Galvanic isolation interfaces now dominate the market, they work on the same principle¹ as Zener barriers with the addition of isolation between input and output circuits.

There are essentially 3 general configurations of Galvanic isolator2
loop powered where the isolator is powered from the signal itself, the most common being for 4-20mA loops and digital output signals. Output voltage can be limited as the loop current has to power the interface
2 Port isolated where a separate power supply (usually 24vdc) connection is required but this is not galvanically isolated from the signal lines. Consideration must be given to the electrical configuration of any circuit the non-hazardous area signal terminals are connected
3 Port isolation is probably the most common with signals and power supply isolated offers a greater flexibility and reduces the possibility of hook up problems

Due to their complexity they must be designed specifically for the application circuit or signal type, Analogue, Digital, input, signal type etc.
These are conventionally shown as 'positive' voltage but, unlike Zener barriers, they are isolated, so the concept of positive and negative barriers becomes irrelevant which could become critical if two or more circuit loops are joined.
Unless allowed in a specific certificate, as fault conditions must be considered i.e. it must be assumed they could be linked in both series and parallel and the parameters added accordingly. e.g two off 28v 93mA isolators linked would result in output parameters for the circuit of 56v 186mA which would almost certainly cause issues.

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