If a person who is positive sleeps with a person who is NOT positive, and the sex is protected and all the precautions are taken to prevent transmission, is the positive person legally required to let the negative partner know before sexual contact? What if the condom breaks?

I am not a lawyer and am not very familiar with all of the legal issues associated with this topic. I do know however, that these things are State regulated and differ State by State, in the United States. I believe some states have laws on the books about this and others do not. And I imagine when there is some law or regulation written about this, the language probably differs. In other words, depending on how it is stated there will be differences as to whether the regulation speaks specifically about "disclosure" and/or "putting somebody at risk" or actually "transmitting the virus," etc. If you really are interested in knowing what the law says you will have to check with a legal authority in your State (or country, if it outside the US).

On the issue of a condom breaking during sex between an HIV+ and an HIV- person, it is important to familiarize yourself with "post-exposure treatment" (PET), also known as "post-exposure prophylaxis" (PEP). This is where the uninfected person who has been clearly exposed to HIV can take a course of antiretroviral therapy following the exposure in order to prevent infection. However, it must be initiated very quickly after the exposure - the sooner the better. And the drugs need to be prescribed by a medical provider. You can read about this on this website and other internet sites as well.