3.6 Selection of external Short Circuit Protection Device (SCPD)
The protection devices (thermal and short circuit) must be coordinated with the SPD to ensure reliable operation, i.e.
- ensure continuity of service:
– withstand lightning current waves;
– not generate an excessive residual voltage.
- ensure effective protection against all types of overcurrent:
– overload following thermal runaway of the varistor;
– short circuit of low intensity (impedant);
– short circuit of high intensity.
3.6.1 Risks to be avoided at end of life of the SPDs
In the case of natural end of life due to ageing, protection is of the thermal type. SPD with varistors must have an internal disconnector which disables the SPD.
Note: End of life through thermal runaway does not concern SPD with gas discharge tube or encapsulated spark gap.
The causes of the end of life due to a short-circuit fault are:
– Maximum discharge capacity exceeded.
This fault results in a strong short circuit.
– A fault due to the distribution system (neutral/phase switchover, neutral
– Gradual deterioration of the varistor.
The latter two faults result in an impedant short circuit.
The installation must be protected from damage resulting from these types of fault: the internal (thermal) disconnector defined above does not have time to warm up, hence to operate.
A special device called “external Short Circuit Protection Device (external SCPD) “, capable of eliminating the short circuit should be installed. It can be implemented by a circuit breaker or fuse device.
3.6.2 Characteristics of the external SCPD (Short Circuit Protection Device)
The external SCPD should be coordinated with the SPD. It is designed to meet the following two constraints:
Lightning current withstand
The lightning current withstand is an essential characteristic of the SPD’s external Short Circuit Protection Device.
The external SCPD must not trip upon 15 successive impulse currents at In.
Short-circuit current withstand
- The breaking capacity is determined by the installation rules (IEC 60364 standard):
The external SCPD should have a breaking capacity equal to or greater than the prospective short-circuit current Isc at the installation point (in accordance with the IEC 60364 standard).
- Protection of the installation against short circuits
In particular, the impedant short circuit dissipates a lot of energy and should be eliminated very quickly to prevent damage to the installation and to the SPD.
The right association between an SPD and its external SCPD must be given by the manufacturer.
3.6.3 Installation mode for the external SCPD
The SCPD is described as “in series” (see Fig. J33) when the protection is performed by the general protection device of the network to be protected (for example, connection circuit breaker upstream of an installation).
The SCPD is described as “in parallel” (see Fig. J34) when the protection is performed specifically by a protection device associated with the SPD.
- The external SCPD is called a “disconnecting circuit breaker” if the function is performed by a circuit breaker.
- The disconnecting circuit breaker may or may not be integrated into the SPD.
Note: In the case of an SPD with gas discharge tube or encapsulated spark gap, the SCPD allows the current to be cut immediately after use.
Note: S type residual current devices in conformity with the IEC 61008 or IEC 61009-1 standards comply with this requirement.