Earthing systems are required
to manage the transfer of fault energy via a low impedance path to limit the
risk to people, equipment and system operation to acceptable levels. An
earthing system is required to perform this function for the life of the
electrical plant for which it is installed, for the range of configurations of
the network and nearby infrastructure that are foreseeable. The earthing system
may need to be augmented over time so as to continue to fulfill this function.
The energy which earthing
systems must manage comes from a range of sources and system events, including:
Generating plant,
Conductively coupled earth fault current,
Inductively coupled earth fault current,
Lightning discharges,
Transient discharges (e.g. switching surges)
Capacitively coupled induction
Identification of these
sources and any special considerations is part of the earthing system design
and is
outside the scope of this
document. Any requirements that impact on the earthing layout design will be
provided as an input to the earthing layout design.
The substation earthing must
achieve the following objectives:
Successful Protection Operation
Safety Compliance
Interference Minimisation
Robustness
Protection of Electrical Plant
The earthing system is
required to ensure proper operation of protective devices such as protection
relays and surge arresters to maintain system reliability within acceptable
limits. It is intended to provide a potential reference for these devices and
to limit the potential difference across these devices. The earthing system is required
to achieve the desired level of system reliability through: Facilitating the
proper and reliable operation of protection systems during earth faults. This
entails reliable detection of earth faults and either clearing the fault or
minimising the resulting fault current. Limiting equipment damage (by enabling
protection systems to operate correctly and thereby limiting the duration of
earth faults) and the consequent need for repair or replacement,Limiting or
reducing interference to substation secondary system equipment (e.g. SCADA).
The earthing system is
required to manage any hazardous potential differences to which personnel or
members of the public may be exposed. These potential differences include:
Touch Voltages (including transferred touch voltages)
Step Voltages
Hand-Hand Voltages