Combination of MV/LV system can be dangerous in some cases. IEC 364-4-442 states that the earthing system in a MV/LV substation must besuch that the LV installation is not subjected to an earthing voltage of:
- Uo + 250 V: more than 5 s;
- Uo + 1,200 V: less than 5 s (Uo√3 in IT).
This means that the various devices connected to the LV network must be able to withstand this constraint.
The same standard states that if Rp > 1 Ω, the voltage Rp IhMT
must be eliminated, for example:
- in under 500 ms for 100 V;
- in under 100 ms for 500 V.
If this is not so, Rp and RN
must be separate whatever the LV earthing system. This rule, not always complied with in certain countries, often leads to the separation of the two earth connections (for MV networks with a high zero sequence fault current). If all the earth connections (substation neutral- applications) have been grouped into a single one, a rise in potential of LV frames may be observed which can be dangerous
There are a good example in " Practical Grounding, Bonding, Shielding and Surge Protection, G. Vijayaraghavan , Mark Brown , Malcolm Barnes " text book which shown as following.
Though it is theoretically possible to have a combined ground at the transformer for both MV and LV, such a practice may lead to unsafe conditions in the event of an MV to LV fault. Figure below shows the reason.
The total impedance for a fault between HV and MV winding (neglecting the line impedance and the leakage impedance of the transformer windings) is the substation ground mat resistance of 10 Ω, the NGR (neutral grounding resistance) value and the MV/LV combined ground electrode resistance assumed as 1 Ω. For a 22-kV system (with line to ground voltage of 12 700 V) the current flow is:
Ig = 12700/(35+10+1)= 276 A
This gives a figure of 276 A. This current will cause the potential of 276 V to appear on the transformer tank and through the neutral lead to the enclosures of all equipment connected in the LV system with respect to true earth potential (this is because in TN-C-S type of systems, the neutral and equipment ground are one and the same). This value is unacceptably high. In actual practice, the line and transformer impedances come into play and the value will therefore get restricted to safe values. Use of combined MV and LV grounding is therefore possible only if the ground resistance can be maintained below 1 Ω.