Methods for Isolating circuits  - Page 1
Forum section: electronics / beginners

author: GoatZero
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:48:43 pm
content: i know there is a thread about isolation transformer that i just started reading, but i doubt it answers this question so please be patient with meI just got my 1st oscilloscope, and i just saw Daves video about how not to make it explodebut i ended up with many questions that nor the video nor the comments answered im wondering, whats the difference between using any of the following in the test circuit-Using an isolation transformer-Isolated ground outlet-Common outlet without ground wire connected-Ground Live/Neutral Adaptor-GFCI outleti still dont see how can i justify an isolation transformer when i can just place an outlet with no mains earth wire, could someone explain this to me?

author: GoatZero
author_rank: Regular Contributor
timestamp: February 16, 2014, 07:57:41 pm
content: Added the GFCI outlet to the list, since when shorting out using the alligator clip in the Live of the test circuit, equals to short, it should be able to sense it and cutoff the current

author: GoatZero
author_rank: Regular Contributor
timestamp: February 16, 2014, 09:54:37 pm
content: anyone?

author: rdl
author_rank: Super Contributor
timestamp: February 16, 2014, 09:58:19 pm
content: Quoteedit: couldn't get the url to post correctlyhttp://www.tek.com/document/technical-brief/floating-oscilloscope-measurements-and-operator-protection

author: VK5RC
author_rank: Unknown
timestamp: February 16, 2014, 10:16:14 pm
content: In Australia, the neutral is connected to ground at the distribution board at every house /building, so even if you don't connect earth to your DUT, you have if neutral has a dc connection with the circuit under investigation. The best way to understand is like Dave did, draw out the circuit, very helpful understanding 'ground', remember that Isolation transformers shouldbe used only when appropriate. They, like RCD, are not a panacea.

author: GoatZero
author_rank: Regular Contributor
timestamp: February 16, 2014, 10:27:34 pm
content: Quote from: rdl on February 16, 2014, 09:58:19 pmThe idea its not to cut it from the oscilloscope it will always be groundedbut placing an new outlet without ground where i can plug the device where ill be using the test leads All this in order for the circuit in test to become a floating ground, that way there is 0 chance of me blowing up the oscilloscope, by missplacing the alligator clipOtherwise a GFCI, this one will also be used only for the circuit being tested, not for the oscilloscopeQuote from: VK5RC on February 16, 2014, 10:16:14 pmWhat does DUT mean? (sorry i have seen it several times today and ignored it until now)

author: mc
author_rank: Regular Contributor
timestamp: February 16, 2014, 10:49:26 pm
content: Quote from: GoatZero on February 16, 2014, 10:27:34 pmDevice Under Test

author: GoatZero
author_rank: Regular Contributor
timestamp: February 16, 2014, 10:53:35 pm
content: Quote from: mc on February 16, 2014, 10:49:26 pmnice i have been having a hard time trying to express that, until now,what i dont understand its why its the isolation transformer the only solutionIf i cut the ground earth from a new outlet, and place it where i can plug my DUT in theory it should work as a floating ground avoiding the short circuit due to mains earthi understand the why i should not cut it, and the hazards however, seems to be an easier and way cheaper solution than the isolation transformerthe only drawback i can see is that if i make an actual short in my DUT and im touching the circuit i will become mains earth, but, i still cant see how the isolation transformer stop that from happening

author: KDC
author_rank: Contributor
timestamp: February 16, 2014, 11:47:12 pm
content: In nearly every mains installation, the neutral of the system is connected to ground. Either at the panel, the meter or the transformer.Thus, if you measure the voltage from hot to ground you'll get the same as hot to neutral. Thus, your neutral is considered your "grounded conductor". (the only time it's truly a neutral is when it's used with all your hot conductors, and is only carrying the difference between them))What most people call "ground" at the plug is just a way to link all the normally non current carrying components together, and back to the neutral/ground point, so if they become energized, hopefully enough current will flow to trip the breaker."Isolated ground" aka what you see in hospitals is a bunch of hooey for the most part, because ground is ground (and at least in canada, you're suppose to have all your grounding systems tied together).The only way to make a system float, is by getting rid of that grounded conductor. An isolation transformer has no electrical connection between the primary and secondary (as opposed to say an auto transformer).

author: VK5RC
author_rank: Unknown
timestamp: February 16, 2014, 11:47:48 pm
content: By not connecting the earth to your DUT, there is still an earth connection via the neutral mains lead, this then allows the circuit, mains active - you - floor etc - real earth - junction box - neutral, with an Isolation transformer there is no connection between mains active and the mains coming from the transformer, the only circuit that can form is if you touch both mains (not really active neutral) coming from the transformer. But remember an Isolation transformer is a safety device for the equipment not you. A RCD (mostly negated by an Isolation transformer) is a device for your safety!

author: GoatZero
author_rank: Regular Contributor
timestamp: February 17, 2014, 12:52:56 am
content: I believe i just came out with a solution, please leave some feedback,I know this is a violation of the code and stuff but please hear me out for a second,What if i install a 2nd ground rod 3m under the floor, this one near my laboratory, of course i will pull out of the rod a wire for the lab and use it exclusively for it, especially for the oscilloscopeAll of this while leaving a DUT outlet using the original mains earth system this way i would overcome the short circuit problem and stop the oscilloscope from blowing upAny feedback its appreciatedwhile still using the phase and neutral of the actual system and ditching pour the mains earth wire that and its own wires for my lab, that way everything and every outlet in the house will be grounded even if i leave this house and dont leave the electric diagram

author: KDC
author_rank: Contributor
timestamp: February 17, 2014, 01:29:13 am
content: While the ground rods are not connected directly together, they're still not isolated and all the issues still exist.Power companies use ground rturn regularly.

author: C
author_rank: Super Contributor
timestamp: February 17, 2014, 02:15:26 am
content: GoatZeroSafety equipment can give you a false sense of safety. Much better to test if that safety function is still functioning then counting on it saving you.Learn how you can test to see if something is safe to do, Do the test.Don't assume that that meter or scope is functioning properly, test it. They both can fail or go out of calibration.For example most scopes have a calibration test point. It's not there just to look pretty. It there to give you a ruff idea that the scope is functioning.Note that the ground clip for a probe is there to get a better reading, not using the ground clip will still give you something.For example for an Unknown circuit and only knowing that the circuit is in the max safe range of the scope(If not sure you should test to see that this is true).1. Turn scope on2. Select the highest X probe you have, Remove ground clip.3. Check scope test signal output4. Set the scope to it's highest Volts/Division5. Check the test signal output. Do you still see a hint of the test signal?6. Touch just the probe tip to the circuit.7. From what you are seeing on the scope,is it safe to go back to #4 and change to a lower setting?If so go back to #4 and repeat all the steps with a lower Volts/Division setting.Would it be safe to a probe with a lesser X rating?If so go back to #2 and repeat all the steps with a lower X probe.May sound extreme but each step is doing something.Think about what is happening here. Each time you touch just the probe tip to the circuit you are adding a new current path. For most scopes and a 1X probe that is a 1Meg resistance( + some Capacitance & inductance) path to the scope's ground. The currents in the circuit will change to try to supply the current that flows when you touch just the probe tip. This very well could be enough to stop the circuit's function.The result is having a good idea if it is safe to connect the ground clip of a probe to the circuit.Now if you still not sure if you can connect the ground?Do you have a two channel scope?If so look for a mode channel One - channel Two.A difference between the two channels mode.If you have this mode then you can connect the ground lead for one probe to the ground lead for the other probe.Use one probe tip for your signal and the other probe tip for the signal's ground.Note that each signal that a probe is connected to will need to be in the safe input range for that channel of the scope. This can change due to the probes( 1x, 10x, 100x, 1000x) and may change with the Volts/Division setting.When you use the difference mode between two channels of the scope you are adding two current paths to the circuit. This may not be the same as just one probe tip creates. Would be a good idea to do the above procedure but touching both probes. For safety use a probe tip that you can clip on to the circuit so you can use just one hand.Note: you can damage some equipment if you switch ranges while under load. This is why the above says touch and not connect. You are changing the Volts/Division with out a signal connected.I would recommend spending more time learning how safety equipment can protect you,How does it do it's protection and what can it not protect you from.Along with this is learning the safe limits of any equipment and how to safely operate that equipment.When you know this, then figure out a way that is safe to test if something is safe to do.The first step for an isolation transformer is knowing that you need to use one and what the risks are if you do use an isolation transformer.You should find this out by a safe to do test.A simple insulated handle screw driver can be dangerous.Put the proper perspective on this, Which is more important1. Your life.2. Some test equipment or circuit.Good test procedures can try to save #2, could save #1 and when you screw up and miss something, the safety equipment might save you but will not be able to if you have already defeated it.C

author: Monkeh
author_rank: Super Contributor
timestamp: February 17, 2014, 02:40:06 am
content: Quote from: GoatZero on February 17, 2014, 12:52:56 amSoil conducts. Why do you think it's referred to as 'ground' and 'earth'?All this would achieve is bypassing safe operation of overcurrent protection.

author: Nerull
author_rank: Frequent Contributor
timestamp: February 17, 2014, 03:07:13 am
content: I don't think you're understanding what an isolation transformer does. It doesn't isolate ground, it floats the entire circuit.The mains voltage coming in over your outlets is an AC voltage, often 120 or 230V, referenced to neutral. Neutral is connected to ground at the distribution point. There is nothing you can do to ground on your end to make that voltage float. The only thing your ideas are likely to accomplish is to make your DUT more dangerous, not less.An isolation transformer completely isolates your DUT from hot, neutral, and ground - allowing you to connect any one voltage level in your DUT to ground - or any other voltage - without creating a return path. This means when you to connect your ground clip to the high voltage rail in a power supply, it doesn't explode because that rail floats to ground. All other voltages in the power supply will also now be referenced to that rail.This means, for example, that your DUT chassis may now have exposed lethal voltages on it and touching it may kill you. The instant you ground any part of the DUT - by clipping a ground lead, or touching, etc., it is no longer isolated and it is critical that you understand what that means before relying on one for safety.

author: Shock
author_rank: Super Contributor
timestamp: February 17, 2014, 01:18:38 pm
content: A proper isolation transformer reduces the risk if you accidentally touch the live.You still have to be aware of electrical safety and best practices, and how to avoid damaging your equipment at all times.