The sizing of the cable trays and devices follows the maximum load currents expected at this level of energy distribution systems.
As a rule:
je b max = Capacity installed capacity x simultaneity factor
The switching/protection device and the connection line must be suitable with regard to overload and short-circuit protection.
In order to ensure protection against overloads, the standard conventional non-tripping currents relating to the devices used must be observed. A check based solely on the rated current of the device or the setting value I r would be insufficient.
Basic rules for ensuring overload protection:
Current rule noted
Non-adjustable protective equipment
i b ≤ I n ≤ I z
The nominal current je n of the selected device must be between the calculated maximum load current je b and the maximum admissible load current je z of the selected transmission medium ( cable or busbar ).
Adjustable protective gear
i b ≤ I r ≤ I z
The rated current i r of the overload release must be between the calculated maximum load current i b and the maximum allowable load current i z of the selected transmission medium ( cable or busbar ).
Triggering the current rule
i 2 ≤ 1.45 x I z
The maximum allowable load current i z of the selected transmission medium ( cable or busbar ) must be greater than the conventional tripping current i 2 – / 1.45 of the selected device.
The test value i 2 is standardized and varies depending on the type and characteristics of the protective equipment used.
Basic rules for ensuring short-circuit protection:
Short circuit energy
K 2 S 2 ≥ I 2 t
( K = Material coefficient; S = Cross-section)
The amount of energy released when a short circuit occurs – and until it is automatically eliminated – must be less than the energy that the transmission medium can carry to the maximum otherwise irreparable damage will be caused. As a rule, this basic rule applies in the time range up to max. 5 sec.
Below 100 ms short-circuit breaking time, the passing energy of the protective device ( according to the specifications of the equipment manufacturer ) must be taken into account.
When devices with a tripping unit are used, compliance with this rule over the entire characteristic curve of the device must be verified. A simple check in the range of the maximum applied to short-circuit current ( i k max ) is not always sufficient, especially when delayed releases are used.
Short circuit time
t a (JE k min ) ≤ 5 s
The cut-off time resulting from the selected protective equipment must ensure that the calculated minimum short-circuit current i k min at the end of the transmission line or protected line is automatically cleared within 5 s at most.
Overload and short-circuit protection need not necessarily be provided by one and the same device. If necessary, these two protection targets can be achieved by a combination of devices. The use of separate switching protection devices could also be considered, i.e. at the start and at the end of a cable run. Typically, devices applied at the end of a cable tray can only provide overload protection.
The method for coordinating overload and short circuit protection is virtually identical for energy distribution systems and final circuits. Besides overload and short circuit protection, the protection of human life is also important for all circuits.
Protection against electric shock
t a (JE k1 min ) ≤ t a permanent
If a single-phase earth fault (I k1 min ) occurs, the resulting current breaking time t one for the chosen protection equipment must be shorter than the maximum allowable breaking time t one permanent this is necessary for this circuit in accordance with standard IEC 60364-4-41 / DIN VDE 0100-410 to ensure the protection of people.
The required maximum current outage time varies depending on the rated system voltage and the type of load connected ( stationary and non-stationary loads ), protection requirements regarding minimum outage time t a perm can be transferred from a circuit of load to another circuits. Alternatively, this protection objective can also be achieved by observing a maximum contact voltage.
As end circuits are often characterized by long supply lines, their sizing is often affected by the maximum allowable voltage drop.
Regarding the choice of switching protection Regarding devices, it is important to keep in mind that long connection lines are characterized by high impedances, and therefore strong attenuation of short-circuit currents calculated.
Depending on the operating mode of the system ( open coupling, closed coupling ) and the supply medium (transformer or generator), the protection equipment and its settings must be configured for the worst-case scenario for short-circuit currents.
Unlike power supply or distribution circuits, when the choice of a high-quality trip unit is considered very important, the protective equipment of the final circuits is not subject to any special requirements with regard to the degree selectivity to be achieved.
The use of a trip unit with LI characteristics is normally sufficient.
summary of energy distribution systems
Basically, the sizing process itself is easy to understand and can be done using simple means. Its complexity lies in the acquisition of technical data on products and systems Mandatory fields. This data can be found in various technical standards and regulations as well as in many product catalogs.
An important aspect in this context is the cross-circuit handling of the dimensioned components due to their technical data. One of these aspects is the aforementioned inheritance of the minimum power outage times of the non-stationary load circuit compared to other stationary load or distribution circuits.
Another aspect is the mutual impact of network sizing and calculation (short circuit), for example, for the use of devices limiting the short circuit current.
In addition, the complexity of the issue increases when different national standards or practices need to be considered for sizing.