European Classifications
Source: http://www.engineeringtoolbox.com/hazardous-areas-classification-d_395.html
You can connect and disconnect intrinsically safe circuits, even when live, since IS guarantees that they will be safe even in the event of a short circuit.
In most of the world, intrinsic safety has replaced other protection schemes because it provides a level of operational freedom that is not matched by other schemes. The biggest drawback of IS as a protection scheme is that it is limited to low-power devices.
Bacically, the concept of intrinsic safety is to use lower-power field devices and connect them through a system of barriers (either using zener diodes or opto- or galvanic isolation), so that the energy available is less than the MIE.
No field device is intrinsically safe by itself. The entire system of wiring to which it is attached must be considered and made safe. This required a tiresome round of re-engineering each installation until the concept of "Entity" appeared. Under this concept, field devices and barriers are certified individually, and can be interconnected fairly easily Just match the parameters for voltage, current and capacitance, and make sure that the device and its barrier are installed in approved fashion.
Intrinsic Safety for Analog Devices
Making analog field devices intrinsically safe and using them in IS applications is pretty simple, as shown in Figure 2. You start with a low-power device, and with the emergence of digital field devices, most transmitters qualify. You install it with a barrier matched to the Entity parameters in the input and/or output wiring, and you're golden.