It does not say where the CB should be, so if it sits in the "sourcing DB" then only having an isolator for the enclosure /DB is adequate.

But be careful for you still need to know the kA rating from this source supply because all switchgear must be able to withstand the PSCC in the circuit.

Conditional short circuit current  > 17kA

or

Aux circuits connected to transformers >10kA - THE PANEL kA (PSCC) VERIFICATION PROCESS APPLIES for the panel!

409.22 Short-Circuit Current Rating. An industrial control panel shall not be installed where the available fault current exceeds its short-circuit current rating as marked in accordance with 409.110(4).

*The NEC is much much stricter...by UL witnessing tests, how many witnesses do they have around the world then?

Based on reference design - the manufacturer of the panel

Based on testing

Based on calculation

Distribution boards that form part of a fixed electrical installation shall
comply with SANS 1973-1 or SANS 1473-1 or SANS 1973-8.

From  a SANS point of view.

6.6.6.1 Alterations or extensions on site to distribution boards with a short circuit
withstand current above 10 kA shall comply with the requirements of
6.6.6.2, and where reports and similar documents are available, apply

a) the rules given in SANS 1973-1 or SANS 1473-1 for the maximum
permissible deviations allowed for a PTTA, or Amdt 5; amdt 6
b) the rules for derivations from the minimally tested ASSEMBLY as in

SANS 1973-8 or from an STA as in SANS 1473-1.

Also

6.6.6.2 When a distribution board is modified or extended, the following
requirements shall apply: Amdt 3
a) the mechanical and electrical integrity of the distribution board shall not
be infringed;
b) the integrity of the area in a distribution board or a section of a distribution
board that comprises the conductors (including distribution busbars)
between the main busbars and the supply side of functional units and the
components included in these units, which is regarded as fault free on the
basis of the reduced short-circuit stresses that occur on the load side of
the prospective short-circuit protective device in each unit, is still
applicable; Amdt 3
c) extensions to the busbar systems shall not adversely affect the electrical
and mechanical performance of the complete busbar system;
d) components shall be selected for their suitability for application taking into
account information available from the component manufacturers. It might
be necessary to derate the components depending upon environment
and application conditions; Amdt 3
e) any changed properties due to alteration or extension of the distribution
board shall be marked indelibly on a supplementary nameplate; and
Amdt 3
f) the required IP rating shall not be reduced. Amdt 3
161

NOTE The term over current protection includes both overload protection (see 6.7.2) and short-circuit protection (see 6.7.3).

Unless fully tested in accordance with SANS 60439-1/ IEC 60439-1, the current density of copper busbars shall not exceed 2 A/mm2 for currents < 1 600 A, or 1,6 A/mm2 for currents > 1 600 A.

SANS 6.6.4.2.4 Unless tested in accordance with SANS 60439-1/IEC 60439-1, the minimum clearance between phases and between phase and earth
shall be at least 8 mm.

Equipment and components shall comply with the clearance distances given in the relevant product standards.

Also 6.6.4.3.2 Unless tested in accordance with SANS 60439-1/IEC 60439-1,
the minimum creepage distance between phases and between phase and earth shall be at least 16 mm. Equipment and components shall comply with the creepage distances given in the relevant product standards.

***The electrician will love you for this!

You will not believe how important this can be when you need it.

Lets just take one example: Not having a method to manage priority between the supplies or simple phase sequence monitoring.   

PS: I said flexible - but simplicity still reigns supreme.

4.4.2.3 explicitly says - if you break the neutral you better do it last (after the phases) and close it before or at the same time as closing the phases.

That's gonna be a challenge with a contactor assembly, because it does not give us licence for neutral disconnection at the same time.

Utilisation categories according to IEC – 60 947
The standards for low-voltage switchgear and controlgear.

*As complete as I can find to date.

What does this table mean?

To assist in the correct selection of all switchgear according to set conditions, the type of load and the duty of switching of the load.

How does this table work?

Why can you have multiple similar applications listed? – Well, one has to consider that the conditions for the use is first determined by the section covered by the IEC standard so under different specifications the same description can be listed.

For instance: -One can switch the load with different characteristics (e.g. frequent switching or not) or switch the same circuit with the load connected or not.

Thank you to the following references:

Rockwell Automation - Allen Bradley

Wikipedia

ABB

but their is an Annex D to assist...but it must not be seen as an exact method - I can see right through this text that whoever was involved in this text had no confidence in their doings whatsoever. Really?

A very nice and complicated way to do this...what is annex D?

I remember when I did contracting we used to have council inspectors to assist and guide us...He looked at the complex I was doing and at my proposed calculation...way over sized he says...this is a much more realistic supply. Wow...experience is priceless. Is there no way to incorporate experience into regulations like this?

We know for a flat complex consisting of this level of load (some complexes are for the wealthy and comes with under floor heating and such...others are for the average person...just the basics) My point is:- Can we not work on the bases of a basic load requirement per unit and add any "frills" with diversity factors to ascertain the current capacity of a specific load. that would be cool!

Fig. A12: Rated diversity factor for distribution boards (cf IEC61439-2 table 101)

I do not have this code... ref from SE guide 2016

Diversity factor - Coincidence factor (ks)
It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use  of a factor (ks).
 

 Vocabulary, as follows:
b Coincidence factor:
b Diversity factor:

I got some base information from the SE guide 2016 - but as a bottom line it seems this diversity factor rules is very much not a local regulation drive. They mentioned the way the French does it.

I actually had a case where a meter was taken as suspect because it registered strange neutral currents. Wow, If only I knew then!  

So I presume this neutral current component is a function of the amount of harmonics as well.

But this killed me:- The magnitude neutral current due to the third harmonic may exceed the POWER FREQUENCY PHASE CURRENT...and it might affect the cables...for instance if exaggerated by an unbalanced load the neutral conductor might be undersized. This highly abnormal situation might arise where a cable has to be sized according to the neutral current because it might be higher then the phase currents.

Over Current Protection Device“(listed component to UL 489/248, power circuit-breaker or fuse).

After the last protective device of a current path with the above described features the branch circuit starts with reduced creepage distance and clearances.

Everything before the protective device corresponds to feeder circuits.

The test connections consist of a three phase link on the outgoing circuits (one of each circuit rating) and a phase to earth link on one outgoing circuit (the largest outgoing circuit).

A three phase link and a phase to neutral link on the incoming terminals or busbars will be fitted as required.

---------===========--------

Voltages in South Africa according to the code 5.3.2

standard voltages:

230Vac single phase

230/400V three phase four wire

Tolerance of +-10% - someone should show Escom this!

Declared voltages:

525V 3phase 3wire with tolerance of +-5%

Preferred Dc voltages:

12, 24 and 48Vdc

Other :

60,80, 110, 220, 250 and 500Vdc

Tolerances determined by application.

Control circuit voltages preferred as 230Vac or 110Vdc

Transient fault[edit]

A transient fault is a fault that is no longer present if power is disconnected for a short time and then restored. Many faults in overhead power lines are transient in nature. When a fault occurs, equipment used for power system protection operate to isolate the area of the fault. A transient fault will then clear and the power-line can be returned to service. Typical examples of transient faults include:

In electricity transmission and distribution systems an automatic re-close function is commonly used on overhead lines to attempt to restore power in the event of a transient fault. This functionality is not as common on underground systems as faults there are typically of a persistent nature. Transient faults may still cause damage both at the site of the original fault or elsewhere in the network as fault current is generated.

Persistent fault[edit]

A persistent fault does not disappear when power is disconnected. Faults in underground power cables are most often persistent due to mechanical damage to the cable, but are sometimes transient in nature due to lightning.^[1]

Symmetric fault[edit]

A symmetric or balanced fault affects each of the three phases equally. In transmission line faults, roughly 5% are symmetric.^[2] This is in contrast to an Unsymmetrical fault, where the three phases are not affected equally. In practice, most faults in power systems are unbalanced. With this in mind, symmetric faults can be viewed as somewhat of an abstraction; however, as Unsymmetrical faults are difficult to analyze, analysis of asymmetric faults is built up from a thorough understanding of symmetric faults.

Unsymmetrical fault[edit]

An Unsymmetrical or unbalanced fault does not affect each of the three phases equally. Common types of asymmetric faults, and their causes:

Arcing and bolted faults[edit]

Where the system voltage is high enough, an electric arc may form between power system conductors and ground. Such an arc can have a relatively high impedance (compared to the normal operating levels of the system) and can be difficult to detect by simple overcurrent protection. For example, an arc of several hundred amperes on a circuit normally carrying a thousand amperes may not trip overcurrent circuit breakers but can do enormous damage to bus bars or cables before it becomes a complete short circuit. Utility, industrial, and commercial power systems have additional protection devices to detect relatively small but undesired currents escaping to ground. In residential wiring, electrical regulations may now require Arc-fault circuit interrupters on building wiring circuits, so as to detect small arcs before they cause damage or a fire.

When calculating the prospective short-circuit current in a circuit, to maximize the value, the impedance of the arc is neglected. Notionally, all the conductors are considered connected to ground as if by a metallic conductor; this is called a "bolted fault". It would be unusual in a well-designed power system to have a metallic short circuit to ground but such faults can occur by mischance. In one type of transmission line protection, a "bolted fault" is delibrately introduced to speed up operation of protective devices.

Analysis[edit]

Symmetric faults can be analyzed via the same methods as any other phenomena in power systems, and in fact many software tools exist to accomplish this type of analysis automatically (see power flow study). However, there is another method which is as accurate and is usually more instructive.

First, some simplifying assumptions are made. It is assumed that all electrical generators in the system are in phase, and operating at the nominal voltage of the system. Electric motors can also be considered to be generators, because when a fault occurs, they usually supply rather than draw power. The voltages and currents are then calculated for this base case.

Next, the location of the fault is considered to be supplied with a negative voltage source, equal to the voltage at that location in the base case, while all other sources are set to zero. This method makes use of the principle of superposition.

To obtain a more accurate result, these calculations should be performed separately for three separate time ranges:

An asymmetric fault breaks the underlying assumptions used in three-phase power, namely that the load is balanced on all three phases. Consequently, it is impossible to directly use tools such as the one-line diagram, where only one phase is considered. However, due to the linearity of power systems, it is usual to consider the resulting voltages and currents as a superposition of symmetrical components, to which three-phase analysis can be applied.

In the method of symmetric components, the power system is seen as a superposition of three components:

To determine the currents resulting from an asymmetrical fault, one must first know the per-unit zero-, positive-, and negative-sequence impedances of the transmission lines, generators, and transformers involved. Three separate circuits are then constructed using these impedances. The individual circuits are then connected together in a particular arrangement that depends upon the type of fault being studied (this can be found in most power systems textbooks). Once the sequence circuits are properly connected, the network can then be analyzed using classical circuit analysis techniques. The solution results in voltages and currents that exist as symmetrical components; these must be transformed back into phase values by using the A matrix.

Analysis of the prospective short-circuit current is required for selection of protective devices such as fuses and circuit breakers. If a circuit is to be properly protected, the fault current must be high enough to operate the protective device within as short a time as possible; also the protective device must be able to withstand the fault current and extinguish any resulting arcs without itself being destroyed or sustaining the arc for any significant length of time.

The magnitude of fault currents differ widely depending on the type of earthing system used, the installation's supply type and earthing system, and its proximity to the supply. For example, for a domestic UK 230 V, 60 A TN-S or USA 120 V/240 V supply, fault currents may be a few thousand amperes. Large low-voltage networks with multiple sources may have fault levels of 300,000 amperes. A high-resistance-grounded system may restrict line to ground fault current to only 5 amperes. Prior to selecting protective devices, prospective fault current must be measured reliably at the origin of the installation and at the furthest point of each circuit, and this information applied properly to the application of the circuits.