Contravening Regulations

[bergie]

fault level and wire size...now this is an interesting topic...you need to do your fault level calculation at the point where the cable is to be connected then only can you determine whether or not it is capable of carrying the fault...an example i installed a 200 kw motor with a soft start...i could have used 2 x 70 mm ecc cables according to my cable size calculations ...but because of the fault level at the point of connection was higher than normal because of the transfomer feeding the panel and the distance ...i had to use a 185 mm cable...aberdaire cables has a booklet which explains this in more detail...i think this is one of the least understood parts of electrical installations...in fact at the time i had another contractor who has installed a couple of panels already on the same site to the left of the transfomer but just as close...and he used 2 x 70 mm cables...i know my installation is correctly designed and installed i have all the documentation to prove it..so i sleep well at night...

mmm,makes sense,but what if you order pre wired panels?

[Justloadit]

Another way of looking at the fault level. If the cable you have used is too small, and there is a short circuit at the most furtherest point in the cable from the supply, the cable may act as a resistor, where the current flowing in the cable may not trip the main breaker. This in turn causes the cable to act like a wire wound heater, and start glowing red hot, which could cause a fire hazard. A good example is to take a 1 meter length of 5 amp wire, and short your car battery with it, ensure you have protective gloves, as you will see the wire get so hot, that the plastic cover on the wire melts off. Continuation of the short will make the wire glow red hot.

Sizing the wire to be able to take the fault current for the main breaker to trip, will force the breaker to trip because of the over current.

[bergie]

i,m presuming the wires are rated for there overload protection. for example the panel has a 60 amp breaker, 16mm wire to a 20 amp breaker feeding a 2,5mm wire,etc if the fault current is 1000 amps ,must we wire the panel with 1000 amp wire?

[AndyD]

The size of the cable doesn't get larger necessarily if the potential short circuit current is higher. If anything it might go the other way where multiple smaller cables are used to limit the short circuit fault current. Short circuit currents damage the device that disconnects the power, as the contacts separate there is much arcing and heat which can cause irreparable damage or even an explosion in extreme cases. Circuit breakers must be specified to handle the potential short circuit current of that particular circuit. The PSCC rating of a breaker is given as an Ics value in kA usually.

[AndyD]

I had a scratch around for some light bedtime reading on PSCC. There's a lot of very involved articles and white papers around but you can start with this which is fairly entry level and it's in an easily downloadable format.

[mikilianis]

Adrianh in reply to you query

In the current wiring code, reference to the standard test finger is made four times.
Fundamental requirements
Clause 5.1.1 Live parts
“It shall not be possible to touch any live part within arm’s reach with the standard test finger (SANS 60529/IEC 60529/SABS IEC 60529)”
Wireways
Clause 6.5.1.1 (k)
“Where it is accessible to the public, protection shall be such that it is not possible to touch any live parts with either the standard jointed test finger or the 2,5 mm diameter 100 mm long rod (see SANS 60529/IEC 60529/SABS IEC 60529). This protection also applies after opening any door or cover that can be opened without the use of a tool or key; ..”
Bathrooms, showers, spa’s, etc.
Table 7.1 Conditions under which electrical equipment may be installed in a bathroom
“B1 denotes that the equipment shall be so enclosed in insulating material that it is not possible to touch current-curing parts with the standard test finger;..”
Explanation of IP ratings
Annex J - Table J.1 - International protection ratings – IP ratings
If you look at the first section under “Protection against solid foreign objects”, IP 2, the following appears:


“Full penetration of 12,5 mm diameter of sphere not allowed. The Jointed test finger shall have adequate clearance from hazardous parts.”
The importance
From the SANS 10142-1 2003 Wiring Code the importance of such a test during installation and/or construction of electrical apparatus and the testing of electrical installations, this test is clearly very important.
How it’s done
This test simulates contact with live parts by a finger. It is done by using a metallic standard test finger, connected by an incandescent lamp or buzzer to one pole of a supply of least 40 V, while the other pole of the supply is connected to the parts intended to be live in normal service. The poles must be electrically connected together. The protection is deemed satisfactory if the lamp does not light or the buzzer does not buzz when an attempt is made to touch the bare or insufficiently insulated live parts with the test finger. The finger must be placed in every possible position and pushed without undue force.
Note: Conducting parts covered only with varnish or deemed to be enamel or protected by oxidation or by a similar process are insufficiently insulated. Such parts should, for the purposes of the test, be covered with metal foil that is in contact with parts that are alive in normal services.