Short circuit rating of Switchgears - Terms

Hi Nick,

What do you mean by "the Switchgear"? Do you mean the 'switchgear assembly' (i.e. the switchboard or panel or distribution board,etc.) or do you mean 'the installed circuit breaker'?

A rating of 50kA @ 1s is generally a rating that could be either.

In most contexts I would take it to be the short circuit rating of the assembly (i.e. the busbar rating) but if there is the slightest doubt I would suggest asking the supplier/manufacturer for confirmation.

Switchgear (as an assembly) will have a number of current ratings. For short circuits, you are concerned with:
Rated peak withstand current (Ipk)
The rated peak withstand current is the “peak value of the short-circuit current, declared by the assembly manufacturer, that can be withstood under specified conditions. The rated peak withstand current shall be equal to or higher than the values stated for the peak value of the prospective
short-circuit current of the supply system(s) to which the circuit(s) is (are) designed to be connected.”
(IEC61439-1)


and
Rated short-time withstand current (Icw) (of a circuit of an assembly)
The rated short-time withstand current is “the r.m.s value of short-time current, declared by the assembly manufacturer that can be carried without damage under specified conditions, defined in terms of a current and time. The rated short-time withstand current shall be equal to or higher than the prospective r.m.s. value of the short-circuit current at each point of connection to the supply.
Different values of Icw for different durations (e.g. 0,2 s; 1 s; 3 s) may be assigned to an assembly. For a.c., the value of the current is the r.m.s. value of the a.c. component.”

and
Rated conditional short-circuit current of an assembly (Icc)
The rated conditional short-circuit current of an assembly is “the value of the prospective short-circuit current, declared by the assembly manufacturer, that can be withstood for the total operating time (clearing time) of the short-circuit protective device (SCPD) under specified conditions. The rated conditional short-circuit current shall be equal to or higher than the prospective r.m.s. value of short-circuit current (Icp) for a duration limited by the operation of the short-circuit protective device that protects the assembly.”


It should be apparent that conditional ratings are based on specific device cut of characteristics whereas rated short time withstand is only limited by the available current and the stated time duration. 1 second is a long time to have a 50kA fault on the board, 3 seconds is a lifetime  - but essentially the board must safely contain the fault energy (ie, doors don't blow open, etc)


In the absence of a time rating, the minimum time would be 4 full cycles.


Regards


OMS

Anderson,

I meant the switchgear assembly

Hi OMS,


Isn't all this parameters elated to the circuit breaker?


When someone says, what is the short circuit rating of the switchgear assembly (the whole structure), what does it usually refers to?

Is it the busbar rating?

Is it the Main Incomer circuit breaker breaking capacity?

Is it the whole assembly withstand capacity?

Isn't all this parameters elated to the circuit breaker?

Not necessarily. Just taking domestic consumer units as an example, it might contain MCBs with a rated 6kA breaking capacity, but the overall assembly is likely to be rated at 16kA (at least if it complies with annexe ZB of BS EN 61439). The manufacturer might take any of several approaches to achieve that increase - perhaps even relying on the outer case to contain any arc products or explosion that escapes an individual MCB.


    - Andy.

To carry that further, to an extreme that lucklily is only occasionally required, very large breakers are designed to spit hot gas and molten metal in certain directions when it all goes wrong. (the good ol'  I² t, (or joules per ohm if you prefer an easier unit), have to go somewhere, ideally away from the switchgear operator!!) The correct enclosure is essential to making this happen safely - large cabinets have part cut sheet flaps at the back or the top that bend outwards like louvres in a controlled way to spread the explosive shock out over a longer time, and for things like a substation next to  a building, the outer containment may be carefully designed to blow out one wall or a take off a light roof, without affecting the remaining structure. 

The 'rating' for such a system is complex, and depends how much needs to be "reset" (or rebuilt) before it can be switched on again.