Test Bench - Isolation Transformer grounding question. - Page 1
Forum section: electronics / projects

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 01:55:42 pm
content: Hello, I've been lurking on the forum for a while but this is my first post.I have some questions about the right way to ground a test bench Isolation Transformer to isolate the circuit of the device under test.I'm in the USA, so the AC line is around 115-120Vac/60Hz, and the transformer I have for this is a Hammond 169WS (1:1, 250VA). The enclosure I'm building this into will have a 3 wire connection to the AC mains, and the outlets provided on the secondary side will be 3 hole outlets (Hot, Neutral, Ground). Devices under test powered from the secondary side will have 2 or 3 prong plugs.I'm attaching a schematic for what I have in mind, and was wondering if I could get a sanity check or some feedback on the layout. The attached schematic shows the transformer, the Hot/Neutral/Ground terminations, and a ground-lift switch on the secondary side.Thanks, any feedback or guidance is appreciated.-Rob

author: Terry Bites
author_rank: Super Contributor
timestamp: March 07, 2022, 02:07:54 pm
content: Ground is ground is ground- the iso side ground is the same as the incomming ground. Dont connect it to the secondary windings or you will defeat the object.https://sound-au.com/articles/iso-xfmr.htm

author: david77
author_rank: Frequent Contributor
timestamp: March 07, 2022, 02:23:50 pm
content: Do not connect Ground & Neutral on the primary side. That is a big no-no! Best case is you will trip the GFCI, worst case you will defeat it. If the socket you plug your transformer in is wired backwards you will pop the breaker.Commercial isolation tranformer sold here in Germany also NEVER provide a connection to the ground terminal on the secondary side. Many even only have two contacts in the socket and omitt the ground tabs. You certainly do not connect the ground from the primary side to the ground terminal of your load as that defeats the isolation barrier the tranny provides.Disregrad what I said if common practice in the US is different.

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 02:52:23 pm
content: You should never connect ground to neutral anywhere other than your main service entrance, and certainly never within a device. You also should never present a user with a 3-prong outlet that does not have a ground connection unless it is clearly labelled and there is some unusual reason to do this. (US) Isolation transformers always either pass the ground through or have two-prong outlets. A clearly labelled ground-lift switch might not be so terrible--certain audio products have them--but there should also be a safety circuit that reconnects the ground if the voltage at the DUT ground terminal exceeds 42 volts or so. What I do is keep a cheater plug handy and only use it when I am actually testing something like ground leakage that requires this isolation. I have a customized cheater plug that allows me to easily measure any leakage.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 07, 2022, 03:18:29 pm
content: It depends what you're usuing it for and why you're isolating.There shouldn't normally be any connection between the output windings and "real" ground, but there are times when the output ground pin and output neutral would be linked, and others where the output ground pin would go to "real" ground.If I were building a transformer box for bench use, I'd use an on-off-on type toggle switch to be able to connect the output ground pin to neutral, float or real earth.

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 03:21:35 pm
content: Thanks everyone - I've updated the attached drawing, omitting the primary side ground connection entirely.I'm putting this together from a bare Hammond 169WS transformer - it's a small (250VA) isolation transformer with 2 wires in and 2 wires out. My thought was the ground lift switch on the secondary would essentially allow the outlets to work as a "cheater" type of connection, with a floating ground plug.Is connecting the neutral/ground on the secondary side a completely bad idea? I had thought that would work best with DUTs with 3 prong plugs, and that the ground could be lifted when necessary or for devices with 2 prong plugs.Thanks.-Rob

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 03:40:34 pm
content: Quote from: RacerR on March 07, 2022, 03:21:35 pmMy question would be why you think it is a good idea? What are you trying to accomplish? Perhaps there is some situation where you would want to do this, but I don't know what it might be offhand.

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 03:46:03 pm
content: I've updated the drawing again - incorporating the 3-way switch for secondary grounding per @mikeselectricstuff's suggestion.@bdunham7 - I was thinking the secondary 3 hole outlets would behave more like an ordinary mains-side 3 hole outlets with neutral tied to ground... could be I'm mistaken about this though.-Rob

author: TheMG
author_rank: Frequent Contributor
timestamp: March 07, 2022, 04:57:51 pm
content: Linking the secondary neutral and ground could present a potentially hazardous situation if the DUT has an external path to "real" ground (an example would be a USB cable connected from the DUT to a computer that is connected directly to mains). In such a case, it would completely defeat the isolation from a safety perspective.For that reason, best to leave the secondary floating. Earth ground can be connected through but don't connect it to the secondary.

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 05:12:04 pm
content: Quote from: RacerR on March 07, 2022, 03:46:03 pmAll that depends on why you are using an isolation transformer in the first place. I use one quite often and am sort of an isolation transformer proponent, whereas some other forum members think the dangers outweigh the benefits--mostly due to the concern of accidentally 'doing it wrong'. Despite using one routinely, I can't think of an actual example where I would want to have isolation and then tie the ground lead to one side of the output.The three obvious benefits of an isolation transformer are (one-handed) touch safety, the ability to arbitrarily select a point in the circuit to be the ground reference for measurement purposes (within limits), and limiting fault current (a very underappreciated feature). Your proposal cripples or eliminates the first two, which are the primary reasons for having an isolation transformer in the first place. I would encourage you to stick with a three-in three-out arrangement with the ground simply passed through. If you absolutely want a ground-lift switch for certain measurements (ground leakage), that alone wouldn't be a problem.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: March 07, 2022, 07:50:02 pm
content: Quote from: RacerR on March 07, 2022, 03:21:35 pmOne thing to check - transformer output voltages are typically specced at their full load, so especially in the case of smaller ones, you may find that at light loads, the output voltage can be noticeably higher than the input. This caught me out a few years ago - it didn;t occur to me that my circuit was seeing about 270V from 240v mains!

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 08:22:36 pm
content: Quote from: bdunham7 on March 07, 2022, 05:12:04 pmWell - I'm primarily interested in learning/understanding audio circuits.The projects I have in front of me and ready to go now are - I think I have these listed in order of difficulty:1. A bunch of FX pedals (kits, DIY, repairs) for musical instruments.2. JLH stereo transistor stereo power amplifier - looking to scratch build, not kit/PCB.3. A small Class-A tube hi-fi stereo power amplifier build, one of the inexpensive kits available online.4. AX-84 Moonlight (Low power tube guitar amp) build - I got all the parts to put this together literally 20 years ago, and have never had the bench set up to do it.5. Heathkit AG-9A Audio signal generator - it's really old, plan is to restore it to functionality.6. Zenith K731 AM/FM Radio (tube) - no idea if it works or not - would like to restore to functionality.7. Marshall 9000 series rack mount power amplifier repair - right channel is red plating the EL-34s almost immediately after power-on.Not all of these will require isolation - and some of them are too large for the Hammond 169WS, so I'm building this IT mostly as proof of concept. Once I've settled on a grounding scheme, I need to sort out fusing, switching, primary/secondary metering, etc.. along with physical mounting/construction in the enclosure.

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2022, 08:27:04 pm
content: The whole point of an isolation transformer is, that the secondary side is potential-free.Just your use of the terms "Hot" and "Neutral" on the secondary side indicate that you're in over your head here.The secondary needs to float. Period.Any grounding on the secondary side should be done by the DUT (device under test) or your test equipment. Nothing else.That's the only way to make a safe test setup.One good idea is to connect the transformer core and the enclosure to "ground". The correct term is "protective earth" or PE and has this symbol:(the only ground/earth symbol that's internationally standardized, btw.).

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 08:33:57 pm
content: Quote from: RacerR on March 07, 2022, 08:22:36 pmThat is an excellent case for an isolation transformer. I have a similar model that I converted to transformer drive rectified DC operation.

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2022, 08:40:18 pm
content: Quote from: bdunham7 on March 07, 2022, 08:33:57 pmI don't know that particular one, but tube circuits very often have a live chassis.I agree with you.

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 09:21:20 pm
content: Quote from: Benta on March 07, 2022, 08:27:04 pmHello - I'm just using the hot/neutral/ground terminology on the secondary side because the secondary outlets I'll use are standard US AC outlets - which have 3 polarized inputs. (Small slot is hot, large slot is neutral, round hole is ground or PE). In the US, Ground and Neutral are directly connected to each other in the mains panel within the residence.At this time, I'm trying to get to a grounding scheme that will work for my needs, though I'm not against adding additional flexibility. The drawing at the top of the thread shows a ground switch on the secondary that can change between "Tied to Neutral", "Connected to mains ground", and "Floating", and I'm looking for feedback as to whether or not that is viable.Thanks.-Rob

author: bdunham7
author_rank: Super Contributor
timestamp: March 07, 2022, 09:54:57 pm
content: Quote from: RacerR on March 07, 2022, 09:21:20 pmIn theory there's no reason that configuration won't work. It's just a matter of understanding what the consequences of operating in each mode are.First is touch safety. In the floating mode, you should not be shocked if you touch any single point in the DUT circuit (touch safety). In the grounded mode, certain issues or defects in the DUT (such as one of the power legs having leakage or continuity to ground) will defeat touch safety and you might be electrocuted by touching a single point within the DUT, but not ground. Note that every 'live chassis' device I've ever seen is two-prong--there is no ground--so you are automatically in floating mode if you use an isolation transformer. A device like my BK 1655 allows me to measure ground leakage directly, which is helpful in these cases. In the tied-to-neutral configuration we are trying to talk you out of implementing, touch safety is entirely defeated and your wall or panel GFCI is defeated as well.Second is selecting a ground point for measurements. In a live chassis DUT connected to normal mains, for example, if you connect your oscilloscope ground to the chassis you will have a fire or explosion and something will be damaged. If you use floating mode, you can connect that scope ground to any point in the circuit (within reason) and your probe will measure relative to that point. If you use the grounded mode, it depends on your DUT, but unless the circuit you are testing is completely isolated from ground internally, you won't be able to connect a scope ground to anywhere except things already at ground potential. If you use the tied-to-neutral mode, you might still be able to connect the scope ground without a fire, but you now may have the ground wire--and possibly external metal surfaces--at a hazardous potential. That is bad.I don't know what it costs you to build your own, but you might be better off with one of these:https://www.ebay.com/itm/255417056774?hash=item3b780afe06:g:0QsAAOSwYU1iIXWL

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2022, 10:40:48 pm
content: Quote from: RacerR on March 07, 2022, 09:21:20 pmEverything is viable. But your scheme defeats the purpose of an isolation transformer. Save your money, get the electric shock you didn't expect, and burn off your 'scope at the same time. My opinion is clear in post #12.This issue has been thrashed to death, and Dave has super videos on the theme.

author: nctnico
author_rank: Super Contributor
timestamp: March 07, 2022, 11:05:06 pm
content: Quote from: Benta on March 07, 2022, 08:27:04 pmNo. The DUT shouldn't become grounded at all because that will result in a dangerous situation. So even with an isolation transformer, you still need to use isolated test equipment (like a differential probe). Otherwise you defeat the purpose of the isolation transformer. The biggest pitfall with an isolation transformer is that it is very easy to ground the DUT again through test equipment. And no, floating test equipment is not a good idea!

author: RacerR
author_rank: Contributor
timestamp: March 07, 2022, 11:41:16 pm
content: Quote from: bdunham7 on March 07, 2022, 09:54:57 pmThanks - I'm trying to wrap my head around this stuff.Quote from: bdunham7 on March 07, 2022, 09:54:57 pm- This part I understand, at least as far as it applies to tube circuits with two prong plugs and a potentially hot chassis.Quote from: bdunham7 on March 07, 2022, 09:54:57 pm- This makes sense to me too, as it applies to a DUT with a 3 prong plug. If I understand correctly, this mode should not change the behavior of a DUT with a 2 prong Plug. I don't have a use case in mind for this configuration.Quote from: bdunham7 on March 07, 2022, 09:54:57 pm- This is where I'm uncertain, and it's probably because I don't fully understand things.Take this example (attached image). This schematic shows a simplified outline of the power supply transformer on the Marshall 9005 power amp I have.- This DUT has a 3 prong power connection.- The AC mains ground connection goes to the device chassis.- The AC mains hot and neutral connections go to the primary side of the DUT power transformer.I do not know if my line of thinking is correct here, but this is the use case I had in mind for the "isolation transformer secondary ground tied to isolation transformer secondary neutral" mode of operation. - - Would a floating ground work better here, or does the circuit design rely on some connection between Neutral and Ground in the mains panel?I also updated the schematic in the original post with labels to clarify which side is primary and which is secondary.Thanks - any insight you folks can share is appreciated.-Rob

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2022, 03:30:02 am
content: Quote from: nctnico on March 07, 2022, 11:05:06 pmOnly if you assume the sole purpose of the isolation transformer is touch safety. If you want to do a measurement with a ground-referenced instrument, you just need to be aware of all that and keep your fingers away. Even if you are primarily concerned about touch safety, the biggest hazard that isolation transformers were designed to mitigate were the live-chassis designs like the OPs Zenith radio. In those cases, the typical way to handle them was to use an isolation transformer and then deliberately ground the chassis. This eliminates the large danger of the live chassis and still permits measurements and testing of almost anything you need to do. The freedom of poking your fingers just anywhere in a live circuit doesn't seem to me to be a very wise goal.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2022, 03:39:16 am
content: Quote from: RacerR on March 07, 2022, 11:41:16 pmWhat you have there is a fairly common situation--a device that has its own isolation. Unless you are working on the primary side of the transformer, everything else is isolated from the mains by that transformer. However, that doesn't mean that there aren't hazards, even single touch electrical potentials. In this case fully floating, isolated with ground fed through or just plugged into mains would all be fine. If I were working on it, I would test it for ground leakage and then plug it in normally to the isolation transformer w/o the cheater plug.Devices aren't supposed to allow any current on the ground lead other than EMI reduction (Y-capacitors) or very small leakage currents from static drain resistors. 500uA max, IIRC, or possibly less these days. I've not run into anything that needed its ground to be referenced to neutral in order to work properly, other than perhaps certain devices related to actual mains wiring, like circuit testers and GFCIs.

author: m k
author_rank: Super Contributor
timestamp: March 08, 2022, 09:54:15 am
content: Quote from: RacerR on March 07, 2022, 11:41:16 pmWhat is a floating ground, is it Earth floating in the space?It doesn't mean that GND is somehow related to soil of Earth just because it uses same letters.I presume you know the name Protective Earth.Like the name says, it's a protectivity issue, not an electricity issue.Your first problem is that you know that one thing you clearly shouldn't, that these N and PE are connected somewhere.Second problem is that you are fixated to name ground.Draw a measurement situation of grounded meter to your last picture.

author: RacerR
author_rank: Contributor
timestamp: March 08, 2022, 11:45:49 am
content: The drawing in the original post has been updated for clarity, and it now shows a 2-way toggle for the secondary side ground lift.

author: nctnico
author_rank: Super Contributor
timestamp: March 08, 2022, 01:06:08 pm
content: Quote from: bdunham7 on March 08, 2022, 03:30:02 amThat is true but you can also make sure to connect the chassis to neutral (in case that is equal to ground). That way you don't defeat GFI protection. Either way working on high voltage circuits is something that needs to be done with care and thought. A Darwin award is easy to obtain.

author: bdunham7
author_rank: Super Contributor
timestamp: March 08, 2022, 02:07:45 pm
content: Quote from: nctnico on March 08, 2022, 01:06:08 pmThat often won't work as planned as the chassis may not simply be connected directly to one prong of the cord. In some designs, the chassis is connected to the negative of a bridge rectifier or full-wave doubler, resulting in there being a complex but exciting voltage present no matter which way the plug is inserted. In others, like the OP's Zenith radio, the chassis is connected to one prong, but through the power switch. The result is that if you insert the plug so that the chassis is at neutral during operation, it will still become live (through the filaments and other circuitry) when you turn the unit off.

author: m k
author_rank: Super Contributor
timestamp: March 08, 2022, 03:14:52 pm
content: Quote from: RacerR on March 08, 2022, 11:45:49 amWhy there is a switch, why you want to lose the protection?

author: IanJ
author_rank: Unknown
timestamp: March 08, 2022, 07:51:30 pm
content: Here's my input.I think it's good practice to call the two conductors on the primary side LIVE & NEUTRAL for obvious reasons, but on the secondary side of an isolation transformer I think it's best to call them L1 & L2. Thats what is done in the offshore industry anyways.I use my own isolation transformer all the time.I do have the option to connect over the earth connection to the secondary side 'earth' connections of the output power sockets, but I don't use a switch....too easy to have it in the wrong position.....rather I have two banana sockets where I need to physically add a short jumper to achieve it.My isolation transformer is originally a medical unit which I modified. It had the incoming earth connected right through to the outputs permanently.I also added a fuse to the output L1 which it didn't have originally.It also has 240VAC/110VAC output voltage selection.....which is handy.Extra precaution as you'll see in the pics is that the UK output socket that you can see plugged in is a metal case (usual for UK), but it's insulated with a 3D printed cover. Why?.....imagine sitting the metal socket on top of a metal cased bench test instrument.....instant connection of incoming to outgoing earth.

author: m k
author_rank: Super Contributor
timestamp: March 09, 2022, 12:12:29 pm
content: Explain situation or two where Protective Earth is dangerous.Counter example.Faulty device has L1 live case and other case has L2.I didn't understand metal case (usual for UK) part.

author: IanJ
author_rank: Unknown
timestamp: March 09, 2022, 12:28:12 pm
content: Quote from: m k on March 09, 2022, 12:12:29 pmHi,If you are referring to my post, I never said anything was dangerous. To me it's just about having options, to know exactly what's going on at all times and being safe to ourselves and safe to our bench equipment.Regarding the metal case on my flying dual socket:-So, the flying metal case is ultimately connected to the output earth banana, and lets say I am using the isolation transformerwithoutthe earth jumper (bananas) in place, but then sit the flying metal case on top of a different instrument (not the one I am testing) in my workshop that has it's own case that is earthed, then the output earth would be connected to the incoming earth.....as if I had the banana jumper in place. So, to avoid this I 3D printed a cover for it. I want to be sure that what I set up is what I get at all times.Ian.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: March 09, 2022, 02:55:42 pm
content: just to add my 2 cents worth from the lands down under...note: we use the term ACTIVE for 'hot' here (both in NZ and across the ditch in Australia), but i will use the term HOT so as to not confuse things further. we also have NEUTRAL connected to GROUND via a single linkat the distribution board.1. a cardinal rule of mains wiring (here in NZ) is that swapping HOT and NEUTRAL shouldnevercreate an unsafe situation. ifanypiece of equipment were to contain an internal connection between EARTH and NEUTRAL, that would violate this rule if said equipment were plugged into an outlet with HOT and NEUTRAL reversed.2. with the one exception of double-insulated equipment,allexposed metal partsmustbe connected to EARTH. this ensures that you cannotreceive an electric shock via contacting said metal parts.the above are enshrined in law.applying the above two rules, (a) your isolating transformer should have the metal core along with any metal casing components connected to EARTH. and (b) the incoming HOT and NEUTRALmustbe kept isolated from EARTH. if these conditions are not met, the equipment has the potential to create a hazard, as well as being illegal.as for thesecondary, well the terms L1 and L2 are an apt choice here, indicating the secondary 'knows'nothingabout HOT and NEUTRAL. L1 and L2 are not a part of the mains supply system, however do need to be treated with care as a hazardous voltage exists between them.EARTH, while advisable to be connection to the DUT, is optional as your isolating transformer creates a safety barrier. while current can flow between L1 and L2, it cannotflow from the DUT down the EARTH wire.having said this, there is no sensible reason to connect either L1 or L2 to EARTH that i can think of, as there is simply no path for the secondary current to flow through apart from between L1 and L2. as others have said, with L1 and L2 kept floating, you are free to work on 'live chassis' equipment with that chassis connected to the same EARTH as your oscilloscope or other expensive/fragile equipment. said 'fragile equipment' includes the service-person!cheers,rob :-)

author: RacerR
author_rank: Contributor
timestamp: March 09, 2022, 03:20:26 pm
content: Quote from: IanJ on March 08, 2022, 07:51:30 pmI've updated the labels on the drawing in the original post.Quote from: IanJ on March 08, 2022, 07:51:30 pmI like this idea - I was thinking about using a switch with a "warning" lamp, the jumper is simpler.

author: m k
author_rank: Super Contributor
timestamp: March 11, 2022, 12:22:05 pm
content: You're still live and kicking, good.More meaningful would be that fuse IanJ have added.Bad area for your gear is between out from isolator and any means to limit power.My room is, I guess, fused by a single 10A and I can't always turn on my variac.But all smaller stuff are still cooked without a notice.For test purposes the isolation transformer is allowed to have a single output only.Medical area is different, there generally everything must be isolated.

author: RacerR
author_rank: Contributor
timestamp: March 11, 2022, 01:16:01 pm
content: Quote from: m k on March 11, 2022, 12:22:05 pmThanks - yeah, I still need to decide on fusing, power and standby switching, indicator lights, and the actual device layout. I'm using a 250VA transformer with a secondary current rating of 1.7A, and I think a 2.5A fast blow fuse is typically used on the secondary side - though maybe I could get away with something smaller.-Rob

author: TheBay
author_rank: Unknown
timestamp: March 11, 2022, 03:54:21 pm
content: Earthing is often a topic on this and a lot of confusion, it totally depends on where you are in the world, what rules apply, use cases and what type of mains sockets/plugs are used too.I'm in the UK and on a PME system (TN-C-S) so the Neutral and Earth are combined and then split at my property.I have an isolation transformer in the workshop, I only use it when I need to use it.The mains earth is connected to the metal chassis of the isolation transformer as it is a Class 1 appliance, the mains earth is also connected to the socket that is on the secondary side of the transformer. In the UK this is how they are sold. Neutral and Earth are never connected together at the appliance.I have no reason to remove the mains earth connection to the secondary. It is beneficial for me to leave that connected for various reasons, such as filtering etc.There is no risk of additional danger by having that connected here, the only condition that could arrise which could cause an issue is having an appliance with the plug wired wrong such as a N/E or L/E reversal.As I am fully aware of this scenario it is extremely unlikely this would ever happen here. Firstly the UK BS 1363 plug is polarised and cannot be put in to the socket on the transformer incorrectly. If I encounter something that does not have a moulded plug I always pop the lid off and check that the wiring is okay, if it's a moulded plug I check the pins back to the internal wiring of the appliance. Every IEC C13, IEC C5 lead that enters here is checked, regardless of where it came from or even if it came with a product brand new, that is something I have always done regardless of using an isolation transformer or not.Lastly if I personally do need to use the isolation transformer all the covers will be removed on the DUT and everything throroughly checked before hand. I cannot think of a scenario here where I would want to use an isolation transformer without doing so.Isolation transformers can have their place and should never ever be taken for granted, they can give a false sense of security and should only be used when absolutely necessary. They are quite often purchased and used for the wrong reasons where other test equipment would be more suitable, such as differential probes etc or methods for testing and correct safety precautions.Regardless of whether you have a mains earth passed through to the DUT or not, there are many situations that they can become dangerous. And the RCD/GCFI on the primary side will not save you, it is completely oblivious to anything connected to the secondary side.

author: m k
author_rank: Super Contributor
timestamp: March 12, 2022, 04:56:46 pm
content: Quote from: RacerR on March 11, 2022, 01:16:01 pmFuse here is a bit different.Normally it's preventing fire hazards, like it still is in the primary side and in DUT, but when wall side can give practically what is wanted this secondary can't give more than it can, you can think it as a power plant of its own.So secondary fuse is like power company protecting their distribution network.For transformer the main problem is heat, and literally melting insulation, so it's not very picky.More picky it can be if you're switching it off and on when it's already hot.Transformer can also heat up quite a bit and still be totally happy.Class A, first insulation design temp is 105C and if core is 30C below it's still pretty warm.If you use things that are overloading your gear much then the fuse must stand that.Means that voltage goes down and current goes up, for a moment, and fuse must be happy all the time.So slow fuse is possibly better.For other nuances I'd say that no switches, no lights, just a plug and a jack.http://www.smartgauge.co.uk/iso_wire.htmlhttps://www.eevblog.com/forum/beginners/isolation-transformer-102532/

author: bdunham7
author_rank: Super Contributor
timestamp: March 12, 2022, 05:14:10 pm
content: Quote from: RacerR on March 11, 2022, 01:16:01 pmYou typically don't need a fast fuse for a transformer. A slow-blow like the MDL 2A would probably work best.

author: TaylorD93
author_rank: Regular Contributor
timestamp: March 12, 2022, 05:24:45 pm
content: I think it is always a tricky one with Isolation transformers.The Incoming Earth should ideally be connected, and it is in my unit, but It does complicate matters sometimes. Having a few Analog CRT Scopes im in the middle of repairing, the Chassis (Earth) is also the 0V of the Power supplies etc. So probing any of the HV circuits referenced to Earth can be difficult as most probes arent rated for the high voltages.Even some Differential probes arent quite man enough.My "work around" as my Isolation transformers Earth connection is hardwired, is to have a separate mains lead with Red heatshrink sleeving along its length to clearly identify it. That way its much clearer than usual.

author: dannybeckett
author_rank: Contributor
timestamp: March 12, 2022, 05:37:37 pm
content: How bizzare... I've just finished building myself an isolation transformer too!Learned a lot about inrush current and remanent magnetization. Earth will be passed thru to the socket but not conected to the iso transformer(s) in any way.Check the thread for simulation and schematics of inrush limiter:https://www.eevblog.com/forum/chat/1000-va-230-vac-50hz-how-much-inrush-current/