I have to connect two analog signals (4 to 20 mA) from field device approved as Ex nA nC [ic] IIC T4 Gc to DCS.
is there a need to install zenner barriers or Ex ia isolators on DCS side?
The answer is a definite maybe!
There is apparently no Code of Practice for specific Ex na lower power devices.
It was a few years ago one end user asked for proof that an IS barrier was in fact intrinsically safe.
It was a case of - simply go back to basics.
In your case - closely check the specification of the field equipment you have.
If loop powered, can more than 20ma be supplied to the field device under any possible fault condition (fail to danger)?
Come back to us with details if unsure.