was expecting nothing or possibly a huge, sunburn-making arc flash. Instead I got a flash from a completely different direction. What??

So I flashed it again, looking at the flash this time. Teensy tiny blue flash. And the crib lighting came back on. Buh duh??? So I tried flashing neutral to ground, should do nothing right? Big mistake!

Turns out my neutral-ground bonding in the panel had failed, and a totally different circuit had a hot-ground fault.

The upshot: you can never count on the circuit being off.

And by the way, the reason I got so far into the work is, I treat all wires like they're energized unless the work absolutely requires touching them.

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    My wife sleeps easier since I always use a contactless voltage detector to double-check it's off. Guessing if it's off is a good way to get killed. – Machavity May 4 '18 at 17:53
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    @Machavity I use one religiously but it's important to realize that they can give false negatives if two out of phase currents are present like in a shared neutral situation. One tip I read about was to take the back of your pinky and touch anything you are unsure of with the nail (after checking with the EMF sensor). If it's live your finger will close toward your palm and away from the conductor, instead of closing on it. – JimmyJames May 4 '18 at 18:07
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    I think most electric surprises come from "off" circuits. – blacksmith37 May 4 '18 at 21:27
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    @supercat, the correct way to use an NCV is to use it on a powered-on circuit, then on the circuit you're interested in, then on the powered-on circuit again. This protects you against a wide range of faults in the NCV, including the possibility that the detector circuit is bad but the rest of the NCV isn't. Your system would alert you to the detector not being turned on, but not to the failure of the detection circuit. – Mark May 4 '18 at 23:25
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    The firearms analogy is apt. One of the basic rules of firearms safety from Jeff Cooper is "always treat every gun as if it were loaded". Applied here that would be "always treat every circuit as if it were live". This is a clever reapplication of the same principle. – Freiheit May 7 '18 at 14:25
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Not all insulated screwdrivers are the same

Those that are UL® tested up to 1000V and also conform to IEC 60900:2012, ASTM F1505-10 & NFPA 70E standards, are specifically designed and intended for working on live applications 1000V or less.

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    At least for the VDE equivalents, they are typically tested to 10kVAC, and approved for use at 1000VAC or 1500VDC. – Someone Somewhere May 7 '18 at 9:45
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Part of your comments actually answer part of your question. First let me point out NEC 100 "Definitions" and the term Qualified Person. That's one who has the skill and the training ... to recognize and avoid the hazards involved. So as an admitted amature you need to make doubly sure any work you do does not involve you working on an energized circuit.

Second: Even though a qualified person has removed covers and isolated and turned off the circuit he is working on, he must acknowledge that there are still live parts which he could come in contact with. The insulation on his tools and meters are part of a safety protocol to prevent accidental contact of those live parts, not the circuit he is working on.

Hope this helps.

6

It's not an engineering or practical issue, it's a legal issue.

Let's say this "insulated" screwdriver's packaging could be read to imply that it's OK for a user to stick it into a live breaker panel. And now, let's say Joe Bozo buys one, enthusiastically sticks it into a live breaker panel, and (perhaps due to his own negligence) gets electrocuted. It's quite likely that his estate, or his life insurance company, would sue the manufacturer for a whole lot of money, and even quite possible that they'd win, with the rationale being that, as you say, the manufacturer had "tempt[ed] people to work on live circuits."

Now instead, say the screwdriver's packaging said quite clearly "don't do this". It won't have much effect on the knowledgeable user's use of the tool. However, a court would see this as a caution to the inexperienced user, and thus as a defense against responsibility for such a user's harming himself.

For more information, you can look into tort law, most enthusiastically as practiced in the United States.

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I feel like this would be better as just a comment, but I do have something small to add and no reputation for commenting. It's not necessarily only about safety. If you're working on a delicate circuit sensitive to static discharge, an insulated screwdriver protects the circuit from you. This is the reason my research group had them.

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    Of course, that's a double-edged sword, since the higher the resistance, the higher a charge can build up before being discharged. Presumably you were also grounded to keep that from happening. – Acccumulation May 7 '18 at 21:45
  • True. My dad who was a telephone equipment installer sometimes brought me into work at the central offices (with all the routing electronics, battery backup and such) and all around the place there were static discharge bracelets for wearing when installing equipment for this exact reason. (connected to ground to prevent charge accumulation) – Mr. HelloBye May 9 '18 at 1:08
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Some circuits that are not live still have (potentially deadly/harmful) charges stored up. PC power supplies are a common example: the capacitors can pack a punch hours after the PSU is unplugged. The insulated screwdriver still protects the electronics inside, not just you.

When working in tight spaces, an insulated screwdriver help protect against those accidental shorts that are not a safety issue. An accidental short on a board under isolation can still matter; a real-time-clock with a small battery backup for example: shorting the power won't cause a noticeable physical effect like sparking, but it would reset the clock.

2

If you are working on RF circuits or any sort of precision circuit, you may or may not have to adjust trimmer potentiometers. If your screwdriver is not insulated, it will contribute some rather significant parasitic capacitance as well as act as an antenna, making tuning the circuit nearly impossible.

  • And the screwdriver may have to be inserted in close proximity to a wire carrying anode voltage. – user207421 May 6 '18 at 19:45
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    This is wrong. A screwdriver with a metallic body will act as a capacitor whether or not it is covered by an insulating shield. Worse, the insulating material will have an higher dielectric constant than air, so it will add more capacitance when performing the trimming. Proper HF trimmer screwdrivers are made entirely of non conductive material (at most they have tiny metallic bits when the plastic material is not strong enough). – Lorenzo Donati May 7 '18 at 12:49
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Like many things in life, they are for doing what you are not supposed to do a little more safely.

The good quality versions will actually be intended for the professional user, to whom working on live circuits is considered something to avoid and to take special precautions about, not something to never do. Cheap versions are copies of what the professionals use, since a) non-professionals will buy it and b) marketing them is not prohibited. However, you do not want the amateur user to consider using insulated tools as a safe and harmless way to work on live circuits. The professional and the knowledgeable enthusiast/amateur (eg a radio ham tuning a vacuum tube PA, a DIY hifi builder doing live adjustments to a powerful amplifier) will not approach this kind of work as "safe and harmless" just by using an insulated tool, pay a lot of attention, and take further safety precautions.

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Insulated screwdrivers work in protection, at least my Klein screwdrivers have. The point is to just not take the risk if you don't know what you're doing. A while back, I wasn't paying attention to where my insulated screwdriver was, moved it to the side, and arc'd out on the old school metal one gang boxes. Didn't shock me through the insulated part of the screwdriver, but left a big gash in the screwdriver.... lesson learned...

Use an insulated screwdriver

Use one hand to use the screwdriver whenever you have to work on a live circuit

Use insulated electrical pliers as your other hand where you need grip

Don't make contact with anything other than the thing you're working on

These are good, basic methods of practicing safety. Besides turning of the power to whatever you're working on in the first place, when possible.

1

Firstly because while it is obviously discouraged sometimes there is no real choice but to work live. Obviously a bunch of precuations should be taken in such cases and such work should only be attempted by suitablly competent people but sometimes it has to be done.

Secondly because shit happens and real electrical installations are full of horrible bodges.

Sure you think that circuit is isolated, you may have tested it with a test instrument and then followed up by testing the test instrument. If you are lucky you may have been there during the isolation procedure and able to verify that turning off a given breaker turned off the circuit. You may see the padlock hanging on what you think is the only breaker supplying the circuit.

But that won't save you from the dumbass that fed the same wiring from two different breakers in different places one of which happens to be turned off right now. It won't save you from the dumbass who took the live and neutral from different circuits when implementing the multiway switching of your landing lights. It won't save you from the open-circuit drain/balance resistor in an old SMPSU. It won't save you when you have to work on an installation that has already been isolated and the dumbass telling you where to add your lockout padlock pointed at the wrong breaker. It won't save you from other circuits in the vicinity of the circuit you are working on.

Hence why many electrians preffer to as much as possible treat electricial installations as-if they are live. Even if they have taken reasonable steps to assure themselves they are dead.

protected by ThreePhaseEel May 8 '18 at 3:54

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