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…

 

[cireland]

Isn't the type s time delayed?

 

[cireland]

30mA trip current would trip up to 7666 ohms on 230v.
100mA would require 2300 ohms. Anything above 200 ohms should be considered unstable.

100mA is there for fault protection, that is line - neutral faults.

 

[ppelec100]

30mA trip current would trip up to 7666 ohms on 230v.
100mA would require 2300 ohms. Anything above 200 ohms should be considered unstable.

100mA is there for fault protection, that is line - neutral faults.

I did some digging around on some forums last night and managed to work it out and it made sense to me what you meant on your first reply…

Correct me if I’m wrong…

So 0.2s disconnection time is for fault protection and when using type A/Ac Rcd then providing that the Ze is low enough you will get a fault current high enough to trip the Rcd at 0.2s…

Rcd tripping at 1x tripping current is 30ma which should trip within 300ms which wouldn’t comply with the 0.2s even though most likely it probably will trip within that time…

But Rcd tripping at 5x the tripping current should trip within 40ms/0.04s which will meet the 0.2s required disconnection times and as the fault current will be more than 0.15A/150ma it will trip within 0.04s.

So if all circuits are on Rcd/rcbo then no need for S type Rcd

If there are some on Rcd/RCBO and the other circuits don’t require additional protection ie distribution circuit or cooker circuit 40amp but they require fault protection providing it meets the requirements not to be on additional protection then a up front S type RCD time delayed Rcd set to 500ms would be fine as the disconnection time will need to be 1s as stated in 411.3.2.4

 

[cireland]

The 100mA RCD is for fault protection. The 30mA is for additional protection.

TT systems have a 1 second disconnection time for fault protection.
0.2 seconds for earth fault protection.
The reason for time delayed is to give time for 'local' disconnection.
That is my understanding of it though I may not be 100% right.

 

[ppelec100]

The 100mA RCD is for fault protection. The 30mA is for additional protection.

TT systems have a 1 second disconnection time for fault protection.
0.2 seconds for earth fault protection.
The reason for time delayed is to give time for 'local' disconnection.
That is my understanding of it though I may not be 100% right.

Okay thank you this makes sense…

Was I right about how to meet the 0.2disconnection time?
The EFLI on TT being low enough to have a high enough earth fault current which will trip the Rcd at 5x the tripping current which will cause it to trip at no more than 40ms which meets the 0.2s requirement

 

[PS Electrics]

Okay thank you this makes sense…

Was I right about how to meet the 0.2disconnection time?
The EFLI on TT being low enough to have a high enough earth fault current which will trip the Rcd at 5x the tripping current which will cause it to trip at no more than 40ms which meets the 0.2s requirement

Yes

Given that an RCD may take up to 0.3 seconds (300 ms) to disconnect it would appear at first glance that some disconnection times in Table 41.1 may not be achievable, for example, the maximum disconnection time for some final circuits on a TT earthing system is 0.2 seconds (200 ms), as shown in Table 3. However, a note under the table indicates that earth faults are of negligible impedance and it follows that disconnection times would be commensurately higher since a value of, for example, a residual current of 60 mA would be expected to provide a disconnection time of 150 ms and 40 ms (0.04 seconds) for a residual current of 150 mA, as shown in Table 2. Therefore, an operating time of 300 ms would be considered acceptable for a TT earthing system.

 

[ImpededLoop]

30mA trip current would trip up to 7666 ohms on 230v.
100mA would require 2300 ohms. Anything above 200 ohms should be considered unstable.

100mA is there for fault protection, that is line - neutral faults.

That's incorrect. RCDs don't operate on line-neutral faults, only earth faults.

 

[ppelec100]

Yes

Given that an RCD may take up to 0.3 seconds (300 ms) to disconnect it would appear at first glance that some disconnection times in Table 41.1 may not be achievable, for example, the maximum disconnection time for some final circuits on a TT earthing system is 0.2 seconds (200 ms), as shown in Table 3. However, a note under the table indicates that earth faults are of negligible impedance and it follows that disconnection times would be commensurately higher since a value of, for example, a residual current of 60 mA would be expected to provide a disconnection time of 150 ms and 40 ms (0.04 seconds) for a residual current of 150 mA, as shown in Table 2. Therefore, an operating time of 300 ms would be considered acceptable for a TT earthing system.

Thank you

 

[Tom256]

Thank you

Yes Table 3A that was in Appendix 3 of the 17th edition of BS7671 and in BS 7671:2018 (Blue Book) up to the release of BS7671:2018 +A2:2022 (Brown book) when it was deleted.
Read along with the note that accompanies Table 41.1 are key to understanding this. If you only have the 'Brown' book you may not be aware.
Oversimplified explanation is this is for Earth fault not additional protection as such under Earth fault a resistance of negligible impedence is assumed at the point of fault and larger current flow will take place.
What must also be taken into consideration in equal measure is that the sum of RA x I delta n must be less than or equal to 50 volts
Regulation 411.5.3 refers

(RA = earth electrode resistance and Earth conductor / Protective earth arrangment to exposed conductive parts)