Any reason for 3 kV vs 1 kV isolation with no voltage higher than AC supply? - Page 1
Forum section: electronics / projects

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 06:13:07 pm
content: Hi,I am working on a circuit with an isolated RS-485 transceiver for home automation, and I have to decide if the trades off in terms of efficiency, cost and components availability are worth it if I want to stick to 3 kV vs 1 kV isolation. My circuit also has ESD protection and I don't see an application where it would be in contact with anything that goes above 240 V AC. Unless I want to start protecting the circuit against a lightning strike or maybe the solar storm of the century, I assume there is no advantage of 3 kV isolation beyond what is provided by 1 kV isolation? 3 kV isolation would be more for a heavy industrial or outdoor application?

author: floobydust
author_rank: Super Contributor
timestamp: February 15, 2023, 06:33:03 pm
content: Your project circuit has hazardous live on one side? What is the danger if the isolation fails?Mains has transients and spikes that routinely go above the 240VAC you speak of, easily past 1kV. So electrical safety standards require and test to much more.I understand your need to keep it cheap but please first keep it safe. Also for long life you would not run insulation near its max. rating.There are many isolators and DC-DC converters rated for 3kV isolation and more.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 06:57:08 pm
content: Quote from: floobydust on February 15, 2023, 06:33:03 pmIsolation for my application is basically to protect the device connected to the non-isolated part of the RS-485 transceiver, my statement was not based on safety concerns. The transceiver will not touch mains unless there is something really wrong with the wiring of the house (e.g. another connected RS-485 device whose neutral is in fact live, in which case there is much more to worry about than my device). Otherwise the deviation of the ground level between the isolated side of my device and the non-isolated side will be due to a floating neutral or due to inductance effects.There are lots of DC-DC converters rated for 3kV, but typically the 1W 5V 200 mA ones have significantly lower efficiency and their availability is not great. I have 2W to supply my whole circuit, which the transceiver is only a part of.

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2023, 08:25:53 pm
content: Quote from: kreyszig on February 15, 2023, 06:13:07 pmThe 240V specification is completely irrelevant to your question.What matters is the peak voltage due to noise/lightning/whatever, and related parameters.In addition, on would do well to define what is included/excluded in "efficiency".

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 08:39:41 pm
content: Quote from: tggzzz on February 15, 2023, 08:25:53 pmYes I agree, it is at least mostly irrelevant, unless we are dealing with house wiring issues.Yes what matters more is a voltage induced by other factors, mostly noise (induction) and ESD. ESD can be taken care of by TVS diodes, so the isolation rating of the transformer does not really matter for this. For noise, I fail to see a residential situation where more than a few volts would be generated in most cases. For kilometer long lines within an industrial setup the noise would be higher. For lightning strikes not much is generally done (the equipment I am attaching the device to would probably fry regardless on its own), and 3 kV vs 1 kV would probably not make much of a difference I assume.I should also mention that one of the reasons for isolating my transceiver is to prevent ground loops between the devices attached to the RS-485 lines. My understanding is that the best setup would be to have the RS-485 line ground attached to the house neutral at a single point, through the usage of isolated transceivers (either all transceivers isolated plus a separate connection to neutral, or through the usage of all but one isolated transceivers, with the non-isolated one having a non-floating ground).By efficiency I meant the rated power output vs input efficiency at max load. Many of these regulated and isolated 200 mA DC DC switching converters are about 70% efficient at max load. Better ones are about 78% efficient and are more likely to use 1 kV vs 3 kV isolation transformers due to lower leakage. I have found a single one at a non-outrageous price for what it is, MTC1S1205MC (https://www.murata.com/products/productdata/8807030358046/kdc-mtc1.pdf) with 78% efficiency and 3 kV isolation, but availability is poor. This aspect of my project is addressed inthis thread.

author: Someone
author_rank: Super Contributor
timestamp: February 15, 2023, 09:20:02 pm
content: This has fractured:https://www.eevblog.com/forum/projects/efficient-regulated-5v-and-isolated-reg-5v-supply-from-unreg-2w-12-7-v/https://www.eevblog.com/forum/projects/minimum-load-circuit-for-switching-power-supply/

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 09:29:55 pm
content: Quote from: Someone on February 15, 2023, 09:20:02 pmYes these are related to the same project, but they are on different topics (isolated + non-isolated output from same switching converter, circuit providing a minimum load for a supply, respectively). This thread deals with the choice of isolation level. Trying to keep it as shorter threads with well-defined topics that could be more useful to others instead of a long project's thread that strays into all kind of tangents and that is difficult to follow without reading the whole thing

author: tszaboo
author_rank: Super Contributor
timestamp: February 15, 2023, 10:32:15 pm
content: IEC 61000-4-4 pulse train immunity tests compliance for example. As I recall it can be up to 2KV.

author: tggzzz
author_rank: Super Contributor
timestamp: February 15, 2023, 10:47:04 pm
content: Quote from: kreyszig on February 15, 2023, 08:39:41 pmA lot of good practices are there to protect against gross "should never happen" faults. Such faults are, of course, rare and it is unlikely that competent engineers would be able to think of them all - and could infer them from reading the regs.To invent a few, what might happen if a substation transformer spontaneously cremated or disassembled itself? Or if there are two phases in the house? Or if one neutral connection starts to have a higher than normal resistance joint? Or if there's a loose connection in an overhead line that chatters when the wind is high?QuoteI wondered if "efficiency" might include "producing a one-off predictably reliable device in the shortest time"

author: kreyszig
author_rank: Regular Contributor
timestamp: February 15, 2023, 11:04:41 pm
content: Quote from: tggzzz on February 15, 2023, 10:47:04 pmThis device is intended to be connected with other devices that are all contained in the same house. The higher resistance for one neutral one applies as a concern, but this would contribute to a voltage increase that is a small fraction of the mains, so still a few volts, which would be well contained by the 1 kV isolation.Quote from: tggzzz on February 15, 2023, 10:47:04 pmFor my application the efficiency factor is mostly motivated by the limited 2W supply I have for the whole board.For context, most RS-485 transceivers that are commonly available for residential use don't even have isolation, a common ground or even built-in selectable termination it seems. I am intending to include all these features for mine.

author: DavidKo
author_rank: Frequent Contributor
timestamp: February 16, 2023, 09:03:49 am
content: Quote from: kreyszig on February 15, 2023, 11:04:41 pmIsolated and non isolated versions are commonly available on aliexpress. Non-isolated version is cheaper, but that is not a question of availability.

author: fchk
author_rank: Frequent Contributor
timestamp: February 16, 2023, 11:54:50 am
content: Quote from: kreyszig on February 15, 2023, 06:13:07 pmPlease check your local regulations. This is a good introduction:https://toshiba.semicon-storage.com/info/application_note_en_20210315_AKX00765.pdf?did=61477fchk

author: tggzzz
author_rank: Super Contributor
timestamp: February 16, 2023, 02:39:26 pm
content: Just so.He doesn't have to convince anybody on this forum.He does have to convince the electromagnetic fields and wave functions, and possibly insurance companies and legal authorities.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 16, 2023, 02:52:52 pm
content: Functional isolation to break ground loops: 1kV isolation is usually enough, at least within a house.Isolation from mains live nets: 3kV obviously, but that rating alone is not enough; see compliance testing (UL numbers and so on) done by the manufacturer and verify they are compatible with the standards you are required to follow. Highly recommended to hire a consultant.

author: peter-h
author_rank: Super Contributor
timestamp: February 16, 2023, 04:27:34 pm
content: The answer depends on whether this product will be sold, and if so what will the spec be.For 220/240V AC isolation you generally need to use 4500V AC RMS test voltage isolators, plus there are specs for creepage distance, etc. I don't think any 2500V AC RMS parts are legal for 220/240V isolation.In the medical and industrial market there are additional complications e.g. "reinforced isolation" for 3 phase mains (~400V AC) can require 12kV isolators; I know this because I build custom products to these specs. But my customers take care of which safety specs they quote; I don't get involved. In industry in particular there are many standards you can choose from.Obviously a 2500 AC RMS isolated device is OK for 220/240V mains but you cannot sell it.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 04:35:17 pm
content: Thanks. So 1kV is definitely good enough then. Again I am not designing a circuit meant to be wired to live mains. I just mentioned mains in the sense that what would happened if someone ended up wiring it to live mains instead of neutral through human error. It would only get in contact with mains if someone wired the RS-485 lines to live mains voltage or the house was wired wrong (neutral and live swapped). It looks like I was a bit too safety cautious in my initial statement and it caused more confusion than anything else.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 04:45:05 pm
content: Quote from: fchk on February 16, 2023, 11:54:50 amThanks for the link. It looks like it does not apply for this particular application, but it does for another project I am working on.

author: tggzzz
author_rank: Super Contributor
timestamp: February 16, 2023, 05:13:52 pm
content: Quote from: kreyszig on February 16, 2023, 04:35:17 pmI've seen "entertaining"/WTF wiring in houses, especially as some bits were added over the decades and some bits slowly deteriorated over the decades.What are you going to say to the insurance company if there's a fire and they see "non-compliant" electronics? If you aren't already aware of it, insurance assessors will look for anything that might enable them to avoid a payout!Example: my car was rear-ended while stationary in a traffic queue. The first thing the adjustor did was to look at my tyres, even though they had obviouslyzero relevanceto the crash. If theyhadbeen faulty, theywouldhave denied a payout on the grounds that my car was on the road illegally.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 16, 2023, 05:27:53 pm
content: Quote from: tggzzz on February 16, 2023, 05:13:52 pmWe are talking about a fuse-protected 12V DC 1W device here, powered through a compliant supply. There is a limit to design against stupidity. Anyone can stick a fork in an outlet or stick a pen in their eye. Something as simple as a USB cable is not designed to protect the user in case the outlet powering the device it is connected to has swapped live/neutral or even worse if a live wire is connected to ground. I don't understand where you are going with this.

author: tggzzz
author_rank: Super Contributor
timestamp: February 16, 2023, 07:52:32 pm
content: Quote from: kreyszig on February 16, 2023, 05:27:53 pmI have no idea of your design; you are the designer and it is up to you to understand all the failure modes. All I can do is nudge you to consider the possibilities.One failure mode might be that some external event destroys (subtly or overtly) part of your device so that it no longer operates as designed. One possibility is that is catches fire, or sends signals that cause something else to catch fire. I expect that, if I was responsible for your design I could come up with a failure mode that is more specific and/or more likely.

author: Someone
author_rank: Super Contributor
timestamp: February 16, 2023, 11:36:41 pm
content: Seems like alotof effort being spent to avoid describing the concept/design/overall picture.

author: kreyszig
author_rank: Regular Contributor
timestamp: February 17, 2023, 01:57:29 am
content: Quote from: Someone on February 16, 2023, 11:36:41 pmOMG stop trolling and get a life. This thread can be closed.