Impedance

[johnnyjb]

Could any body help me im stumped.Im in ireland and im doin a city guilds course B im not worried bout the tiny differences between our irish regs and the iee regs but i wanna know whats an exceptable earth fault loop impedance what would be too much in a tncs or tt system houses. regs say Zs < Uo/Ia could anyone explain better please , i know mcbs have impedance limits what happens if you exceed the level of impedance do they not operate properly just any general knowedge will help please

 

[Spark123]

If you exceed the maximum permissable efli for a given protective device for a given disconnection time then the protective device may not operate quickly enough (or maybe not at all) to remove the danger. With a TN system the preference is for the overcurrent protective device to disconnect the supply in the event of a fault. In a TT system owing to the increased impedance given by the resistance of the general mass of earth, not as much current can flow and hence the protection needs to be afforded by means of RCDs.

 

[johnnyjb]

so in a tt system say you had a 32 amp mcb which had a load wired to it that was leaking current to earth but the mass of earth had a high impedance which kept the fault current below the mcb rating.The mcb would not trip ,it has a fault current but is not enough for the mcb to detect

Am i right

 

[Spark123]

Yes, which is why in a TT system RCDs need to be used. BS7671 IEE regs (don't know what you use) requires the RAIa<50v, where RA is the sum of the resistances of the earth electrode and the protective conductors and Ia is the current causting automatic disconnection of the protective device. If you are using a 30mA RCD to provide this protection in theory you can have a max loop impedance of 50/0.03=1666ohms!! This isn't acceptable in the real world and here we would not accept a figure greater than 200 ohms, most sparks would want it a lot lower anyway. (More than likely this figure is going to be reduced to 100 ohms in the near future).
The equation RAIa<25v needs to be used for some special locations.

 

[johnnyjb]

Am i right in thinking that fault currnts in tn cs systems would be alot higher since the impedance is alot lower but this in turn should activate your protective devices alot faster ( or whatever times their rated for)In the uk what is aceptable efli for tncs domestic house

By the way were using the BS 7671 :2001

 

[Spark123]

With a TN system the exposed conductive parts are connected directly to the supply transformer by metal i.e. with a TN-S it is via the metallic sheath of the cable/separate core, with a TN-CS the neutral and earth combine at the service head and are connected directly to the distribution transformer. This is often also referred to as PME (Protective Multiple Earthing) as the CNE is tied to earth at various positions along its length. The fault currents (Phase - Earth) in a TN system will more than likely be a lot higher than that of a TT system. The maximum Ze for a TN-CS is 0.35ohms and a TN-S is 0.8ohms
http://en.wikipedia.org/wiki/Earthing_system

 

[johnnyjb]

Thanks alot ,sometimes you need to hear (or read) someone say it before it makes sense