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Technical Information for Earthing and Short Circuiting
When working under the absence of voltage VDE 0105 part 100: 1997 – 10 requires the working place first of all to be clearly defined. Next the requirement to obtain and maintain a voltage free state have to be fulfilled under the observance of the 5 safety rules:
1. Switch off
2. Ensure supply cannot be re-energised
3. Verify the absence of voltage
4. Apply earthing and short circuiting device
5. Apply cover or partition against neighbouring live sections
Any deviation from these 5 rules must have a substantial cause.
When using earthing and short circuiting devices the following must be observed:
The devices must be thoroughly examined for perfect condition before use. Damaged cable insulation or protruding bare wires will exclude further usage.
The devices must only be used in switchgear where their short circuit rating is not exceeded. The maximum short circuit current is given on the short circuit and earth leads and on each short circuit bar.
Any devices which have been subjected to a full short circuit must not be re-used.
Short circuit devices, cables and busbars are dimensioned to be short circuit proof. Earthing cables such as the mutual earth cable of a 3-phase earthing and short circuiting device do not need to be short circuit proof in 3 phase balanced systems since they only divert residual currents.
Connections on earthing and short circuiting devices are either compressed or bolted. Welding or soldering is no longer applied due to the possibility of hardening of the conductor wires.
Uninsulated leads for 3 phase earthing and short circuiting devices must not be used, due to the danger of sintering if the leads are contacting parts of the switchgear in the case of a short circuit, due to electro dynamic forces. Leads are insulated with PVC, which has been found is the best compromise for cost and durability. Leads with Hypalon insulation are more flexible at low temperatures but they tend to fracture when hitting parts of metal framework. Furthermore they do not allow visual inspection of the copper wires due to the colouring.
The length of cable between two connections must not be less than 1.2 times the distance between the two connections. Excessively long short circuit cable will cause unnecessary movements and inadmissibly high voltages. The dynamic force generated in case of a short circuit is considerable and must be taken into account.
When connecting short circuiting devices with cables in parallel the following conditions must be fulfilled:
1) cables must be of identical length
2) identical lead type (cross section, stranding, material)
3) identical connection parts and pieces
4) any devices inserted must be close to each other, leads in parallel
5) loading capacity per lead must be reduced to 75% in the case of uncertainty as to current sharing
General notes, current rating, calculation of cross section
Current rating and determination of cross section
(to DIN VDE 0683 part 1 : 1988 –03)
The current rating of the short circuit cables and bars depends on the material, the cross section A, and the short circuit time T k.
Earthing and short circuiting devices must have a current rating according to the data in the following diagrams.
Depending on the material, short circuiting bars must meet the current rating according to the diagrams in figs. 4 and 5.
The formulae for calculation of minimum cross sections A in sq mm are including each a numerical value (4.1/5.07/5.54/8.62) the maximum initial short circuit alternating current l k” in kA and the short circuit time T k in seconds. The indicated rating allows for temperature reducing influences and refers to lead end temperatures of 250°C or 400°C for devices for railway earthing.
In all calculation formulae the reference short circuit current is the initial short circuit alternating current l k” which equals the sustained short circuit current l k” resp. the disconnection alternating current l a. This complies with the most critical case, when the short circuit is most remote from the generator. It is not permissible to reduce the minimum times T k for the thermical rating of the leads or busbars stated in the tables as the dynamic effect of the instantaneous short circuit current must be considered. For this reason the curved shape in the diagrams for lower values is limited by horizontal lines.
The family of curves in the current load capacity diagrams is based on an initial short circuit alternating current lk” in case of a short circuit most remote from the generator (K = 1.8).
The highest peak value of the instantaneous current ls is calculated as follows:
I s” = K • √2 • l k ” = 2.54 • l k”
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