FT232R (USB to serial) mains powered project finished but still have a question

I have built a project the uses the FT232R USB to serial converter. My project is mains powered and is working fine but I keep worrying that if I plug my project into different computers and laptops that the ground between my project and the PC could be at a different level.

Is this possible or even likely to happen? or will the PC USB ground always the same level as my mains powered device?

Looking at the FT232R data sheet, there is no provision for this but I keep thinking that if for some reason the ground on the PC USB is higher or lower, I will get a current flowing through the USB cable and into my project, maybe blowing it up or something. 8-O

Its better not to connect the mains ground directly to the GND of FT232.. If you can't avoid this, try using an optocoupler at the serial port part.

The problem of ground loops in electric circuits with multiple protective earth connections doesn't only apply to USB interfaces. In so far, it's unlikely to find specific hints in the FT232 datasheet. Usual electrical installations in buildings have a central earth point and separate protective earth and neutral conductors (TN-C-S or TN-S earthing system, see Earthing system - Wikipedia, the free encyclopedia). You shouldn't observe voltage differences between protective earth contacts of different wall outlets. This may be different in industrial power distribution systems, if you connect to different sub-distributors. Also VFD inverters can possibly induce compensating currents in the protective earth conductors.

So I think, using USB or RS232 interfaces without optocouplers for galvanic isolation is O.K. for home and laboratory but should be avoided in industrial systems with unclear earthing situation or widespread installation.

Thanks for the very informative replies

I should mention, when the project is unplugged from mains it switches over to being USB powered. (I am using 2 diodes for the power supply selection)

Could I still opto-isolate the FT232R and still have it power the board when mains not turned on?

Would there be any point in placing polyfuses (Resettable fuses) in the VCC and GND lines coming from the USB?

Could I still opto-isolate the FT232R and still have it power the board when mains not turned on?

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You would need a DC/DC converter for USB powered operation with galvanic isolation.

Would there be any point in placing polyfuses (Resettable fuses) in the VCC and GND lines coming from the USB?

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Placing a PTC fuse in the GND line would requre aditional protection of the USB data lines. VUSB will be current limited at the host side anyway. I think, the alternatives are full galvanic isolation (with at least a few 100 V isolation voltage rating) or USB GND ties to circuit GND.