Earthing and Protection – SS1 Solar Photovoltaic Installation & Maintenance Lesson Note (Second Term)

Earthing and protection are the methods used to keep a solar system and its users safe from faults, lightning and surges.

• Earthing: gives dangerous electricity a path into the ground through an earth rod.
• Surge protective device (SPD): sends surges to the earth wire before they reach the inverter.
• Circuit breaker: switches off the circuit when the current is too high.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State two reasons why a solar system needs protection.
2. Explain what earthing is and how an earth rod works.
3. Describe what a surge is and how an SPD protects a system.
4. Explain how a circuit breaker protects against overload and short circuit.
5. State why DC breakers must be used on the solar panel side.

Why does a solar system need protection?

A solar system needs protection because lightning, surges and wiring faults can damage equipment and harm people.

• Lightning: a strike near a house sends a huge jump in voltage (a surge) down the wires.
• Damage without protection: the surge can destroy the inverter, damage the batteries and start a fire.
• Faults: a live wire touching a metal body can make that body dangerous to touch.
• Too much current: overload and short circuit make wires overheat.

Two reasons for protection

1. Safety for the machines: stops high voltage from “killing” the inverter, batteries and other electronics.
2. Safety for people: if a wire touches the metal frame of a fridge, you do not get a shock when you touch it.

What is earthing?

Earthing (or grounding) is the connection of the metal parts of a solar system to the ground through an earth rod.

What parts are earthed?

• Solar panel frames: the metal edges of each panel.
• Inverter casing: the metal body of the inverter.
• Metal mounting structure: the frame that holds the panels on the roof.

What is an earth rod?

An earth rod is a long copper rod driven deep into the soil to carry fault current into the ground.

How does earthing work?

1. Drive the copper earth rod deep into the soil.
2. Connect a green/yellow earth wire from the panel frames and inverter casing to the rod.
3. If a fault leaks current into a metal part, the current flows along the earth wire.
4. The current goes into the soil instead of through the body of a person.

• Why it works: current takes the path with the least resistance, and the copper wire resists far less than a human body.
• The Nigerian factor: very dry, sandy ground conducts electricity poorly.
• Common local practice: installers put charcoal and salt around the earth rod to help the current escape into the soil more easily.

What is a surge?

A surge is a sudden, large and short-lasting rise in voltage in a wire.

• Lightning: a strike on or near the house or power line.
• The national grid (“NEPA”): the light comes back suddenly with too much force.
• Effect: sensitive parts such as the inverter electronics can burn out.

What is a surge protective device (SPD)?

An SPD is a device that sends surge current to the earth wire so that it does not reach the inverter.

• The shield: think of the SPD as a “security gate” fitted across the wires.
• Normal conditions: the SPD stays inactive and electricity flows to your TV and other loads.
• The sacrifice: when a huge surge arrives, the SPD instantly “opens a side door” and sends the dangerous power into the earth wire.
• Needs a good earth: an SPD can only do its job if it is connected to a good earth.

How do you read the SPD indicator?

• Green window: the SPD is working.
• Red window: the SPD has “taken the hit” to save the house and must be replaced.

What is a circuit breaker?

A circuit breaker (or MCB) is an automatic switch that cuts off a circuit when too much current flows.

Overload protection

• Overload: too many appliances on one circuit, such as an AC, an iron and a kettle together.
• Danger: the wires become dangerously hot.
• Action: the breaker senses the heat and “trips” (switches itself off) to prevent a fire.

Short circuit protection

• Short circuit: a positive wire and a negative wire touch by accident and cause a huge spark.
• Action: the breaker shuts off the circuit almost instantly, faster than you can blink.

Why should you never fit a bigger breaker to stop tripping?

• Wires have a limit: each wire in the wall can carry only a certain current safely.
• A bigger breaker lets more current through: it will not trip when the wire is already overheating.
• Result: the wire can melt its covering and cause a fire.
• Correct action: reduce the load on the circuit.

What is the difference between DC and AC breakers?

A DC breaker is made for direct current from the solar panels and batteries. An AC breaker is made for alternating current on the house side.

• Reason: a DC spark (arc) is harder to put out than an AC spark.
• Risk: a regular house breaker might not be able to stop a DC spark from the solar panels.

Where are the protection devices placed?

The DC breaker sits between the panels and the inverter, and the AC breaker sits after the inverter.

What are the key safety terms?

The three main protection tools are the earth rod, the SPD and the circuit breaker.

Evaluation

1. The visual check: you see a solar panel on a roof with no green wire attached to the metal frame. Is that system safe? Give a reason.
2. The “why”: why is it a bad idea to replace a “tripping” circuit breaker with a bigger one just so you can use your electric iron? (Hint: think about the wires in the wall.)
3. What does it mean when the window on an SPD turns red?
4. Why must DC breakers be used for the solar panels?
5. Define earthing and state the job of an earth rod.

Assignment

Scenario: It is a rainy day in Port Harcourt. Lightning strikes a tree near a house. The house has solar panels but no SPD. What do you think will happen to the inverter? Name the device that should have been fitted and explain how it works.