For panel boards addressed to Hazardous Areas, ‘Ex d’ and ‘Ex e’ executions Many customers advise us the need to have a dimensional...
Many customers advise us the need to have a dimensional criteria for enclosures used as distribution panels, motor starters, control, signaling and marshalling boxes.
Since this is a topic that covers many different aspects of dimensional character/construction, with this first part of the topic we intend to highlight the variables inherent to the proper sizing of enclosures suitable for installation in hazardous areas, from the mechanical and electrical point of view.
In order to make a correct sizing of enclosures for the uses described above, it is essential that all the parameters are systematically taken into account and used as a method of calculation for determining the size of such enclosures.
Considering that these enclosures must be suitable for installation in hazardous areas (‘Ex d’, ‘Ex de’) and they are, therefore subject to certification by qualified entities recognized at international level, is of utmost importance that these sizing meet all the parameters set by the reference Standards, such as:
This task belongs mainly to the manufacturer of such equipment, as prescribed by the regulations listed above; however, we intend to provide our customers a wide description of the criteria that we usually take to realize the sizing.
These types of enclosures, in succession called "Panel Board", which must be suitable for installation in explosion-proof areas, industrial plant, on drilling/production plant (Off-Shore), on flotation devices used in the drilling and/or production (FPSO), are divided mainly into the following types:
The types of construction for these Panel Boards consist essentially of:
The "directly" execution provides for the entry of cables directly to the Panel Board in ‘Ex d’ execution, with the use of sealed cable glands that does not allow for a possible explosion within the Panel Board to spread outside of it. In this case, the choice of the cable or sealing fitting shall meet the requirements of the Standard, as in Figure 1.
Typical of “directly” entry in ‘Ex d’ execution.
The "indirect" execution involves the use of an increased safety "Ex e" junction box for input/output cables, within which are housed the increased safety terminal blocks. From this enclosure are derived all the links from and to the explosion-proof enclosure in "Ex d" execution. The passage between the two enclosures will be through sealed locking devices and/or sealed bushings, always in ‘Ex d’ execution. In this case, the choice of the sealing fittings and/or the sealed bushings has to comply with the requirements of standard, such as from the above Figure 1.
Typical of “indirectly” entry in ‘Ex de’ execution.
One of the primary problems for the mechanical dimensioning is to verify that the quantities of the entries of the in/out cables from the panel are correctly dimensioned and arranged, according to the parameters of the certificate. This problem is attributable to the maximum possible drilling of each wall and/or of the cover that are a focal point for this dimensioning.
Since many types of enclosures require the verification of the maximum allowable quantity of entrances, please refer to the manuals for installation, use and maintenance of our enclosures series EJB..., EJB .. A..., EJBX ..., GUB ..., EJBE ..., EJBXE ...,... GUBE..., GUBE.. H..., CCA..C... and CCAI... leaving the specific description of this sizing to a following dissertation.
Another problem is to check the quantity of devices installed inside of these enclosures and the minimum respect distances, always in accordance with the parameters of the certificate and, not least, to the specifications required by customers. Also for this test, refer to the manuals for installation, use and maintenance mentioned above and a following dissertation.
The problems related to a proper electric dimensioning are numerous and can be summarized as follows:
Let's look for each issues which actions must be developed.
The rated operating current (Ie) is the value of current (in amps) that the components absorb in operation, continuous operation 24 hours and it is the value that will be considered for a correct dimensioning of the electrical conductors inside of the enclosure. It should be considered that this value must be communicated by client at the time of dimensioning request and, depending on the temperature and the temperature class in which the Panel Board will operate, the designer will have to properly size the conductors and components based on these data.
The breaking capacity (Icn) in kA and time of intervention (s) are basic values for the correct sizing and must be communicated by client at the time of dimensioning request.Such values determine all the electrical dimensioning and, therefore, are considered fundamental for the analysis of calculation.
The choice of trip curves of thermo-magnetic or magnetic circuit-breakers is also an important part in the sizing and it’s a parameter, such as the breaking capacity (Icn), which must be communicated by client at the time of dimensioning request.
The sizing depending on the environment temperature, according to temperature class and as per the subdivision of IEC 60079-0 or EN 60079-0 standards, is a further important parameter for a correct analysis. In fact, the current capacity of the devices is strongly influenced by the variation of temperature, which in turn undergoes a reduction in function of the temperature class, class of temperature that must be communicated by client at the time of dimensioning request.
temperatures which must not be exceeded due to overheating of internal components.
The coordination of electrical/filiation and selectivity is another of the dimensional variables that must be considered for the calculation. In fact, if you opt for sizing all Thermal-magnetic circuit breakers according to the breaking capacity required, provided that it would be technically correct, you would have a overheating of components and electrical conductors, according to this value. However, the construction technologies have made that, with a proper coordination of the circuit breakers, you can get, downstream of the main circuit breaker, a lower short circuit current and, consequently, adopt downstream circuit breaker suitable for the underlying value. This, in addition to allowing a suitable dimensioning, leads to a not negligible economic saving.
The sizing according to the maximum allowable power dissipation (W) is essential for a proper sizing because it affects the electric parameters of the certificate. In fact, in order to get the certificate, a series of tests have been performed in order to verify the maximum allowable thermal dissipation in the enclosures, as a function of both the temperature class and the ambient temperature. For this test, therefore, have to be taken into account the equipment individual dissipation that will be installed inside the enclosures and their sum, which mustn’t, in any case, exceed the expected value of the certificate. Obviously these values, detectable from the technical documentation of each manufacturer, will have to be remeasured as a function of the temperature and the downgrading for installation under a prevented ventilation.
The verification of the energy which pass through the electrical conductors (I2t = K2S2) is of great importance for the purposes of a correct dimensioning because all the currents caused by a short circuit that is present in any point of the circuit, must be interrupted in a time not greater than that which brings the wires to the allowable temperature limit. Therefore, it will be necessary to test the conductors based on that value. Such value of I2t must be specified by the manufacturer of the protective device.
The coefficient of contemporaneity is another of the parameters necessary for the correct electrical sizing. This coefficient must be communicated by the clientat the time of dimensioning request. It determines, in function of the simultaneous operation of the equipment, the value of the current that must be considered for sizing the electrical conductors. For contemporary, we mean the ability to be operative in continuous parallel service.
The dimensioning of the auxiliary transformer for secondary circuits, where necessary and in the presence of auxiliary circuits generated within the Panel Board, is another of the analysis parameters. This calculation is performed by summing the loads and the contemporaneity factor that they can have, considering also the peak of maximum absorption of the biggest start-up device in addition to the sum of the total loads.
The nominal voltage (Ue) is the value of the operating voltage that defines the characteristics of the use of a specific circuit. All electrical equipment that are going to be used, shall be constructed in order to be capable of operating at this voltage value, with the tolerances set by the reference standard which in principle correspond to ± 10%, unless otherwise specified in the customer request.
The insulation voltage (Ui) is the efficient value of the insulation test voltage assigned by the manufacturer of the component and/or equipment, which determines the resilience of its isolation and surface distances (the shortest distance measured along the surface of an insulator interposed between two conductive parts).
The nominal frequency (Hz) is the frequency value at which report the operating conditions of the system. All electrical equipment that are going to be used, shall be constructed in order to be capable of operating at this frequency, with the tolerances set by the reference standard which in principle correspond to ± 5%, unless otherwise specified in the customer request.
The electromagnetic compatibility of the components and/or electrical equipment is one of the requirements in the use of electrical equipment which must be taken into consideration.
We conclude this first part saying that all activities of electro-mechanical sizing are the prerogative of the manufacturer of the Panel Boards in explosion-proof execution: analysis, calculations and the resulting executive project are of its responsibility, putting the plate certifying the compliance with the relevant standards.