Installation of photovoltaic systems in places with poor grounding

Every building must have a grounding system. However, given the low average quality of Brazilian electrical installations, especially residential ones, it is not difficult to come across a building where there is no type of grounding. 

Grounding is essential for protection against electric shocks, in addition to being part of the protection system against atmospheric discharges for installations and equipment, especially the inverter, which is the most sensitive component of the photovoltaic system.

Responsibility for installation

Our commercial quotations will not always find suitable electrical installations that have a grounding system, in accordance with standards. 

Although there is no legal item that directly requires new work on an existing installation to be retrofitted, as we will be offering an engineering service when installing a photovoltaic system, we must follow the corresponding ABNT standards.

The standards alone do not have the force of law, but engineering and equipment installation services must comply with the consumer protection code, which characterizes the installer as a service provider. 

The consumer protection code requires faithful compliance with ABNT standards in this type of service, with legal punishment for non-compliance with this obligation, meaning that damage caused by the installation may be the legal responsibility of the installer. Also NR10, which brings safety requirements in electrical work, gives the force of law to ABNT standards.

When faced with the need to install a system in a place where there is no grounding, we cannot simply see the lack of grounding as a problem beyond our system and that would not be our responsibility. 

As grounding has essential safety functions, there is no way to install a solar system in this condition without it posing a risk to the owner or the installation itself.

Basic Grounding Concepts

The grounding system has 3 main functions:

• Ensure the protection of people in situations of electrical failures (energized device housing, earth faults in the medium voltage network, provide paths for atmospheric discharges, etc.);
• Electrostatic charge flow;
• Ensure the proper functioning of protections against short circuits and overvoltages.

In a residential rooftop photovoltaic system, we can exemplify some situations in which the grounding system would act to guarantee protection:

• If there is any conductor on the AC side exposed and in contact with the metal housing of the inverter and a person touches that device, they could be electrocuted when closing the circuit between the conductor, the housing and the ground;
• In the event of a lightning strike on the roof, if the system was properly equipotentialized and grounded, there would be a path of descent for the lightning, minimizing impacts on the rest of the installation;
• The DPS needs to be connected to earth to be able to divert voltage surges and protect the equipment.

In the examples above, the lack of a grounding system would pose a risk of damage to the installation and people. Therefore, we cannot install a photovoltaic system in a residence that is not grounded or has poor grounding. 

In these situations, we must inform the customer of the precariousness of the installation and we must wait for the customer to correct the situation with another professional or for the service to be carried out by the PV system installation company itself.

Grounding systems

A grounding system is basically composed of a grounding electrode, which is the element that will physically be at the interface between the electrical system and the earth, the grounding conductors, which connect the electrode to the switchboards, busbars and the protection and equipotentialization conductors. , which connect the equipment and ground pins of the power take-offs to the equipotentialization bars of the switchboards.

Every installation contains at least one grounding point, which is incorporated into the installation's power input pattern, where the utility's neutral is grounded by a rod. This is necessary for the proper functioning of the electrical energy distribution system, but it cannot be the only grounding solution.

If the installation does not even have a neutral that comes from the dealership network, the customer or designer must contact the distributor to regularize the input standard.

Leaving this neutral grounding point, we usually have a conductor called PEN, which will be connected to a distribution board where it can be separated into a neutral bar and a main ground bar (also called BEP – main equipotential bus). At BEP we connect the grounding electrode.

The NBR 5410 standard defines which elements can be used as grounding electrodes. They are, in preferred order of use:

1. use of the foundations’ own concrete reinforcement;
2. use of metal tapes, bars or cables immersed in the concrete of the foundations;
3. use of buried metal mesh, at the foundation level, covering the building area, which may contain rods for complementation;
4. at least, use of a buried metal ring, surrounding the perimeter of the building and complemented, when necessary, by vertical rods and/or cables arranged radially (crow's feet).

Note that of the 4 available options, none of them only supports the neutral grounding rod of the input standard without other elements to complement it.

The NBR 5410 standard also classifies the ways in which protection and equipotentialization connections can be made, the main ones being: the TN scheme, the TN-S scheme, the TN-CS scheme, the TT scheme and the IT scheme. 

In residential systems, the IT grounding scheme is not permitted. If the TT scheme is chosen, the circuits must be protected by a DR device (residual differential), as the functioning of circuit breaker protection is impaired.

Example of PV systems in precarious installations

What should we do when the work only has grounding for the input standard? 

In the paragraphs below we suggest a way of adapting the grounding system based on an assembly with a PEN conductor between the input standard to the general panel, and from the general panel onwards a TN-S type assembly. TN-C, TN-CS and TT type assemblies can also be implemented in a residence.

TN-S grounding system

To compose a minimum residential grounding system, we must have the PEN conductor in the general installation framework coming from the grounding point of the input standard. 

This conductor goes to the general switchboard, where it is separated into two bars: a neutral bar (isolated from the switchboard housing) and a main equipotential bar (BEP). 

The neutral conductors for all circuits will come from the neutral bar. The equipotentialization conductors and the conductor that will connect to the grounding mesh (known as the grounding pigtail) will come from the BEP bar.

A large proportion of small electrical installations do not have an appropriate grounding grid. Among these, some do not even have a main equipotential bar that distributes the protective conductor throughout the installation, as shown in Figure 4.

As seen previously, the entry standard rod alone is not sufficient to comply with the applicable ABNT standards. 

When possible, we must compose our grounding electrode using the 4 alternatives presented in NBR 5410 (foundation armature, bare conductors integrated into the foundation, grounding mesh on the ground or grounding ring surrounding the installation). 

However, we will not always be able to add these elements to the customer's electrical installation. For these cases where the building is already built, cannot be altered or we cannot access the elements, the standard presents flexibility on the type of grounding electrode, allowing (for example) simpler electrodes, with the addition of rods and sections of buried cable. If these rods are added, they must be connected to the BEP.

We cannot fail to equipotentize the modules with the inverter and the inverter with the rest of the installation. If the installation point of our inverter does not have access to the protection conductor, we must provide this connection to the panel where the BEP or BEL (local equipotentialization bus) is located. 

This conductor must go along with the inverter output circuit (AC). The sizing of this protective conductor can be simplified by the table below, taken from the NBR 5410 standard:

It is always worth highlighting that we should not have separate grounding systems, that is, a grounding system only for the photovoltaic system circuit and another grounding for the rest of the installation. 

The inverter circuit and other circuits in the installation must have equipotential grounding, whether connected directly to the same grounding electrode or connected to a BEP or BEL bar.