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I am going to use a magnetic mechanical relay to switch 240v AC.

I am using MPC250D6Z05APT, in its specification it did not say that the 5V signal is isolated from its output upon failure of the product.

My question is by nature of mechanical relays are the signal pins/ coil pins isolated from its output terminal?

If not, how would you create a protection circuit, without having external power supplies?

Here is what I though up with using traditional opto isolator, but I am not confident since the 5V supply is still coming from the same board.

enter image description here

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  • \$\begingroup\$ traditional opto isolator Read that 3rd word again: ISOLATOR it means it does isolate. The fact that the designer of the schematic you show above decided to connect both sides of the opto isolator to the same ground does NOT mean that the device does not isolate, because it does. That schematic simply isn't using that property. Nearly all mechanical relays are similar, the coil and switch(es) are isolated. To be sure, look in the datasheet at the maximum isolation voltage. If there's no such value, the device has no isolation. \$\endgroup\$ – Bimpelrekkie Oct 30 at 12:24
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    \$\begingroup\$ I am using MPC250D6Z05APT Include a link to it's datasheet so we know what we're dealing with. \$\endgroup\$ – Bimpelrekkie Oct 30 at 12:27
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    \$\begingroup\$ A relay's coil is not normally connected to its contacts in any way. But "failure of the product" is far too broad, and could include scenarios in which a connection is made by bent parts, etc. What exactly are you concerned about? \$\endgroup\$ – Dave Tweed Oct 30 at 12:34
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    \$\begingroup\$ The MPC250D6Z05APT appears to be something exclusively sold by Aliexpress. Nuff said! \$\endgroup\$ – Andy aka Oct 30 at 12:36
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    \$\begingroup\$ I have a simple rule, "No Datasheet = No Sale", generally applicable, but especially so when dealing with mains! \$\endgroup\$ – Dan Mills Oct 30 at 13:12
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If "MPC250D6Z05APT" is this one's 24V version, then the schematic on the label shows that the coil is insulated from the contacts, as is normally the case with a relay.

enter image description here

The datasheet should specify the maximum voltage the isolation can withstand. Buuuuuttt... since this is aliexpress... whether there is a datasheet, or if it corresponds to the product being sold, or if it can be trusted... are all left as an exercise to the reader...

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  • \$\begingroup\$ you can never directly trust anything on aliexpress. but i would like to add another layer of protection for my uC in the event of the switch will come in contact with live wire. In the discroption it says safe isolation between coil and contact ,withstand voltage up to 4KV but who know if this is true. I am opening and closing a live wire by the way, which another device uses that main voltage signal to do its stuff \$\endgroup\$ – Jake quin Oct 30 at 13:34
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The relay seems to be the same design as the below one, made by Xiamen Hongfa:

enter image description here

I believe the original design was Japanese, maybe Omron. Your guess is as good as mine how good the Chinese implementation is. If the approval markings are real it's probably safe enough. 4kV is actually pretty good isolation, better than the typical sugar cube relay (eg. Matsushita JS series) which is more like 1.5kV.

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