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

Interesting, very interesting. So the graphs aren't actually defined in BS EN 60898 then? (and furthermore 10x Idn doesn't guarantee 5s disconnection time for devices over 32A or under 10A). I wonder where the charts in BS 7671 came from then?


   - Andy.

Thanks OMS, so reading that it looks then as if the worst case is going to be at the PSSC = 11*In end of the curve, and only 10, 16, 20 and 25A devices have to be fast , for the rest T diss could be up to 8 seconds. Now in terms of let-through energy and potential to do cable damage, 8 seconds and 5 seconds of heating time really are  significantly different. 

I wonder how many installations have D type breakers, and are thought OK  because they meet volt drop considerations at normal running current, but would not actually operate the magnetic part  into a dead short - these may need to be re-checked assuming  8 seconds for the  adiabatic heating time . Which is awkward, as the adiabatic calc loses validity for long disconnection times as there is time for the heat to spread into the local volume.

Maybe we should assume that the breaker itself is operating as an adiabatic  thermal device, that the curve is such that at 10* In it is 8 seconds, and a thermal fuse like law (i2t is constant ) applies thereafter, so at

11In Tdiss = 8 seconds* (10/11)^2 = 6.6 secs

12In Tdiss = 8 seconds* (10/12)^2 = 5.5 secs

13In Tdiss = 8 seconds* (10/13)^2 = 4.7 secs

14In Tdiss = 8 seconds* (10/14)^2 = 4.1 secs

15In Tdiss = 8 seconds* (10/15)^2 = 3.6 secs


If the breaker is not adiabatic, but has significant cooling over the 8 seconds, then it speeds up faster with increasing fault current  than this assumption suggests, so these maximum breaking times will tend to err on the safe side.


If so the Zs in the table for 5 seconds would need reducing to something like 10/13 of the current value.


Another one to add to the growing list of potential gotchas.

That's about the size of it Andy, Mike


See below, the various aspects of Table 7 I posted above

8.6 Automatic operation
8.6.1 Standard time-current zone
The tripping characteristic of circuit-breakers shall be such that they ensure adequate protection of the circuit, without premature operation.
The zone of the time-current characteristic (tripping characteristic) of a circuit-breaker is defined by the conditions and the values stated in Table 7.
This table refers to a circuit-breaker mounted in accordance with the reference conditions (see 9.2) operating at the reference calibration temperature of 30 °C, with a tolerance of C 0 + 5 °.
Compliance is checked by the tests specified in 9.10.


Basically, the manufacturer then defines the time current curve and publishes those curves


The only curves in the standard are those for discrimination purposes depending on if the MCB is a current limiting (zero crossing point) or not


The message is use Type D MCB's at your peril


You might be shocked at the requirements for Type B and Type C
Type B - 3 x In:

0,1 s < t < 45 s
(for In ≤ 32 A)
0,1 s < t < 90 s
(for In > 32 A)
Type C - 5 x In

0,1 s < t < 15 s
(for In ≤ 32 A)
0,1 s < t < 30 s
(for In > 32 A)



Regards


OMS

Probably worth noting the data in the original post is taken from Table 41.3 for CB's exceeding 125A


For MCB's operating in conventional 0.4 second time, it should be clear that the MCB will be at or around 20 x In if the Zs data is complied with - so will be out in 0.1 seconds (from the standard) for 0.4 seconds compliance (from BS 7671)


Regards


OMS

Sounds like things would be far simpler if we gave up attempting to have time-current graphs for circuit breakers and just had a simple table of currents and times like we do for RCDs. (Of course the numbers would have to be correct, apparently unlike the ones accompanying Fig 3A6 it seems).

 

It's on my list of things to gripe about for the next DPC, if no-one else gets there first.


  - Andy.

I'd go along with that - with sensible margins adopted, it would be a foolproof method of showing compliance with 0.4 and 5 second disconnections, based on data already required in BS EN 60898-1 - but we have something very similar with limiting Zs values so it may well not warrant the change of approach


Regards


OMS

Ha, I don't like to be British about this, so does it really matter if it is 6 secs or 5 secs, or my cable rating is 33A or 34A? We think it does, but the EU probably doesn't?


Regards