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I am currently revising a plug-in power meter (i.e. a power meter that you plug into mains, then plug the DUT into it) for CE compliance. It features power switching using a latching relay, only on one pole. Because Schuko is reversible, do I need to add an extra relay so that both poles are switched?

user36129
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3 Answers3

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Assuming that EN60950 applies to your power meter, in a case where the mains plug is reversible then EN60950 requires that a disconnect device "shall disconnect both poles simultaneously" (section 3.4.6). However, pulling the power meter out of the mains socket will disconnect both poles simultaneously and so it is quite valid to consider this to be a disconnect device. Most domestic electrical equipment sold in the EU has a mains switch which only switches one pole of the mains supply and relies on the mains plug as the EN60950 disconnect device.

For CE compliance your relay only needs to switch one pole.

Steve G
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  • That makes a lot of sense. Actually, that makes tons of sense, given that I have a few devices in the home that do exactly this, so that explains why they're able to bear a UL certification. I'd like to wait for possibly some more people to weigh in and get a consensus (maybe somebody who has dealt with this exact section in a certified product?) but thanks already for the explanation. – user36129 Jun 21 '16 at 08:54
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    The DUT socket is also Schuko. Switching just one pole will violate EN60950 IMHO. Think of a user that wants to change e.g. a lightbulb - he does **not** expect the socket to be "hot" in OFF state. – Turbo J Jun 21 '16 at 09:02
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    I see one reason why you cannot use one pole only: The output is another Schuko, and a user may well assume the second Schuko to be "off" completely when it may just appear to be "off" (in case it's plugged in such that you don't disconnect L, but N only). Thinking about it a bit more, I can't remember having seen a device with a switchable mains output that did not disconnect both L and N. Except when it's hard-wired (e.g. a switched wall outlet) and you know you always switch L and never N. – zebonaut Jun 21 '16 at 09:20
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    @TurboJ: the problem with that line of reasoning is that even my fully certified plug-in lamp fittings are all single pole switched even though the plugs are reversible. They even say on the inside of the lamp shade: unplug before changing light bulb. Apparently, this is an acceptable practice. – user36129 Jun 21 '16 at 09:20
  • @user36129 Yes, lamps are an exception to the rule. Devices that feature a mains outlet for other devices to be connected to are, IMHO, not an exception to the rule. – zebonaut Jun 21 '16 at 09:23
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    Which section of EN60950 does switching just one pole violate? En60950 does not even require you to have a switch on the product, you can rely on pulling out the plug on the power supply cord to serve as the disconnect device. (section 1.7.2.2) – Steve G Jun 21 '16 at 09:34
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    @SteveG It boils down to the question whether the presence of the relay makes the power meter a disconnect device or not. The fact that the outlet is yet another Schuko may put you in the situation that anything but a real, two-pole disconnect device is so ambiguous that it's illegal. No switch at all may be OK, a one-pole switch may not be OK, because it may lead a user to wrong assumptions (seemingly off vs. really off). – zebonaut Jun 21 '16 at 09:44
  • Yeah, that's what I'm worried about here. If relays were free and my design wasn't space constrained I wouldn't care, but in this case I'd really like for the answer to be 'I only need one relay'. Wishful thinking, perhaps. – user36129 Jun 21 '16 at 09:47
  • @zebonaut switching both poles should be important only for service personnel who need to know for certain when they open up the equipment that it is truly disconnected. If the product has been designed correctly and passed CE certification then it should not matter to the user which pole is switched, since it should not be possible for the user to touch either Live or Neutral conductors. That is the reason why reversible mains plugs are allowed. – Steve G Jun 21 '16 at 10:01
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    @user36129 if you go down the path of considering that the relay is a EN60950 "disconnect device" then the relay has to meet more stringent requirements than a purely functional relay. For example it will need a contact separation of at least 3mm (see 3.4.2). – Steve G Jun 21 '16 at 10:03
  • @SteveG, switching both poles should be important not only for service personnel, because this device provide the power to another device and the user should be aware that if he open the switch the downstream device is yet powered. – Antonio Jun 21 '16 at 10:27
  • @Antonio the downstream device is not powered, but it may have Live still connected inside the equipment. But unless the user opens up the equipment why does that matter? – Steve G Jun 21 '16 at 10:33
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    @SteveG, it does matter if the user open the downstream device. This is, IMHO, the heart of the matter. – Antonio Jun 21 '16 at 10:39
  • @Antonio it is indeed the heart of the matter. But once a user opens the equipment using a tool then that person is now a "service person" in EN60950 terminology and should know what they are doing. They should know to disconnect the equipment by using the disconnect device, ie pulling the plug from the socket and not relying on a switch or relay. – Steve G Jun 21 '16 at 10:50
  • Uhm, maybe you are right, the definition talk about a _skilled_ person. But if i'm a maker, and I connect my home-made circuit to the power meter I'm a service personnel or not? I think there's a little ambiguity. You've insinuated a doubt in me... – Antonio Jun 21 '16 at 12:16
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enter image description here

Your power meter has a shuko (wall side).
It ALSO has a switch.

Then the EN60950 say, in section 3.4.6 that:
"Three examples of cases where a two-pole disconnect device is required are:

  • [...]
  • on PLUGGABLE EQUIPMENT supplied through a reversible appliance coupler or a reversible plug (unless the appliance coupler or plug itself is used as the disconnect device);
  • [...]_ .

Then if you use the plug as disconnect devices it's ok, it unplug two poles at the same time, else, if you put switches as disconnect devices the switch must be bipolar.
I agree with the Turbo J's and zebonaut's comments above.

The heart of the matter is: If I have a switch, this switch power off the device connected to my power meter or not?

I think that is not so obvious (for the user) that the switch is a functional switch and not a disconnecting device for the downstream device. If the switch remove the power to the device connected to the power meter it is a disconnecting device and must be bipolar, if it only stop the power measurement, leaving the device powered then we can say this is a functional switch.

The user must be aware that if he open the switch, and this is only a functional switch the downstream device may be connected to the hot wire, and there's the risk of electric shock.

Antonio
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  • Unfortunately, the question isn't just about 'does it work' - it's 'would this be acceptable for CE certification'? I know it works - I've already got a working device. However, it's not immediately clear from e.g. EN 60950 whether my type of device is permitted to only do single-pole switching. It's permitted for light fixtures etc., but this? Idk, I'm hoping somebody here knows. – user36129 Jun 21 '16 at 08:30
  • @user36129, Ah, well, it seemed too simple. :-) I edited the reply. – Antonio Jun 21 '16 at 09:30
  • @Antonio if you rely on pulling the plug as the disconnect device then the switch does not need to be 2-pole. In this case the switch would be classed as a "functional switch" and does not need to comply with all of the requirements for an isolation switch. – Steve G Jun 21 '16 at 09:48
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    @SteveG, I'll agree if the device was a blender or hair dryer, but in this case I think that is not so obvious (for the user) that the switch is a functional switch and not a disconnecting device for the downstream device. **If the switch remove the power** to the device connected to the power meter it is a disconnecting device and must be bipolar, **if it only stop the power measurement**, leaving the device powered then we can say this is a functional switch. What is the case? – Antonio Jun 21 '16 at 10:11
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Disclaimer: Sorry, my copy of EN60950 is not in English, so the exact words are not a quote, but my translation.

However, I come to the conclusion that you must switch off both poles, because you say that your meter "features power switching", which sounds much like a true disconnect switch and is not just some sort of stand-by or functional switch:

3.4 Disconnection from the supplying branch

[...]

3.4.6, Number of poles for [...] single phase devices:

[...] Three examples of cases where a two-pole disconnect device is required are:

[...]

2) on PLUGGABLE EQUIPMENT supplied through a reversible appliance coupler or a reversible plug (unless the appliance coupler or plug itself is used as the disconnect device)

3) on equipment supplied from a socket-outlet with indeterminate polarity

There must not exist any ambiguity whether or not your relay may act as a disconnect device. If there is a chance that a user may think it is possible to switch off the outlet using the relay (because: "Why is it there in the first place, anyway?"), you are, IMHO required to use a two-pole relay or two relays acting simultaneously.

  • Example for a very simple device that does not require a two-pole switch: Power strip or extension cord without a switch.

  • Example for a very simple device that does require a two-pole switch: Power strip with a switch.

Also, quoting Steve G's comment below his own answer: "if you go down the path of considering that the relay is a EN60950 "disconnect device" then the relay has to meet more stringent requirements than a purely functional relay. For example it will need a contact separation of at least 3mm (see 3.4.2)."

Late edit, (sorry!): I have just learned that there are power strips with funcional switches (as opposed to disconnect switches). These must be labeled: "Disconnected only when unplugged from outlet!" (my own translation of "Spannungsfrei nur bei gezogenem Stecker!"). So yes, it's all about building something that leaves no room for error on the user side.

zebonaut
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  • What version is your copy? In my revision (2001-02) there isn't this example; my copy say: "Three examples of cases where a two-pole disconnect device is required are: 1) on equipment supplied from an IT power system; 2) on PLUGGABLE EQUIPMENT supplied through a reversible appliance coupler or a reversible plug (unless the appli- ance coupler or plug itself is used as the disconnect de- vice); 3) on equipment supplied from a socket-outlet with inde- terminate polarity." – Antonio Jun 21 '16 at 10:35
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    @Antonio It's EN 60950-1:2014, and it looks like there is no difference in the examples. It's just that my translation was quite loose. (I knew why I put the disclaimer on top). Thanks for your comment, I have edited your text into my answer. – zebonaut Jun 21 '16 at 10:39
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    @Antonio Hmmm... It's all in 3). Schuko is without known polarity, so it's really quite clear. – zebonaut Jun 21 '16 at 10:48
  • I'm convinced, I'm going to have to find a way to fit in a second relay. That was quite a bit more complicated than I expected. – user36129 Jun 21 '16 at 10:58
  • @zebonaut, it apply both conditions, 2 for the plug and 3 for the socket. I underlined condition 2 because of the plug used as disconnecting device (I thought the reversibility of the Shuko was well known). – Antonio Jun 21 '16 at 13:08
  • @user36129 I suspect that you can't use 2 separate relays because of the "simultaneously" requirement. You need 1 relay with 2 poles. – Agent_L Jun 21 '16 at 13:56