Updated : 19/01/2017
YES — for the whole time you're working there, your employer is obliged to comply with that country's basic rules on employee protection. These include the minimum wage, working hours, minimum rest periods, etc.
Find out more on your rights by contacting that country's liaison office for posted workers.
If you want to work in another EU country for a few months only, the best option for you is to post yourself abroad.
This enables you to work abroad while remaining covered by the social security system of the country where you usually work.
Before leaving you should:
If you don't need to move to the country during the time you will be working there (but will just be making short visits), all you need is a European Health Insurance Card. You can get one from your home healthcare authority.
When you arrive in the country, you should:
YES — you can apply for an extension, if your work lasts longer than originally estimated due to unforeseen circumstances and if the total posting period including the additional extension is no longer than 2 years.
Which authority should you contact? Ask the liaison office for posted workers in the country where you're working.
You need to prove that additional work became necessary due to unforeseen circumstances — otherwise the host-country authority can refuse the extension.
If this happens, you can still remain abroad, but your status changes to expatriate— meaning you'll have to switch to the host-country social security system (paying contributions there and no longer in your home country).
MAYBE — When it's clear from the start you'll be working abroad longer than 2 years, you can request an exemption from the host-country's laws in this field, allowing you to remain covered by your home social security for the duration of your work abroad.
Exemptions of this type vary from case to case, require the agreement of the authorities in both countries and are valid for a defined period only.
Which authority should you contact? Ask the host-country liaison office for posted workers.