I have an unusual TT set up for a large multiple unit filling station. By the time I get to the final circuits there will be five RCDs in "series". 400A main intake, 200A to separate buildings on site 200A incomer to panel board in each building, various in the panel board 160 to 63A to the final circuit distribution boards then individual MCB/RCD for each final circuit.
On the 400 and 200A units I have adjustability 0.03 to 10000mA and time delay 0,60,150,310mS but on the 160A-63A units 0.03 to 5000mA and 0,60,150,500,1000mS. Schneider Compact NSXM all 4-pole.
I am unsure how much reliance can be placed on the exactness of the time-delay but the maximum time delay on the larger units is only 310mS. The larger units are already in place by others. We are responsible for one of the units thus commencing with a 200A module.
The 400A module is set at set at 10000mA /310mS and first 200A at beginning of distribution circuit is 3000mA and 150mS. So I need a 200A incomer to the panel in our building and 160/63A for the distribution circuits in the panel in our building. I am struggling to see how I can achieve appropriate earth leakage selectivity. Earth loop is currently 2.36ohms at our unit intake and much the same at main intake so around 100A of possible earth fault current.
I also need 2-pole or 4-pole RCD protection for my final circuits to avoid neutral/earth faults being left in. Cost is mind-boggling!
Just thought this was an interesting set up that probably few would experience.
