How do Solar Distribution Boards Differ from Regular DB’s?

With the steady decline in solar panel prices it has become an attractive option for business owners to install solar panel power systems to ensure a stable power supply. Solar distribution boards, sometimes called combiner boards, form an integral part of this solar supply system. The working of Solar distribution boards and the differences between regular distribution boards are discussed further below.

The basic construction of a distribution board

The basic distribution board (DB) consists of a steel enclosure which contains circuit breakers connected to DIN rails or bus-bars. This allows for the distribution of power within an electrical power system. In a normal distribution board the system is fed from a main circuit breaker (CB) which in turn feeds other smaller circuit breakers which control their respective load circuits. In the below image the main circuit breaker feeds the smaller line circuit breakers in a hierarchical fashion, this is a good example of a traditional distribution board.

[Diagram of a typical distribution board. Image created by Author.]

The basic working of Photovoltaic panels

Photovoltaic (PV) panels or solar panels, as they are commonly known, generate electrical power when exposed to sunlight. A group of solar panels are usually used to produce power and are arranged in a PV array. The PV array outputs a DC voltage and current corresponding to the amount of solar radiation which reaches the PV panels. This voltage depends on the arrangement of the panels, but it is usually 12V. This DC power is then converted to AC power with the use of a DC to AC inverter to produce 220VAC. The inverter then feeds this power to the solar distribution board. It is sometimes necessary to implement more than one PV array. Fortunately the solar DB can accept more than one inverter output.

As one would expect the power produced by the PV array is highly sensitive to changing weather conditions. This is why electric power systems utilizing PV power usually rely on grid based supplies as well. The power system thus relies on a dual power supply. These two power supplies need to be synchronised in order to protect the electric circuitry from damage due to mismatched voltages. This is done by synchronising the inverter output voltage to the grid voltage. The PV array can thus not completely supply an entire business without the support of grid based power, but it greatly reduces the amount of grid based power the business uses. Your kWh meter will thus have a much lower reading at the end of the month.

For an entirely off-grid solution it is wise to invest in generators and backup battery supplies to ensure that a continuous and steady supply of power is available.

Why is a Solar Distribution board upside-down?

In a solar distribution board the flow of power is sometimes referred to being ‘upside-down’. This is due to the fact that the feed of power is in the reverse direction compared to regular distribution boards. Multiple smaller lines of power feed into a bigger line of power. The smaller lines come from the PV array inverters. Thus in the solar distribution boards a collection of smaller circuit breakers feed into a large main circuit breaker, which feeds a load.

The direction of power flow is important as this determines the direction in which measurement devices such as current transformers (CT) and energy meters should be placed. In the solar distribution board the direction should thus be reversed. Certain circuit breakers are also direction specific and should thus also be installed in the correct direction of current flow.

The below image displays the flow from the small CB’s into the main CB. The solar distribution board can then feed into another DB which has feeds from a grid supply or other sources of power.

 

[A schematic representation of a Solar Distribution Board working in conjunction with utility power. Image created by Author.]

The solar distribution board is thus used to distribute power from separate PV arrays into a single main line which supplies a load.

For more information on Distribution Boards, visit the Switchman Website or email joshb@switchman.com .

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand.

jannes9000@gmail.com

 

How to Interpret a Single Line Diagram (SLD)

Single Line Diagrams (SLD)

When designing an electrical distribution board, it is common practice to use a simplified notation in order to represent the electrical system. This graphical representation is called a single line diagram (SLD).

[Single Line Diagram (SLD) for a triple change over distribution board – Drawn by Sheik Essop, Switchboard Manufacturers]

The single line diagram can represent an entire system or a small part of an electrical circuit. Electrical elements such as circuit breakers, transformers, capacitors, busbars, contactors and conductors are shown by standardized schematic symbols. Single phase or three phase systems can be easily represented as a single conductor with horizontal line strokes indicating the number of phases.

[A SG factory staff member assembling a distribution board based off a Single Line Diagram, Switchboard Manufacturers, JHB]

Universally accepted electrical symbols such as in the SLD’s above are used to represent the different electrical components and their relationship within the circuit. To interpret SLD’s one must familiarise oneself with these symbols. The table below shows some of the most common symbols used when designing distribution boards.

Individual electrical symbols
Symbol Identification Explanation
Transformer Represents a variety of transformers from liquid filled to dry types. Additional information is normally printed next to the symbol indicating winding connections, primary / secondary voltages and KVA or MVA ratings.
   Generator Represents a generator often used for backup supply of essential loads.
   Circuit Breaker Represents a circuit breaker. The 3 horizontal lines indicate that the breaker is a 3 phase breaker. 2 lines would indicate double pole and 1 line, single pole.
Isolator Represents a 3 phase isolator.
Power Factor Correction Represents capacitors that make up a PFC panel.
Surge Arresters Represents a 3 pole + Neutral Surge Arrester for lightning protection.
Fuse Represents low voltage and power fuses.
Earth Leakage Represents an RCD/ Earth Leakage unit with no overload protection.
Meter Metering device with Current Transformers (CT’s) .
  Ground (earth) Represents a grounding (earthing) point.
  Contactor Represents a contactor usually triggered by an external occurrence such as a day/night switch.
  Normally open (NO) contact Represents a single contact or single pole switch in the open position for motor control.
  Normally closed (NC) contact Represents a single contact or single pole switch in the closed position for motor control.
   Change Over  Can be automatic or manual. Can be mechanically or electrically interlocked.
A few more Symbols often found as a legend on the side of an SLD:
 

Simple Electric Circuit

Now that you are familiar with electrical symbols, let’s look at how they are used in interpreting single line diagrams. Below is a simple electric circuit. As you can see, the layout is like a branched structure where we have a main incomer feeding smaller breakers that regulate the current to the load. Even though the main incomer is a double pole, the diagram does not require two lines throughout the circuit to indicate this. Only one line is required, hence the name “Single” Line Diagram.

[Single Line Diagram for an RDP house DB – Drawn by Raj Rangasami, Switchboard Manufacturers]

You can tell by the symbols that this single line diagram has a Double Pole (DP) 63A Isolator as the main incomer feeding a 63A DP Earth Leakage, a Single Pole (SP) 32A MCB (STOVE), a SP 16A MCB (GEYSER) and a SP 10A MCB (LIGHTS). The Earth Leakage in turn is feeding a SP 20A MCB which is the supply to the plugs as labeled.

Ready Board

[Ready Board available for purchase on the Switchman Products online store]

The components from the above single line would most likely fit into a polycarbonate enclosure like in the image above. This could be used for an RDP board for low cost housing. A ready wired board fitting this description can be purchased here at ‘Switchman Products’.

Drawing Single Line Diagrams

Single line diagrams are often drawn using CAD software such as Autocad. However, these programs are costly and for someone looking for a free program with a full list of components, ABB offers a program called “e-Design”, which can be downloaded here.

I have a SLD, what now?

Once you have an SLD, your accredited board manufacturer can begin the design work for the General Arrangement (GA) of the physical components. The designer must have a full understanding of the physical aspects of the components in order to place them correctly. These drawings can get complicated and an experienced designer or engineer can prevent potential hazards and design faults. In addition, an experienced designer can draft a board in the most cost effective manner, saving you money along the way.

[A general arrangement drawn by one of Switchboard Manufacturers Experienced Designers, Sheik Essop]

Need help understanding or drawing singles line?

Send us a mail at cad@switchman.com and one of our experienced staff will assist you in designing your board.

Article by Josh Berman, BSc. Elec. Msc Wits,

Electrical Engineer at Switchboard Group.

Joshb@switchman.com

How to Select the Right Circuit Breaker for your Installation?

Selecting the correct Circuit Breaker (CB) for your distribution panel is crucial  for the longevity of the installation as well as the safety of those maintaining and occupying the premises. This article addresses the selection of key breaker attributes such as voltage, current and kA rating.

Rating Considerations:

Circuit Breaker Voltage Rating

The voltage rating of a CB is determined by the highest voltage that can be applied  across any two conductors in the circuit.  It is important to select a circuit breaker with enough voltage capacity to meet the end application. A single phase AC circuit in South Africa is generally rated at 230V  and a single pole CB rated at 230V can be used. A 3 phase AC circuit operates at 400V and requires a Triple Pole CB rated at 400V.

Circuit Breaker Current Rating

The next rating to consider is the amperage or ‘operating current’ of the breaker. CB’s are designed to operate at 100 percent of the required load . However,  in order to offset the effects of heat generated by the system, it is good practice to select a CB at approximately 125 percent of the required load.

For example: If a supply of 250A is available from the transformer, the breaker of choice for the main incomer should be rated at 250A in order to protect the transformer. However, the feeder breakers feeding a 25A load should be rated at 32A.

[Photo Taken at Switchboard Manufacturers Johannesburg]

Circuit Breaker kA Rating

Finally the ‘kA rating’ or ‘fault level’/’rupturing capacity’ of the CB should be taken into account. The kA rating of the CB indicates the maximum short circuit current that the CB can withstand without arcing or catastrophic failure. This current can be upwards of 100 times the required load and has the potential to  cause major damage to property and personnel.

For Example: A circuit breaker rated at ‘6kA’ means that the circuit breaker can withstand 6,000 amps of current during the brief time it takes to trip.

Why is it so important to choose the correct kA rating?

If the short circuit current is greater than what the CB can withstand, the contacts in the CB can weld together,  preventing it from tripping.  Another possibility is that the CB can explode, spewing dangerous plasma.

Under Rated Circuit Breaker

[Breaker fitted to a DB with an Under rated fault level]

So how do I calculate the correct kA?

The maximum current that can flow through a circuit is determined by the size of the transformer feeding the circuit as well as the length of the cable run from the transformer. This is often called the downstream short circuit current. This will determine the maximum kA rating required for the main circuit breaker.

For example: A 500kVA transformer that has a short circuit current of 35kA at its terminals. The cable run from the transformer to the main breaker is 10m and is run with 90mm2 cable. The resistance in the cable limits how much current comes from the transformer, and so after calculations it was determined that the short circuit current at the end of the cable would be 26kA. In this case, a 20kA circuit breaker cannot be used in the installation.

Switchboard Manufacturers
Distribution Panel

[Photo Taken at Switchboard Manufacturers Johannesburg]

SABS Approved Dealers:

When selecting a CB, it is vital for it to be SABS or IEC approved. This provides the assurance that the CB’s have been tested to strict quality standards and will operate in a safe manner as required. Well known brands such as ABB, Schneider and CBI are all SABS approved and are regarded as high quality devices. Switchboard Group is a registered supplier of these products and the leading manufacturer of LV panels is South Africa.

Conclusion:

In conclusion a CB should be selected based on the nominal current, kA rating, number of poles required and whether the CB is SABS approved.

Author: Brendon Swanepoel

2nd Year Electrical Engineering Student, University of the Witwatersrand

Brendon is completing Switchboard Group’s 6 week Learnership and Training program offered to students looking to further their practical skills.

Empowering South Africa’s youth.