If your vehicle is subject to EU & AETR (European Union and European Agreement Concerning the Work of Crews of Vehicles Engaged in International Road Transport) rules, then you need to use a tachograph at all times. The weight of your vehicle is important, as the law now requires any vehicle weighing over 3.5 tonnes to be fitted with a tachograph.
This includes your vehicle and trailer if your using for your business.
Thankfully, the majority of drivers are well informed about whether a vehicle needs a tachograph or not.
Tachograph systems, both analogue and digital, are used to record driving information such as time spent driving, speed and distance travelled, as well as being used to ensure that both drivers and employers follow the rules in regards to drivers hours.