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EF: How is 0.2s disconnection time met on TT?

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EF: How is 0.2s disconnection time met on TT?

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ppelec100

How is 0.2s disconnection time met on TT?

Hi all, studying for 2391 and confused on disconnections times for TT systems

Now I’m comfortable understanding the disconnection times for TN systems that need to be met which is 0.4s and because the Zs will be low enough to cause a high Ipf to cause the MCB to operate within 0.4s… also from Rcd/RCBO which disconnects in 300ms (0.3s)

But what I’m baffled on is for a TT system. The Ze will be too high which will make the Zs too high also and will not meet max Zs for the MCBs which is why RCD protection is required… but if the max disconnection is 0.2s type A Rcd/RCBO will not meet the disconnection times. Spoke to a friend and he said 100ma type S RCD which has a disconnection time of 200ms but then that will disconnect before all the rcd /rcbos meaning the rcd/rcbos will be pointless and the S type will cause all to trip at the same time??

How do you achieve separation of circuits to avoid nuisance tripping?

Please correct me if I’m wrong and help me get my confused brain around this.

Thanks

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