The performance of a cable under fault conditions is an important factor in cable selection if the cable is to withstand the effects of a short circuit.
The primary concern with cables under a fault condition is the heat generated and the potential damage this may have on the cable insulation. The fault rating is based on the principle that the protective device will isolate the fault in time to limit the temperature rise within the cable.
Prysmian publish cable short circuit ratings for both the conductor and the screen/armour wire:
- the conductor fault level is known as the symmetrical or three-phase rating;
- screen/armour fault level is known as the asymmetrical or single-phase (phase to earth) rating.
Prysmian use the methods outlined in the International Electrotechnical Commission standard 60949 to calculate short circuit ratings. Values published are adiabatic. This is a worst-case scenario because it assumes the heat transferred to the surrounding insulation and sheath, should a short circuit occur, is not taken into account.
For the conductor fault calculation Prysmian uses a 90-250°C temperature rise for thermosetting polymer insulated cables and a temperature rise of 70-150°C for cables with a thermoplastic insulation. The higher temperature is the limiting temperature of the insulation in a fault. For both cable types the lower temperature is the maximum operating temperature of the cable. In theory the value quoted for the lower temperature will be conservative, since a fault is unlikely to occur when the cable is at its full load and maximum temperature.
For the screen/armour fault calculation the cable is again assumed to be at its maximum operating temperature. For armoured or screened cables this will normally be 90°C with the temperature in the screen/armour approximately 10°C lower. The limiting temperature of the insulation is 200°C for PVC and LSOH sheaths, giving a temperature rise of 80-200°C. For MDPE sheaths the limiting temperature of the insulation is 250°C, giving a temperature rise of 80-250°C, when calculating the fault rating.
When calculating faults, we quote the most frequently requested value of one second. Alternative fault values can be calculated by dividing the 1 second fault rating by the square root of the time required in seconds. This calculation is valid for times between 0.2 seconds and 5 seconds.
For example: The 3 second value of the screen would be
6.2/square root 3 = 3.5kA
Where 6.2 = the 1 second fault value and
3 = the time in seconds.
Fault sharing for aluminium screened MV cables
Prysmian quote the fault rating of the armour layer for its medium voltage armoured cable range in compliance with their specifications.
- Cables in this range have either one or three stranded copper conductors; extruded crosslinked polyethylene (XLPE) insulation, a metallic screen of overlapped copper tapes and a single circular layer of aluminium wire armour. Aluminium has single and steel has the three stranded conductors.
- The armour wire and copper screens will be bonded together at every joint and termination positions so that the external fault currents will preferentially take the parallel armour path.
- In the case of an internal cable earthfault the breakdown arc will connect through to the armour wires, which will take the current to earth.
In the event of a breakdown of the XLPE insulation, the conducting arc produced would vaporise the cable armour bedding at the fault position and bring the armour wires into circuit as a parallel path.
- The fault current will preferentially flow in the armour wire as the resistance of the copper tape will be higher
Copper Wire Screen MV cables
Direct Network Operators (DNOs) generally use copper wire screened cables manufactured in accordance with BS7870-4.10 rather than armoured cables.
Each DNO has a defined fault level on their network and will specify the fault level that the wire screen must meet.
Cables are typically manufactured to a range of copper wire screen sizes, with 50mm2 being the most commonly specified. The table below provides the one second, adiabatic rating assuming an MDPE sheath with a final sheath temperature of 250°C