Short circuit disconnection times

[Ian1981]

I like these sort of threads

Maybe my interpretation of BS EN 61439-3 is wrong......I see it that any device or factory wiring within the enclosure is covered for 16kA Breaking Capacity. I don't see that it would cover for thermal constraints of outgoing wiring from circuit breakers.

This is from Beama:

BS EN 61439-3 Annex ZB prescribes a specific conditional test arrangement relating to the use of
an upstream BS 88-3 (formerly BS 1361 type II) 100 A fuse-link. This 16 kA conditional test verifies
the performance of the incoming device, its connections, busbar, cable links, circuit-breakers,
residual current operated circuit-breaker and any other item in the consumer unit not separately
rated at 16 kA or higher. BS EN 61439-3 requires this rated conditional short-circuit current to be
identified by the symbol Icc and BS 7671 Regulation 536.4.201 references this symbol.

No that’s fair enough but I can’t imagine a case where the thermal constraints of a door bell transformer will be exceeded if protected by a class 3 limiting type B 6 amp 60898 device especially domestic installations if the csa is at least 1.0mm

 

[TJ Anderson]

No that’s fair enough but I can’t imagine a case where the thermal constraints of a door bell transformer will be exceeded if protected by a class 3 limiting type B 6 amp 60898 device especially domestic installations if the csa is at least 1.0mm

The I2t for 1.0mm is 13225 A2s. The Beama guide for 60898 Class 3 limiting states <35000 A2s for In <16A. I know in reality they are lower but manufacturers data would needed to confirm to confirm the energy let through of device. I have found some which are below this for their B6 devices.

 

[Pretty Mouth]

It's a bit complicated isn't it? While we're on the subject of class 3 energy limiting devices...

Table B7 in the OSG gives minimum conductor sizes for class 3 mcbs, according to In and fault level. For a 6A with <= 3kA fault, minimum is 1.0mm. The beama guide you mentioned TJ gives I²t = 15000 for the same 6A/3kA, using adiabatic gives minimum 1.065mm². Do you think the difference is so small that it is ignored, margin of error?

(subtle difference - OSG gives fault levels whereas Beama guide give SC capacity, does it make a difference?)

 

[Pretty Mouth]

I found manufacturer's I²t data for chint, wylex and hager MCBs, and put them though the adiabatic. As you said they give a considerably smaller conductor size compared with using model fault curve data, or beama data for class 3 devices. For all 3 brands, 1mm² was fine on a 16A breaker with the maximum 6kA fault

 

[Charlie_]

Do you realize it’s bank holiday weekend?

 

[Pretty Mouth]

Yup. But I'm a saaad git with no women or friends who's quit drinking. This is what I do for 'fun' when I'm not sitting wondering where it all went wrong

 

[Pretty Mouth]

I guess I'm in good company though, it seems that I'm not the only one here

 

[Baddegg]

Quit drinking?....what’s that like then?

 

[Pretty Mouth]

For me, it's great. But I suspect it's not for everyone.

 

[Pretty Mouth]

The penny is beginning to drop I believe. Please jump in and correct me if any of my assumptions are wrong:

The disconnection times don't apply to short circuits because they don't need to - the danger is from burning up the cables, not from electrocution. Energy let through and thin conductors are more of a concern.

Minimum conductor size can be calculated for a given fault using the adiabatic, and manufacturer's data is best as it will allow the smallest conductor size. For MCBs and RCBOs, the let through energy increases with fault current.

Sound about right?