Conexión a tierra, tierra y seguridad eléctrica

La conexión a tierra (toma de tierra o puesta a tierra) constituye una vía de retorno común para la corriente eléctrica de un circuito eléctrico
6 de marzo de 2020 por
Conexión a tierra, tierra y seguridad eléctrica
Techno Sun, SLU, Hugo Rodrigo Zapata :

Ground connections

Grounding is necessary for electrical safety and also constitutes a reference point in a circuit for measuring voltages.

In general, there are three types of grounding, which are: ground (earth), chassis ground (or earth) and earth ground.

  • Grounding is a direct physical connection to the earth. It is usually done by putting a copper rod (ground spike) in the ground soil. But depending on the age and location of the system, it can also be a copper plate or strip buried in the ground, or the water mains or water pipes of a house.

  • The chassis ground or earth ground is a connection to a metal structure such as a vehicle or the metal hull of a ship. It can also be the metal casing of electrical equipment.

  • Grounding is a common reference point for measuring voltages in a circuit. As a result, a voltage can be above ground (positive) or below ground (negative).

Electrical safety

Electricity is dangerous; it can kill, injure or burn a person. The most dangerous part of electricity is the current. A small current passing through a person can already be very dangerous. See the table on the right.

Current will flow when an electrical circuit is closed.

For example, imagine two loose AC wires: one live and one neutral. If the wires are just there, no current will pass through because the circuit is not closed. But if you touch one live wire with one hand and the neutral with the other, you close the circuit yourself and electricity will flow through the live wire back to the neutral wire through your body and your heart. The current will continue to flow until the fuse blows, but by then it will probably be dead.

Apart from touching a neutral and a live wire at the same time, there are other dangerous situations that can occur, for example when electricity passes through the ground. This is more common than someone touching a phase and neutral conductor at the same time.

The neutral conductor is connected to ground at some point. This can be in the domestic installation, in the distribution network or in the power generator (star point).

If there is a fault in electrical equipment, metal parts on the outside of that equipment may become live. This can happen if there is an internal shortcut between the live electricity and the metal casing of the equipment.

Think, for example, of a malfunctioning washing machine. It may be due to an electrical fault, mechanical damage or damaged electrical cables touching the metal housing of the electrical equipment.

As soon as you touch the faulty washing machine, electricity will flow from the phase to the metal casing and to earth through its body. From the ground the electricity will flow to the neutral of the mains supply. The circuit is complete.

Electricity will continue to flow until the mains supply fuse blows. But as in the previous situation, it will probably be dead by then.

To make electrical installations safer, the grounding conductor was introduced. The grounding conductor connects the metal casing to the ground.

Now. If you touch the faulty equipment, the electricity will pass through the grounding conductor and not through your body. This is because electricity chooses the path of least resistance. The path through your body and the ground is more resistive than the path through the ground conductor. But keep in mind that it is still possible for a small amount of current to pass through a person. A current of more than 30 mA is already dangerous.

Only one grounding conductor is not sufficient. A residual current circuit breaker (RCCB) is also required in the installation. More information on residual current circuit breakers can be found in the section Residual current circuit breaker (RCCB).

Ground wiring

Grounding or earthing cables are yellow/green. In older installations and in other countries it can also be found with green wire.

Good ground wiring is essential for electrical safety. The cable and ground connections must have low electrical resistance.

Remember that electricity will go through the path of least resistance. So you have to make sure that the ground wire is thick enough and that the connections are tight.

The ground wire can have high currents when faulty equipment is present, and must be able to carry this current until the system fuse blows. So it is important that the ground wire is thick enough.

Differential circuit breaker (ID)

Electricity can be very dangerous. Incorporating a grounding conductor into a system makes it safer, but installations can be made even safer with a residual current (RCD) circuit breaker.

Its use is mandatory in all AC installations.

The ID detects that electricity is passing to ground and disconnects immediately. Electricity will go to ground if there is a fault in the system or, more importantly, when current is passing through a person. The IDs are designed to disconnect as soon as a current flow to ground is detected.

Residual current circuit breakers may be given different abbreviations according to their English names:

  • Residual Current Device (RCD) Residual Current Circuit Breaker (RCCB).

  • Ground fault circuit interrupter (GFCI).

  • Ground fault current interrupter (GFI).

  • Appliance leakage current circuit breaker (ALCI).

  • Safety switch.

  • Earth leakage device

An ID measures the current balance between the phase and neutral conductor. The device will open its contact when it detects a current difference between the phase and neutral.

In a safe system the supply and return currents should add up to zero. If this is not the case, there is a fault in the system, the current is leaking somewhere in the earth or into another circuit.

IDs are designed to prevent electrocution accidents by detecting this current leakage, which can be much smaller (typically 5 - 30 milliamps) than the currents required to trip conventional circuit breakers or fuses (several amps). They are designed to operate in 20 - 40 milliseconds. This period of time is less than that required for the electric shock to cause the heart to go into ventricular fibrillation, the most frequent cause of death by electric shock.

A safe system protects against short circuits, overloads and earth leakage currents.

Earth leakage detection is only possible in systems where the neutral conductor is connected to the earth conductor, such as a TN or TT system. It is not possible to detect earth leakage in an IT network.

Where to mount an ID (residual current circuit breaker)

In an electrical installation, the ID must be mounted before the loads. In reality, this means that the ID has to be mounted before separating the installation into two different groups. If an inverter or inverter/charger is used, the ID would have to be placed after, otherwise there would be no earth protection when the inverter is operating. Electrical consumers that only operate when connected to the port socket will need their own ID.

Accidental tripping of the ID (residual current circuit breaker)

In some installations, the ID will jump early. This may be due to the following:

  • The system has a double MEN link (neutral to ground) and this will cause the ID to jump due to a potential difference in ground.

  • The system has equipment that introduces a small amount of 'below threshold' neutral ground leakage, but the cumulative effect can cause unpredictable ID activations. Some of the appliances that often give problems and are worth checking and disconnecting first when problems arise are: old refrigerator compressors and electric heaters (due to their own ground differential from the main ground spike).

Inverter and inverter/charger neutral to ground bonding

An AC power supply must have a neutral to ground (MEN link) for the ID to operate.

This is the case for the grid, but also if the AC source is a generator or an inverter.

  • If the AC power source is the mains, the MEN link will be connected at the control panel at the point where the mains enters the installation.

  • If the AC power source is a generator, the MEN link will be connected to the AC connection terminals of the generator.

  • If the AC power supply is an inverter, the MEN link will be connected at the AC connection of the inverter or at the control panel of the installation.

But when combined inverter/charger units are used, the MEN link is not so simple.

The inverter/charger has two different operating modes:

  • In inverter mode it operates as a stand-alone inverter and is the main power source for the system.

  • In charger mode it will be powered through mains or generator power in the system.

When the inverter/charger is inverting and acting as a power supply, it will have to make a separate MEN link. But when it is fed through a generator or the grid supply, the incoming supply will have to have the MEN link instead of the inverter/charger.

Victron inverter/chargers include an internal ground relay. This relay automatically makes or breaks the connection between ground and neutral.

Inverter/charger is in charger and power mode

When the inverter is connected to the AC source, the AC input relay is closed and at the same time the ground relay is open. The AC output system depends on the AC supply to provide the neutral-to-ground bond. The bond is necessary for the AC output circuit ID to function.

Inverter/charger in inverter mode

When the AC power supply is disconnected, has been turned off, or has failed, the AC input relay opens. When the AC input relay is open, the installation no longer has a neutral to ground link. This is why at the same time the ground relay is closed. As soon as the ground relay is closed, the inverter/charger makes an internal neutral-to-ground bond. The bond is necessary for the AC output circuit ID to function.

Mobile installations

A mobile installation is an installation that operates independently of the grid. When it is connected to AC power it normally does so via the grid at different locations or from generators. For example, ships, vehicles or mobile auxiliary power systems. In this section a ship installation is used, however this information can be applied to any mobile installation.

Mobile installations do not have a ground spike. So something needs to be in place that creates a central ground potential. All touchable metal parts of the ship or vehicle must be connected together to create a local ground. Some examples of metal parts on a boat or vehicle are: the chassis, hull, metal fluid lines, railings, engine, power point ground contacts, light conductors, and ground plate (if present).

A mobile system is usually connected to different power sources and it is often not clear which of the port or pontoon supply cables is grounded or if the grounding is connected. On the other hand, the phase and neutral may not be properly connected. Connecting such a source to a moving system could create a short to ground. Or there is no ground at all.

It is also important to consider whether the mobile system is connected to the power supply or whether it is disconnected from the power supply and operates autonomously.

Isolation and Grounding of Victron equipment

This section explains the isolation of different Victron products between AC and DC, or between DC and DC.

This information is necessary for a system with Victron equipment to be properly grounded.

Isolation of all Victron inverters and inverter/chargers:

  • Between AC circuits and chassis: basic insulation. The chassis must be grounded.

  • Between AC and DC: reinforced insulation. Once the chassis has been grounded, it is considered safe to touch DC if the rated voltage is 48 V or less.

  • Between DC circuits and chassis: basic isolation. Therefore, negative or positive DC grounding is allowed.

In case of positive grounding, uninsulated interface connections will be referred to DC negative and not to ground. Grounding such a connection will damage the product.

The AC grounding terminal of all inverters and inverter/chargers is connected to the chassis.

AC neutral grounding for Victron inverters

The neutral of all inverters 1600 VA and above and the Phoenix Inverter Compact 1200 VA inverter is connected to the chassis Grounding the chassis also grounds the AC neutral. A grounded neutral is required for proper operation of an ID (or RCD, RCCB, RCBO or GFCI).

If a reliable earth ground is not available and/or if an ID (or RCD, RCCB, RCBO or GFCI) is not installed, the AC neutral connection to the chassis should be removed to improve safety. Warning: Such an installation may not comply with local regulations.

The AC neutral of lower power inverters is generally not connected to the chassis. However, a neutral-to-ground connection can be established: refer to the product manual.

AC neutral grounding for Victron inverter/chargers

The output AC neutral of all inverter/chargers is connected to the input AC neutral when the feedback relays are closed (AC available at the input). When the feedback relays are open, a ground relay connects the outgoing neutral to the chassis. A grounded neutral is required for proper operation of an ID.

The ground relay can be disabled on almost all models. Refer to the product manual.

Isolation of MPPT solar chargers

No isolation between PV input and DC output.

Basic isolation between input/output and chassis.

Isolation of other products

Battery chargers: reinforced insulation between AC and DC. Basic insulation between AC and chassis, except for IP65 Smart chargers, which have reinforced insulation between AC and plastic housing.

DC-DC converters, diode splitters and FEC splitters and other DC products: the housing is always isolated from the DC (basic isolation).

System grounding

So far we have talked about AC grounding in AC installations, but it is also needed in the DC components of an installation. This section describes some common installations that contain not only an inverter/charger, but also a battery bank, a solar charger and a PV array.

Grounding of a system disconnected from the grid

Do not ground either the positive or the negative of the PV array. The negative PV input of the MPPT is not isolated from the negative output. Therefore, grounding the PV will cause ground currents.

PV frames can however be grounded, either close to the PV array or (preferably) to the central ground. This provides some protection against lightning strikes.

Put the ground connection close to the battery. Touching the battery poles is supposed to be safe. Therefore, the battery ground should be the most reliable and visible ground connection.

The DC ground wiring should be of sufficient thickness to carry a fault current at least equal to the rated current of the DC fuse.

The inverter chassis or Multi/Quattro must be grounded. There is no basic isolation between AC and chassis.

The MPPT solar charger chassis must be grounded. There is basic isolation between AC and chassis.

Note that the grounding of the AC distribution with fuses or MCB, as well as that of the PV array and frame, is not shown.

Off-grid generator

Use a single ground, close to the battery. Touching the battery poles is supposed to be safe. Therefore, the battery ground should be the most reliable and visible ground connection.

The DC ground wiring should be of sufficient thickness to carry a fault current at least equal to the rated current of the DC fuse.

Similarly, AC ground wiring should be able to carry a fault current at least equal to the AC fuse current rating.

The ID will only work if the Multi/Quattro chassis is grounded.

Off-grid high power generator

Ground the generator directly to the central ground.

Grid-connected energy storage system (ESS)

The DC ground wiring should be able to carry a fault current at least equal to the rated current of the DC fuse.

Connect the inverter/charger chassis to the grounding bus bar.

The output AC ground can be removed from the center bus bar or from the AC output terminal.

Source: Wiring Unlimited de Victron Energy