Think your side-tracking from the original question.
If the Ze on your TT system was 200ohmn your Zs is going to be above max. for any protective device.
Ze can never be higher than Zs.
100ma TD RCD`S are used as main switch to discriminate with 30ma rcd`s for final circuits.
30ma rcd`s are only required for cables buried in plaster at depth of less than 50mm or special locations
It is well-known that most circuits feeding 13A socket-outlets now require RCD protection but equally important are requirements for additional protection of wiring concealed in walls or partitions, which includes lighting circuits and their concealed switch wiring. This means that the vast majority of domestic circuits, power and lighting now require 30mA RCD protection.
There are also important requirements to minimise nuisance tripping and collateral risks due to a tripped RCD affecting other circuits, such as lighting.
Socket-outlets
Rule 411.3.3 calls for additional protection by means of a 30mA RCD for all socket-outlets with a rated current not exceeding 20A for use by “ordinary persons”. The only exceptions allowed are for socket-outlets for use under the supervision of skilled or instructed persons (e.g. some commercial/industrial locations) or a specifically labelled socket provided for connection of a particular item of equipment (such as a freezer).
This rule clearly covers the vast majority of domestic circuits feeding 13A socket-outlets, and any other socket-outlets (5A, 15A etc.) up to 20A. There is also a requirement for RCD protection of circuits feeding mobile equipment with a current rating up to 32A for use outdoors.
Cables in walls or partitions
Rule 522.6.7 calls for RCD protection of wiring concealed in walls or partitions. All concealed wiring at a depth of less than 50mm now requires protection by a 30mA RCD unless it is provided with earthed mechanical protection, for example by metallic conduit or trunking. This applies to many lighting circuits and their switch wiring, including those installed in previously defined a “Safe Zones”.
There is a further requirement for protection by means of a 30mA RCD where cables are concealed in walls constructed with metal stud partitions, irrespective of the depth from the surface, unless provided with protection in the form of earthed metallic covering, trunking, conduit or other mechanical protection so as to avoid damage to the cable during installation or construction of the wall.
Special locations
Rule 710.411.3.3 calls for RCD protection of all circuits in specific locations such as those containing a fixed bath or shower. This means that, in bathrooms or bedrooms with en-suite facilities, circuits feeding lighting, heating and showers must have RCD protection.
Other “special installations and locations” as defined in Part 7 of the Regulations are also required to have RCD protection. These include swimming pools and saunas, agricultural premises, caravans and caravan sites, floor and ceiling heating systems.
Sub-division of circuits
Rules 314.1 and 2 require that every installation should be divided into circuits as necessary to avoid danger and minimise inconvenience in the event of a fault. Designers are required to reduce the possibility of unwanted RCD tripping due to excessive protective conductor currents but not due to an earth fault.
Separate circuits may be required for parts of the installation which need to be separately controlled in such a way that they are not affected by the failure of other circuits. The appropriate subdivision should take account of any danger arising from the failure of a single circuit, for example an RCD trip causing the disconnection of an important lighting circuit.
This affects the configuration of the protective devices in a consumer unit. For example if a number of circuits are protected by a common 30mA RCD, lighting circuits need to be spread over more than one RCD.
Rules 314.1 & 2 also call upon the designer to take steps to reduce the likelihood of unwanted RCD tripping due to excessive protective conductor currents, other than earth faults.
Typical situations would include IT equipment with certain types of radio frequency interference suppression, and certain types of heating equipment. The cumulative effect of such loads can produce a standing earth leakage current that is beyond the threshold point of a normal 30mA RCD. However this situation is becoming more common in residential environments.
Earth loop impedance
It should also be noted that Chapter 41 of the Regulations includes revised earth loop impedance tables based on a nominal voltage of 230V (Previously 240V). This results in slightly lower values of earth loop impedance and could, in some situations, mean that RCDs will be required to achieve the required disconnection time where previously overcurrent protection devices would be considered adequate.
Solutions
A number of different consumer unit configurations are possible to satisfy these requirements. The ideal solution is an in-line arrangement with an RCBO (combined MCB and RCD) providing overcurrent and residual current protection of each final circuit; in this case tripping of an RCBO will not affect other circuits. However, this is also an expensive solution. The most economic solution is a dual-RCD unit with individual circuits protected by miniature circuit-breakers divided between two RCDs.
