"technicians's" GFCI idea - Page 1
Forum section: electronics / projects

author: djacobow
author_rank: Super Contributor
timestamp: March 14, 2014, 09:00:12 pm
content: I've been thinking a bit about isolation transformers for more safely working on mains powered equipment, and how a GFCI can provide similar service. (It doesn't let you stick your scope's ground clamp anywhere, but it does stop current from flowing from the DUT through you to ground, by quickly shutting down if the hot and neutral currents are not equal.)I was wondering if it would be an interesting or useful think to make a "super" GFCI. Standard ones shut off if they detect a 4 or 5mA current differential. They trip in 50-150ms, as far as I can tell by googling. In my mind, a better one for the lab would be one where you could set the trip current, perhaps all the way down to a few uA, and you could shorten the trip time to a cycle or two, or maybe a fraction of a cycle. The fraction of a cycle seems problematic though for lagging/leading loads.Such a device would be to twitchy to be used as a normal GFCI, but on a bench where it's just another piece of gear with buttons, maybe a useful thing. And potentially much cheaper and less bulky than a true isolation xformer.Opinions?-- dave j

author: denelec
author_rank: Regular Contributor
timestamp: March 14, 2014, 09:37:11 pm
content: The hot-ground short circuit current will be hundreds of amps.You will need to interrupt it very quickly to prevent damage to your scope.A mechanical relay may not be fast enough. And it will probably not be able to interrupt the short circuit current.You will need to find a device able to interrupt hundreds of amps in a very short time.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2014, 09:54:10 pm
content: uA certainly won't be possible; the leakage capacitance from hot to the surroundings will draw more current.Likewise, transient loads -- nominally balanced -- will show up as spikes and bumps in the common mode current.If you're in the habit ofsticking your probe where it shouldn't go, I suggest putting a "condom"(isolation transformer) on your probe or your device-under-test...Tim

author: calzap
author_rank: Frequent Contributor
timestamp: March 14, 2014, 09:59:54 pm
content: I'm curious if anyone has ever seen a scope or probe fried by connecting a ground lead to mains "hot" (aka ungrounded mains conductor). The barrels of the BNC's on my scope appear to have a hefty connection to the cord ground wire, so perhaps the GFCI or panel breaker would trip before damage to the scope itself occurred. Am I going to test the concept? Not a chance. But I would like to hear any disaster stories. Anyone remember VTVM's? They had the same potential (no pun) problem.Mike in California

author: Nerull
author_rank: Frequent Contributor
timestamp: March 14, 2014, 10:09:01 pm
content: I've seen a few posts where people have blown traces out of their scopes by doing that. I certainly wouldn't trust a breaker to blow first - they're often designed not to trip immediately on overcurrent so they don't constantly trip when starting electric motors and such.

author: Rerouter
author_rank: Super Contributor
timestamp: March 14, 2014, 10:22:40 pm
content: If the device has an EMI filter on the input you literally cannot even get away with 1mA, due to the 2 little capacitors shunting to ground, which tend to be 1-10nF,at 10nF on 50Hz mains your impedance is about 320K which means its letting through about 750uA from active to earth at all times,at 10nF on 60Hz mains your impedance is about 260K which means its letting through about 900uA,What your after is trip speed, which is generally accomplished with a magnetic breaker, (magnetic impulse pulls or pushes the trigger pin free) the actual time it takes to then remove power comes down to how powerful the spring (the bigger it is the harder it is to set) and how you make it hard for an ark to be sustained while its travelling (e.g. have a shutter that moves across once the metal contact plates have 1mm of seperation)Also i should probably point out there are different grades of circuit breakers as i feel this will be more relevant if this thread goes further,The common breakers are a C curve, but you can get all the way down to A curve, which means the magnetic trip goes off the moment you exceed the breakers rating,

author: djacobow
author_rank: Super Contributor
timestamp: March 14, 2014, 10:29:15 pm
content: I'm sorry, I wasn't clear. I did not think this would allow me to place my scope ground onto the circuit willy-nilly. I thought I was saying that I knew that wouldn't work, only that it could provide similar human safety benefit as isolation.I am convinced from posts above and thinking about it that reasonable capacitance and inductances on the DUT input make the instantaneous measurement of differential current not work. It has to be over a period of cycles. That's probably why a conventional GFCI uses between 50 and 150ms to trip (time depends on size of differential). That's 3 to 15 complete cycles. It actually seems like quite a long time to have mains current coursing through your body, but I guess it still provides reasonable safety as is.Anyway, it's not a great idea. They can't all be.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2014, 11:36:20 pm
content: Most important thing to remember is, protection circuitry is there either for the wiring (fire hazard) or the people (shock hazard). Sometimes both (breakers on grounded circuitry; GFCI).GFCI being what it is, I suppose it's already good enough for that (touching things). You'd want to read the respective standards to see what the goals are, if any. Doesn't mean you won't get a zap off it, but you *probably* won't die at least (3-15 cycles is definitely enough to feel something!).I don't know what the rates are for, like... if you got shocked this bad for this long, how dead are you; so, whether that's a bad shock or a fatal one.Tim

author: Monkeh
author_rank: Super Contributor
timestamp: March 14, 2014, 11:54:52 pm
content: Quote from: Rerouter on March 14, 2014, 10:22:40 pmWhat standard is this under? IEC 60898-1 explicitly excludes an 'A' curve because of the potential confusing with current.

author: David_AVD
author_rank: Super Contributor
timestamp: March 15, 2014, 12:33:29 am
content: Keep in mind that nothing will save you if you hook yourself between active and neutral.

author: TerraHertz
author_rank: Super Contributor
timestamp: March 15, 2014, 01:11:59 am
content: Quote from: David_AVD on March 15, 2014, 12:33:29 amHmm... actually that and the thread title gave me an idea, that might be useful for industrial electrician safety.What about a wearable device, in the form of a wrist of ankle band, that included two skin contact pads spaced somewhat apart. Electronics in the gadget would monitor the differential DC & AC voltage between the pads. If that suddenly spiked upward, the device would emit a coded burst radio signal.The site's main breakers trip control circuit would include a receiver for that coded burst, and trip the breakers rapidly if received.Just get the OH&S wonks to legislate that such things must be worn by everyone going anywhere near industrial electrical equipment and hey, instant multi-million dollar captive market.

author: David_AVD
author_rank: Super Contributor
timestamp: March 15, 2014, 01:36:10 am
content: Finding appropriate points on the person to monitor might be tricky.Grabbing the back of a 240V switch (or earth-less connector) would mean the points are really close together. In that situation I'm not sure how good the detection would be and how lethal the shock will be (no path across the chest).I'd say that most of the shocks I've gotten involved earth contact as well, even if there was active-to-neutral contact. My parents invested in an ELCB (older version of RCD) when I was a teen (~30 years ago) and into pulling things apart.In the field, falling off a ladder (after getting a shock) could result in more personal damage than the shock itself.

author: ajb
author_rank: Super Contributor
timestamp: March 15, 2014, 01:41:32 am
content: Quote from: calzap on March 14, 2014, 09:59:54 pmAt one point I was using a parallel port-based scope (it seemed like a good investment at the time!) along with an SSR at way-too-late-o-clock and clipped the probe ground to the wrong screw terminal. There was a pop, and the lights in my lab went out, and I'm left sitting there, naturally, thinking obscene thoughts in the dark about what I just did to my computer. After resetting the breaker and my UPS to my surprise the computer booted right back up, and the scope worked just fine, despite putting sufficient fault current through the parallel port to trip a 15A mains breaker. Only evidence of that moment of stupidity is a little bit of copper plating on the plastic case of the SSR and a tiny divot in the alligator clip.So there you go. I did the experiment so you don't have to!

author: Monkeh
author_rank: Super Contributor
timestamp: March 15, 2014, 01:45:32 am
content: Quote from: ajb on March 15, 2014, 01:41:32 amThe probe ground was probably connected to the shield of your parallel port, which is connected firmly to the ground plane on the motherboard, which is connected to mains ground very firmly by every mounting screw on the board and every ground lead in the machine.Not quite a comparison to a modern scope design.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2014, 01:47:46 am
content: The worst I ever did with a computer was connect an offline tube amplifier of my own design (hey, I was young..) to the sound card via a headphones cable. Fortunately the cable was quite low quality, so that when I plugged in the amplifier (did I mention I hadn't used a polarized plug, either?..), it burned out. The way it burned was quite interesting: there were little puffs of smoke, from little burn marks, evenly spaced along the cable!My scopes also have their inputs firmly grounded via chassis, so they would probably survive the same sort of fault. However, I doubt the same is true for every brand name unit -- remember those early crappy TDS100s and stuff with the surface mounted BNC connectors?Tim

author: David_AVD
author_rank: Super Contributor
timestamp: March 15, 2014, 01:59:14 am
content: Quote from: ajb on March 15, 2014, 01:41:32 amThis is why you should have the lights on a separate breaker (and RCD) to the power. Being plunged into darkness after an incident can be the start of a secondary incident.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 15, 2014, 04:30:19 am
content: Quote from: ajb on March 15, 2014, 01:41:32 amI did the same experiment years ago,when I forgot for a moment that the I wasn't using the isolation transformer.( Another Tech was using it,servicing a very messy switch mode in a piece of TV Studio gear,so I elected to "wing it" without one)Grounding clip on (allegedly) Neutral,turned it on,& "Zap!" the bench RCD dropped out.Damage?--small burn mark on the Grounding clip.The 'scope was,I think,a Tek 453,or something like that--solid connection between BNC outer & chassis frame,& hence to the Earth conductor.So even "oldtimers" can be caught out!Luckily, I was nowhere hear the connection when it was turned on,as I follow the following principle:Dealing with high voltages,connect first ,then turn the DUT on & measure,if this if at all possible.Thefirsttime I encountered an RCD was when I came in on afternoon shift,grabbed an AVO 8 & began troubleshooting a piece of gear on the bench."OK,let's see if it is getting any Mains power"The Neutral tag at the input to the power transformer was obscured,so I went the "slack" route of measuring between Active & chassis,which was firmly attached to Earth.The RCD dropped out,turning off the fluoros above the desk,& leaving me in darkness.I reset what I thought were just a new set of circuit breakers,& tried again----again it dropped out.This time I read the label on the "new breakers",& the light dawned:The AVO drew enough current from the Mains on its AC volts range to trip the RCD!Going back to 'scope probes---on one occasion,I was chasing a problem with a 5kW Broadcast Transmitter,& had been poking low level RF into the PA grid circuit (with the Tx off) to see if I could see it.It was a bit inconclusive,so I opted to wind the drive level back,leave the probe in place,& fire the Transmitter up,reasoning that the probe had pretty good ratings.(I can only plead extreme fatigue,as it was around 2:30am!)Nota good idea!A blinding flash,& the Tx dropped off.All of the probe forward of the fingerguard had been vapourised!Damage to the Oscilloscope?---nil!

author: Pillager
author_rank: Unknown
timestamp: March 15, 2014, 07:50:20 pm
content: Quote from: TerraHertz on March 15, 2014, 01:11:59 amThat concept is interesting, but not really practical.In Austria (most of the world, I'd wager, but I don't know for a fact) we learn1.) You NEVER work on a live mains circuit unless there are no other options.2.) There are standard procedures to follow, even when working on a circuit that is switched off.3.) If you must work on a live mains circuit, there are special procedures to follow, special tools for the job and even special clothes (depending on the current involved).If you have the trainig for the job, you should follow the rules. If you do, normally nothing can happen (if all the equipment is in order). If you don't follow the rules, then1.) you're either foolish or careless2.) if something happens it's your own fault.Safety concerns ALWAYS trump operational concerns, so if you don't have the equipment, you don't work on the circuit.And if you have neither trainig nor equipment, give my regards to Darwin...