Surge Protective Device (SPD) and Residual Current Device (RCD)

Best practice for using surge protective device and Residual Current Device

Created by: Glen Zhu | Updated Date: June 8th, 2024

Surge Protective Device (SPD) and Residual Current Device (RCD)

Best practice for using surge protective device (SPD) and Residual Current Device (RCD) together

Where the power distribution system incorporates Residual Current Device (RCD) transient activity could cause Residual Current Device (RCD) to operate and hence loss of supply. Surge protective device (SPD) should be installed upstream of RCD wherever possible to prevent unwanted tripping caused by transient over-voltages.

Where surge protective device (SPD) are installed in accordance with BS 7671 534.2.1 and are on the load side of a residual current device, an Residual Current Device (RCD) having an immunity to surge currents of at least 3 kA 8/20µs, shall be used.

Important notes: S type Residual Current Device (RCD) satisfy this requirement. In the case of surge currents higher than 3 kA 8/20µs, the RCD may trip causing interruption of the power supply.

If the surge protective device (SPD) is installed downstream of the Residual Current Device (RCD), the RCD should be of the time-delayed type with an immunity to surge currents of at least 3kA 8/20µs. Section 534.2.2 of BS 7671 details the minimum SPD connection requirements (based on the SPD modes of protection) at the origin of the installation (typically a Type 1 SPD).

If you are not familiar with surge protective devices operation and types, you better read the basics of surge protective devices first.

Surge Protective Device (SPD) connection type 1 (CT1)

An surge protective device (SPD) configuration based on connection type 1 (CT1) is for TN-C-S or TN-S earthing arrangements as well the TT earthing arrangement where the surge protective device (SPD) is fitted downstream of the Residual Current Device (RCD).

Figure 1 – Surge protective device (SPD) installed on the load side of residual current device (RCD)

In general, TT systems require special attention because they normally have higher earth impedances which reduce earth fault currents and increases the disconnection times of Overcurrent Protective Device (OCPD).

Therefore in order to meet the requirements for safe disconnection times, RCDs are used for earth fault protection.

Surge Protective Device (SPD) connection type 2 (CT2)

An surge protective device (SPD) configuration based on connection type 2 (CT2) is required on a TT earth arrangement if the SPD is upstream of the residual current device (RCD). The RCD being downstream of the SPD would not operate should the SPD become defective.

Figure 2 – Surge protective device (SPD) installed on the supply side of residual current device (RCD)

The surge protective device (SPD) arrangement here is configured such that the SPDs are applied between the live conductors (live to neutral) rather than between live conductors and the protective conductor.

Should the surge protective device (SPD) become defective it would, therefore, create a short circuit current rather than an earth fault current and as such would ensure that the overcurrent protective device (OCPD) in-line with the surge protective device (SPD) safely operate within the required disconnection time.

A higher energy surge protective device (SPD) is used between neutral and the protective conductor. This higher energy SPD (typically a spark-gap for a Type 1 SPD) is required as lightning currents arise towards the protective conductor and as such this higher energy SPD sees up to 4 times the surge current of the SPDs connected between the live conductors.

Clause 534.2.3.4.3, therefore, advises that the surge protective device (SPD) between neutral and the protective conductor is rated at 4 times the magnitude of the SPD between the live conductors.

Therefore, only if the impulse current Iimp cannot be calculated, 534.2.3.4.3 advises that the minimum value Iimp for a surge protective device (SPD) between neutral and the protective conductor is 50kA 10/350µs for a 3 phase CT2 installation, 4 times 12.5kA 10/350µs of the SPDs between the live conductors.

The CT2 surge protective device (SPD) configuration is often referred to as the ‘3+1’ arrangement for a 3-phase supply.

Surge protective device (SPD) and TN-C-S earth configurations

The minimum surge protective device (SPD) connection requirements at or near the origin of the installation for a TN-C-S system requires further clarification as Section 534 of BS 7671 illustrates (see Figure 3 below) a Type 1 surge protective device (SPD) being required between the live and PE conductors – the same as required for a TN-S system.

Figure 3 – Installation of Types 1, 2 and 3 surge protective device (SPD), for example in TN-C-S systems

The term ‘at or near the origin of the installation’ creates ambiguity given the fact that the word ‘near’ is not defined. From a technical point of view, if a surge protective device (SPD) is applied within a 0.5m distance of the PEN split to separate N and PE, there is no need to have an SPD protection mode between N and PE as shown in the figure.

If BS 7671 would allow the application of a surge protective device (SPD) to the TN-C side (utility side) of the TN-C-S system (observed in some parts of Europe), then it may be possible to install SPDs within 0.5m of the PEN split to N and PE and omit the N to PE SPD protection mode.

However as a surge protective device (SPD) can only be applied on the TN-S side (consumer side) of the TN-C-S system, and given SPDs are typically installed at the main distribution board, the distance between the surge protective device (SPD) installation point and the PEN split will almost always be greater than 0.5 m, so there is a need to have a surge protective device (SPD) between N and PE as required for a TN-S system.

As Type 1 surge protective device (SPD) is specifically installed to prevent the risk of loss of human life (to BS EN62305) through dangerous sparking which could present a fire hazard for example, in the interests of safety alone, the engineering judgment is that a surge protective device (SPD) should be fitted between N and PE for a TN-C-S system as it would in a TN-S system.

In summary, as far as Section 534 is concerned, TN-C-S systems are treated the same as TN-S systems for the selection and installation of surge protective device (SPD).

The basics of surge protective device (SPD)

A surge protective device (SPD) is a component of the electrical installation protection system. This device is connected to the power supply in parallel with the loads (circuits) that it is intended to protect (see Figure 4). It can also be used at all levels of the power supply network.

This is the most commonly used and most practical type of overvoltage protection.

The principle of Surge Protection Operation

Surge protective device (SPD) are designed to limit transient over-voltages due to lightning or switching and divert the associated surge currents to earth, so as to limit these over-voltages to levels that are unlikely to damage the electrical installation or equipment.

Figure 4 – Principle of protection system in parallel

Types of surge protective device (SPD)

There are three types of surge protective device (SPD) according to international standards:

  • Type 1 Surge Protective Device (SPD)

Protection against transient over-voltages due to direct lightning strokes.

The Type 1 surge protective device (SPD) is recommended to protect electrical installations against partial lightning currents caused by direct lightning strokes.

It can discharge the voltage from lightning spreading from the earth conductor to the network conductors.

Type 1 Surge Protective Device (SPD) is characterized by a 10/350µs current wave.

  • Type 2 Surge Protective Device (SPD)

Protection against transient over-voltages due to switching and indirect lightning strokes.

 The Type 2 Surge Protective Device (SPD) is the main protection system for all low-voltage electrical installations.

Installed in each electrical switchboard, it prevents the spread of over-voltages in the electrical installations and protects the loads.

Type 2 SPD is characterized by an 8/20µs current wave.

  • Type 3 Surge Protective Device (SPD)

Type 3 Surge Protective Device (SPD) is used for local protection for sensitive loads. These SPDs have a low discharge capacity.

They must therefore only be installed as a supplement to a Type 2 Surge Protective Device (SPD) and in the vicinity of sensitive loads.

They are widely available as hard-wired’ devices (frequently combined with Type 2 Surge Protective Device (SPD) for use in fixed installations).

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