What does the x mean on MCB?
The x symbol at the end of the line actually denotes the type of switching device. These are defined in IEC 60617 (Also adopted in many countries as a European norm, such as BS EN 60617 or SFS-EN 60617 for Britain and Finland respectively). They relate to the current that the device is able to switch and the fault current that the device is capable of closing onto and breaking. 

No symbol (IE just a switch symbol) is just a generic switch. No information is given on isolation, load or fault switching capability.

 

A horizontal dash at the contact indicates an isolator. Just a dash indicates a device which when open has sufficient space between the contacts to withstand load and fault current without an arc developing across the contacts; however, this kind of device has no on-load or fault switching capability, just the ability to isolate off load. 


A horizontal dash with a circle below it indicates this device is an isolator with on-load isolation capability. The device will be able to switch on load, and also to switch onto a rated fault current. These devices are often called switch disconnectors. A switch disconnector however is not rated to open on a fault current. 



An x at the end of the line indicates full fault make and fault break capacity. 
There are then additional symbols for the type of interrupt capability depending on the type of trip characteristics are used, but these can be outlined in many different ways depending on the type of diagram. 



It is very important to get the symbols correct in specifications and design drawings. I was involved with a project where a claim was made for several million euros due to incorrect use of these symbols. There's a big cost differential between a 4000A Disconnector and breaker. If this is specified incorrectly there are cost and safety implications.