Power supply safety relais triggers during startup of Electron

[ScruffR]

What pins are you using to control the relay?
You may need to stay away from the Dx pins that also double as JTAG pins during boot-up

There are some threads like this one

Pin state on reset?

I believe you are running into the JTAG port, which is doc’ed here (see the footnote in particular): https://docs.particle.io/datasheets/photon-datasheet/#jtag If you can’t change pins, you can do what the footnote says.