E54,I'm interested to know what value of Ra you would accept.given that a 30ma RCD will provide earth fault and additional protection up to 1667 ohms..(in theory!)...why is an Ra of 200 ohms not acceptable?
Unless you achieve an Ra of <5 ohms ,for example,the typical OCPD,s on final circuits will not reliably operate in an acceptable time under earth fault conditions. Therefore you are likely to be reliant on the RCD.
It seems to me that the RCD will operate in exactly the same way whether the Ra is 5 ohms,or 200 ohms.....so why bother with 5 ohms?
The question I am asking is what is your maximum Ra....and why that value?...discuss....:smilewinkgrin:
Sorry for the delay in replying wirepuller, i fell asleep as i'm also up-dating my monthly project report, i just keep the forum running in the background, and come back from time to time when i need a break. ..lol!!!
Let me start of by saying that i don't particularly like to see a TT system on domestic installations full stop, especially in the UK!! The art of creating a good TT system there, seems to have been lost since it was common practice in the rural and suburb areas of towns. Then it was common practice to aim for a 10 to 25 ohm Ra and depending on the sparks, ...often achieved better. These day's it's just not economically viable in many cases on a domestic installation, But that's not to say that a decent attempt should not be made, by at least using a DEEP driven rod, of a suitable depth and not just bunging in a super thin 1.2m rod in the ground, that in all honesty is a total waste of time!! ...
What i personally try and achieve on a TT system is a low enough Ra to trip a socket circuit on say a 16A/20A MCB, in several seconds and lower rated MCB's in 5 sec, (although an MCB can trip anywhere on the instantaneous portion of it's T/C characteristic) which was a little easier on the older type 1 MCB's and BS 1361 fuses, than it is with the new type B MCB's.
I'm not trying to meet the new or old TT system disconnection times, based on RCD protection. I would however be trying to give the installation a degree of protection should a RCD fail for whatever reason, ie ''Addition Protection''. We must also remember that disconnection times rely on the installations Zs and not necessarily on the Ra/Ze alone when a RCD fails.
I've never said that installing a good/decent TT system is going to be cheap, it's not, certainly not in the time taken to install one. Depending on the soil resistivity and conditions, could further add to costs, in terms of materials... However, i'm fairly confident that given say 4 X 3m 3/4'' rods i could achieve an Ra value of 3 to 5 ohms in many parts of the UK, and in some area's that i know, below 1 ohm, with less rods. Depending on the soil conditions, that may well also require electrode enhancers and/or modern soil conditioning chemicals.
So what does a 200 ohm Ra give you?? Absolutely nothing really, ...It achieves a 50V touch/threshold voltage, only while a 30mA RCD is functional!! The lower the Ra, the better the chance of limiting the touch/threshhold voltage that will be present in a fault condition... I can't say i know of any other national code or Reg, that has such a high max Ra value of 200 ohms. The States and Canada require 20 ohms, even France wants to see a max of 100 ohms, and they use 500mA RCDs on domestic installations....
As it so happens, we are in the final throws of completing a number of ground fields on my project, with a contract specification level of max 1 ohm for each of the ground fields, which was achieved on all of them, before all of the rods being placed. The highest ground field to date, coming in at 0.38 ohms and when all are connected together by a site ring (before connecting all project buildings) of 240mm, i've roughly calculated a final Ra of around 0.17ohms. OK, so i'm far happier working with extensive TT systems, than the much smaller ones, but even they can be made/designed to provide a decent and stable measure of protection. As mentioned earlier, it just depends on how much time and of course money you, and the customer wants to invest in the installation. I know what i've managed to achieve with relatively little monetary cost, but with a fair bit of time at my house in Cyprus and with pretty bad soil conditions too, (soil resistivity of 650+ ohm/m).
(in
I know full well that you disagree on the value a good TT system can bring to an installation, based on the time and cost, and that you would prefer to trust that RCD totally, to provide your earth fault current protection, whereas i wouldn't. Which is why it is better to agree to disagree, because i'm sure we are not going to change our thinking on the matter...lol!!!
