According to the standard, if the installation has anything to do with human safety (i.e. a hospital or surgery or care home) or public service (i.e. library, IT data centre, transport) or commercial activity (i.e. trade park, industrial unit, workshop, hotel, farm etc.) then fitting surge devices is a default, yes it must be done, there is no need to do a risk assessment. Surges occur not just because of lightning strikes but often due to other electrical devices in the neighbourhood. This is because insurance companies have identified these areas as being the highest risk and most costly to pay out, a surge device mitigates against damage due to those damaging surges.
Firstly, we need to ask if the supply is Single Phase or Three Phase.
Is the supply current less than but equal too 125A or more than 125Amp? This will help determine the type of backup protection required.
For overhead supplies, a Type 1 surge device must be used. The supply is likely to be a TT network and if it has been upgraded it may be TNCS, in any event, the same type of SPD can be fitted.
The latest IET standards show two connection types. The CT1 for TN and CT2 is for TT the CT2 can also be used for TN supplies.
The devices that can be used on overhead lines are for the CT2 connection method. They have a gas tube in the neutral pole to stop earth faults.
The other applications for TT supplies, but which are not overhead, are caravan parks and yacht marinas- in these cases, you just need a type 2 surge device.
In all other cases, the UK is served mainly via PME or TNS/TNCS supplies, in this case, a surge device can be used that meets the CT1 connection see the latest IET standards.
Please note:if there is any doubt as to the supply type then the CT2 surge devices above are suitable for the TNS/TNCS supplies. Also, note that the devices for TNS/TNCS or CT1 cannot be used in TT supplies or the CT2 method, this is not safe to do as the surge device may trip the RCD if it had a fault and there is also the chance of risk to human life with leakage currents from the surge device on the wrong side of the RCD.
For the most common supply, TNS/TNCS the options are:
The last type of installation is if the building has a lightning protection system fitted. If the quality of this fitted LPS system is not known and the supply is TNS/TNCS then the default surge devices are:
This is the situation for most retail parks, industrial units and other metal framed structures.
No, the surge device at the incomer protects from induced lightning strike currents onto the supply lines or man-made surges down the grid. A Type 2 is always fitted after the Type 1. The Type 2 protects internal installation from local inductive load surges/ switching events conducted on the supply lines within a building up to ten metres, but one SPD is not 100% effective. Even the best surge devices at the incomer leave a let-through voltage and, after 10m of cable inductance, this voltage could rise requiring a second SPD closer to the end equipment. Therefore, a second surge device is required i.e. Type 2 in a sub-distribution board or the SPD can be a Type 3 device fitted nearer to the equipment to be protected. Typical Type 3 surge devices are either in the 13amp surge-protected trailing socket, or it may be OK to fit the surge device into an extra deep socket box.
The compact Type 3 device 924 396 is ideal for alarm panels and cash machines etc, otherwise, there are options for DIN mounted surge devices.
Please note:As the Type 1 and 2 surge devices are wired across the supply, no current flows through them. The Type 3 surge devices can be series wired with “dirty” as the input from the supply and “clean” from the SPD to the end equipment, it is important to ensure the maximum load current rating of the SPD is not exceeded. If it helps, consider the installing of SPD’s or over voltage devices as you would for fuses or MCBs as overcurrent devices.
The Type 1 and Type 2 SPD is with the main incomer, Type 2 is with the sub-board and the Type 3 where you may expect to see a plug top fuse or similar device. This means that there will be many SPDs within the installation, as there will already be fuses or circuit breakers. This also follows the LPZ (lightning protection zone) concept of breaking down the electrical system into zones where SPDs are to be fitted. It is also important that all SPDs within the installation are from the same supplier to ensure coordination in use (see latest IET Standards). This may mean removing the existing SPD in installation if you come across an old device not complying with the standard.
Do not forget:You may have other cables/services coming into the building and these may require SPD protection such as Telephone lines, IT lines and Solar Panels.
According to the IET standards a risk assessment must be carried out to determine if protection against transient overvoltages is required. If the risk assessment is not performed, the electrical installation shall be provided with protection against transient overvoltages, except for a single dwelling unit where the total value of the installation and equipment therein does not justify such protection.
Note:Always refer to the latest updated IET Standards.
Notes*
Part number: The 951/952 XXX series is pluggable with remote monitoring options. The 941 XXX series is not pluggable but do have visual status indication on the front of the device as per 534.2.8. For the non-remote monitored 951/952 XX5 series just replace the 5 at the end of part number with a 0. Other non-LV mains SPDs are available.
For any further guidance please contact the technical team at Europa direct on 01582 692 444.
