Cable sizing with MCCB tolerance band and "I2≤1.45xIz" rule

[DarrowZan]

Hi!

As per AS/NZS 3000:2018, a cable is protected if Iz≥In and I2≤1.45xIz, where In is the MCCB nominal current, Iz is the derated current capacity of the cable and I2 is the effective tripping current of the MCCB.

What I don’t understand is the I2≤1.45xIz part. As per AS/NZS 60898, 1.45xIn was tested to trip circuit breakers within a time <2hr for In>63A. So for currents below this level the MCCB may not trip? In that case, shouldn't the cable be sized for >1.45xIn?

The I2≤1.45xIz seems to imply that if my I2=100A (effective tripping time is 100A), then a cable with derated capacity of 70A is perfectly okay… but for any loading between 70A and 100A in that cable my MCCB may never trip and my cable will burn eventually?

I'm looking to use a TemBreak PRO with a +5% to +20 tolerance band, so my I2 would be 1.2xIn as a worst case I assume, not 1.45xIn.

I have a client asking for a 630A MCCB to feed a 630A switchboard. If I have a 600A load with the 630A MCCB and a cable derated for 690A we pass all items, but if someone keeps adding loads to the switchboard until I have 720A then my MCCB may never trip as the effective tripping current is 630x1.2=756A and my cable will slowly burn? So do I size my cable for >756A?

Thanks!

 

[Julius Right]

The Standard IEC 60364-4-43 Electrical installation of buildings - Protection against overcurrent specifies coordination between conductors and overload protective devices (normally placed
at the beginning of the conductor to be protected) so that it shall satisfy the two following conditions:
Ib ≤ In ≤ IZ ( Condition 1 )
I2 ≤ 1.45 × IZ ( Condition 2 )
where:
Ib design current of circuit
Iz continuous current carrying capacity of the cable under working conditions
In rated current of the protective device; for adjustable protective releases e.g. MCCB, the rated current In is the set current I1
I2 current ensuring effective operation in the conventional time of the protective device.
What is Condition 1
Practically the standard requires that the selected protective device, has a rated (or set) current that is:
higher than the load current, to prevent unwanted tripping.
lower than the current carrying capacity of the cable, to prevent cable overload and Condition 2 and MCB:
The Standard allows an overload current that may be up to 45% greater than the current carrying capacity of the cable but only for a limited period.
The verification of condition (2) is not necessary in the case of circuit-breakers because they automatically tripped if:
I2 = 1.3xIn for circuit-breakers complying with IEC 60947-2 (circuit-breakers for industrial use);
I2 = 1.45xIn for circuit-breakers complying with IEC 60898 (circuit-breakers for household and similar installations).Therefore, for circuit-breakers, if In ≤ Iz, the condition I2 ≤ 1.45⋅Iz is self-verified.