This article looks at the factors to be considered when providing protection against overvoltage in a domestic premises having a service fuse of rating not exceeding 100 A per phase.
Once it has been determined that overvoltage protection is required at a domestic premises it is essential that the system put in place is sufficiently robust to provide the required level of protection throughout the electrical installation.
Factors to consider
The following factors have to be taken into account when designing and installing an overvoltage protection system:
- Does the structure have an external lightning protection system?
- What is the system earthing arrangement at the premises?
- Is there sensitive equipment within the installation that may require local protection?
- What are the distances between overvoltage protective devices and the equipment to be protected?
Does the structure have an external lightning protection system?
Where SPDs are required to protect the electrical installation in a building having an external lightning protection system (LPS) or other methods of protection against direct lightning strike, Type 1 SPDs, also known as equipotential bonding SPDs, must be installed as close as possible to the origin of the electrical installation (534.4.1.3).
These Type 1 SPDs are installed to prevent dangerous sparking which could lead to fire or electric shock hazards. However, such SPDs, installed alone, do not provide protection against failure of sensitive electrical and electronic systems. As a result, it is necessary to install additional SPDs of Type 2 and/or Type 3 to protect equipment within the installation (see Note to 534.4.1.1). Typically, most modern equipotential bonding SPDs will be combination Type 1 + 2 devices.
Where Type 1 SPDs are installed at, or near, the origin of the installation, each live conductor shall be connected to either the main earthing terminal or the main protective conductor, whichever route is shorter. This is known as Connection Type 1 (CT 1), or common mode protection. See Fig 1.
Where the Type 1 SPDs are selected using BS 7671, rather than carrying out a risk analysis in accordance with BS EN 62305-2, the impulse discharge current of the SPD (Iimp) shall not be less than the values stated in Table 534.4 (534.4.4.4.2). For Connection Type 1, the minimum permissible Iimp is 12.5 kA whether connection is between L-PE or N-PE for both 1-phase and 3-phase supply systems.
Where a structure has an LPS, a coordinated SPD system connected in both power and signal lines is required (Clause 7 of BS EN 62305-41) as part of an overall surge protection management plan (Clause 9.2 of BS EN 62305-4). As a result, where a structure has an LPS, the surge protective measures should preferably be designed in accordance with the BS EN 62305 series and not Sections 443 and 534 of BS 7671.
System earthing arrangement at the premises
Where SPDs are required by Section 443 to protect the electrical installation in a building not having an external LPS, or not requiring protection against the effects of direct lightning, Type 2 SPDs must be installed as close as possible to the origin of the electrical installation (534.4.1.4).
Where the building is not protected against direct lightning strike, the Type 2 SPDs for use at or near the origin of the installation shall be selected based on their minimum nominal discharge current (Inspd) and the Connection Type employed (534.4.4.4.1).
Where Type 2 SPDs are installed at, or near, the origin of the installation, the choice of Connection Type that is required is dependent on the system earthing arrangement.
Combination Type 2+3 devices are readily available.
TN system
Where the installation forms part of a TN system, in general both Connection Types CT1 and CT2 may be employed.
Whereas Connection Type CT1, described earlier in this article, sees line and neutral conductors connected to protective earth (PE), with Connection Type 2 (CT2) the SPD is connected between line and neutral and between neutral and PE via either the main earthing terminal or the protective conductor, whichever route is shorter. Connection Type 2 (CT 2) is also referred to as differential mode protection (534.4.2 & 534.4.3). See Fig 2.
TT system
Typically, an RCD providing fault protection will be situated at or near the origin of an installation forming part of a TT system. The presence of this device affects the choice of permissible Connection Type.
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Where Connection Type 1 (CT1) is used, the SPD arrangement should be installed downstream of any RCD at or near the origin to provide fault protection. See Fig 3.
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Where Connection Type 2 (CT2) is used the SPD arrangement should be installed upstream of any RCD at or near the origin to provide fault protection. See Fig 4.
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It should be noted that it is not recommended to install Type 1 SPDs downstream of an RCD (534.4.6).
Table 1 is based partially on Table 534.5 of BS 7671 and summarises the possible modes of protection that may be required for various LV systems in domestic premises.
Is there sensitive equipment within the installation that may require local protection?
In a typical domestic premises, containing more commonplace, readily available and, relatively speaking, moderate value electrical equipment the overvoltage protection installed at the origin will generally be sufficient regardless of circuit length.
However, where high-value items such as high-end home cinema and HiFi equipment or hard to replace items of equipment are present, additional overvoltage protection from Type 2 + 3 SPDs should be installed close to the equipment to be protected. These may be situated in fixed socket-outlets or in mobile socket-outlet trailing leads conforming to relevant product standards, see Fig 5 (534.4.1.1) or within consumer units serving the equipment to be protected (534.4.1.5).
What are the distances between overvoltage protective devices and the equipment to be protected?
If the distance between an SPD and the equipment to be protected exceeds 10 m, it may be necessary to install additional SPDs between those at or near the origin and those situated close to sensitive equipment (534.4.4.2). Additional SPDs may also be required if the protection level (UP) of the SPD at/near the origin of the installation is higher than the impulse immunity of the equipment.
In general, neither of the above factors is applicable in most domestic premises, as there are typically no inductive loads within the structure which could be a source for switching transients downstream of the SPD installed at the origin of the installation.
What about home offices or workshops external to the dwelling?
Many people now work from home either fully or partially in small offices or workshops. Where these are situated in the main body of the domestic premises, as mentioned previously in this article, the overvoltage protection provided at the origin of the installation is likely to offer sufficient protection against overvoltage.
However, if the home office or workshop is in a building separate to, but supplied from, the dwelling overvoltage protection may be required as a result of the distance between the SPD at the origin of the dwelling exceeding 10 m and where the damage caused by an overvoltage could result in significant financial or data loss (443.4.1).
This overvoltage protection may be incorporated in a small locally installed consumer unit (534.4.1.5).
Summary
In general, for a 'typical' domestic dwelling, a single surge protective device (SPD) installed in or close to the consumer unit serving the property will be sufficient. However, in some cases, such as where connected equipment of significant value is present or where a supply extends to buildings separate to the dwelling, it may be necessary to install further suitably coordinated SPDs of the correct type situated close to the items to be protected.
We would like to acknowledge the kind assistance of Samad Khan of ABB Furse in the production of this article.
Footnote
1 BS EN 62305-4:2011. Protection against lightning Part 4: Electrical and electronic systems within structures.
