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There is no need to be so hostile.

always on:

So the magnets in an MRI are electromagnets yes, however they're not quite the same thing as a nail with some coils of wire and a 6 V battery. They're super conductors cooled with liquid helium to get them below the critical point. Because of the extreme temperatures and magnetic fields cycling them up and down is a very involved process that shortens the life.

Hospitals basically never turn them off, and certainly by the time patients are being seem they're on and remain on. They don't quench them between scans. The pulsing heard during a scan is not the main magnet turning on and off, a common misconception.

strength:

Generally the magnets are powerful for magnets, but they are not that strong in terms of making a buttplug become a bone snapping projectile. Most are about 1 T inside the chamber which is around as strong as the field strength of powerful magnets in consumer devices. The field is larger, and consequently they have more time to accelerate things but you can remove buttplug sized ferromagnetic objects from them by hand. For instance see: https://www.youtube.com/watch?v=I6OHy3MipS4 playing with a keyring at the start. There is not enough force involved, a person would stop approaching when they felt extreme discomfort of buttplug moving and walk backwards.

danger with metal filings:

Safety assessments are absurdly cautious. read for example the SDS for conc. hydrochloric acid. you can pour it over your hand and get a mild itchy rash.

Blinding someone isn't worth it so they're very paranoid about largish (e.g. mm sized) filings in eyes. People with iron in their skin frequently get MRIs and report mild pulling and heat (see keyring example). They don't experience the shards ripping out of their flesh or anything dramatic.

Ah seeing your edits.

So the question about magnetic strength limits given zero resistance is very interesting! Glad to see interest as it's an obvious question very few people ask.

All superconductors have a property we call critical field. Above this limit the material is no longer able to compensate for and exclude magnetic fields, and the superconducting properties collapse.

So if we made a coil of say YBCO (a common type 2 in lab examples) and we pumped more and more current through it, eventually it overwhelmed the material and suddenly resistance would skyrocket, generating heat causing a feedback cycle and it would probably melt to slag.