My old Clipsal circuit breaker P/N 4CB116 16A which was rated at 8Ka is broken. I need to replace it but Clipsal now only seem to make a 6Ka or a 10Ka which one should be used ?

 

I would getting the 10Ka switch.

 

Certainly getting the breaker with the 10,000 amp interrupting capacity is the more conservative choice, but it is undoubtedly more expensive and probably physically larger. Looking at the wiring there is no way that much current could ever flow in the event of a zero-resistance short circuit. Thus it would be completely safe to use the breaker with the 6kA rating.
What made the original breaker fail? There is more to this question that we don't know, isn't there?

 

6kA may be ok too depending on specific circuit and installation. however without knowing details, 10kA is a way to go

 

The wires in the photo do not look like they could deliver much current for even a single millisecond. AND that rating is for interrupting capacity.Power delivered through the wires in a short-circuit condition. So supply impedance is the limiting factor.

 

Source impedance, breaker co ordination, and hardware configurations. It’s largely an engineered solution.

 

kA rating of an MCB or an MCCB is the maximum current it can safely interrupt in case of a short circuit. If the current goes beyond this value, the circuit breaker could be damaged. kA rating is known as the short circuit withstand capacity or ultimate breaking capacity of a circuit breaker. It is denoted by Icu.

walgreenslistens

 

kA rating of an MCB or an MCCB is the maximum current it can safely interrupt in case of a short circuit. If the current goes beyond this value, the circuit breaker could be damaged. kA rating is known as the short circuit withstand capacity or ultimate breaking capacity of a circuit breaker. It is denoted by Icu.

walgreenslistens

I am quite aware of that, which was the basis for my remark that the lower rating should be adequate, given that the fed is a wire and not a bussbar in a panel fed by much larger conductors. So that is the basi for my engineering analysis. It was notat all a random guess.

 

That may be true but picture alone is not good enough to make that determination. Size and length of supply conductor is not known (that is thicker wire connecting to bottom left). Better safe than sorry.

 

The fact that the supply connection is a wire and not a busbar is adequate verification. The limitations and impedance of wire are known, much less than a buss bar. Experience and knowledge beats fear!

 

noted but... based on what i see, i still would not agree.

could it be used? possibly. it seem to be low voltage.
would it violate code? also possible. cannot tell for sure with limited info provided.

reason for conservative stance?
i used to design and commission custom equipment for all types of industry for many years. i recall instance where an automotive client insisting on use of specific standard parts on a project. very common for any larger company. in this case that also meant using 480V 8kA 3-pole MCBs. so i asked for the list and their standard, and did as they requested. equipment was tested and approved before delivery. but installation also need to meet standards so as equipment was being installed, electricians run usual conduit and pulled wires pulled from disconnect at bus bar to the disconnect on our equipment.

and as usual i met with field inspector and we checked everything. and it became evident that despite some 12ft (4m) long wires, fault current was possible in excess of the rating of the MCBs which happened to be the weakest link. as a compromise to make changes on the spot, everyone agreed (and inspector accepted) workaround to add more wire (another 4m) which was placed inside section of flexible conduit placed inside cabinet and before main disconnect.

the point is that things like this needs careful consideration and when something (including replacement) is below rating of the original, this would require (at least here) new approval process before replacement is installed and equipment is allowed to be powered up. field inspection cost alone would be way above the price difference of the two replacement options.
therefore based on my own experiences and fact that picture shown does not indicate where the wire comes from, what the size it is, what the available fault current at the supply is, what the actual length of the wire is etc. i can only suggest to practice caution. at this point i would not recommend going for the 6kA substitute. that would not be a wise thing to do here, but may be acceptable in some parts of the world.

 

This will undoubtedly offend some individuals, but the inspectors that I have dealt with only follow a script. Understanding and insight are not part of their process, at least not while I was dealing with them.
In this instance we see the feed to this group of three breakers a being a wire lead, and we see no bus bar at all. Given that reality, if the installation includes protection of that wire from another breaker, is it reasonable to think that the other breaker will have the greater interrupting capacity?? That would be standard practice in most parts of the world.

 

is it reasonable to think that the other breaker will have the greater interrupting capacity?? That would be standard practice in most parts of the world.

what you suggest is quite worrisome.

SCCR rating of some downstream equipment is not based on a rating of the upstream disconnect.
even if the breakers or disconnect are connected as you suggest (without low power transformer or whatever bottleneck such as known impedance of the circuit), they are still merely two devices connected in series. And you are still wrong - it is the device with lower rating that is the weak point or bottleneck.

https://electricalengineeringresour...current-ratings-in-industrial-control-panels/

"A panel with an incorrectly calculated SCCR could fail or cause an arc flash, with the potential to cause serious injury or death, as well as significant damage to the facility."

"Rather than “calculating” the SCCR of a panel, it actually only requires that you investigate the fault capacity of the relevant components and then identify the component with the lowest capacity … the weakest link in the circuit. The SCCR of that component is the SCCR of the entire panel assembly."

one picture, showing only small part of the circuit is not enough to even identify those relevant components, not to mention circuit construction, component rating etc.

enforcement of safety standards is there for a reason. please do not offer advice on things that involve safety based on a speculation.

 

My apologies to all for not responding sooner:

the reason for replacement is that the breaker trips OFF without any connections being present. It does not remain latched to ON.


As per MrBill2

In this instance we see the feed to this group of three breakers a being a wire lead, and we see no bus bar at all. Given that reality, if the installation includes protection of that wire from another breaker, is it reasonable to think that the other breaker will have the greater interrupting capacity?? That would be standard practice in most parts of the world.

I have checked the main board and these 3 MCB are supplied by a 40amp 6ka breaker .
from this can I assume the 6ka replacement would be fine?

 

Coming in late: Consider what the wiring rated for.

 

My apologies to all for not responding sooner:

the reason for replacement is that the breaker trips OFF without any connections being present. It does not remain latched to ON.


As per MrBill2

I have checked the main board and these 3 MCB are supplied by a 40amp 6ka breaker .
from this can I assume the 6ka replacement would be fine?

Indeed! That the device is broken was stated in post #1, and at that point I resumed that the TS was correct in the evaluation. That sort of failure mostly comes from using the breaker a an on/off switch, resulting in mechanical wearing out. I've seen that before.
And now, with the additional information we can see that matching the upstream interruption rating will be adequate.

 

MisterBill2...many thanks for that confirmation!