Grounding system

A grounding system creates a low-resistance connection between system equipment and/or a system conductor (wire) - called a neutral or grounded conductor - to the earth by using a grounding electrode. A grounding system is not necessary for an electrical system to function, the electrical distribution systems in some countries lack any type of grounding and others have complex grounding systems with additional measurement devices to protect users. It is common for many small-scale off-grid to lack a grounding system as it significantly increases costs and installation time and may not yield significant benefits. As system size, voltage, and cost increase the benefits of a grounding system grow. The electrical code for every country will contain information about the requirements and appropriate equipment for that location.

Additionally, it is necessary to consult the manual for any charge controller or inverter before deciding upon a grounding scheme as they may have different requirements for grounding. There are a few cases that are worth noting here:

• There are some small inverters - modified sine wave and square wave - that will be destroyed if they are connected in a system that has both an AC and a DC system ground as they do not properly isolate their DC input from the AC output.
• There are some charge controllers that are intended for specific applications, like remote telecommunications installations, that are positively grounded.

Grounding terminology

Wiring diagram of a stand alone PV system with a charge controller with DC lighting control and an inverter for AC loads. The system is designed using a TN-S grounding scheme.

Most off-grid systems are designed for use with a TN-S grounding scheme, although they can be used in a variety of configurations. This scheme has a grounded conductor (neutral) and a seperate equipment ground that is run with each circuit to bond together non-current carrying metal parts of the system (enclosures or major system components). The term conductor is used commonly in the electrical field as it encompasses all wires and cables that may be used to create a circuit. A TN-S grounding scheme has has two major functions that work together:

• System grounding - System grounding is created by connecing one current-carrying conductor of an electrical system to ground. A proper system ground provides a means to dissipate excess static electricity created by friction or lightning. This helps to create a stable system voltage and protect system equipment from damage.
• Equipment grounding - Equipment grounding is created by connecting all non-current carrying metallic components of a system to ground. Thise creates a path for any current created by a fault - due to failure of insulation or a loose connection - to return through the equipment grounding or the earth to the grounded conductor.

A proper grounding system requires various different components that are connected together in order for the system to work effectively and safely.

Component	Function
(1) Grounding electrode	The connection point between the earth and the electrical system. It is important that a grounding electrode have sufficient surface area in contact with the earth in order to eablish a good connection. There are many different types of grounding electrodes - copper rods, steel rods, copper plates, the metallic pipes of a building, or a proper connection to the rebar used in the foundation of a building. The appropriate grounding electrode will vary based upon the electrical code, building and soil type.
(2) Grounding electrode conductor (GEC)	The connection that runs from the grounding electrode to the location of the rest of the electrical equipment - typically a wire that runs from the grounding electrode to a grounding busbar in the main distribution panel.
(3) Equipment grounding conductor (EGC)	The connection that runs from the grounding busbar in the main service panel to all of the non-current carrying metallic components of a system (conduit, inverter housing, charge controller housing, enclosures, etc.).
(4) DC system ground	The connection between a DC current carrying conductor in an electrical system and the grounding electrode conductor - commonly made through a ground fault protection device.
(5) AC system ground	The connection between an AC current carrying conductor in an electrical system and the grounding electrode conductor - typically made immediately after the inverter output at a busbar used for the inverter output circuit or in the main distribution panel. The connection is made from the main grounding busbar to the busbar of the grounded conductor. There are some inverters that come with a pre-established internal system ground, for example any that come with an integrated ground fault protection device (GFPD). A second system ground should not be created.

There are several terms that are used when discussing off-grid PV systems and grounding that are important to understand:

Term	Definition
Non-current carrying conductor	A conductor that is not intended to regularly carry current as a part of the normal functioning of an electrical system. All grounding conductors fit into this category.
Current carrying conductor	A conductor that is intended to regularly carry current as part of the normal functioning of an electrical system. All circuits related to the power generation, energy storage, and distribution to loads fit into this category.
Grounded system	A system that has an AC or DC system ground. The system may be a DC grounded system, an AC grounded system, or an AC/DC grounded system. There is at least one grounded current carrying conductor in the system.
Ungrounded system	A system that does not have any system ground. There is no grounded current carrying conductor in the system.	Grounded conductor	A current carrying conductor that has a connection established to ground by an AC or DC system ground.
Ungrounded conductor	A current carrying conductor that does not have a connection established to ground by an AC or DC system ground.

Grounded and ungrounded conductors in practice

The current carrying conductors in a circuit will behave differently depending upon whether they are grounded or not. Grounded and ungrounded systems each carry different advantages and disadvantages.

=Ungrounded system

An ungrounded conductor in an ungrounded system will only have a voltage relative to the other ungrounded conductors in the system, but will not have a voltage relative to ground. For a #Ground fault to pass current and trip a overcurrent protection devices or a residual current device, two seperate ground faults will be required. This means that a person could touch either single conductor of the system - as long as there was no connection to ground at any point - and be safe.

Ungrounded systems do not readily identify single ground faults, which represents a significant safety hazard as a user or technician may not discover that a fault exists until they make contact with a conductor that would have had no voltage to ground had it not been for the fault. They can also create other additional double-fault scenarios that do not exist in grounded systems that require an overcurrent protection device on all positive and negative wires in a circuit to be properly mitigated.

Component	Voltage relative to other conductors
Ungrounded conductor 1	Will have full circuit voltage relative to the other ungrounded conductor. Will have no voltage relative to ground.
Ungrounded conductor 2	Will have full circuit voltage relative to the other ungrounded conductor. Will have no voltage relative to ground.
Grounding conductors	Will have no voltage relative to either ungrounded conductor 1 or ungrounded conductor 2.

Grounded system

Component	Voltage relative to other conductors
Ungrounded conductor 1	Will have full circuit voltage relative to the grounded conductor, ground, and any grounded equipment.
Grounded conductor	Will have the full circuit voltage relative to the ungrounded conductor. Will have no voltage relative to ground or any grounded equipment.
Grounding conductors	Will have full circuit voltage relative to the ungrounded conductor. Will have no voltage relative to the grounded conductor.

When a DC or AC system ground is created, a grounded current-carrying conductor is created. A grounded conductor that has a solid connection to ground should maintain a voltage of 0 V relative to ground. This means that coming into contact with a grounded conductor or a #Ground fault does not represent the same hazard that it does if the conductor were ungrounded. Grounded conductors are still hazardous though as they are carrying current in a circuit. The grounded conductor in a system will have the full circuit voltage relative to the ungrounded conductor in the circuit.

with one ungrounded conductor in a system with no system grounds will - regardless of which conductor - will have no effect. But a second ground fault with a conductor that has a voltage relative to the other conductor that experienced a ground fault.

Ground faults

Wiring diagram of a stand alone PV system with a charge controller with DC lighting control and an inverter for AC loads. This system is experiencing an AC ground fault.

A ground fault occurs when a current carrying conductor of an electrical system has an insulation failure or comes loose from a connection and makes contact with somethat that has a connection to ground. Ground faults can be very dangerous because they can lead to electrocutions and fires, therefore it is important to identify them as quickly as possible and disable them. When there is a ground fault, current escapes the wires of a circuit and seeks other paths to be able to complete the circuit. A person who grabs something that has been energized by a fault to ground can become a path to complete the circuit and be electrocuted. In a properly grounded system the most common ground fault scenario is a ground fault between an ungrounded current-carrying conductor (black wire) and a grounded piece of equipment. A ground fault can occur with a grounded conductor but it will not typically be apparent until there is a second ground fault involving the ungrounded conductor (black wire).

System grounding and equipment grounding work with overcurrent protection devices, ground fault protection devices, residual current devices and inverter electronics to identify faults and disable them as quickly as possible by opening (disconnecting) the circuit. This system functions because a fault to an equipment grounding conductor will create a short circuit if there is low enough resistance, which will activate these devices and cause them to open (disconnect) the circuit.

An AC ground fault involving an ungrounded conductor typically occurs as follows:

1. A ground fault occurs between a grounded piece of equipment and an ungrounded conductor (wire).
2. The equipment grounding conductor has a connection - due to the system ground - to the grounded conductor. The current follows it because it is a low resistance path to the grounded conductor, which completes the circuit.
3. The current reaches the grounded conductor and this effectively creates a short-circuit which causes the inverter to supply large a amount of current to the newly created low-resistasnce circuit.
4. This high current flow will be identified by the inverter electronics, residual current device (RCD) or an overcurrent protection device (OCPD) as a fault, which will cause the one of them to trip and open (disconnect) the circuit. A RCD or inverter electronics will react quicker than an OCPD.

These devices only temporarily remove the hazard, thus after a ground fault occurs its is necessary to carefully troubleshoot the system to locate the fault and fix it.