Chapter A General rules of electrical installation design

B 0
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B - Connection to the MV public
distribution network
Supply of power at medium
voltage
The strategy in this case, is to reduce the resistance of the substation earth
electrode, such that the standard value of 5-second withstand-voltage-to-earth for
LV equipment and appliances will not be exceeded.
Practical values adopted by one national electrical power-supply authority, on its
20 kV distribution systems, are as follows:
b Maximum earth-fault current in the neutral connection on overhead line distribution
systems, or mixed (O/H line and U/G cable) systems, is 300 A
b Maximum earth-fault current in the neutral connection on underground systems is
1,000 A
The formula required to determine the maximum value of earthing resistance Rs at
the
the
substation,
substation,
to
to
ensure
ensure
that
that
the
the
LV
LV
withstand
withstand
voltage
voltage will
will
not
not
be
be
exceeded,
exceeded,
is:
is:
Uw �Uo
Rs = in ohms (see cases C and D in Figure B10). C10).
Im
Where
Where
Uw = the lowest standard value (in volts) of short-term (5 s) withstand voltage for the
consumer’s installation and appliances = Uo + 1200 V (IEC 60364-4-44)
Uo = phase to neutral voltage (in volts) at the consumer’s LV service position
Im = maximum earth-fault current on the MV system (in amps). This maximum earth
fault current Im is the vectorial sum of maximum earth-fault current in the neutral
connection and total unbalanced capacitive current of the network.
A third form of system earthing referred to as the “IT” system in IEC 60364 is
commonly used where continuity of supply is essential, e.g. in hospitals, continuousprocess
manufacturing, etc. The principle depends on taking a supply from an
unearthed source, usually a transformer, the secondary winding of which is
unearthed, or earthed through a medium impedance (u1,000 ohms). In these cases,
an insulation failure to earth in the low-voltage circuits supplied from the secondary
windings will result in zero or negligible fault-current flow, which can be allowed to
persist until it is convenient to shut-down the affected circuit to carry out repair work.
Diagrams B, D and F (Figure B10)
They show IT systems in which resistors (of approximately 1,000 ohms) are included
in the neutral earthing lead.
If however, these resistors were removed, so that the system is unearthed, the
following notes apply.
Diagram B (Figure B10)
All phase wires and the neutral conductor are “floating” with respect to earth, to which
they are “connected” via the (normally very medium) insulation resistances and (very
small) capacitances between the live conductors and earthed metal (conduits, etc.).
Assuming perfect insulation, all LV phase and neutral conductors will be raised by
electrostatic induction to a potential approaching that of the equipotential conductors.
In practice, it is more likely, because of the numerous earth-leakage paths of all live
conductors in a number of installations acting in parallel, that the system will behave
similarly to the case where a neutral earthing resistor is present, i.e. all conductors
will be raised to the potential of the substation earth.
In these cases, the overvoltage stresses on the LV insulation are small or nonexistent.
Diagrams D and F (Figure B10)
In these cases, the medium potential of the substation (S/S) earthing system acts on
the isolated LV phase and neutral conductors:
b Through the capacitance between the LV windings of the transformer and the
transformer tank
b Through capacitance between the equipotential conductors in the S/S and the
cores of LV distribution cables leaving the S/S
b Through current leakage paths in the insulation, in each case.
At positions outside the area of influence of the S/S earthing, system capacitances
exist between the conductors and earth at zero potential (capacitances between
cores are irrelevant - all cores being raised to the same potential).
The result is essentially a capacitive voltage divider, where each “capacitor” is
shunted by (leakage path) resistances.
In general, LV cable and installation wiring capacitances to earth are much
larger, and the insulation resistances to earth are much smaller than those of the
corresponding parameters at the S/S, so that most of the voltage stresses appear at
the substation between the transformer tank and the LV winding.
The rise in potential at consumers’ installations is not likely therefore to be a problem
where the MV earth-fault current level is restricted as previously mentioned.
Schneider Electric - Electrical installation guide 2008