1. requesting a MHA assesment
a/ family member being concerned: your nearest relative (there is a hierarchy who and under what circumsnances may become your nearest relative, usually: if you're married or living together for over 6 months- you partner or spouse; if you're single - this will be your parent or your sibling or your children if you have adult children) may request a MHA assessment by contacting an Approved Social worker.
b/ if you're caught in public behaving in a way that suggests a mental disorder, a policeman/woman can bring you to an appropriate place (place of safety) under a section called 136, which gives them power to detain you up to 72 hours for an assesment of your mental health-> in this case the police requests MHA
c/ your doctor if believes that your mental state warrants a MHA assesment may requests one
2. MHA assesment: An ASW iniciates the MHA assessment first by gathering necessary information (usually by phone; ringing the relative/family/local MH services if there is any evidence of them being involved).
3. the ASW calls two doctors, one of them has to be Section 12 approved (usually consultant psychiatrist), the other - a doctor who has previous knowledge of the patient, it's usually your GP. if the ASW can't fing your GP, the other doctor has to be another section 12 approved doctor (usally another consultant psychiatrist); under special circumstances if the ASW has difficulty inn finding 2 doctors, she/he can use only one doc (in this case: sec 12 app= cons psychaitrist);
4. they go out and assess you mental state. if your sectionable - they can section you
you're sectionable if you meet certain criteria (someone has already given you a link and some explanation for it, I see, so I omit that part)
There are 3 main sections you can be forcible brought into a mental hospital (you can be sectioned and forcibly treated only for your mental disorder): section 2 which is for assessment but also gives the power to impose treatment (lasts up to 28 days); section 3 (for treatment; lasts up to 6 months), and section 4 (1 doctor needed for this one; emergency admissions up to 72 hours )
- the MHA assessment is usually carried out at your home (during the day only), A&E department, Police station (secion 136) or in special premises within a hopsital/mental health unit etc ('place of safety'); if you're at home or any other place outside a hospital - you need to be transported to the MH hosp; this depends on the level of your cooperation, but usually an ambulance is called and the ASW goes with you; sometimes, if there is a risk of violence, the police is involved; if you are co-operative even your relative or the asw him/herself/your GP can bring you to the local hosp.
5. at the hospital you need to be assessed by a doc within certain time (depends on the secion, usually 72 hours), diagnosed and decision on treatment is made. you have right to apply against your secion (time limit depending on the section), you have a MH panel hearing (depending on the section).
6. while on a section you're still have right to a leave, but it's up to your RMO (responsible medical officer) to decide on that (only if your mental health allows), this is called section 17.
7. 'up to..' doesn't mean that you will be detained for the exact time; you may be discharged from section earlier (if your MH improved or by a MH tribunal/ hearing if found not sectionable); the section can also be prolonged if necessary (i.e. your MH still meets the criteria for treatment and you don't agree to receive it voluntairly).
i hope it helps. It's the UK Mental Health Act. Every country has its own MHA, usually similar in the main ideas, but details different.
details in the link provided.