Type D MCB 5 Second Tripping Current

Table 41.3 is giving you the case for both operation between 0.4 seconds and 5 seconds - if you check the edge of the vertical section for Type D you should find 10 x In for 0.4 seconds - this section is vertical so for no change in current, the CB may be operating both magnetically and thermally


Basically, once you reach 10 x In the CB is going to operate in definite minimum time (around 0.01 seconds) - however, it is only required to operate (for BS 7671 compliance at 0.4 seconds or 5 seconds)


Regards


OMS

That looks rather like a mis print, and is a dead copy of the same advice for C types.

However, note that it will be less than 20 times because at 5 seconds we might be tripping on the thermal part for D type, though not always for C type, where even at 10 times, the magnets kick in.


In the end the maker's data trumps the generic appendix to the regs in this case.

Try comparing the table with the generic D-type chart in appendix 3 of BS 7671 (3A6) - that's looks to be a reasonable match to 10x to me for 5s. The appendix 3 graphs are meant to be the slowest possible for devices complying with standards - so I'm a bit surprised that manufacturer's data can show something slower.

  - Andy.

@ AJJewsbury


Yes chart 3A6 comes in at 10In for a 5 second trip time for a type D MCB.


However it still does not marry up with what ABB state is actually required to trip their type D MCBs within 5 seconds.


I found a more detailed time current graph below from ABB specifically for their type D MCBs to 60891-1.


As you can see the tripping current is around 13In, not 10In as stated in table 41.3 in BS 7671 17th Edition A3.


Going off the ABB graph below it would seem that table 41.3 in BS 7671 17th Edition A3 has values stated in it that potentially would fail to activate the breaker trip mechanism within the required 5 seconds.

Which is rather surprising.

It is perhaps telling that the Schneider Data  indicates that their breakers would be well within the spec.

Given the figures in the BS are lifted from the BS EN 60898 for the breaker, they should match or be better, and it would be interesting to see if ABB claim full complaince to the breaker standard, or if it is a printing error on their chart - do you have the breaker no to go with that chart please. Equally in practice there are very few situations where breaking in 6 seconds instead of 5 will matter - it cannot be safety of life against shock or it would be 0.4 or 0.2 seconds we would be looking, so it is lower risk.

@‍mapj1 


I'm not so concerned with the practicalities of 6 second trip time, I'm simply concerned as how we adhere to BS 7671 17th Edition A3 clause 411.3.2.3 if the values stated in table 41.3 are below that required to activate the ABB MCB trip mechanism within the required 5 seconds.


Below linked ABB document, page 59 for the graphs :

https://library.e.abb.com/public/b0b41850655145d09f4dd77b6c43acb4/2CCC451059C0203.pdf

Well, if you need to always guarantee a 5 second trip time with that  s400 UC ABB breaker, then one should use the maker's curve, and deduce your own lower max Zs than the generic one BS 7671 recommends.


Equally I note that the only current time points that are actually tested are on page 60, presumably all the trip times for intermediate currents are only an interpolation, so I am not sure how accurate the graph is as a representation of production spreads. May be worth giving ABB a ring if you need an accurate value for some reason.
Abb UK tech support.

@mapj1‍ 


I emailed ABB and they basically stated that the graph is taken from 60898-1. If what ABB are stating is correct it seems a bit odd that 60898-1 would allow a 5 second tripping current of 13In and BS7671 17th Edition A3 only allows 10In.


"That Time/Current curve is the tripping curve as defined by the standard EN60898-1, and as such details the upper and lower limits allowable to conform to that standard"


"The actual trip curve of the ABB MCB is of course much narrower and closer defined. Specific curves are available and published in our freely downloadable software called Curves, link below :
https://new.abb.com/low-voltage/launches/selectivity/tools-support/curves"

That was quick moving.  Hmm. '7671 claims to be based on the original EN curves anyway. I begin to suspect  the fat finger of cut and paste. I do not have the original EN here unfortunately. Perhaps some one else on here does.

OK - probably worth a bit of clarification

Instantaneous tripping current - For Type D  - Above 10 In up to and including 20 In


For test requirements (with the MCB cold, and again for Type D), for test currents of 10 In then:
0,1 s < t < 4 s

(for In ≤ 32 A)
0,1 s < t < 8 s

(for In > 32 A)

Where the MCB is rated less than 10 A then  t < 8 s is permissible


For test currents of 20 In then t < 0,1 s


The standard requires the tripping characteristic of circuit-breakers to be contained within the zone defined by the parameters above


Regards


OMS