Class II (Double Insulated) Power Supply with Output Tied to Earth: FE or PE?

[lauxt]

Hi everyone,

I'm completely confused on this one. I've been thinking about this for a long time (a year or two) with no clear answer.

Our company designs medical switch-mode power supplies. Everything we build is fundamentally Class II (reinforced/double insulated) construction. The transformers, creepage/clearance, etc. all are met with just two AC input conductors, no earth needed. These days, true Class I switch-mode power supplies are becoming increasingly more rare with widespread use of triple insulated wire (TIW).

There was essentially the same question raised here: Separation requirement for Class II equipment with functional earth

..but with no clear resolution (for me).

In our case, the earth connection to the power supply is made via the 3rd pin of an IEC inlet. The standard seems to imply that when using the AC power supply cord to supply earth to the system, that the earth connection cannot be made to an accessible part, and that a connection to an AP somehow invalidates the FE designation. For instance, if we insert a ferrite core in the earth lead (internal the power supply) to improve EMI, the wire may fail during a 40A current test since there's a lot of wrapped around the ferrite core, causing greater temperature rise than just a length of wire alone.

In this case, it seems logical to designate the power supply as Class II with FE. And from my view, the 40A earth bond test does not even make sense for a Class II power supply in the first place. After all, it doesn't need this earth conductor to provide two means of protection.The power supply is fundamentally class II construction, and according to Subclause 8.6.9, "The allowance does not degrade the degree of protection against electric shock"

1. How do you explain the below lines that say a FE connection made by power supply cord cannot be connected to AP (i.e. the power supply output)?

Subclause 3.35 – FUNCTIONAL EARTH TERMINAL
In ME EQUIPMENT, functional earth connections can be made by means of a FUNCTIONAL EARTH TERMINAL that is accessible to the OPERATOR. Alternatively this standard also allows a functional earth connection for CLASS II ME EQUIPMENT via a green and yellow conductor in a POWER SUPPLY CORD. In this case the parts to which this conductor is connected cannot be ACCESSIBLE PARTS (see 8.6.9), and have to be insulated from ACCESSIBLE PARTS.

2. How do you explain the lines on functional earthing of "internal screens"? These lines seems to imply that if the functional earth (FE) is not separated by at least two means of protection (from an AP) that it becomes a protective earth (PE)...how?

The current flowing through the FUNCTIONAL EARTH CONDUCTOR has to be limited in the same way as current in the PROTECTIVE EARTH CONDUCTOR. To prevent the FUNCTIONAL EARTH CONDUCTOR from becoming a PROTECTIVE EARTH CONDUCTOR, the insulation between internal screens including internal wiring connected to them and ACCESSIBLE PARTS needs to provide two MEANS OF PROTECTION.

3. If you earth the output common of a (fundamentally) Class II ME power adapter, it Class I or Class II?There are many instances that a Class II power adapter can be incidentally become earthed. I don't see why it would need to meet Class I earth bond requirements when it fundamentally does not need to reply on earth for means of protection.

Thanks very much. I hope someone can give some feedback. I'm going crazy!

Tim

 

[Peter Selvey]

I've looked into this a few years a ago and it is definitely a head spinning subject.

I believe the original idea for a "functional earth" was a terminal on the outside of the equipment, accessible to the operator, for purpose of helping with noise reduction in sensitive equipment. Obviously this should be double insulated from mains parts, since it is not guaranteed to be earthed at all, let alone being able to reliably carry fault currents.

Next, it was considered that some manufacturers may want to have an "isolated internal screen", which is undefined but assumed to surround the mains parts and limit noise reaching the secondary circuit. This screen was allowed to be connected to the earth terminal in the appliance inlet or power cord. Manufacturers now routinely use this inlet/cord based functional earthing in Class II equipment to provide better EMC performance, i.e. not only for screening but bypass caps, ferrites etc as would be normal for a Class I system. This often has nothing to do with sensitive medical equipment, just run of the mill medical devices. More on this later.

In the second edition, it stated that such a screen should be double insulated, but it never said to what. Mindless test engineers (i.e. me) blindly assumed this meant double insulation from mains parts. This lack of clarity was also copied to Ed 3.0, and it was only in Ed 3.1 that it was finally made clear that the 2MOP was between the FE part and accessible parts.

After some head spinning, the penny finally dropped that the standard assumes that such an isolated screen only has 1 MOP to mains parts. For a part to be "protectively earthed", the earthing needs to have a protective function. This criteria is often forgotten, and test engineers routinely test anything that's earthed with the 25A test, leading to burn out of AWG32 cables that are just provide a signal ground. By ensuring 2MOP between the screen and accessible parts, the screen may be earthed but is not protectively earthed and hence does not need the 25A test.

It might be thought that a screen with 2MOP from mains parts is a reasonable alternative. However, since the supply earth is used (cord/inlet), at some point the insulation system must transfer from a system of [1MOP insulation + 1MOP earthing] or (I+PE) to a system of 2MOP using insulation only (2I). This transition would consist of three sequential zones starting at the supply side:

Zone 1 is (I+PE), earth passes 25A test [cable, terminals, inlet etc]
Zone 2 is (2I+PE), earth passes 25A test [crossover zone]
Zone 3 is (2I+FE), no 25A required for earthed parts (functional only)

In this construction, it is OK to connect the FE to accessible parts, secondary ground etc.

But, as you can see, this is complicated and it's getting beyond reasonable to write a standard to handle this.

Also, and critically, it's impossible to avoid doing a 25A test (Zone 1, Zone 2). Once the 25A test becomes applicable, in theory the equipment becomes Class I. Full stop. You cannot call a device Class II if there is any reason to do a 25A test.

In practice, Zone 1 and 2 can be tiny, and compliance is usually obvious by inspection, as the parts are certified, especially for appliance inlets. So, it might feel that Zone 1 and 2 can be ignored. If this fudge is applied, it's OK to call the equipment Class II and the FE can then go on to connect to secondary parts without any 25A testing. And a lot of manufacturers/labs do this.

There is a complication with cord systems (not appliance inlet) for the loose strand and other faults around the terminals.

So, a possible compromise (future amendment) could say that if an appliance inlet is used, provided that 18AWG or higher wiring is used for earthed wiring for at least 5cm from the inlet (obviously passes the 25A, no test required), and 2MOP is provided between the earth wired and mains parts from 2cm from the inlet. This provides the 3 zones above.

But ... for me there is a third issue which is EMC/EMI. If earthing is required to pass normal condition EMC tests, then in my opinion a functional earth should be not be allowed. A Class II device should pass all EMC tests without the function earth connected. Although it might be covered by an instruction manual statement ("This device must be connected to earth for compliance with EMC regulations"), such a statement would have no practical effect and would easily fail useability testing.

In a hospital it might be assumed that devices with 3 pins will always be connected to earth anyway. But in home use and clinics, and in particular 100-120V systems, 3 pin outlets are not standard, so the possibility for not connecting to earth is high. Hence EMC should really pass without the earth. Which then begs the question: why both with an functional earth at all?

It really goes back to the origin: a functional earth should be reserved as an optional connection to help reduce noise in sensitive equipment such as EEG, ECG and so on.

Long story!

 

[lauxt]

Wow. Thank you Peter for your exhaustive and well-informed post.

I've looked into this a few years a ago and it is definitely a head spinning subject.

I believe the original idea for a "functional earth" was a terminal on the outside of the equipment, accessible to the operator, for purpose of helping with noise reduction in sensitive equipment. Obviously this should be double insulated from mains parts, since it is not guaranteed to be earthed at all, let alone being able to reliably carry fault currents.

Next, it was considered that some manufacturers may want to have an "isolated internal screen", which is undefined but assumed to surround the mains parts and limit noise reaching the secondary circuit. This screen was allowed to be connected to the earth terminal in the appliance inlet or power cord. Manufacturers now routinely use this inlet/cord based functional earthing in Class II equipment to provide better EMC performance, i.e. not only for screening but bypass caps, ferrites etc as would be normal for a Class I system. This often has nothing to do with sensitive medical equipment, just run of the mill medical devices. More on this later.

In the second edition, it stated that such a screen should be double insulated, but it never said to what. Mindless test engineers (i.e. me) blindly assumed this meant double insulation from mains parts. This lack of clarity was also copied to Ed 3.0, and it was only in Ed 3.1 that it was finally made clear that the 2MOP was between the FE part and accessible parts.

1. It makes sense that a standalone, external "functional earth terminal" should be isolated from the applied parts and also from the mains part, because who knows what will actually get connected to that terminal and the quality of that connection. On the other hand, if the functional earth connection is made via the yellow/green conductor in a power supply cord, then it seems the risk of connecting a non-earth potential to PE is greatly mitigated.

Yet, Subclause 3.35 (above), seems to say the opposite: If using a yellow/green conductor in a power supply cord (for FE), there must be isolation from accessible parts, and further, gives no explicit mention of the requirements when using a standalone functional earth terminal. Hmm.

2. By your explanation, it seems that the term "screen" really means any part, or collection of parts, which might be used for some noise/EMI mitigation purposes. If the screen can be separated from accessible part by 2xMOP, then the earth connection can just be considered functional. Correct? But, if the screen can only be separated by 1xMOP or no MOP, then the earth connection to the screen must meet the requirements of a protective earth? Why? I'm spiraling again...

Example: I know medical power supplies often place a single Y2 capacitor between output common and earth (for EMI purpose). Since one Y2 capacitor with about 4-5mm creepage/clearance only gives 1xMOPP, then are you saying that the earth connection to that Y cap would need to satisfy protective earth requirements. And again, why? What's the practical safety rationale there?

3. The standard seems to be concerned with isolating FE from accessible parts, but very often accessible parts are purposefully earthed. In this case, we would not consider an earthed secondary as a "screen"? So what rules apply? Do we only care about the screen concept for Type-BF or Type-CF applications where we need to maintain isolation?

4. **And to my original question, if we connect the output of a medical power supply to earth (via IEC inlet and earth conductor in power cord), but the wire from inlet to earth cannot meet protective earth requirements (25/40A), then can we call it Class II with FE?**

Zone 1 is (I+PE), earth passes 25A test [cable, terminals, inlet etc]
Zone 2 is (2I+PE), earth passes 25A test [crossover zone]
Zone 3 is (2I+FE), no 25A required for earthed parts (functional only)

In this construction, it is OK to connect the FE to accessible parts, secondary ground etc.

But, as you can see, this is complicated and it's getting beyond reasonable to write a standard to handle this.

Also, and critically, it's impossible to avoid doing a 25A test (Zone 1, Zone 2). Once the 25A test becomes applicable, in theory the equipment becomes Class I. Full stop. You cannot call a device Class II if there is any reason to do a 25A test.

I am struggling to understand what you mean by the 'zones' above. Are these zones for a Class II system with FE? Why is it impossible to avoid doing a 25A test if you define the earth connection as functional only? If you could explain the zone system in terms of a specific example, that might be helpful for me (and others).


Sorry my response is a total mess. It represents what's going on in my mind. If you can answer even a few of my questions, it would be really helpful to me, and hopefully others too.

And again, thank you for the detailed response. Really appreciate it. Helps me feel less alone on this.

 

[Peter Selvey]

The question "Why is it impossible to avoid doing a 25A test if you define the earth connection as functional only?" might hint at the heart of the confusion.

It's not a matter of the designer "defining" or designating an earth connection as functional or protective. The designation arises out of the physical location of the part with respect to mains circuits in the actual equipment.

For a part to be "protectively earthed" it must meet three distinct criteria:

C#1: it is connected to the mains earth (i.e. to the earth in the power supply cord)
C#2: it is close to mains parts (<2MOP)
C#3: the part is accessible, or connected to parts that can be accessible

If all three are met, the part is deemed to be a protectively earthed part and must pass all PE related requirements such as the 25A test.

If not, the part is just functionally earthed.

Even in a Class I device, there are large parts of earthing that are functional only. For example, if there is a metal frame that is well removed from mains parts, even if it is solidly earthed with a 14AWG green/yellow wire and even has a PE symbol for good luck, and the production test engineer insists on pumping 40A into it ... none of that makes it "protectively earthed". It is just functional earthing. Usually earthing in secondary circuits is functional only. It would not be unusual that 90-95% of the earthing in a Class I device is functional.

Now, with this understanding, we come to a Class II device that uses the 3rd terminal in the supply cord or inlet for functional earthing.

Power supply cords, appliance inlets and terminal blocks only have 1MOP between mains and earth. While sometimes they might (just) pass the tests for 2 MOP between mains and earth, they are not designed, manufactured or production tested for 2MOP. So up to and including the terminal is just 1MOP, not 2MOP.

So now somebody solders a wire to this earth terminal.

Say this wire goes to an EMC filter with 1 MOP capacitors, but stays well away from any accessible parts and the secondary circuit (as per the standard). This meets criteria C#1, C#2, but not C#3. Thus, the circuit is functionally earthed. No PE related testing is required.

But let's say the wire connects to an EMC filter designed with 2MOP (2 x Y2 caps), and then connects to the secondary as well, which can be accessible.

Away from inlet/terminals, we can see that even though C#1 and C#3 are met, C#2 is not since there is 2MOP to mains parts. So that part is definitely functional earthing. Again this is nothing to do with designer's designation, it is due to the physical separation and/or insulation from mains parts being 2MOP or more.

But ... at the terminals, all three criteria are met: C#1, C#2 and C#3. So that area still needs to be protectively earthed. And if any part of the device is protectively earthed, it automatically becomes Class I equipment.

See how that goes. I should make a little video, it could help to explain better than words!

 

[Mietto]

Very helpful discussion! It's rare to find such high level of details elsewhere on the internet.

See how that goes. I should make a little video, it could help to explain better than words!

I totally agree with the video!