Understanding short circuit currents

[sysengineer]

I'm confused about short circuit currents and how they are used to determine the ratings of equipment within an electrical system.

My own intuition would suggest that that a system must be designed such that any equipment upstream of the short circuit must be able to withstand both the current and energy produced as a result for sufficient time for the protective device to interrupt.

So for example if a single phase feeder is supplying some sort of control panel and a short occurs on this cable everything upstream of this fault i.e. the dist. board or whatever must be able to withstand the fault conditions until the protective device interrupts.

On the other hand if the control panel is supplying some sort of external heater and a short occurs within the heater, everything upstream of the heater must be rated i.e. up to the protective device inside the control panel.

My reasoning is that if a short occurs within a supply system, (ignoring back emf from motors and such for now) everything downstream of the short will not be affected i.e. very little current will flow as the resistance path of the short circuit is tiny compared to that of the downstream load.

The confusion is that IEC 61439: Low Voltage Switchgear & Controlgear Assemblies states:

"6.2 Prospective short-circuit current at supply terminals Icp (kA)
The prospective short-circuit current is the current that would flow if the supply conductors to
the ASSEMBLY were short-circuited with negligible impedance at the supply terminals of the
ASSEMBLY."

If the supply conductors to an assembly are short circuited then in terms of the assembly what is the problem? Surely due to the negligible impedance of the short very little current would flow?

 

[dpc]

It is just a definition. I believe it represents the maximum fault current that any device in the ASSEMBLY must be able to withstand and/or interrupt.

 

[sysengineer]

So "supply conductors to the assembly" must mean any load downstream of the assembly correct? and what is meant by "negligible impedance at the supply terminals"?

 

[magoo2]

Supply conductors to the assembly means the conductors on the source side of the assembly (between the source and the assembly).

Negligible impedance at the supply terminals means you can't take advantage of any source impedance.

 

[waross]

Negligible impedance means a short circuit. The current is limited only by the source impedance.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[sysengineer]

So effectively the prospective short circuit current at the supply terminals (Icp) affects only the equipment upstream of the assembly?

I'm still not understanding what is meant by "negligible impedance at the supply terminals" is this referring to the supply terminals of the source or of the assembly. I'm assuming the source because the impedance of the assembly if I am correct will not affect the Icp?

 

[FreddyNurk]

An example of what the clause means:
If you have a transformer, say 2MVA with Y connected LV, 400V at around 2% impedance (I didn't state it was typical). This would give about 150kA.

The fault current you'd get by connecting your assembly to it and shorting out the incoming terminals is the same as what you'd get on the low side if you shorted the terminals of the transformer (no cable impedance is included for this exercise). This is then the figure that you use to appropriately rate the assembly.

If, of course, you have multiple sources (say, embedded generation) then you need to consider this.

 

[juleselec]

You're right, if there was a fault at the supply terminals, the downstream assembly wouldn't be significantly affected. But I don't think that captures the intention of the standard

From a short circuit analysis perspective, the switchgear assembly can be represented in a simplified form as a single node. This means that we forget about the impedances of the terminal connections, circuit breaker stabs, busbar joints, etc and call it a single equipotential node (or bus or terminal).

So from the assembly's point of view, the worst case prospective short circuit current will occur when the fault is at the assembly itself (i.e. at the node) and the fault has zero (=negligible) impedance, i.e. it is a bolted three-phase short circuit.

A fault at the supply terminals is effectively a fault at the point of connection, and in our simplified model, it is a fault at the assembly itself.

I think this interpretation is consistent with the text a few paragraphs after your original quote:
"An ASSEMBLY is assigned a short-circuit rating by the ASSEMBLY manufacturer, defined in terms of the maximum prospective short circuit current applicable at the point of connection to the system."

 

[sysengineer]

Ok I think i'm starting to follow. So the short circuit at the point of connection to the assembly is the basis for all switchgear inside the assembly?

So would it be correct to suggest that the short circuit current (Icp) at the point of connection is applicable to all circuits feeding off the assembly?