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XB2/ZB2 are the old pushbutton and pilot light range replaced by the current XB4/ZB4 range, also known as Harmony 4 range
By general Telemecanique/Schneider convention: Those numbers beginning with "X" are a complete assembly whilst those beginning with "Z" are a component part of that assembly, e.g.:
XB4BS8442 is a complete Emergency Stop button with one normally closed contact. This is made up of ZB4BS844 (operating head) and ZB4BZ102 (collar and contact block).
The significance of this is important as XB4 assemblies are mostly direct replacements for XB2 assemblies but ZB4 parts are not interchangeable with ZB2 parts. If a part of an XB2/ZB2 assembly has to be replaced (with ZB4), then the whole assembly must be replaced, either with an XB4 assembly or all ZB4 components to make up a complete XB4 assembly.
Further to this, there are some instances where different methods are used to achieve the same ends, e.g.:
IN XB2/ZB2 a latching pushbutton is created by using a standard momentary pushbutton and adding a latching adaptor ZB2BZ21 between the button and the contacts. In ZB4 a specific latching pushbutton head is used so there is no ZB4 equivalent of ZB2BZ21 and a momentary can not be converted to latching or vise versa. Incidentally, in much Schneider documentation latching push buttons are referred to as "push-push to release buttons".
There are a number of similar instances so do not assume that there is always an equivalent ZB4 part for every ZB2 part.
For the XB2 to XB4 parts only, due to the constraints discussed above, the Substitution Tool available on the Schneider Electric web site can help with cross reference between old an new product.
It is essential to check the form and function of the original and replacement products to ensure that they meet the required specifications.