Surge Protective Devices In TN Systems

Fault conditions

For “protection against electric shock under fault conditions“ in TN systems, overcurrent and residual current devices have been approved. For the use of surge protective devices (SPDs) this means that these protective devices may only be arranged downstream of the devices for “protection against electric shock under fault conditions” in order to ensure that the measure to protect against life hazards also operates in the event of a failure of an SPD (surge protective device).

Surge Protective Devices In TN Systems (Guide For Low Voltage Electricians); photo credit: ienergytech.com.au

If a Type 1 or 2 surge protective device (SPD) is installed downstream of a residual current device, it has to be expected that, because of the discharged impulse current to PE, this process will be interpreted as residual current by a residual current device (RCD), and it interrupts the circuit.

Equivalents for SPD classifications

(In the following lines we will use the designation SPD Type 1,SPD Type 2, SPD Type 3)

	Definition acc. to IEC 61643	Definition acc. to EN 61643
SPDs which withstand the partial lightning current with a typical waveform 10/350 μs require a corresponding impulse test current I_impThe suitable test current I_imp is defined in the Class I test procedure of IEC 61643-1	SPD class I	SPD Type 1
SPDs which withstand induced surge currents with a typical waveform 8/20 μs require a corresponding impulse test current I_nThe suitable test current I_n is defined in the Class II test procedure of IEC 61643-1	SPD class II	SPD Type 2
SPDs that withstand induced surge currents with a typical waveform 8/20 μs and require a corresponding impulse test current I_scThe suitable combination wave test is defined in the Class III test procedure of IEC 61643-1	SPD class III	SPD Type 3

Moreover, if an SPD Type 1 is loaded with partial lightning currents it must be assumed that the high dynamics of the lightning current will cause mechanical damage on the residual current device (Figure 1 below).

This would override the protective measure “protection against electric shock under fault conditions”.

Figure 1 – RCD destroyed by lightning impulse current

Of course, this must be avoided. Therefore both lightning current arresters Type 1 and SPDs Type 2 should be used upstream of the residual current device. Hence, for SPDs Type 1 and 2, the only possible measure for “protection against electric shock under fault conditions” is using overcurrent protective devices.

The use of SPDs must therefore always be considered in conjunction with a fuse as the overcurrent protective device. Whether or not a supplementary separate backup fuse must be designated for the arrester branch, depends on the size of the next upstream supply fuse and the backup fuse approved for the SPD.

The following maximum continuous voltages apply to SPDs Type 1, 2 and 3 when used in TN systems (Figures 2 and 3 A to B):

Figure 2 – “3-0” circuit in TN-C systems

Figure 3A – “4-0” circuit in TN-S systems

Figure 3B – “3+1” circuit in TN-S systems

TN-C-S systems

Figure 4 illustrates an example of the connections for use of lightning current arresters and surge protective devices in TN-C-S systems.

It can be seen that SPDs Type 3 are used downstream of the residual current device (RCD). In this context, please note the following:

As a result of the frequency of switching surges in the terminal circuits, SPDs Type 3 are primarily employed to protect against differential mode voltages. These surges generally arise between L and N.

A surge limitation between L and N means that no impulse current is discharged to PE. Thus, this process can also not be interpreted as residual current by the RCD. In all other cases, SPDs Type 3 are designed for a nominal discharge capacity of 1.5 kA. These values are sufficient in the sense that upstream protective stages of SPDs Type 1 and 2 take over the discharge of high energy impulses. When using an RCD capable of withstanding impulse currents, these impulse currents are not able to trip the RCD or cause mechanical damage.

TN-S systems

The Figures 5 to 9 illustrate the use of SPDs as part of the lightning protection zones concept, and the required lightning and surge protective measures for a TN-C-S system.

Use of SPDs in TN-S systems // Example

SPDs used in TN systems // Examples

Example 1 // Office Building – Separation of the PEN in the main distribution board

Figure 6 – SPDs used in TN systems – Example: Office Building – Separation of the PEN in the main distribution board

Example 2 // Office Building – Separation of the PEN in the subdistribution board

SPDs used in TN systems – Example: Office Building – Separation of the PEN in the subdistribution board — Example: Office Building – Separation of the PEN in the subdistribution board

Example 3 // Industry – Separation of the PEN in the subdistribution board

Figure 8 – SPDs used in TN systems – Example: Industry – Separation of the PEN in the subdistribution board

Example 4 // Residential building

Figure 9 – SPDs used in TN systems – Example: Residential building

Reference // Lightning protection guide – DEHN + SÖHNE

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.

One Comment

Eleazar Dile
Feb 26, 2020
thank you for the information i really like to know more…
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Surge Protective Devices In TN Systems (Guide For Low Voltage Electricians)