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RCD disconnection times on a TT system

Volt Hunter
Hi Guys, first post after a long time reading the forums.


I have a client questioning the disconnection times provided by a 30mA type A RCBO on a TT system, tripping times provided are 221ms at 30mA and below 40ms at 150mA.


It protects a 16A socket in an outdoor environment, so must comply with the disconnection time provided in Table 41.1 of BS7671 of 0.2 seconds

Looking at table 3A on page 363, that states that a 30mA RCD must trip within 300ms at 1x, which is obviously outside the limits above, and 40ms at 5x.


Can we state that because the 5x test trips at less than 40ms it complies with fault protection, or does the 30mA disconnection time also have to comply?


Thanks in advance for any guidance you can offer.
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  • 0

    Can we state that because the 5x test trips at less than 40ms it complies with fault protection, or does the 30mA disconnection time also have to comply?



    As with a TN system you need to ensure that disconnection will occur within the required time for a L-PE fault of negligible impedance. So things depend a little on the actual loop impedances involved - but in a typical setup where Ra is under 200 Ohms you'd have an earth fault current of in excess of 1A - so the 5x figure should be adequate with a large margin of safety.


       - Andy.
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  • 0

    Can we state that because the 5x test trips at less than 40ms it complies with fault protection, or does the 30mA disconnection time also have to comply?



    As with a TN system you need to ensure that disconnection will occur within the required time for a L-PE fault of negligible impedance. So things depend a little on the actual loop impedances involved - but in a typical setup where Ra is under 200 Ohms you'd have an earth fault current of in excess of 1A - so the 5x figure should be adequate with a large margin of safety.


       - Andy.
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