Ics and Icn: debunking the myth about those little boxes on MCBs

[sunray]

So... if there hasn't been a fireball and it still stays up and it passes a current test and Henry still works on it. How do you know if Icn has been exceeded and it needs to be replaced?

 

[2nd best]

ive always been taught the following
Icn - Maximum breaking capacity but may not be suitable for continued use
Ics - service breaking capacity without loss of performance
A good way to remember it Ic-n not serviceable
Ic-s serviceable

 

[KeenPensioner]

Eh up

There are often two boxes on the side/front of BS EN 60898 circuit breakers, BS EN 61009 RCBOs etc, e.g.:

View attachment 30871

The bottom number is often misunderstood.

The top number is Icn (in amps). In this case, Icn = 6kA.

The bottom number is the energy limiting class. This is referenced in Tables ZA.1 and ZA.2 in BS EN 60898 and Tables ZD.1 and ZD.2 in BS EN 61009. It ultimately refers to the permissible I[SUP]2[/SUP]t (let-through) values for circuit breakers up to and including 63A (types B and C only). In this case, from table ZD.1, because it is a type B 16A breaker and Icn=6kA, class 3 corresponds to a permissible let-through value of 35,000 A[SUP]2[/SUP]s. Without access to these tables, I'm not sure how you'd ever know.

The main thing is...:

The single digit number in the square, underneath the large number in the rectangle, is not Ics (in kA).

For once, despite its faults, the On-Site Guide gives correct information about Icn and Ics.

Table 7.2.7(i) (page 74 in the yellow version) lists values for Ics for a range of values of Icn.

For Icn <= 6kA, Ics = Icn
For Icn >6kA and <=10kA, Ics = (0.75 x Icn) (minimum value: 6kA)
For Icn >10kA, Ics = (0.5 x Icn) (minimum value: 7.5kA)

This is in the BS EN standards ("... the ratio shall be in accordance with [this].")

So:
If you see a rectangle with "6000" in it, then Ics = Icn = 6kA.
If you see a rectangle with "10000" in it, then Ics = 7.5kA, Icn = 10kA.

There have been several posts over the years on this forum and others that propagate the myth that the bottom number, in the square, is Ics (in kA). I've also seen it printed in at least one text book. I will spare the author's blushes as it is otherwise a super, top-notch book (which taught me a lot).

Are we all clear?

(A click on "thanks" would be appreciated if this is useful.)

View attachment 30871

Saved my life!! I've been studying this and could not get to an understanding. Even Chris Kitcher's book.."Practical Guide to Inspection, Testing....etc" Page 146 is a bit vague. Your explanation is crystal clear, thanks again.