IET
 Topic Title: 11kV Substation Earthing Topic Summary: HV/LV earthing and touch voltage calculations Created On: 17 April 2014 08:11 AM Status: Read Only Linear : Threading : Single : Branch
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 17 April 2014 08:11 AM sime55 Posts: 2 Joined: 23 November 2004 Hi all, Could someone shed some light on the following please. I've got a modelled 11kV feeder with calculated earth fault currents of 1.2kA, with a feeder disconnection time of 0.2s the maximum touch voltage using Fig 44.2 in the regs is around 550V. If the HV and LV earths are combined then using a calculation for touch voltage I am getting = 550/1200 = 0.46ohms. This value seams very low and difficult to achieve in practice. I don't want to go down the route of segregating the earths. Am I doing this calculation correctly or is there an alternative option to investigate before recommending segregation? Cheers 17 April 2014 08:41 AM dlane Posts: 690 Joined: 28 September 2007 Hi, If you want to combine your HV / LV earthing systems then it becomes quite complex and more onerous compared to keeping them segregated. When you combine them you will transmit a higher earth fault potential around all of your earthing system so you will need an earth mat installed around your installation that bonds all of the metallic aspects of the installation together to minimise the touch potential between them. You also need the mat and earth rods placed strategically to reduce the step potential and touch potential to earth. You then need to be careful regarding the boundary of the installation and guard against the earth fault potential being transmitted outside of your substation and endangering other aspects of your site or the general public. Generally under these scenarios a specialist earthing company is called in and they will carry out the required modelling and system design so you end up with a touch and step potential map of the installation. The system would then need testing under a fall of potential test which can be awkward and potentially dangerous to carry out depending upon the installation and its location. There used to be a general guidance that to combine your earthing systems you had to have less than a 1 ohm resistance anyway, not sure if this guidance still exists. Personally I would look to keep HV and LV earthing systems segregated unless the nature of the installation prevents that in which case I call a specialist earthing company to design the installation to achieve the required protection levels. Kind regards Donald Lane 17 April 2014 09:02 AM Jaymack Posts: 5393 Joined: 07 April 2004 Originally posted by: sime55 "I've got a modelled 11kV feeder" "If the HV and LV earths are combined" Firstly, is 11kV HV? I've been accustomed to keeping separate earthing systems for such voltages, also any earth system for fencing. Regards 17 April 2014 10:02 AM sime55 Posts: 2 Joined: 23 November 2004 Originally posted by: Jaymack Originally posted by: sime55 "I've got a modelled 11kV feeder" "If the HV and LV earths are combined" Firstly, is 11kV HV? I've been accustomed to keeping separate earthing systems for such voltages, also any earth system for fencing. Regards The 11kV will have a 11/0.4kV transformer in the substation compound. hence the need to interconnecting the HV and HV earths. 17 April 2014 10:54 AM dlane Posts: 690 Joined: 28 September 2007 Hi, As long as your substation is physically big enough to maintain a safety distance between the HV and LV earths then it will be perfectly acceptable to keep the 2 earthing systems separate and this is probably how it is installed in the majority of occassions. Combining the earthing systems will have a big impact on the rest of the installation that you need to take into account. We tend to combine earthing systems where you have large complex installations that contain a mixture of HV / MV / LV apparatus physically located amongst one another such as multiple transformers / switchgear / motors installed around the site. A site with a couple of HV feeds into transformers that then feeds out into the LV distribution can usually be kept separate. Kind regards Donald Lane
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