Sometimes questions are raised when designing a product with regard to a part of the product being class II while the product is classified class I. This is sometimes difficult to grasp for some people so in this article I will try to explain a bit more how this works. First of all we need to start at the basics.
Every product is designed with regard to electrical shock hazard to have two lines of defense. This means that when one breaks down the other will be sufficient to still maintain the safety and than it is expected that that will be sufficient. Chances of both breaking down in one time is not very likely.
In electrical safety (europe) we have basically 4 classes. These classes can all be defined with regard to electrical safety as having two lines of defense but they all accomplish this in a different way.
This class is not acceptable anymore in many countries. It is based on two insulations, being the first one in the equipment and the second one in the environment. This was very acceptable in the past when people had carpeting, wooden floors, not many electrical devices, and no central heating in the form as we know it now. The environment was more os less insulating and when a fault would occur in the basic insulation (first line of defense) the environment would serve as the second line of defense.
In class I insulation a different solution is chosen. In this case there is still the basic insulation in the device but the second defense comes from an earthed housing. This earthing will take care of blowing the fuse or in nicer words the safe discharge of the fault current. When the insulation breaks down and the housing would become live the current will be lead away through the earth conductor and that will in turn blow the fuse or nowadays at least trigger the residual current device. In this way again 2 lines of defense have been established.
This one has be developed later keeping in mind that insulation materials had become a lot better and plastics where taking more and more the place of metals. In class II there are also 2 lines of defense. The first is not different as mentioned before, just the basic insulation. The second line of defense is another insulation. This is called the supplementary insulation. It must comply with more stringent requirements but thinking that that will be the defense that you will have with the protection against electric shock it is not more than logical.
Now in class II insulation you can also have a third kind of insulation. This insulation can take over the basic insulation function and the supplementary insulation function. This insulation is called reinforced insulation. Reinforced insulation must comply with the added requirements of basic and supplementary insulation.
This is the last class we know. It does assume that the electric shock hazard has already been taken care of and the supply voltage is safe. That does mean that the supply voltage must be insulated from the 230 V supply voltage by at least one of the above classes and that the voltage is low enough not to pose a safety hazard. In this case the insulation in the equipment is only functional and does not provide protection agains electric shock. In cases it is allowed that it can be touched.
So what about a class I product that is made out of plastics for a large part. Lets assume as an example street lighting. In street lighting you see sometimes that the housing that connects to the pole is made of metal and has an earth connector but the lamp compartment is completely made out of plastics. So how to view this in that case. The lamp compartment should comply with class II requirements. This will take care of the electric shock hazard. The luminaire however is classified class I since this is the lowest class for a part of the luminaire. It is always needed to mark the lowest class since that can limit the allowed use of a specific luminaire.
As an example of this limitation you can look at bath room equipment. Nearing zone 0 the acceptable class of insulation goes up. Meaning Class II product can be used in more zones than class I Class III can be in zone 0 (in the water). So it is important in designing to decide where you want to use the equipment. Making use of Class III or Class II constructions in a Class I product is very common.