if the devices are connected 1-phase, 1 device should be sufficient (see the instructions).
the pro variant has a different design and therefore fits better on a standard tableau.
the pro version is not fully compatible with the protocol and offers less possibilities to read out the values. so check what is supported by your software.
Solder on the corresponding plugs. Otherwise, the supplied "flip sensors" are calibrated with the EM3 to the respective phase and are therefore labelled separately. If the phase is mixed up or a 50A version is used, at least the calibration (with a measuring load) must be started. However, I cannot say whether the EM3 also tolerates other "flip sensors".
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The Shelly 3EM is a first generation device.
From the second gen. are all the newer products like the i4, Pro Series (1, 2,1PM,2PM,4PM), the Plus Series (1PM,2PM), and as far as I know also Motion2
Unfortunately I'm not satisfied with it, at the beginning it worked great, but after a while it doesn't work anymore, I can't get into the interface 192.168.33.1 anymore, it always asks for admin and password, which I never set, but now I can't get in, password unknown 12345678, it can't be reset in the factory settings?
Is there a way?