Equipotential Earth Bonding
1 OVERVIEW
In certain situations, DSE modules may be susceptible to damage caused by a potential difference (pd) across earth
connections. This leads to the creation of an unforeseen fault current path through the module. A typical example of
such a scenario would be:
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A DSE module connected to a 3 phase, 4 wire genset system.
Star point and battery negative connected to the same earth point, both being well terminated with
adequate sized cable.
4 current transformers (CT’s) in use, measuring L1,L2,L3 and N. S2 of each is connected to CT common
on the DSE module and a common earth point. The termination to the earth point is sufficient, however a
relatively high impedance is present between this earth point and the star point earth connection.
In normal operation the pd between all earth points is close to 0 V, and the system operates correctly.
An earth fault causes current to flow through the earth circuitry. The star point and battery negative earth
terminals remain at a near-equal potential, even as the fault current rises. Conversely, impedance
between the CT common earth and star point earth causes a pd across the circuit, which rises with
increasing current flow.
The pd is seen across the CT common and battery negative terminals of the DSE module. This gives rise
to a fault current path through the module, causing erroneous sensor readings and damage, the extent of
which is solely limited by the duration in which the current is allowed to flow.
2 INSTALLATION CONSIDERATIONS
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Each component of the earthing circuit must be adequately specified to handle the demands of the
system it is designed to protect, including calculated fault currents.
The layout of the system must be considered. Cables, terminals etc. must be in locations suitable for the
function and serviceability of the system.
There must be no pd across all earth terminals and the S2 connection of each CT must be connected to
earth.
The impedance of the earthing circuit must be low enough to negate the possibility of fault currents
causing significant pd’s, such as the condition described above.
Continued overleaf
Author: J.Z.
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056-091 ISSUE: 1
3 EXAMPLE SYSTEM
NOTE: The diagram shows a generic earthing configuration. For further information on earthing
conventions, such as floating or positive earth, refer to the relevant DSE operator’s manual.
NOTE: For further details of earth fault protection, refer to DSE Publication: 056-019 Earth Fault
Protection
The diagram below shows a typical 3 phase, 4 wire, star (wye) generator system connected to a generic DSE module
S2 of each CT must
be connected to earth.
If not connected
dangerous voltage
levels could be
induced across the CT
windings, leading to
incorrect current
readings, damage to
the CT and, possibly
the DSE module.
Examples of earth faults include:
Breakdown of conductor
insulation creating a leakage path
to earth, a fault on the load or a
lightning strike to the system.
A high potential at the
CT common terminal
with respect to the
star point earth may
cause damage to the
DSE module.
Voltage difference across the earth circuit
may also lead to incorrect sensor readings
and possibly sensor damage.
056-091 ISSUE: 1
Page 2 of 2
Author: J.Z.