Earthing

Introduction

1.1 Evolution of needs

Today the 3 earthing systems such as defined in

IEC 60364 and French standard NF C 15-100,

are:          

·         exposed-conductive parts connected to

neutral -TN-;

·         earthed neutral -TT-;

·         unearthed (or impedance-earthed) neutral -IT-.

The purpose of these three systems is identical as regards protection of persons and property:

mastery of insulation fault effects. They are considered to be equivalent with respect to safety of persons against indirect contacts.

However, the same is not necessarily true for dependability of the LV electrical installation with respect to:

• electrical power availability;
• installation maintenance

1.2 Causes of insulation faults

In order to ensure protection of persons and continuity of service, conductors and live parts of electrical installations are «insulated» from the frames connected to the earth.

Insulation is achieved by:

• use of insulating materials;
• distancing, which calls for clearances in gases

(e.g. in air) and creepage distances (concerning

switchgear, e.g. an insulator flash over path).

Insulation is characterised by specified voltages

which, in accordance with standards, are applied

to new products and equipment:

• insulating voltage (highest network voltage);
• lightning impulse withstand voltage (1.2; 50 ms

wave);

• power frequency withstand voltage

(2 U + 1,000 V/1mn).

Example for a LV PRISMA type switchboard:

• insulating voltage: 1,000 V;
• impulse voltage: 12 kV

1.3 Hazards linked to insulation faults

An insulation fault, irrespective of its cause,

presents hazards for:

• human life;
• preservation of property;
• availability of electrical power;

the above all depending on dependability.

Electric Shock of persons

A person (or animal) subjected to an electrical voltage is electrified. According to the gravity of the Electric Shock, this person may experience:

• discomfort;
• a muscular contraction;
• a burn;
• cardiac arrest (this is Electrocution

2 Earthing systems and protection of persons

This section defines the Electric Shock and

Electrocution hazards for the various earthing

systems, such as specified by the International

Electrotechnical Committee in standard

IEC 60364.

The LV earthing system characterises the

earthing mode of the secondary of the MV/LV

transformer and the means of earthing the

installation frames.

Identification of the system types is thus defined

by means of 2 letters:

·         the first one for transformer neutral connection

(2 possibilities):

·         T for "connected" to the earth,

·         I for "isolated" from the earth;

·         the second one for the type of application

frame connection (2 possibilities):

·         T for "directly connected" to the earth,

·         N for "connected to the neutral" at the origin of

the installation, which is connected to the earth  Combination of these two letters gives three possible configurations:

TT: transformer neutral earthed, and frame earthed,

 TN: transformer neutral earthed, frame

connected to neutral,

  IT: unearthed transformer neutral, earthed

frame.

Note 1:

The TN system, as in IEC 60364 includes

several sub-systems:

  TN-C; if the N and PE neutral conductors are

one and the same (PEN);

 TN-S: if the N and PE neutral conductors are

separate;

  TN-C-S: use of a TN-S downstream from a

TN-C (the opposite is forbidden).

Note that the TN-S is compulsory for networks

with conductors of a

2.1 TN system

2.2 TT system

2. 3 IT system

3 Earthing systems confronted with fire and electrical power unavailability hazards

3.1 Fire

It has been proved, then accepted by standard makers, that contact between a conductor and a metal part can cause fire to break out, in particularly vulnerable premises, when the fault current exceeds 500 mA.

3. 2 Electrical power unavailability

This hazard is a major one for operators, since it results in non-production and repair costs which can be high. It varies according to the earthing system

Chosen. We remind you that availability (D) is a statistical

quantity   equal to the ratio between

two periods of time:

4 Influences of MV on LV, according to the earthing systems

LV networks, unless a replacement uninterruptible power supply (with galvanic insulation) or a LV/LV transformer is used, are influenced by MV.

This influence takes the form of:

·         capacitive coupling: transmission of

overvoltage from MV windings to LV windings;

·         galvanic coupling, should disruptive

breakdown occur between the MV and LV

windings;

·         common impedance, if the various earth

connections are connected and a MV current

flows off to earth.

This results in LV disturbances, often

overvoltages, whose generating phenomena

are MV incidents:

·         lightning;

·         operating overvoltages;

·         MV-frame disruptive breakdown inside the

transformer;

·         MV-LV disruptive breakdown inside the

transformer.

Their most common consequence is

destruction of LV insulators with the resulting

risks of Electric Shock of persons and

destruction of equipment

5 Switchgear linked to the choice of earthing system

Choice of earthing system affects not only dependability (in the largest sense) but also installation, in particular with respect to the switchgear to be implemented.

5.1 TN system

In this system the SCPDs (circuit-breaker or fuses) generally provide protection against insulation faults, with automatic tripping according to a specified maximum breaking time (depending on phase-to-neutral voltage Uo:

5.2 TT system

With this system, the small value of the fault currents (see previous section) does not allow the SCPDs to protect persons against indirect

contacts. RCDs   need to be used, associated with circuit-breakers or

switches (see IEC 60364 - paragraph 413.1.4.2).

These devices must meet the following standards in particular:

5.3 IT system

Remember that in the event of a double fault,

safety of persons is provided by the SCPDs.

When the first insulation fault occurs, the

calculation proved there was no risk (contact

voltage lower than limit safety voltage).

Automatic de-energising is therefore not

compulsory: this is the main advantage of this

system.

To retain this advantage, standards recommend

(IEC 60364 - paragraph 413.1.5.4) or stipulate

(NF C 15-100) the use of an Insulation

Monitoring Device (IMD) and locating of the first

fault. In point of fact, if a second fault occurs,

automatic breaking is vital due to the Electric

Shock risk: this is then the role of the SCPDs

backed up by the RCDs if required.

Locating the first fault for repairs (curative

maintenance) is considerably simplified by the

use of a Ground Fault Location Device (GFLD).

Predictive maintenance, based on the monitoring

(recording) of variations in insulation impedance

of each circuit, is also possible.

LV networks, using the IT system, which take

their origin at a MV/LV transformer, must be

protected against risks of insulation faults

between MV and LV by a «surge limiter».

Finally, to fix the potential of the LV network with

respect to the earth (short network supplied by a

MV/LV transformer), an impedance can be

installed between the transformer neutral and

the earth. Its value in 50 Hz, of the order of

1,500 W, is very high in DC and in very low

frequency so as not to obstruct insulation

measurement and fault locating.