I ripped a transformer out of an old wall wart (120 to 30 VAC).
I want to mount it in a project with a metal enclosure which will be grounded to mains earth.
The wall wart was plastic and had no ground whatsoever. There is about 8 volts AC potential between the transformer core and ground.
Is it good practice to ground my transformer's core or should i try to isolate it?
Isolate. The cores are all laminated so they do not short out to each other or to ground.
Am surprised you were even able to measure a voltage on them.
The cores are indeed laminated however they are soldered/welded to attach the E and I plates. This obviously means they are not isolated from each other.
But I assume this doesnt change your answer so I will isolate the transformer the transformer core from ground.
Thanks.
I'd suggest you do a resistance check between core and the primary windings. If you have an HV insulation tester, conduct the test at 250volts. If any show a relatively low resistance, say less than 1 megohm then your best bet is to dump the transformer.
It's not critical, but I would ground the core, because that will reduce the degree to which a transient on the live wire of the mains gets capacitively coupled to the secondary. The 8V you are measuring is probably caused by capacitance between the primary and the core.
Do not do it if not certified, mind legal implications when not using approved components !!
If you rip up a transformer from a probably approved wall wart power supply and you put it into your equipment, the equipment goes into other category of electrical devices (at least in Europe) and you might be responsible for any direct or indirect damages and losses caused by the malfunction of your equipment 