Main power supply voltage (Um) #
Um (max. rms AC or DC voltage) refers to the maximum voltage that can be applied to the non-intrinsically safe terminals of intrinsically safe unit (supply side of associated apparatus). The permitted value of Um is given in the documentation and is usually marked on the associated apparatus label.
The documentation will also show the maximum output voltage and current that can be taken, and the allowable inductance and capacitance for the circuit to be connected to the interface (Uo, Io, La & Ca).
The documentation (and possibly the labelling) of the field equipment will give values for maximum input voltage and current that can be applied on the connection facilities. The inductance and capacitance of the equipment will be given and all this data will be considered when specifying the associated apparatus (interface) and the cable requirements (Ui, Ii, Li & Ci).
Equipment that can be connected to I.S. interfaces #
Only the apparatus specified in the system documentation may be connected to the output of the interface (safety barrier).
In addition to cabling conforming to the system requirements, the documentation will include certified intrinsically safe field equipment and simple apparatus.
Analysis of Intrinsically Safe Systems and its parameters #
The safety barrier label and/or documentation will indicate the maximum voltage and current that it can deliver under fault conditions, and the maximum permitted inductance (L) and capacitance ( C) that may be connected to its hazardous area terminals.
The field device label and/or its documentation will indicates he maximum voltage and current it can receive, and the values of its own inductance and capacitance.
The electrical voltage and current parameters must be matched and the capacitance and inductance should be verified that they are within the safety limits.
The field device may then be used safely with any barrier whose maximum voltage and current are equal to or less than the field device values.
The table shows the required relationship between the parameters.
| Safety Barrier parameters | Field equipment (FE) parameters | ||
| Voc = Uo | Voltage | Ui = Vmax | Volatge |
| Isc = Io | Current | Ii = Imax | Curent |
| Ca | Capacitance | Ci | Capacitance |
| La | Inductance | Li | Inductance |
Voc ≤ Vmax Barrier output voltage not greater than FE max input voltage
Isc ≤ Imax Barrier output current not greater than FE max input current
Ca > Ci + C cable Barrier allowable connected C more than FE C + cable C
La > Li + L cable Barrier allowable connected L more than FE C + cable L
Example:
