SPM- Safety Protection Measures involves various protection techniques. One of the main protection technique is Equi potential bonding. Earthing and bonding should always be ensured, in particular, bonding of every conductive service directly or via an equipotential bonding SPD, at the point of entry to the structure. To explain the concept in a simple manner, earthing arrangement & equi potential bonding in a house is explained below. The same can be extrapolated to complex industrial applications.
Lightning equipotential bonding (EB) in accordance with IS/IEC 62305-3 will protect against dangerous sparking only. Protection of internal systems against surges requires a coordinated SPD system in accordance with IS/IEC 62305-4.
Equipotential bonding SPDs are always required at the entrance of the LPZ to bond incoming lines, which are connected to the internal systems within the LPZ, to the bonding bar. Using an interconnected or extended LPZ can reduce the number of SPDs required.
Existing power-frequency earthing systems might not provide a satisfactory equipotential bonding plane for lightning currents with frequencies up to several MHz, because their impedance may be too high at these frequencies.
Even an LPS designed in accordance with IEC 62305-3, which allows mesh widths typically greater than 5 m, and which includes lightning equipotential bonding as a mandatory part of the internal LPS, might not be sufficient for sensitive internal systems. This is because the impedance of this bonding system may still be too high for this application.
Equipotential bonding for lightning currents with frequencies up to several MHz requires a meshed low impedance bonding network having a typical mesh width of 5 m. All services entering an LPZ should be bonded directly, or via a suitable SPD, as closely as possible to the boundary of the LPZ.
If, in existing structures, these conditions cannot be fulfilled, other suitable protective measures should be provided.