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?
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.
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.
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.