I have been asked a question by a guy who is presently preparing to undertake a C&G 2391 practical assessment. I can see solutions but not ideal so I wonder if other minds can suggest something diferent and maybe simpler. The situation is this
100A three phase supply TN-S with Ze of 0.8 ohms by enquiry (actual measured value 0.76 ohms) The first DB in the chain (DB1) includes a 63A single phase breaker protecting a single phase sub-main supplying a consumer unit in a flat above the workshop. The sub-main consists of 16mm conductors for both live and neutral and a 16mm earth conductor so Zdb at the consumer unit is measured as 0.77 ohms. The circuits protected by the breakers within the consumer unit all comply with the regs relating to max earth loop impedance taking account of rule of thumb. The problem is that the max impedance for a 63A type B MCB is tabulated at 0.71 ohms and rule of thumb takes this down to less than 0.6 ohms required. Considering the requirements as part of the test results of DB1 the circuit does not comply with requirements and the schedule of test results and notes to the intermediate inspection report reflect the situation. If this occurred in real life, rectification would be required. His question to me was how best to modify the circuit to comply. As I see it there is no possibility of gettinig the Zs down to 0.6 ohms as Ze is higher and I cannot think of any way to get a negative value of R1 and R2 for the circuit conductors. An easy solution is to provide RCD protection for this circuit but the RCD will then provide whole house (flat in this case) cover and leave the occupants vulnerable to losing lights and everything else in the event of an earth leak on one of the other circuits in the consumer unit. This is generally considered unsatisfactory by present day standards. Using a 100mA time delayed RCD and 30mA quick acting RCD or RCBO to serve sockets in the consumer unit would probably be satisfactory but can anyone suggest an easier way which woukd comply with the Regs?- posted
17 years ago