The importance of a grounding system

June 2016
Importancia sistema de tierras

SAFEGROUND®, monitoring the grounding system in the surge protection device itself.  No earth, no protection

The grounding system is a basic part of any electrical installation, and aims to:

- Limit the potential difference between metallic masses and ground.
- Make sure the protection devices operate.
- Eliminate or reduce the risk posed by a fault in the electrical equipment used.

There are two main types of protection that depend on the ground connection for their proper operation. These are surge protection (transient overvoltage protection for equipment) and earth leakage protection (protection of personal from indirect contact, i.e. electric shock).

The effects of transient overvoltages on an installation are avoided using surge protection devices (SPD). These work by shunting the overvoltage energy to ground, thus avoiding damage to electrical and electronic equipment.

The quality of surge protection is closely linked to the grounding system, since a high impedance path may increase the exposure of sensitive equipment to the effects of the surge. In fact, in the event of complete loss or lack of a ground connection, the surge protection loses all effectiveness.

foto 4 SAFEGROUND 2016

To understand the relationship between the quality of the ground connection and the effectiveness of surge protection, people often use a very intuitive hydraulic simile. If we equate the energy of the overvoltage to a specific volume of liquid, the ground connection may be thought of like a funnel. There is a direct relationship between the diameter of the funnel opening and the quality of the ground connection. Once the funnel has drained the entire volume of liquid, the surge will have been shunted to ground and the equipment will see a completely normal voltage.

ESQUEMA-EMBUDOS-01

In this graph we compare what happens to a bad ground connection (funnel on the left - small opening) to what happens to a good one (funnel on the right - big opening) in the event of a surge. We can see how a ground connection with higher resistance (funnel on the left) is saturated with the energy of the discharge (the funnel overflows because it is incapable of draining fast enough). Intuitively, this prolongs the time during which the equipment to be protected is subjected to the effects of overvoltage, and which therefore suffer more wear (the funnel takes longer to empty due to its smaller opening).

Especially in systems in which all the equipment is referenced to ground, this can even make the surge current unable to find a better escape route ends up doing direct damage to the equipment and rendering the protection ineffective. The funnel on the left is unable to dissipate energy properly and overflows, which intuitively represents the situation of damage to equipment.

In the extreme case of loss or lack of a ground connection, the surge protection loses all effectiveness.

As for indirect contacts, these occur when a person comes into contact with a live metal part of an installation, usually due to an accidental insulation failure.

Protection against indirect contact is provided by RCDs (residual current devices). Its operating principle is the detection of current leaks above its sensitivity threshold (of the order of milliamps), by comparing the incoming and outgoing current in a given circuit. Any difference between the two corresponds to a leakage current, which means that the RCD will open the circuit to avoid any risk to the users of the system.

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In the case of earth leakage protection, connecting equipment to ground is essential for safety against indirect contacts, because if there’s no ground connection, the leakage needed for the RCD to operate will not be detected before someone touches the metal housing and an indirect contact (electric shock) occurs, discharging the current leakage through it.
The widespread use in industrial installations of lower sensitivity RCDs increases if anything the relationship between grounding and safety, since an indirect contact would represent a much greater potential danger for people.

For the protection to work properly, the correct status of the ground connection of an electrical installation is essential.