Double insulated water level sensor - Page 1
Forum section: electronics / projects

author: b_force
author_rank: Super Contributor
timestamp: July 04, 2023, 11:38:01 am
content: So for a project with a water boiler, I am looking for a water level detecting system.Quickly a very small summary how these water level detectors work:How standard (simple) water level detectors works, is that they have two (or three) probes.Some (high frequency) signal is put on one probe, while the other one works as a "detector" probe.Since water conducts, this will give a small (AC) current on the other probe.(the reason an AC signal is being used has to do with electrolysis)In this case the second probe is the metal of the boiler itself. (that's what the manufacturer decided to do)One of the requirements is to double insulate the entire system.I am having trouble to come up with something that will meet the Class II double insulation standards.Actually, I am even in doubt if this is possible when the metal of the boiler itself is the second probe but has to be earth grounded as well for safety.I am not looking for a circuit or ideas for the level detecting itself (unless it completely solves this double insulation issue)The best way I can come up with, is to use an additional isolated power supply, either from the power supply or an isolated DC/DC converter and connected the detection circuit with a opto to the main PCB.On paper I think this is the most elegant one, but requires quite some redesign in the power supply.Also isolated DC/DC converters add quite some cost on the BOM + the availability is pretty poor atm.Another idea I found is just using some Y caps between the detector as well as a Y cap between functional ground and safety earth ground (= metal chassis of boiler). Which I guess could be something on paper, but in practice this solution feels pretty dirty....I have been looking around what the usual approach is, but I am having trouble finding anything more professional that your standard hobbyist circuit. Where they just connect functional ground to safety earth ground, meaning it's not double insulated anymore.Any ideas are welcome, including the idea of changing the current probe system all together.It has to be ideally compliant with EN-IEC 60335-1 , otherwise additional testing is needed.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2023, 02:26:25 pm
content: Quote from: b_force on July 04, 2023, 11:38:01 amNo longer double isolated then, the case is then undeniably part of the device and exposed.If you just use an non isolated power supply inside the device (ie. rectify + buck) and put a microamp level AC current into the insulated probe, it's not going to trigger any ground fault interruption if you let it go through the earth connection of the external case for return. It's not pretty, but you can still have "reinforced" non conductive insulation and it will work just fine, somewhere in the distance earth is connected to neutral.Only question is if EN-IEC 60335-1 allows intentional earth leakage current, you can make it small enough that there are tons of compliant devices which unintentionally have more, but intent can matter.

author: b_force
author_rank: Super Contributor
timestamp: July 04, 2023, 05:42:34 pm
content: Quote from: Marco on July 04, 2023, 02:26:25 pmWell I am a bit confused about this part.Section 3.3.6 from 60335-1 says that it's allowed to use a protective impedance as a barrier.See attached documents.This would suggest that using some (two) Y caps would be sufficient, since those are considered protective impedances and with two it complies with being double insulated.QuoteThe issue with this solution, is that you need some kind of reference.And functional ground and safety earth ground aren't connected together because of the double insulation.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2023, 05:48:04 pm
content: With a non insulated power supply one of your supply rails is neutral as far as AC is concerned, also connected to earth, in the distance.

author: donmr
author_rank: Regular Contributor
timestamp: July 04, 2023, 06:09:01 pm
content: Or use a different type of sensor. Ultrasonic sensors don't need to be electrically connected to anything external like the tank (except the sensing circuit of course).

author: b_force
author_rank: Super Contributor
timestamp: July 04, 2023, 06:59:52 pm
content: Quote from: Marco on July 04, 2023, 05:48:04 pmThe power supply also has to be 60335 compliant, so I am not completely following you?Earth en reference ground are not connected when things are double insulated.So this is a fully isolated PSU.

author: b_force
author_rank: Super Contributor
timestamp: July 04, 2023, 07:02:08 pm
content: Quote from: donmr on July 04, 2023, 06:09:01 pmYeah, I have seen those.Unfortunately there is no direct control over the boiler design.So it is what it is for now.That being said, if it's absolutely not possible to be 60335 compliant, than this boiler can't be used obviously.Makes you wonder though, how other people do this.But I can't find much online expect for some hobbyist schematics.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 04, 2023, 08:24:06 pm
content: I would think that using a double insulated power supply and having the measuring circuit common connected to the boiler by a capacitor would be sufficient. I'm not even clear it needs to be a class-Y capacitor since the power supply provides the line/neutral isolation. Certainly a lot of devices powered by double insulated power supplies and stamped with ratings saying that are used in a way that provides an external connection to earth ground such as a laptop with USB ports, but the ground connection is not part of meeting the safety requirements.

author: floobydust
author_rank: Super Contributor
timestamp: July 04, 2023, 10:13:26 pm
content: Product safety requirements are dictated by the engineer, not the client. I think you're off on a tangent.How do you think something in contact with the boiler's plumbing and water constitutes a shock hazard? In reality, the boiler's plumbing natural gas, water is earth-grounded, certainly at the pump(s).The source of power is the 24VAC control transformer (Class II already energy-limited, double-isolated) I guess in the boiler controls. You double-insulate to mains or other sources of hazardous live.There are dozens of commercial low water cutoff (LWCO) controls for boilers, some 24VAC others mains powered. None are double-isolated.Mains versions have small power transformer and an output relay to give isolation, so they could cause a shock hazard if they bridged to the 24VAC side, i.e. thermostat going hazardous live.There are also a separate set of safety standards for limit controls due to the boiler safety aspect. It depends on the size of the boiler, BTU rating.The low water, high high temp etc. can have a functional safety requirement as well. UL 353, ASME CSD-1 are older ones.

author: b_force
author_rank: Super Contributor
timestamp: July 05, 2023, 11:50:35 am
content: Quote from: floobydust on July 04, 2023, 10:13:26 pmIn this case the product needs to be compliant to 60335 and will be eventually tested and certified that way. What other companies do is out of the question.Most professional solutions I know and can find also have a complete floating detection circuit because of this.

author: b_force
author_rank: Super Contributor
timestamp: July 05, 2023, 11:53:33 am
content: Quote from: ejeffrey on July 04, 2023, 08:24:06 pmThis could only work when safety earth ground and functional ground will be connected together.Which will violate the 60335 norm that this product has to be compliant with.

author: beanflying
author_rank: Super Contributor
timestamp: July 05, 2023, 12:46:53 pm
content: The other used in Industry which you may or may not have run across is capacitive probes rather than a plain probe (or re inventing your own wheel). Back when I had a real job we specced these for specific chemical and process control systems. I am 15 years out of that industry so likely a lot of different options out there.Worth a look if not for your but for others info.Just one source to get you startedhttps://au.omega.com/pptst/LV3000_LV4000.html

author: ejeffrey
author_rank: Super Contributor
timestamp: July 05, 2023, 05:10:27 pm
content: Quote from: b_force on July 05, 2023, 11:53:33 amWhat I am suggesting is that the circuit ground is capacitively connects to the boiler frame which is connected to earth ground. No connection is made (directly) to the safety ground conductor of the building wiring. And as long as the power supply is isolated the ground leakage current will still be limited even in the case of a fault since you can't form a ground loop through the building wiring, and using a capacitor for the chassis connection will limit the signal current to below the legal thresholds as well.As pointed out above, part of this depends on whether the standards treat an intentional connection differently from "unintentional" leakage through EMI capacitors. I don't know the answer to that.I've never actually certified a product for 60335 so you should definitely find someone who has done so, ideally on a similar product. But definitely I've seen plenty of products listed as 60335 or similar that are used in a way where they are actually grounded.