workbench safety: mains isolation - Page 1
Forum section: electronics / general

author: tweek
author_rank: Contributor
timestamp: January 23, 2011, 02:41:43 am
content: The last year or so has kept me away from the electronics hobby for various reasons. In that time though I managed to get my hands on a house and I've been allocated by the lady of the house 100sq ft of my very own. Workshop time! Sorry Dave and Chris this is pure coincidence, and WOTW had nothing to do with it.Anyhow I've racked up a queue of several projects I want to build and at least one of them (off-line SMPS for my bench) will require work with mains voltage, and potentially require me to poke at the thing with an oscilloscope. While I intend to isolate the output, obviously some of the guts of the power supply aren't.My understanding is that this is a problem for test gear where the ground clip is the actual supply ground, and isolation must be used. Funny thing is I don't really see this discussed much. Does anyone maintain isolated power at their workbench? If so, can anyone comment on whether it's more prudent to isolate the DUT, or the test gear itself. It seems that it would be easier and cheaper to isolate the test gear, but there is an additional potential safety benefit from instead isolating the DUT, on the off chance that you, for some reason, become not isolated if you know what I mean.Recommendations on isolation transformers is also welcome!Any thoughts?Thanks!Phil

author: Mr J
author_rank: Regular Contributor
timestamp: January 23, 2011, 04:32:59 am
content: probably a good idea added safety. Another idea...power kill switch for the bench. On mine I use a DPDT switch to cut both hot and neutral located on the right side of the bench with a big red facepate.

author: SG-1
author_rank: Regular Contributor
timestamp: January 23, 2011, 04:35:18 am
content: I use this for mains isolation, it allows one to dial in the trip current. It was designed to test power tools & such for insulation failure. It was used & abused when I found it, but still working.

author: Psi
author_rank: Super Contributor
timestamp: January 23, 2011, 04:38:15 am
content: An RCD is another option

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 04:38:40 am
content: I isolate the DUT simply because I have a number of pieces of test gear that I would have to isolate, but I'm usually only working on one DUT at a time.BTW ... I've always had a big kill switch on the front of my benches. A really good idea.

author: SG-1
author_rank: Regular Contributor
timestamp: January 23, 2011, 04:43:58 am
content: When working with mains you should consider wearing safety glasses in case something vaporizes.Wear some kind of gloves when probing around.Modify your multi-meter probes something like this to reduce the likehood of causing a short circuit or ground fault.

author: Unknown
author_rank: Unknown
timestamp: January 23, 2011, 06:22:35 am
content: Connect the DUT to an isolation transformer, not the test equipment. You want IEC class I equipment (anything with a grounded plug) to be grounded. The equipment was designed with the assumption that ground is safe to touch under all circumstances, a floating ground may be connected to dangerous voltages. Say you accidentally clip the ground clip of your scope to live (or something that gets connected to live due to a fault), and then touch neutral with your other hand. An isolation will not protect against this, but a grounded scope would. Not grounding test equipment will also compromise signal integrity. If you want test equipment to float, buy test equipment properly designed for this, like a Tek TPS2000 series scope or a Fluke Scopemeters. All DMM's I know are already floating, even bench DMMs with grounded plugs, so no point hooking these up to an isolation transformer.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: January 23, 2011, 06:32:18 am
content: Quote from: tweek on January 23, 2011, 02:41:43 amNo thoughts just an question, how many watts you need ?The copper price are flying sky high , and the transformers price follows that route too.And so I suggest you to build just an " partial " isolation by transformer ,like an special wall plug , so to use it with some devices that can cause problems.Like old radios with tubes , or old televisions with tubes.In this case , you will need something small and affordable. ( lets say 500W Max, so to operate for long time , and remain cold too)

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 06:43:58 am
content: I'm personally in favour of a portable isolation transformer that is specifically used for a specific task at hand.i.e. ie requires the conscious effort to utilise the transformer when you expect it to be required.If you are always isolated, this may lead to complacency should you move elsewhere, and being portable you can easily move it elsewhere if the job cannot be moved to the bench...Nevertheless - RCD - YES YES YES.In Australia it is now mandatory that all houses (with recent wiring work performed) must have an RCD fitted.You can always use another one, local to you bench, just to be sure.Besides all that 1000 square feet - NIRVARNA - my shed at home is only 600 sq ft and that usually houses two cars :-(

author: Psi
author_rank: Super Contributor
timestamp: January 23, 2011, 06:56:46 am
content: For mains power on my desk i built a control box to distribute power to all my gear.With a master on/off switch and neon i can turn off all my gear with one switch and can't miss seeing the bright master neon at night, so it's pretty hard to leave anything on that might burn the house down.(which is good because i seem to attract nearly-burning-the-house-down eventsElectric blanket fire a while ago and that garden slug shorting out the wall socket that i posted on here a week or so back)The master switch powers 4 outputs, each with their own switch and neon.Output A is a cable hardwired to a 4-way powerboardOutput B, C and D are 3 pin mains sockets on the back of the box. So i can have testgear/equipment plugged directly in at the back with their own switch or i can plug in extra power boards and have multiple devices on each output.Its quite useful, i have the built-in power swiches set to ON for all my gear. That way i can just turn on the master switch and whatever output switches i need.A for Lighting, B for Soldering/rework, C for Test gear and D for the Variac and Isolating transformer (which i use if i need 230v for projects).Also included in the control box is a fuse (since i dont need full 10A at the desk) and an earth terminal for my anti static strap.I plan to put a RCD on the input to the control box at some point as well, currently it's just mains from a wall socket.

author: tyblu
author_rank: Frequent Contributor
timestamp: January 23, 2011, 08:36:40 am
content: Fairly long answer here:http://electronics.stackexchange.com/q/8534/2118The short version: isolate DUT, monitor floating chassis voltage relative to Earth ground, and don't touch anything while DUT is powered and/or you are taking measurements. (Turn off power, clip on probe, set trigger, turn on power, step back, turn off power, look at chassis voltage, equalize chassis if needed (if capacitive), then read test gear/'scope.)

author: jahonen
author_rank: Super Contributor
timestamp: January 23, 2011, 10:33:19 am
content: Yes, I have one on my bench. It is actually an isolated variac with output power of 630VA 0-250 VAC nominally. I used to repair all kinds of equipment in my previous life, and for measuring the non-isolated primary side with an oscilloscope safely, the DUT isolation is a must. Having just a RCD does not allow that, although it naturally improves safety otherwise. I still use it whenever working with low-power mains powered equipment, even if I don't necessarily need the isolation. Note that there must beonly oneDUT connected to the isolation transformer output, or the situation becomes hazardous again.I believe the isolation transformer reduces CAT-requirements due to that it increases the resistance so short circuit current is reduced compared to direct mains connection. Of course that depends how stiff transformer you'll have.Isolating just the measuring equipment is dangerous, but if you can afford it, both can be isolated from mains (DUT and mains powered measuring equipment). Also do not use same isolation transformer for both DUT and your measuring equipment. I have isolation on the measuring equipment too, haven't seen any odd behaviour due that.Regards,Janne

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 23, 2011, 10:57:41 am
content: QuoteDefinitely agree with that one. Unless all you do is SMPS design, most of the time you won't need an isolated supply, so bring one out when you need it.A variac (with V and I metering) is also a very useful addition to an iso transformer for debugging mains stuff - you often want to start things slowly to watch for smoke, and at some point will need to test your hardware over a range of mains voltages.One word of caution on isolating transformers, especially smaller ones. It's easy to assume that the output voltage is the same as the input, however this will usually only be the case near full load, and at light loads, the output voltage can be higher - a long while ago I was debugging a mains fan controller and was puzzled for a while as to why it wouldn't work properly on the isolating transformer - turned out it was putting out about 285V at no-load.

author: Psi
author_rank: Super Contributor
timestamp: January 23, 2011, 12:21:12 pm
content: yeah, it can also be an issue with variacsSome countries have 240V mains with houses close to the step down transformer getting near 250V.When building and testing mains gear you normally want to test between 210v and 250V and want a variac that can deliver that. So a lot of variacs are designed to be able to produce 250v from 220v.Setting your variac to max if you already have 250v gives obvious results.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 03:30:15 pm
content: An isolation transformer is a good idea but you don't need a large one, 250VA is fine for most applications and can be used on larger loads for short periods.One thing to bear in mind is that an isolation transformer will defeat an RCD so there's no point in having both although all test equipment should be powered of an RCD.Make sure the device being tested is protected with a suitable fuse. Here in the UK it's not a problem as all plugs are fused but in other countries it's not the case.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 03:45:50 pm
content: RCD?

author: tyblu
author_rank: Frequent Contributor
timestamp: January 23, 2011, 04:27:29 pm
content: GFIin Aussie-talk (residual current device).

author: Simon
author_rank: Unknown
timestamp: January 23, 2011, 06:17:44 pm
content: I always use a portable RCD, that way if you touch a live by mistake your saved but make sure it is a trip at 30mA or less sort. if you make sure you clip your scope ground to neutral you should be ok but an isolation transformer is a good idea.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 06:46:48 pm
content: Quote from: Simon on January 23, 2011, 06:17:44 pmIs that really what you meant to say?Ground is not the same thing as neutral. If, by some freak failure, you loose your neutral connection and you have your scope ground tied to neutral you'll have all of the AC current coursing through ground. Not a good thing.

author: tyblu
author_rank: Frequent Contributor
timestamp: January 23, 2011, 07:46:21 pm
content: Neutral isn't 0V, either, due to unequal split-phase loading. Perhaps you mean isolated neutral?

author: tweek
author_rank: Contributor
timestamp: January 23, 2011, 08:12:17 pm
content: Wow what a great response to my first thread! Y'all sure know how to make a guy feel welcome. Definitely a few angles in here that I had not thought of before.Quote from: Kiriakos-GR on January 23, 2011, 06:32:18 amI figure 400-500VA is probably more than enough for any single device I would want to test. I was definitely looking for a portable unit as some have suggested, which I would only use when I need it (which honestly will likely not be very often). I'm thinking about a unit similar to this:http://www.tripplite.com/en/products/model.cfm?txtModelID=227The MSRP is kind of high but I've seen them for around US $200.I'm also considering the 250VA model which is considerably cheaper.Quote from: RayJones on January 23, 2011, 06:43:58 amHere in the US GFI/RCDs are still only required in specific areas and they generally pop out at 5mA. AFCI breakers are now pretty universally required on branch circuits to all areas of the home where people might be regularly and I think some of these units also provide ground fault protection. My house was built a couple years before that was universal but I'm considering up-fitting at least some of the circuits here with those devices.Wow 1000sq ft awesome! That is way more generous than the 100sq ft my wife actually let me have!Quote from: JohnS_AZ on January 23, 2011, 04:38:40 amDo you use a large rocker switch as Psi showed in his picture or do you have a "big red button" type arrangement with some sort of shunt trip or a relay that kicks out when you slam it? I used to have the latter in the workshop I had as a kid. Big red buttons rock! Bonus if it flashes.Quote from: JohnS_AZ on January 23, 2011, 06:46:48 pmAlso even in the best case where your neutral is intact, if you were using a RCD I think you would likely cause it to trip . The only time it's really OK to consider ground = neutral is generally at the service entrance where all your bonding points are tied together.Reminds me of a story though...this past Thanksgiving I was at my in-laws and we got on the topic of electric work in old farm houses. He told me a story about how one time he lost the neutral up at the barn so he just hooked it to a copper pipe that was going back to the house. Problem solved! Still picking up the rubber bands that popped in my head that day.There was a book I was reading a while back (can't remember the name right now) that talked about how the problem with most safety systems and procedures is that probability-wise there is generally little to no penalty for defeating them.Thanks,Phil

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 08:36:02 pm
content: How about getting a transformer with two 120V secondaries so you can test 240V as well as 120V devices?

author: Neganur
author_rank: Unknown
timestamp: January 23, 2011, 09:03:58 pm
content: Safety regulations here demand both DUT and measuring equipment connected to isolation transformers, if you go anywhere near mains power.It's always a good idea to have your scope connected to one anyways. You end up creating ground loops more often than you think if your signal generator and scope use the same mains for example.I use a fairly inexpensive140VAone for my scope. And an old german variable 1000VA, 0-250V for the DUT.Variable is a must btw, when you intend to repair old gear - slowly increase the voltage to not upset those capacitorsHandheld multimeters are always safe in this regard.

author: Simon
author_rank: Unknown
timestamp: January 23, 2011, 09:11:11 pm
content: I was told once in Italy that if you got a shock off a tap it was because someone was using the earth as neutral to evade logging their consumption on the meter, To be honest I don't know why we don't earth the neutral in buildings ? that way we know that neutral is neutral

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 09:19:45 pm
content: While a little off topic, here is the electrical diagram of my new workbench. Some will certainly say that it is over the top, but I already own everything in the diagram, so why not use it?My isolation transformer is a stand alone, so it's not included in the drawing. I always use it for HV DUT, but I never have for my test equipment. If one side is isolated, never saw the necessity to isolate the other.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 09:20:46 pm
content: Quote from: tweek on January 23, 2011, 08:12:17 pmYes I agree that grounding the neutral beyond a RCD is increasing the likelyhood to trip it, depending upon whatever else is hanging off the RCD.Yes, our domestic arrangement in Australia also is that Earth and Neutral are tied together at the switchboard's main entry point, and is known as Multiple Earth Neutral - MEN.The RCD then monitors the ideally balanced current in Active and Neutral from beyond that point. ie Neutral != Earth after the RCD.There are actually two neutral bars - before and after the RCD, and one just happens to be earthed.If you are using an isolation transformer, by all means you can connect the probe ground to neutral, but solely with an RCD - no.

author: tweek
author_rank: Contributor
timestamp: January 23, 2011, 09:22:51 pm
content: Quote from: Simon on January 23, 2011, 09:11:11 pmI don't know how they do it elsewhere but as a kid I used to do residential electric here in the US (father is a retired electrician) and I'm pretty knowledgeable on how it's done here. Where the supply comes in, usually at the main breaker panel, you bind the neutral, earth ground and water ground (water pipes, including jumping out the water meter and heater) to a single common bus bar. At the main panel all grounds and neutrals come back to that bus bar. In the context of the home electric system, that IS your zero volt reference. As you branch from the pain panel, current in the neutral will cause a voltage drop which makes it at a higher potential than ground and therefore no longer acceptable to be taken as he same as ground. which is why they are always run out separately, and why neutral is insulated whereas ground doesn't have to be. So at least here they do in fact bind neutral and earth at the service entrance. After that all bets are off.Phil

author: Simon
author_rank: Unknown
timestamp: January 23, 2011, 09:31:01 pm
content: as far as i know in the UK neutral is only grouded at the substation but i may be wrong

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 09:40:43 pm
content: I believe in the older days the earth was provided by available copper plumbing.These days a massive copper grounding stake needs to be driven into the ground adjacent to the switchboard.It's over 1.2m long in the ground.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 09:46:03 pm
content: Quote from: Neganur on January 23, 2011, 09:03:58 pmWhy would you need to connect the measuring equipment to an isolation transformer?Modern 'scopes are double insulated which means the outputs are isolated from earth anyway so it gains you nothing.One thing to note is although that devices connected to an isolation transformer don't need to have an earth connection, their metal cases should all be bonded together so they can't float at different voltages.Quote from: Simon on January 23, 2011, 09:11:11 pmIf the neutral wasn't connected to earth the mains could float at any voltage. Here in the UK we have a three phase system so if one live were to become connected to earth, the other lives would float at 400V. Worse still if the mains wasn't earthed and someone connected a high voltage autotransformer which developed an earth fault, the whole mains would float at a dangerously high voltage causing the insulation to fail, smoke and fire. From memory the electrical regulations state that an autotransformer must not be connected to an isolation transformer.Quote from: Simon on January 23, 2011, 09:31:01 pmIt depends on the earthing system used. Where I live, the neutral and earth share a cable until they enter the property where they're separated. The cable is coaxial with the live in the centre and the earth/neutral on the outside. This kind of system is known as TN-C-S.http://en.wikipedia.org/wiki/Earthing_systemA workbench with an isolation transformer is an example of an IT system.

author: tweek
author_rank: Contributor
timestamp: January 23, 2011, 09:51:12 pm
content: Quote from: JohnS_AZ on January 23, 2011, 09:19:45 pmWow, I'm just going to be jealous in advance because I'm pretty sure my setup will not be that nice.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 10:04:17 pm
content: Quote from: Hero999 on January 23, 2011, 09:46:03 pmMy Rigol DS1052E is definitely grounded at the BNC inputs, I just checked it.So unfortunately you cannot make such wide sweeping statements.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 10:10:54 pm
content: The BNCs on my old HP scope are tied to chassis and electrical ground as well.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 23, 2011, 10:13:23 pm
content: Quote from: tweek on January 23, 2011, 09:51:12 pmThere's really nothing special there. It just takes into account 30 years of bitches about other benches. Really, take away the ESD monitor and the EMI filter (which no hobbiest really needs anyway) and it's not even all that expensive.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 10:27:02 pm
content: Quote from: JohnS_AZ on January 23, 2011, 10:10:54 pmMy old Tektronics and Cossor 'scope but I thought modern plastic cased 'scopes were class 2? May be not as I've not got one myself but I've used one before which was.Quote from: RayJones on January 23, 2011, 10:04:17 pmAll right may be I'm wrong about that but how would an isolation transformer change that?Unless the 'scope chassis isn't bonded to the chassis of the device under test there's a risk of a high voltage between them.The 'scope is normally run off an isolation transformer or SMPS anyway, all my old 'scopes certainly and I've seen pictures of newer 'scopes which suggest that to be the case.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 10:32:30 pm
content: The isolation transformer assists greatly with a SMPS.The input circuit is a full wave bridge - active and neutral are the inputs.If you apply your grounded CRO probe to the -ve of the rectifier, you have just shorted half the rectifier out - not good.When you place an isolation transformer on the input, neutral is no longer bonded to earth and you can place you CRO's ground anywhere safely.Note that this should however only be done with the power off, and the bulk capacitors discharged.

author: Zero999
author_rank: Super Contributor
timestamp: January 23, 2011, 10:37:30 pm
content: Quote from: RayJones on January 23, 2011, 10:32:30 pmI know that, in fact I suggested it, a few posts back.You've misunderstood me, I meant using an isolation transformer to power your test equipment is a waste of time.QuoteNot sure what you mean, of if the power supply is switched of then the neutral isn't connected to the capacitor or anything else.

author: tweek
author_rank: Contributor
timestamp: January 23, 2011, 10:45:36 pm
content: Quote from: Hero999 on January 23, 2011, 10:37:30 pmI think the intention is to make sure that the SMPS is not energized when test leads are connected. If the caps aren't discharged, then there is still a ton of energy in the box. Not an isolation issue just a general safety issue.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 10:47:22 pm
content: I have never advocated using an isolation transformer for test equipment. I also agree and personally think that is not the sane thing to do.I was referring to moving the ground clip about in your SMPS. Yes in theory you could move it whilst powered, but I wouldn't.There is a shit load of joules in that cap just waiting to bite.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 23, 2011, 11:00:19 pm
content: I'll try and reword my reasoning.Are you likely to be shocked by your CRO?NO - you'd smash it up and throw it away if that was ever likely to happen, so why isolate it?Are you likely to be zapped by a mains drive SMPS.BY JINGOS YES - damn dangerous devices.If you float the whole lot with an isolation transformer that nasty return path to ground now has an air gap at the transformer.Grab the active after the tranny - no shock.It however still does not protect from direct connection to active AND neutral, or any other double touches like the aforementioned bulk cap.You still must have your wits about you, but the primary benefit is be able to probe with grounded test devices and not blow the living bejesus out of something that may have already been OK.

author: Psi
author_rank: Super Contributor
timestamp: January 24, 2011, 01:15:38 am
content: Quote from: JohnS_AZ on January 23, 2011, 09:19:45 pmThat's a very nice setup.Quote from: tweek on January 23, 2011, 08:12:17 pmYeah, i only used the rocker switch because i already had it and the industrial emergency stop buttons were expensive.Ya should probably use an emergency stop button if you can.However they're mainly for areas where many people are working so one person can hit the button quickly if they see something bad happening to someone else.When it's just you working at your hobby desk your hardly going to be able to press any button while getting a shock. So when there's just one person the switch is more there to shut off power when you see smoke, and by then the damage to your circuit is normally already done. Cutting the power superquick with a emergency stop button is unlikely to save your circuit.Of course with an emergency stop button you do get the advantage of a switch design with safety in mind. You can be sure that if the switch fails it will fail to a safe state

author: tweek
author_rank: Contributor
timestamp: January 24, 2011, 01:54:46 am
content: Quote from: Psi on January 24, 2011, 01:15:38 amI've also been kicking around heading to a marine supply store and getting one of those cut-off switches you find in boats where you can hook something to yourself and it kills the engine if you fall out of the boat. I'm not sure that solves any more problems either, but it would look cool.I've got a couple more weeks before I can actually act on most of this so for now I will just have to satisfy myself by thinking of stuff, and ordering that transformer.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 24, 2011, 02:12:49 am
content: [ Red Mushroom Emergency Stop Push Button Switch ]http://www.sourcingmap.com/red-mushroom-emergency-stop-push-button-switch-p-89803.html?utm_source=google&utm_medium=froogle&utm_campaign=usfroogle8-)

author: Chasm
author_rank: Regular Contributor
timestamp: January 24, 2011, 02:34:27 am
content: The main advantage of an Emergency Stop Button on a workbench is that it can be operated really easily. Even if you are working alone.It is often mounted in a position that can not be obstructed, also an important feature.

author: Psi
author_rank: Super Contributor
timestamp: January 24, 2011, 03:23:45 am
content: Quote from: JohnS_AZ on January 24, 2011, 02:12:49 amim not sure, getting a cheap emergency stop button kinda defeats the purpose of having one.Would want to check the switch quality definitly.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 24, 2011, 03:30:53 am
content: If you go emergency stop, you should go the latching relay version - they probably have a fancier name than that though!That way you can easily have multiple stop locations, and should the power be lost to the bench, you won't get any surprises when it returns.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: January 24, 2011, 04:07:47 am
content: Quote from: Psi on January 24, 2011, 03:23:45 amWell, NO switches fail now and then. NC switches almost never do. However, here ya go ...http://www.omega.com/pptst/OMPBD7_mush2.html

author: Psi
author_rank: Super Contributor
timestamp: January 24, 2011, 04:18:58 am
content: Quote from: JohnS_AZ on January 24, 2011, 04:07:47 amhehe yeah, thats about the price they were when i was designing the power box for my desk.

author: Unknown
author_rank: Unknown
timestamp: January 24, 2011, 05:17:17 am
content: Quote from: Hero999 on January 23, 2011, 09:46:03 pm99% of the scopes I've seen are IEC protection class I (grounded, not double isolated), double isolated would be class II (without ground connection). Class I equipment relies on a good ground connection to protect the user (anything connected to ground is by definition at a safe potential). Most scope vendorsstrongly recommend against floating scopes, it's unsafe. I guess you could use an isolation transformer as long as the ground connection is left intact, but what's the point, the scope already has a transformer? The only exception would be something like a Tek A6901 ground isolation monitor, which continuously monitors the potential between safety ground and scope ground, and will protect the user if it reaches dangerous levels (above 30V or so) by turning off the scope and grounding it.Just use the proper tools: isolated scopes (which is not just a battery powered scope) or differential probes. Messing around with safety ground and isolation transformer without knowing what you're doing will increase risks, not decrease them.

author: Chasm
author_rank: Regular Contributor
timestamp: January 24, 2011, 05:37:25 am
content: Quote from: alm on January 24, 2011, 05:17:17 amAs so often it is about knowing what you are doing.Isolation transformers, HV probes, differential probes, isolated scopes and many other things are tools designed for very specific purposes. As long as you use them as intended they are pretty safe.Once you do something stupid they will do their best to kill you. Simple as that.The problem is usually starts with my first sentence, knowing what you are doing. That is usually pretty hard if you were not trained in the specific aspects.Take the isolation transformer.It will protect you against one fault, touching a hot wire. It will not protect you against a second fault, aka touching another wire. That is pretty obvious. It will also circumvent the RCD device you may be using on your bench. As far as it is concerned the current that goes into the isolation transformer comes back via neutral and it will not trigger.Really, knowing the pitfalls of your tools is essential.

author: scrat
author_rank: Frequent Contributor
timestamp: January 24, 2011, 11:52:04 am
content: Quote from: RayJones on January 23, 2011, 10:04:17 pmIt MUST be earthed,since otherwise, connecting the probe's ground to mains or high voltage would be normally harmful.

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2011, 05:51:24 pm
content: Quote from: scrat on January 24, 2011, 11:52:04 amWhat about battery powered 'sopces? Rigol 'scopes have battery powered option.You won't get a shock providing the device under test is connected to an isolation transformer.Although that won't protect you if you connect other pieces of floating test equipment to the same circuit, which is why you should make sure the chassis of each piece of test equipment are connected together so they can't float at different voltages.

author: Unknown
author_rank: Unknown
timestamp: January 24, 2011, 07:14:57 pm
content: Quote from: Hero999 on January 24, 2011, 05:51:24 pmNot sure about the Rigol. The manual of all IEC class I scopes with battery option I've seen explicitly states that the scope should be grounded by a separate ground connection when used battery powered (my scope has a separate ground screw on the back for this purpose), or only used for potentials below 30V or so.Quote from: Hero999 on January 24, 2011, 05:51:24 pmNow it only takes a single fault to create a dangerous situation, without disconnecting the ground lead of the scope, it would take a double fault (broken ground and broken isolation transformer / connection of person to second conductor).What safety do you gain by connecting a piece of class I test equipment to an isolation transformer? Connecting the ground clip to a dangerous potential will just blow the fuse, inconvenient, but not a safety issue. Why do feel the need to ignore safety advice by scope manufacturers (I quoted Tek, but I'm sure Agilent and Lecroy will tell you the same)?Quote from: Hero999 on January 24, 2011, 05:51:24 pmThis sounds like a bodge job, can you refer to instructions from the past thirty years or so by any reputable test equipment manufacturer that tell you to do this? I referred you to instructions by Tek that tell you to never, ever do it (with normal scopes).

author: jahonen
author_rank: Super Contributor
timestamp: January 24, 2011, 08:46:45 pm
content: Quote from: alm on January 24, 2011, 07:14:57 pmWhy it would be any more dangerous if you have both DUT and scope behind their own isolations? Of course, measuring mains connected equipment would be quite unsafe with isolated scope.And yes, user manual for my Agilent MSO6034 says thatQuoteI have always thought that spirit of that advice is that some people will try to measure potentials galvanically connected to the mains with battery powered scope (or, generally power supplies galvanically connected to surrounding environment), thus creating the hazard with the surrounding environment since everything connected to the scope (like a computer etc.) is now live respect to the environment.However, I see no big problem if I always have both scope and DUT isolated from mains and each other. Of course, that requires care and discipline to always use isolated powering for mains powered DUT.Regards,Janne

author: Zero999
author_rank: Super Contributor
timestamp: January 24, 2011, 09:02:52 pm
content: Quote from: alm on January 24, 2011, 07:14:57 pmA good isolation transformer should have class 2 insulation between the primary and secondary so the chances of that happening are the same as any other piece of class 2 equipment.QuoteMost 'scope manuals are written with the assumption that you're not running it off an isolation transformer and bonding its chassis to that of the device under test.QuoteNo bodge job, it's recommended that the chassis of all equipment running off an isolation transformer be connected together so there's no risk of them floating at different potentials. It's written down in the IEE regulations as protection against electrical shock using a separated supply. The idea is that if all metal parts are bonded together and the power supply is isolated from earth, there's no chance of receiving a shock. It's pretty safe because there would need to be a triple fault in order for you to be shocked: the insulation would have to fail on both appliances as well as the bonding between them. Using an earth bonded 'scope run directly from the mains is more risky because there only needs to be two faults to receive a deadly shock: insulation and earth bond failure.Setting up such a system is easy, just connect a ganged mains socket to the isolation transformer secondary and connect all the devices under test and test equipment to it. All the chassis will be now connected together because even though there's no earth connection, there's still the earth bus inside the gang socket connecting the chassis together.

author: tyblu
author_rank: Frequent Contributor
timestamp: January 25, 2011, 02:27:59 am
content: Quote from: jahonen on January 24, 2011, 08:46:45 pmWhen the 'scope is isolated and has a normal ground (class 1 something-or-other), its chassis and reference floats. The danger is with respect to your voltage. If you are 'Earth' (mains) grounded -- for eg. touching your grounded workbench -- then the oscilloscope may have a "hot chassis", floating its reference high above yours. This is why alm mentions this...Quote from: almI stick a banana plug into the Earth ground in a North American socket and keep a voltmeter between the chassis and Earth ground. Using a cheap meter, I think this actually 'bleeds' any high voltage chassis capacitive charge, but I may be blowing smoke -- regardless, it hasn't been a big deal for me yet, but I'll keep doing it.

author: PetrosA
author_rank: Frequent Contributor
timestamp: January 25, 2011, 04:48:07 am
content: It sounds like there's some misconceptions going around about how floating voltages are or aren't dangerous. Here's my 2 centsIf you have a separate or isolated voltage source that doesn't share a ground connection anywhere in the system with the mains, then floating voltages are safe. Try this experiment - take a UPS, unplug it from the mains so it starts producing voltage on its own and check voltage to system ground. It won't exist. If the UPS switches on while plugged in because power fails, you will get voltage to system ground. Edit to add: If the disconnected UPS does make a second, accidental contact with mains phases, neutrals or ground, especially through your body, there would be a dangerous situation because there would then be a path for current to flow.I don't pretend to know how isolation transformers outside the US are wired, but here they must have their ground connected to the system ground. The only way to know this for sure is to take resistance readings between the ground and neutral on the isolated side, and isolated ground to system ground, and isolated neutral to system neutral.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 25, 2011, 06:02:03 am
content: I agree, I think more is being made out of isolation transformers than really exists.The #1 problem with normal mains is that Neutral is earthed somewhere.This means touching live is very likely to induce a possibly deadly current flow down through your boots to good ol mother earth.The sole purpose of the isolation transformer is to float your neutral (and live) away from earth, and indeed the earth connection is still carried through to the end equipment.So if you grab live with your hand brushing against the metal chassis you won't havethat killer current path.As for test equipment needing to be isolated - total utter bullshit.You still have the protective earth connected through, and that's what the BNC outers on the CRO attach to!

author: tyblu
author_rank: Frequent Contributor
timestamp: January 25, 2011, 06:13:00 am
content: Quote from: PetrosA on January 25, 2011, 04:48:07 amYou are quite right, and I feel silly. This happens more often than is comfortable.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2011, 05:53:29 pm
content: Quote from: RayJones on January 25, 2011, 06:02:03 amI agree, you don't have to power your test equipment from an isolation transformerQuoteThat's not necessary providing the test equipment is connected to an isolation transformer and its chassis is connected to the chassis of the other equipment connected to it.I'm talking about regulation 471-12-01http://www.theiet.org/publishing/wiring-regulations/mag/2006/18-portablesupply.cfm?type=pdfhttp://books.google.co.uk/books?id=Z432pJcac6QC&pg=PA80&lpg=PA80&dq=Regulation+471-12-01&source=bl&ots=bbTjcRH3vG&sig=eo84q5YnT6LuFqPNxMLT62HYrBo&hl=en&ei=Dgw_TevyA9-ShAf37aSLCg&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDQQ6AEwAw#v=onepage&q=Regulation%20471-12-01&f=falseAll it means it that if more than one appliance is running from an isolation transformer, the exposed metal parts need to be bonded together to prevent them floating at dangerous potentials.Of course if you're not using an isolation transformer to power the 'scope you need to connect the chassis to protective earth.

author: Chasm
author_rank: Regular Contributor
timestamp: January 25, 2011, 06:12:24 pm
content: Quote from: Hero999 on January 25, 2011, 05:53:29 pmUsing more than one isolation transformer is widely seen as both unnecessary and a recipe for disaster.Such a setup is rarely warranted.When working on HV circuits apply the KISS principle at all times.And try to think up front.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2011, 06:41:01 pm
content: Quote from: Chasm on January 25, 2011, 06:12:24 pmI agree, which is why I've never said anything about using more than one isolation transformer, read my posts.QuoteIt's not a high voltage circuit, mains is low voltage.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 25, 2011, 10:08:33 pm
content: Quote from: Hero999 on January 25, 2011, 05:53:29 pmSo your CRO develops an internal fault and active mains connects to the internal chassis which should be grounded.Guess what?Your "Bonded to exposed metal" has suddenly become live, connected to the mains, and no amount of isolation transformers will protect you from the inevitable shock.I also suggest we refer to this document on pg4 under " "Ed" and "Ing" "http://www.apcmedia.com/salestools/SADE-5TNQYQ_R0_EN.pdfThis clearly states that the isolation transformer ONLY OPENS ACTIVE/NEUTRAL - ground must be carried through.So by bonding together your exposed metal, you are doing what you should always be doing, connecting that to earth.If you are not, why oh why would you deliberately open the protective ground?Connect all your exposed metals to real earth.You still need to have your wits about you.Stupidity will still be rewarded with a jolt.The isolation transformer allows you connect your test equipment without adversely shorting sections out, like the input full wave bridge of a SMPS.And this goes back to what I stated earlier, turn it off, and allow large caps to discharge before playing about with your test leads, especially the earthy end.

author: Zero999
author_rank: Super Contributor
timestamp: January 25, 2011, 11:35:22 pm
content: Thgis is hard work. . .Quote from: RayJones on January 25, 2011, 10:08:33 pmNo because if it's powered by an isolation transformer (which is what I said) there would be no risk of a shock.If a device is powered by an isolation transformer, connecting its chassis to earth provides no extra protection against electric shock.

author: RayJones
author_rank: Frequent Contributor
timestamp: January 25, 2011, 11:39:46 pm
content: Yes it is hard work.So what are you connecting your exposed metal work to?

author: Unknown
author_rank: Unknown
timestamp: January 26, 2011, 06:39:25 am
content: Quote from: Hero999 on January 25, 2011, 05:53:29 pmGoogle Books link doesn't work, but the PDF states that 'there should be no connection between the separated circuit and any other circuit, or to earth'. I believe this would include connecting a scope connected to an isolation transformer to a DUT on a different isolation transformer. So it doesn't appear to be that the proposed isolated workbench would comply with this.Quote from: Hero999 on January 24, 2011, 09:02:52 pmTriple fault? You connect the scope ground to some potentially dangerous potential in your DUT (eg. live wire), this is safe, because everything is floating. Now a fault occurs and the neutral wire in the isolated DUT gets connected to something you touch (eg. case): you get shocked, since you're likely to be touching the scope ground somehow (all exposed metal on most test equipment is connected to ground). If the scope had been grounded, there would have been sparks and a blown fuse as soon as you clipped it to the live wire, and the ground conductor would have carried all current and the GFI/RCD would trip. Isolation transformer does not make the voltages less dangerous, it just makes it safe to touch one of the wires at a time. If you somehow get connected to the second wire, you can still get shocked.The issue with ground is that there tends to be lots of grounded stuff around, for example piping for heating and water, some floors, workbenches, almost all large fixed metal objects, so it's quite likely that you connect your floating ground to earth ground somehow. From the signal integrity front, there is usually something like 1000pF of capacitance from your scope to ground (in your average lab, obviously less if you suspend it in mid-air somehow), so if there is any connection between your DUT and ground, this can give some nasty coupling.Regarding isolation transformers and ground: some do connect the ground conductor, some don't. Connecting the scope to the former won't do any harm, but it won't do much good either. It takes a double fault for class I equipment to shock someone, so the effort is probably better spent in improving safety some other way. Connecting equipment like scopes to the latter can be dangerous, and is recommended against by the manufacturers. I still don't see how cutting the ground connection would improve safety, the only advantage is that it can be convenient for measurements to non-isolated circuits without proper tools like differential probes, and that's precisely the most dangerous application.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 06:25:56 pm
content: Quote from: alm on January 26, 2011, 06:39:25 amThe PDF is a shortened form of the full standard.The PDF is talking about powering one device off an isolation transformer. The full standard goes on to talk about powering multiple devices off the same transformer which is perfectly safe as long as their chassis are connected together.QuoteNo because the DUT is also powered by an isolation transformer and its chassis bonded to the 'scope.QuoteIt doesn't matter if there's a ground around or not. Just don't power anything on your test bench with a grounded power supply and bond all the chassis together and you'll be fine.I think a drawing is in order to get across what I'm trying to say.

author: Unknown
author_rank: Unknown
timestamp: January 26, 2011, 08:15:07 pm
content: Quote from: Hero999 on January 26, 2011, 06:25:56 pmBut multiple isolation transformers are not permitted, and that's the solution that was suggested by multiple people in this thread. What's the point of the isolation transformer if the DUT doesn't float relative to your scope? I can see why this might be useful in an outdoor environment without any ground nearby (eg. powered from battery/generator), which is what I believe was the intention of the regulations you quote, but what do you gain on the bench except a false sense of security?Quote from: Hero999 on January 26, 2011, 06:25:56 pmIf you make sure there are no grounded objects nearby and never measure anything not connected to the same isolation transformer / ground, this setup is probably no worse than no isolation transformer at all, since you're basically creating your local circuit with local ground, but what's the advantage over the usual setup of grounded test equipment and DUT on an isolation transformer? The latter allows the DUT to float relative to the user and the scope, something that your proposed setup does not (since the scope ground in connected to DUT ground, and the user is likely to be connected to the scope ground). With your proposed setup, there is always the risk of connecting the scope to something not on the same isolation transformer, which can be dangerous.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 08:50:54 pm
content: Quote from: alm on January 26, 2011, 08:15:07 pmWell I agree that multiple isolation transformers are a bad idea, even though bonding the chassis together should eliminate mos of the shock risk there's still the possibility of voltages adding together.QuoteIsolation transformers are routinely used indoors in the UK for shock protection, for example the only sockets allowed in the bathroom are for shavers and are powered by an isolation transformer.QuoteAgain, as I said above, nothing bad should happen if the chassis are connected together but I wouldn't recommend using more than one isolation transformer ever.Did you look at the image I posted? Before making this post it had zero views, the first was me clicking on it to see if it worked, there again it could be the server playing up.

author: Unknown
author_rank: Unknown
timestamp: January 26, 2011, 09:18:49 pm
content: Quote from: Hero999 on January 26, 2011, 08:50:54 pmThis would apply if you just connected the DUT to the isolation transformer, touching both ground (likely in a bathroom) and one of the conductors won't result in you getting fried in that case. Touching 'local ground' and any ground-referenced potential will get you shocked, however, with one isolation transformer and the ground connections together. With just the DUT on an isolation transformer, ground could be floating, so touching scope ground and the now floating potential would be safe. And GFI/RCD would still be functional even if you touch the scope ground and a non-floating potential. I guess especially small isolation transformers (like shaver sockets) will probably limit the current, but this is hardly the case for large isolation transformers for the whole bench, and I'd probably consider a GFI/RCD more important.Quote from: Hero999 on January 26, 2011, 08:50:54 pmI did, but I didn't feel the need to click on it to enlarge it, since it just confirmed that we had the same set up in mind.

author: Zero999
author_rank: Super Contributor
timestamp: January 26, 2011, 09:45:33 pm
content: Yes shaver sockets are normally low power, typically 25VA.I suppose all I'm saying is I feel safer working at a bench which doesn't have an earthed power supply.One of my 'scopes has to be run off a transformer as it's the US model and we have 230V in the UK. Naturally I used an isolation transformer rather than an autotransformer, although the chassis is earthed because the transformer is in a metal box so is not class 2: one of the connections on the secondary could easily make contact with the case, still even then it'd need the insulation on my 'scope to fail too but I decided it wasn't worth the risk. I do use another isolation transformer for the DUT so I suppose having to isolation transformers is a necessary evil, not something I'd do by choice.