Intrinsic safety is all about limiting the power in a circuit to ensure no sparking can occur which, in most cases, results in minimal heating affect. By design intrinsic safety is a system concept and therefore the (power limiting) parameters for compliance of the system (loop) must be defined.
Known as the safety parameters, the key parameters are Voltage, Current and Power with system loop Capacitance, Inductance and Resistances being critical.
Each item in the loop must have its parameters defined1 to assure compliance.
For Associated Apparatus i.e Intrinsic safety Barrier. The parameters represent the maximum output to the Hazardous Area for safety purposes, including safety margins, and therefore more than the working or maximum supplied output in real circuits.
|Voltage||Um||Maximum voltage input from undefined connected equipment2|
|Voltage Out||Uo||Maximum voltage supplied to the field (peak a.c. or d.c.)|
|Current Out||Io||Maximum current output supplied to the field (peak a.c. or d.c.)|
|Power Out||Po||Maximum electrical power that can be taken from the apparatus3|
|Capacitance||Co||Maximum capacitance allowed in the circuit4 connected to the Associated Apparatus|
|Inductance||Lo||Maximum inductance allowed in the circuit to be connected to the Associated Apparatus|
|Lo /Ro||Maximum allowed ratio of Inductance allowed to be connected to the to Associated Apparatus. This, in almost all cases, refers to the the connected cable5|
For the Hazardous Area equipment matching safety parameters are defined:
|Voltage In||Ui||Maximum voltage allowed to be connected to maintain intrinsic safety Integrity|
|Current In||Ii||Maximum current capability of the connected associated apparatus to maintain intrinsic safety Integrity|
|Power In||Pi||Maximum electrical power capability of the connected associated apparatus to maintain intrinsic safety Integrity|
|Capacitance||Ci||The effective internal capacitance presented at the device terminal as part of the intrinsic safety circuit|
|Inductance||Li||The effective internal inductance presented at the device terminal as part of the intrinsic safety circuit|
|Li /Ri||The effective internal ratio of Inductance presented at the device terminal6|
By definition the safety parameters of simple apparatus do not need to be defined as it must have a Po of <1.3 W and negligible capacitors or inductance therefore the safety parameters voltage, current and power for the simple apparatus are not relevant being controlled by any intrinsic safety associated apparatus.
As most process field instrumentation is designed around 24vdc or 12vdc, intrinsically safety barriers at those working voltages have been designed to deliver the maximum power to the field and still keep below the gas curves (with a safety margin).
This results in most barriers having very similar characteristics e.g. for a working voltage of 24vdc the Zener diode voltage would typically have a safety voltage of 28v to ensure no leakage in normal use, a 10% safety margin is applied giving a figure or 30.8v.
From the IIC graph this equates to 140mA.
Traditionally a ⅓ safety margin would be applied giving 93mA as the safety current7 allowed which would require a 300Ω resistor.
However, as it was realised that modern components were accurate and reliable, the duplication of safety margin was deemed no longer necessary so the ⅔ factor could be increased resulting in higher power '240Ω barriers as shown below.
As a guide typical figures are:
Intrinsic Safety System
Compliance for each Intrinsically safe loop loop is demonstrated in the Descriptive System Document (DSD) based on comparison of the above parameters.
Not all parameters are necessarily specified, some may be inferred, not relevant/required to ensure safety. ↩
Um is not related to the field intrinsic safety parameters but is used to establish the power rating of components (primarily Zener diodes) under fault conditions to ensure they can continuously withstand that fault condition. Note that the connected equipment in the non-hazardous area must be supplied from a voltage less than Um. ↩
For a conventional Zener barrier this usually calculated from the Voltage and current parameters *Po = (Uo Io)/4**. However, depending on the output circuitry of an isolation interface this may not be the case. ↩
The circuit included all field devices and cable (see Intrinsically Safe Systems ↩
Both Inductance and resistance are proportional to length the ration L/R of the two is constant for any cable. As the spark potential is proportional to inductance and inversely proportional to resistance in effect the cancel each other out. This of often used when the Inductance alone is a limiting factor in determining the length of the cable allowed. ↩
The Li/Ri is used for cables and is very rarely applicable to any other field devices. ↩
Note This is safety parameter and not the actual current available. ↩