Why the 0.1 sec to 5 sec in table next to Fig 3A4?

[HappyHippyDad]

I was hoping someone would give me an idea of what the 0.1 sec - 5 sec referrs to in the table to the right of Fig 3A4 (BS7671).

Any Fault current that exceeds the straight line I thought meant instantaneous disconnection? (i.e 0.1 sec (probably faster)), but the table infers that it could be anything from 0.1 sec - 5 sec, so without looking at the manufacturers let through data how do we know?

This has cropped up in another current thread but didn't want to highjack it.

 

[Richard Burns]

The table is just being comparable to the other tables that are for 0.4s or 5s disconnection times only.
It is saying that because the thermal magnetic circuit breakers have reached the magnetic (instantaneous) disconnection for 5s current values then the tables are suitable for any regulatory disconnection time required from 0.1s to 5s. So if you have a TN distribution circuit (at 5s) or a TT final circuit (at 0.2s) you can use the same table. It is not saying that the device disconnects in a specific time it is only saying that the table can be used for circuits that have a minimum disconnection time between 0.1s and 5s.

 

[HappyHippyDad]

Thanks Richard,

So does that mean that the only way to get an exact disconnection time for any prospective current past the 'straight line' we have to refer to maufacturers let through data (which never seems very easy to find!)?...............

and if so we cant just use 0.1 sec in the adiabatic (even if high prospective current - say 1000A) unless we have checked this data?

This is all in relation to Fig 3A4.

edit... I think I need to read your answer a bit more as my last post shows that I'm not understanding it properly

 

[HappyHippyDad]

The table is just being comparable to the other tables that are for 0.4s or 5s disconnection times only.
It is saying that because the thermal magnetic circuit breakers have reached the magnetic (instantaneous) disconnection for 5s current values then the tables are suitable for any regulatory disconnection time required from 0.1s to 5s. So if you have a TN distribution circuit (at 5s) or a TT final circuit (at 0.2s) you can use the same table. It is not saying that the device disconnects in a specific time it is only saying that the table can be used for circuits that have a minimum disconnection time between 0.1s and 5s.

I should have looked at this bit more closely!

Still unsure how we actually get a disconnection time though, when prospective current is past the straight line. Can we assume instantaneous disconnection (i.e 0.1s or faster)?

 

[HappyHippyDad]

The table is just being comparable to the other tables that are for 0.4s or 5s disconnection times only.
It is saying that because the thermal magnetic circuit breakers have reached the magnetic (instantaneous) disconnection for 5s current values then the tables are suitable for any regulatory disconnection time required from 0.1s to 5s. So if you have a TN distribution circuit (at 5s) or a TT final circuit (at 0.2s) you can use the same table. It is not saying that the device disconnects in a specific time it is only saying that the table can be used for circuits that have a minimum disconnection time between 0.1s and 5s.

I think you've already answered my last question as well Richard, I just needed to read it a little more slowly :smiley2:

 

[Richard Burns]

So, any questions?:willy_nilly:

 

[HappyHippyDad]

So, any questions?:willy_nilly:

Sorry! I was getting a bit caught up there wasn't I! Just get a bit involved in it :smiley2:

I suppose I would like to be certain that 'past the straight line' is instantaneous? And if so, why does it say to refer to maufacturers let through energy data for those fault currents exceeding instantaneous? And does instantaneous mean 0.1 sec or faster?

 

[Richard Burns]

To be precise about how fast a device disconnects you can read the graphs in BS7671, but if the fault current is off the graph, because it is too high, which is often the case then the only way to find that time is to search for the manufacturers data.
However for adiabatic calculation you can use the highest fault current on the graph and the relevant disconnection time (0.1s) and get a value for the csa, if this complies then a higher current will comply, I think.
The current limiting effects of circuit breakers can only make the situation better.
The best way to use the adiabatic is to use the current let through values (I²t) as these give the lowest csa possible, but there is rarely a need for this in a standard domestic situation.
Circuit breakers generally disconnect in less than 0.1s once it is up to the magnetic trip level, some of the manufacturers state 0.001s!

 

[HappyHippyDad]

Thanks alot for explaining that Richard. I have gone through stages of thinking I understand the straight line and the table, then not, but now I do to an adequate level that satisfies me. Thank heavens for that! :smiley2:

 

[telectrix]

this was what i meant in the other post. if the fault current is on the straight line "drop off" the graph, what value of t would you use in the adiabatic. i've always used 0.1sec.

 

[Richard Burns]

I would use 0.1s if using the graphs, the csas achieved are always less than 1mm² (for a type B MCB) so a simple answer.