S
sparky79
ok so for the people who seem not to know where the values come from. 21 ohms is the value that your local supply authority quote for the earth value at the their substation if you put a rod down there. this is not the reading you are trying to achieve. 1666ohms , this is the reading you need to achieve to stop a touch voltage of 50volts occurring when using a 30ma rcd. 50v/0.03A=1666ohms. also if you are using a 100ma then it would be 50/0.1=500ohms. the reading of 200ohms is the level that is deemed to be a stable reading, however a reading slightly above this as with the original post is neither here or there. you are not relying on the earth rod to operate an mcb, hence why you "have" to use rcds on all circuits on a tt system. the mcb is there to protect the circuit from overload only, in tt case this will be phase to neutral. and when the fault is to earth the rcd is the means of protection to the operator and not to the cable. so the reg of "do not use a rcd as sole means of protection" would mean you do not have a bank of rcds and no mcbs. this is where you use rcbos. so with all this in mind you as the electrician need to use some common sense and decide if you are happy with the reading. if you put in a brand new rod and get a reading of 220ohms then I would deem that as a stable reading. if it was an old bit of corroded water pipe giving a reading of 220ohms, then I would say that it should be replaced with a new rod. both surface area and depth are the factors to improve the reading. but there will be a minimum reading you will not get below as this is the resistance of the earth and substation earth rod.I hope this is of help. andy
