How is 0.2s disconnection time met on TT?

[ppelec100]

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

 

[Risteard]

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

The reality is that it won't only be the minimal operating current in a fault, and even if there is 2 I delta n flowing then disconnection times will be much quicker. Of course we don't actually carry out a 2 I delta n test (or even need to do a 5 I delta n test anymore).

 

[ppelec100]

The reality is that it won't only be the minimal operating current in a fault, and even if there is 2 I delta n flowing then disconnection times will be much quicker. Of course we don't actually carry out a 2 I delta n test (or even need to do a 5 I delta n test anymore).

Your reply has confused me more if I’m being honest… the minimal operating current of the Rcd i.e 30ma/100ma? And what is 2 I delta n? Is that 2x the rated tripping current of the Rcd? Delta being the triangular symbol?

So how would we confirm that 0.2s would be met by selecting the right RCD for TT? What I’m struggling to understand is how to get around the nuisance tripping of all circuits…

S type 100ma will trip in 0.2s which is good but will cause all other circuits that it serves to be knocked out. 30ma rcbos will realistically trip within 0.2s too but they’re designed to trip within 0.3s so they can’t be selected…