So, if you must install a simple electric apparatus in a classified area, how do you verify intrinsic safety?
Mirko Torrez Contreras
Freelance Technical Translator and Writer | Certified Profibus and Profinet Engineer and Trainer I Explosion Protection Consultant and Trainer | Technology evolution storyteller
The initial disappointments
After engineers first know about the possibility of using noncertified simple electrical apparatus in intrinsically safe installations, the initial enthusiasm vanishes in two stages:
The first disappointment comes from the fact that, although simple electrical apparatuses do not require to be certified as intrinsically safe, they still require an associated energy limiting apparatus to work safely.
The second disappointment arrives when they find out that although the simple apparatus does not need a certification, it is still necessary to perform the verification or proof of intrinsic safety?for the circuit.
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The reasons behind
The reason for the first requirement is that even though a simple apparatus cannot accumulate energy, it still must be powered by an energy limiting device to avoid the effects of failures that could happen I the wiring both in the safe area as well as in the classified area.
And the second disappointment appears when the engineer tries to determine what values should be employed as entity parameters because, due to the lack of certification, they are not specified in the device.
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For simple answers, go back to the basics
Fortunately, the answer to this last question is quite simple, at least initially:
Let us remember the basic concepts:
An intrinsically safe circuit is composed by three components:
Each one of these components is capable of energy accumulation. To avoid either of them from becoming into an ignition source, each of the intrinsically safe circuit components must be energy limited by design and assessed with the adequate testing methods and devices.
These testing procedures allow for the determination of the system component’s electrical entity parameters. These entity parameters enable users to perform the proof or verification of intrinsic safety.
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The proof of intrinsic safety
The proof consists in the verification of the following mathematical relationships:
The entity electrical parameters should be informed by the supplier, and they must be obtained by performing the corresponding tests as described by the standards.
To consider a device as intrinsically safe, either as a fully intrinsically safe device or as an associated device, the determination of these entity parameters is required and mandatory.
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The “simple electrical apparatus”
There is an exception for this requirement: the group of devices that comply with the definition of “simple electrical apparatus.”
Simple electrical apparatuses must adhere to the following criteria:
If these conditions are complied with, then the protection against ignition generated by a spark is safeguarded by the source of the intrinsically safe circuit, i.e., the “associated apparatus.”
This associated apparatus shall always be assessed and certified as either Category 1 or Category 2 equipment.
Key details to consider, like temperatures…
It must be taken into consideration the fact that the verification of intrinsic safety, performed using the previous calculations, do not consider additional details such as the maximum admissible ambient temperature, or the measures for limiting or avoiding the creation of electrostatic charges, etc.
For the determination and the classification into the correct existing temperature classes, it can be assumed that the maximum surface temperature of the simple apparatus can be determined – in the simplest case – from the maximum source power (Po) supplied by the associated apparatus and the components thermal resistance to ambience. This parameter may be calculated as follows:
Po = ? Uo x Io for resistive current limitation (galvanic isolators).
Po = Uo x Io for electronic current limitation (zener barriers, optocouplers)
For temperature class T4 it is also possible to use table 3 of EN 60079-0. As the worst-case scenario power matching shall be assumed for all components such as semiconductors, resistors, electrolytic capacitors, etc.?
…, reactance,…
If the issue regarding Temperature classes has been properly taken care of, then the next consideration should be the maximum values of the connectable inductance Lo and capacitance Co.
These values have been determined by experimental methods and the corresponding graphs and tables are included in the 60079-11 standard, where the results of those experiments are condensed into the inductive minimum ignition curve and the capacitive minimum ignition curve.
These curves establish:
This is of course a simplification since any real-life circuit contains both L and C loads at the same time.
… and the type of intrinsically safe circuit.
Intrinsically safe circuits can be classified into the following two groups:
Circuits that feature a concentrated reactance, so the reactance of the components can be considered as lumped in the apparatus.
Therefore, there can be four types of circuits:
Type 1: Circuits with distributed reactance and without lumped Li or Ci
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Type 2: Circuits with distributed reactance and lumped Li without Ci
Type 3: Circuits with distributed reactance and lumped Ci without Li
Type 4: Circuits with distributed reactance and lumped Li and Ci
The verification of intrinsic safety, as described previously in this note, is valid only for the three first types of circuits.
For the fourth, additional considerations must be taken. They will be addressed in a future article.
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Two additional options
Simple electric apparatuses, either they do not feature Li and Ci reactance values, like most of the resistive equipment, or they feature a Li or Ci value, which must be considered along with the cable reactance.
Simple apparatuses usually do not feature a lumped reactance, so the question that appears is the following: how to perform the verification of intrinsic safety for circuits type 1, 2 and 3 in the case of simple apparatuses?
The answer looks trivial, initially. Let us perform the calculation for a simple dry contact, the most frequent case of simple apparatus.
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An example
We will be using a Phoenix Contact intrinsically safe galvanic isolator for digital inputs:?
The values of Ui, Ii and Pi are equal to the ones of the associated apparatus since the simple apparatus does not add additional energy to the system.
The Uo value, in the worst case, is equal to the open circuit voltage, then the maximum value of Ui will be also 9,6 V.
The Io value, in the worst case, is equal to the short circuit current, then the maximum value of Ii will be also 10 mA.
The Po value can be specified or not in the associated apparatus certificate.
If not specified, Po’s value can be determined by the method of current limitation used in the associated apparatus:
Po = ? Uo x Io for resistive current limitation (galvanic isolators).
Po = Uo x Io for electronic current limitation (zener barriers, optocouplers)
In our case:
Po = ? Uo x Io = ? x 9,6V x 10 mA = 24 mW
But if the Po value is specified in the documentation, then that is the value to be employed in the verification. For the Phoenix Contact device, we are using the certificate specified Po value of 25 mW.
Lastly, the remainder details of the proof are determination of the Lc and the Cc values to be used.
This is important, because the allowed values of Lc and Cc will define the maximum cable length admissible, at least from the intrinsic safety point of view. In most cases the maximum allowed cable length is limited by voltage drop due to the cable’s resistance rather than by its reactance values.
In our example, we can find out the cable’s Lc and Cc values by:
If we calculate based on the above values, the following cable reactance result on a maximum cable length of:
Max length due to Lc = 300 mH/1 mH/km = 300 km
Max length due to Cc = 3,6 μF/ 0,2 μF/km = 18 km
These are the maximum cable lengths for a dry contact. The lower value is the one to be selected.
Of course, long before achieving those distances the voltage drop caused by the cable resistance will make your cable run unusable.
As an example, using a cable with a 0,5 mm2 section typical resistance is 72Ω/km.
Since Vdrop = (2 x Wl x Rc) x Io
9,6 V = 10 mA x (2 x Rc x Wl)
Wl = 9,6 V / (0,010 A x 2 x72 Ω/km)
Maximum cable length = 6,6 km
Obviously, common sense must prevail, nobody would use a 6,6 km long cable to connect a dry contact. A typical and practical limit would be 600 m. But you get the idea.
It is simple, but just not that simple…
So, this is the way to perform the proof or verification of intrinsic safety when using a simple electrical apparatus. As William of Ockham ‘s famous “Ockham razor argument” states, "entities should not be multiplied beyond necessity". This is certainly a case when William’s argument becomes relevant.
Mirko Torrez Contreras?is a Process Automation consultant and trainer. He read the question that gives this article its title, and he realized after some time that the answer was really simple, but just not that simple...
Phoenix Contact?sponsors this article. The opinions exposed in this article are strictly personal. All the information required for and employed in this article is of public knowledge.