We conclude the series by wrapping up these requirements, unpacking overcurrent protection, identification of DC conductors and other additional requirements…
The overcurrent protection requirements that are needed for an alternative supply are:
- Overcurrent protection and isolation should be located as close as possible to the output terminals of the alternative supply. This is not applicable in situations where the cable connecting the unit to the distribution board is mechanically protected and is regarded to be within the fault-free zone of the distribution board where protection is installed.
- The magnetic characteristic of a circuit breaker must be of a low threshold value in view of the high impedance.
- Overcurrent protection is also needed for the protection of the conductors.
If an alternate supply system is required to operate in parallel with main supply or where two or more alternate supplies may operate in parallel, then the circulating harmonic effects should be limited to not exceed the thermal rating of the conductor. Circulating harmonic currents can be limited by:
- The selection of an alternate supply system with compensated windings.
- The provision of suitable impedance in the connection to alternate supply system generator star position.
- The provision of switches which interrupt the circulatory circuit but are always interlocked so that protection is not impaired.
- The provision of filtering equipment
- Any other suitable means.
How are DC Conductors Identified?
The positive and negative conductors of a DC circuit are colour coded as red and black or blue, respectively. Conductors can also be colour coded at the end of the conductor by a durable colour marking such as insulating sleeves.
If symbols are used to identify conductors, then:
- The polarity of a positive conductor is identified by a “+” symbol located at either ends of the conductor by printed adhesive tapes or cable markers.
- The polarity of a negative conductor is identified by a “-” symbol located at either ends of the conductor by printed adhesive tapes or cable markers.
Installations may also contain both AC and DC circuits and if these conductors are colour coded then it is required that the DC conductors have symbols added at both ends of the conductor to distinguish them from the AC conductors.
What are the Requirements for a System where an Alternate supply is used as a switched alternative to the main supply?
Precautions are required to be taken so that the alternate supply does not operate in parallel with the main supply. The following are suitable precautions that can be made:
- An electrical, mechanical, or electromechanical interlock between the operating mechanisms or control circuits of the changeover switching devices could be installed.
- A system of locks with a single transferable key can be implemented.
- A three-position break-before-make changeover switch.
- An automatic changeover switching device with suitable interlock.
- An inverter if it complies with IEC 62116 standards.
It is also required that the control panel can be either set mounted or be a free-standing cabinet but in both cases the cabinet will be considered a control assembly.
What are the Additional Requirements for Photovoltaic (PV) or similar installation that are used as an alternative supply to the main supply?
The following are the necessary requirements for PV or other similar systems that are used as an alternative supply to the main supply:
- The PV installation should comply with SANS 60364-7-712.
- Polycrystalline and monocrystalline solar panels must comply with SANS 61215 whilst thin film solar panels shall comply with SANS 61646.
- The rated voltage for each circuit must be indicated at all ends of the circuit. It is required that every circuit be easily reliable in the case of combined circuits. If single core conductors are used, then the conductors for each circuit must be tied together at intervals to ensure identification, unless another arrangement is used.
- Switch-disconnection arrangements are required for installations that consist of an inverter, battery arrangement and PV panels.
The DC component of the installation for PV installations will be investigated further in an upcoming series.
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