056 091 Equipotential Earth Bonding

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Author: J.Z. Page 1 of 2 056-091 ISSUE: 1 
 
 
 
 
 
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: 
 
o A DSE module connected to a 3 phase, 4 wire genset system. 
o Star point and battery negative connected to the same earth point, both being well terminated with 
adequate sized cable. 
o 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. 
o In normal operation the pd between all earth points is close to 0 V, and the system operates correctly. 
o 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. 
o 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 
 
o 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. 
o The layout of the system must be considered. Cables, terminals etc. must be in locations suitable for the 
function and serviceability of the system. 
o There must be no pd across all earth terminals and the S2 connection of each CT must be connected to 
earth. 
o 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 
 
 
056-091 ISSUE: 1 Page 2 of 2 Author: J.Z. 
 
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 
 
Voltage difference across the earth circuit 
may also lead to incorrect sensor readings 
and possibly sensor damage. 
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. 
A high potential at the 
CT common terminal 
with respect to the 
star point earth may 
cause damage to 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.