Yep sorry was just refering to the OP original post
Not quite sure what the OSG means by 21ohms being the usual max stated resistance of the distributors earth electrode at the supply transformer, is this something you would see out on a domestic job as I don't think I know what a TT with distributors earth before if thats the case? How would you tell, would there be an earth following the incoming tails from overhead lines or similar?
the earthing conductor would not be connected to the sheath of a tns system and would not be connected to the neutral in the Head it would run of to a earth electrode somewhere in garden
the 21 ohms is the suppliers side at the earth rod at the transformer end
200ohms is the max from consumers earth electrode to the suppliers
but as Tel say try to get a reading around 100ohms or less and the rcd 30mA is for if you fail to meet the max Zs for the circuit from RA X by i delta n ie 30mA rcd which is 0.03A is less than or equal to 50v
Ra is the sum of reistances of the earth electrode and the protective conductor(s) connecting it to the exposed conductive part
to find the max of any rcd its Ra divided by I delta n Ie 30ma
50 divided by 0.03A gives you 1666.666667 which when rounded up gives you the max Zs of 1667ohms
a 100mA rcd
50 divided by 0.1 = 500ohms
a 300mA RCD
50 divided by 0.3 = 167 ohms rounded up
a 500mA rcd
50 divided by 0.5 = 100 ohms max
idelta n is the rated residual operating current
if it was a TN system then its
Zs xIa is less than or equal to Uo
Ia is the current in amperes causing the automatic operation of disconnecting device within the time specified by the relevent table 41.1 was pg 46 in the BRB now pg 53 BGB
tables 41.2 are for fuses 0.4 disconnection time 41.3 is for Circuit breakers 0.4secs 41.4 are for fuses with 5secs disconnection time
table 41.5 is for rcds and rcbos
A TT system has a max of 200 ohms at consumers earth electrode then it uses General mass Of earth to reach the suppliers Earth elaectrode which they guarantee will not be more than 21 ohms
then the suppliers earth electrode is connected to the star point of the transformer
TN-S uses the sheath and is seperate in consumers side and suppliers side and has max of 0.8
TN-C-S has a seperate earth in consumers side but then connected to the neutral in the suppliers head and from the neutral and earth are combined using a protective earthed Neutral (Pen conductor) all the way to star point and there will be earth electrodes along the way ie PME Protective Multiple Earthing
hope that makes sense and hopefully i have it right as last time i said all that was for me 2391 exam june 2010 lol
