Does an employer have to supply a microwave for staff use?

No
nope, they just have to provide breaks.


Answers:
I don't think so.
No.
No. Sorry this is just a perk that a lot of places offer but they are not required to do so by law.
Under law, of course not.
No. That would be a side benefit, like soft drink and snack vending.
They must, however, provide clean drinking water for the staff.
Do they have to according to who? If you asking according to the government. The answer is no.
What?

Before or after your daily foot rub.
no. why would anyone think that they have to?
No they don't have to supply a microwave for staff use. Under the law they have to provide restrooms, temperate water for washing hands, a safe work environment, Workman's comp insurance. The others are considered a no essential items that have nothing to do with employee performance.
Of course they do. They are also legally obligated to have a tanning bed on the work site...

Honestly, if you and your co-workers need a microwave why don't you all pitch in a couple of bucks and buy one. You can get one for about fifty dollars.
No they do not.