When supplying an outbuilding why do some people say put it on a TT system and others don't. I understand that it's due to potential difference but if the building is in a field with livestock, should the animal I.e a horse or cattle walk in the path of the rod under fault conditions and considering 0.5 milli ohms could kill them surely it would be better to export the earth from the origin I.e the house. You could of course put more than one rod in the ground but as there's very little in the reason this what's everyone else's opinion.
It is my understanding that the issue is exporting PME earth (a term I don't like but hey ho). If the origin is PME then it is safer to treat an outbuilding as a TT and ensure no earth connection from the PME. Its something to do with the potential for a PME earth to rise in potential in the event of a neutral fault because the neutral and earth are combined in a PME supply, causing a votlage gradient with other earths in the vicinity on the same supply earth. (But I am not clear on that point so I might be wrong). However, there is no issue with a TN-S earth being taken in to an outbuilding. its only PME (TN-C-S). Livestock have four corners (lol) so they can tolerate only 25mA or less. 25mA can kill livestock, 50mA is lethal to humans. Message was edited by: unphased
A farm should not utilise a PME earthing terminal anyway. When it comes to PME here with WPD (DNO), they will not provide a PME earthing terminal to a farm, nor a farm house if the house is on a farm. They will either provide a TN-S if they can, or not provide a terminal. Predominantly, farms are remote, and have their own pole mounted TX's, so TN-S is usually provided. You are not meant to export the PME earth as it can float above true earth potential due to volt drop caused by neutral currents. You may find a voltage potential between the exposed conductive parts and true earth. Standing on the soil and touching a metal clad socket could give you a tingle, even if the network is NOT faulty. Ever heard of the many cases of people receiving 'shocks' (tingles) when operating an outside tap bare footed? It comes up occasionally here, more so when surfers come home and wash their boards and themself down using an outside tap before going inside. The tap (if connected with copper pipework) will be at the PME potential, and the person will be at true earth potential (standing on sodden grass or a path). PME can be exported to an outbuilding if you also export the equipotential zone - this means using an earthing conductor in the submain that is sized for both use as an earthing conductor and as a main protective bonding conductor, and then providing main protective bonding to anything thats required in the outbuilding. There has been articles about exporting PME to a wooden garden shed for a socket and light, and this is deemed acceptable - It's no different from an outside socket, or someone running an extension lead outside. It's when you get to larger buildings where you need to start to consider the issue.
Not arguing with the 25 MA but I was informed by a vet it was 0.5. You say a farm shouldn't be pme lectrician but around were I live they all are. In the regs it's say that it's not recommended that farm buildings are on a T.N.C.S but can be allowed if the metal mesh in the concrete is earthed and the mesh welded together to ensure the building is at the same potential, it is however no acceptable for a farm to be on a T.N.S. the rest however is greatly appreciated as it is metal.
Yeah 95% it's says it in the regs. I've never seen one either but loads of T.N.C.S around and not that many TT farms either.
Most farms i've come across (which admittedly isn't many) have been TT, I would have thought that TNCS would be the one not allowed just like building sites and caravan sites etc,--- similar environments.
Hmm well being in the sticks I'm surrounded by them dairy arable and livestock and there all T.N.C.S. like I say there are measures that should be taken to ensure everything within the building is earthed so it's at the same potential. Milking parlours are always fitted with both rcbos and time delayed rcds for the mains switch.
Only have the red book o hand at the moment but reg 705.415.2.1 states a T.N.C.S is not recommended unless the metal mesh of the building can be earthed.
You are wrong. The Regs state that the ONLY earthing not be used is TN-C (which is barely used at the best of times). TN-C-S and TN-S can both be used, but I can assure you that here, WPD, will never provide TN-C-S to a farm. Are you sure they are TNC-S? Are you making assumtions by looking at the service head? Are they fed by their own pole mounted TX's? For example. This looks like it is a TN-C-S PME supply doesn't it? This is on a farm. But you would be very wrong to make that assumption. It may look like a PME, but it is not...... notice the dodgy phase rotation too [/i] And just for good measure, a smaller one too.....Looks like TN-C-S at first glance..... But again, it's TN-S
Ahh yes I am wrong about the T.N.S sorry it is T.N.C didn't read it properly. But no I am 100% sure there T.N.C.S I do remove the covers as its the only way you can tell, there's not much three phase round here either tbh mostly single phase installed only about 50 years ago at the earliest.
Can't see why they would be. Compare the PEFC & PSCC, this would indicate if TN-C-S or not. If TN-C-S then the readings would be the same.
Feel free to watch this, he explains it brilliantly. There is a very valid reason for these approaches.
The biggest mistake many electricians make is when they only install one single earth rod and never actually test it. You need to test but in fairness several earth rods maybe needed to reduce the impedance. Driving in a deeper earth rod is better in some cases but deeper as well as more rods is always better.
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