rcd

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It's for circuits not requiring 30 mA protection.
 
You need to have a revise on the rules Concerning TT systems TBH toptrics, thats the best bet mate as you needed to ask, I promise I am not being nasty, but a refresh regarding it may be of benefit for you.

Mike
 
Morning.
What is the reason for having a 100ma rcd as a main switch on a tt system instead of 30ma

Why, ....because you have this numbty 200Ra ohm figure in BS7671 whichi s no good to man or beast, (certainly not for any protection purposes) and 30mA RCD's looking after several circuits on each side of the CU. The trouble is RCD's are NOT the most reliable protection devices out there, so if one of those normal RCD's fail to operate under fault conditions etc, you have a reasonable back up in a 100mA S type RCD up-front device....
 
There is an interesting article in jan PE mag on this...and it argues that a 100ma S type will fall foul of reg 531.2.9 re- ensuring downstream devices tripping before upstream in the event of a fault. The conclusion was a minimum of 300ma S type would be required for final circuits with 30ma protection.
 
Doesnt make sense, 100ma wont meet the disconnection times for final circuit, also wouldnt 30ma trip before 100ma so why would it need to be s type.
 
Doesnt make sense, 100ma wont meet the disconnection times for final circuit, also wouldnt 30ma trip before 100ma so why would it need to be s type.

Of course it will. It will give you a max Zs of 500 ohms per circuit protected by it. A 30 mA will trip before a 100 mA but what if you get a higher leakage current of say 100 mA you need to then delay the 100 mA.
 
The 30 ma rcd won't see the fault as it will not be protecting the circuits that the 100 ma rcd is protecting. The main this is all circuits on a tt have to be protected by an rcd, does this make it any clearer.
Ben
 

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