Published in Electrical & Power Review, AUGUST 2015
This article discusses earthing of low voltage electrical installations from the point of vide of IEC 60364
Apart from electrical installations, earthing finds importance with electronic or instrumentation and IT due to the usage of VLSI ICs and their associated failures or problems. Because of its application related with safety, it is analysed in various International and National standards related to specific context.
This article approaches earthing from the view point of IEC 60364-5-54-Ed3. The scope of this standard is limited to earthing arrangement and protective conductors including protective bonding conductors of low voltage installations i.e. 1,000 V AC or 1,500 V DC in order to satisfy the safety of electrical installations with cross reference from below standards. IEC 60364-5-2: EMC � installation and mitigation guidelines � earthing and cabling.
IEC 60364-4-41: 2005: LV electrical installations � protection for safety-protection against electric shock.
IEC 60364-4-44: 2007: LV electrical installations � protection for safety-protection against voltage and electromagnetic disturbances.
IEC 60364-5-51: 2005: Electrical installation of buildings � selection and erection of electrical equipment � common rules.
IEC 61140: 2001: Protection against electric shock � common aspects for installation and equipment.
IEC 62305 � 2010 (Ed.2): All four parts.
T � Direct connection of a point with earth (T � Terra in Latin)
I � Connection of a point with earth via high impedance (I � Isolated)
N � Direct connection to neutral at the origin of installation which is connected to earth.
TN is further classified into TN-S (separate)/TN-C (combined) and TN-C-S (Combined at source and separate at load). Earthing is intended to provide connection to earth which:
- Is reliable and suitable for the protection of the installation.
- Can carry earth fault I and protective conductor I to earth, without danger from thermal, thermo-mechanical and electro-mechanical
stresses and from electric shock arising from these I.
- If relevant, is also suitable for functional requirement.
- Suitable for the foreseeable external influences (IEC 60364-5-51) � e.g. mechanical stresses and corrosion.
- Where I with HF are expected to flow (cl 444 of IEC 60364-4-44: 2007).
- Protection against electric shock shall not be adversely affected by any foreseeable change of earth electrode resistance (example: corrosion, drying, freezing etc.)
- Type, dimension and material of earth electrode shall be selected to withstand corrosion and to have adequate mechanical strength for the intended life time.
- Main parameters for corrosion: Soil pH at the site, soil resistivity, soil moisture, stray and leakage AC and DC I, chemical contamination and proximity of dissimilar metals.
- Minimum thickness of protective coating is greater for vertical earth electrode because of greater exposure to mechanical stresses while being embedded than for horizontal earth electrode. Efficiency of earth electrode depends on its configuration and local soil conditions. Depending upon the value of earth resistance required one or more earth electrodes suitable for the soil conditions shall be selected.
- Types of earth electrodes.
- Concrete embedded foundation earth electrode.
- Soil embedded foundation earth electrode.
- Metallic electrode embedded directly in soil- vertically or horizontally (eg. Rods, wires, tapes, pipes, plates).
- Metal sheath of cables (according to local conditions)
- Other suitable underground metal work (eg. Metal water pipes) according to local conditions.
- Welded metal reinforcement of concrete (except pre-stressed concrete) embedded in the earth.
Dos and Don�ts
Earth electrode shall not be directly immersed in water of a stream, river, pond or lake. Earth electrode parts shall be connected together by exothermic welding, pressure connectors, clamps or other suitable mechanical connectors. The same is applicable for connection of earthing conductor to an earth electrode. Cross sectional area of earthing conductors shall be 6 sq.mm. for Cu or 50 sq.mm. for steel. Where lightning protection system is connected to earth electrode, cross sectional area of earth conductor should be 16 sq.mm. for Cu and 50 sq.mm for steel. Aluminium shall not be used as earthing conductor.
Where more than one earthing terminal is provided, they shall be interconnected. It shall be possible to individually disconnect each earth conductor connected to the MET. Disconnection shall be possible only by means of a tool. A protective conductor not forming part of a cable shall be mechanically protected if installed in conduit/trunking.
- Protective bonding conductors
- Earthing conductors
- Protective conductors
- Functional earthing conductors, if relevant
Types of protective Conductors
One of more of the following:
- Conductors in multi core cables.
- Insulated or bare conductors in a common enclosure with live conductors.
- Fixed installed bare or insulated conductors.
- Metallic cable sheath, cable screen, cable armour, wirebraid, concentric conductor and metallic conduit.
Joints in protective conductors shall be accessible for inspection and testing except for:
- Compound-filled joints
- Encapsulated joints
- Joints in metal conduits, ducting and busbar trunking system
- Joints forming part of equipment, complying with equipment standards
- Joints made by welding or brazing
- Joints made by compression tools.
Though, individual earth electrodes are commonly used, concrete embedded earth electrode has several advantages.
Erection of concrete embedded foundation earth electrode
Concrete used for building foundation has lower conductivity and large contact area with the soil. Bare metal electrodes completely embedded in concrete shall be used for earthing purposed except concrete isolated from soil by use of special thermal insulation for special applications. Due to superior chemical and physical effects, bare or hot dip G steel and other metals embedded in concrete to a depth of more than 5 cm are highly protected against corrosion, for the life time of the building. Wherever possible, the conductive effects of the reinforcement of the building should also be used.
By having a concrete embedded foundation earth electrode during the erection of the building is an economical solution to obtain a long lasting (life time), good earth electrode because:
- No need for additional excavation works.
- The depth of erection is free from negative influences resulting from seasonal weather conditions.
- It provides good contact with the soil
- It extends over all of the building�s foundation surface and results in min. Earth electrode impedance which can be obtained with that surface.
- It provides optimal earthing arrangement for LPS.
- From the beginning of the building construction, it can be used as earth electrode for the electrical installation of the construction activities.
Besides the earthing, concrete embedded foundation earth provides good reference for main protective bonding.
Other considerations for use of concrete embedded earth electrodes
Metal reinforcement of foundation of the building shall be used as an electrode which has strong connections. Pre-stressed reinforcement must not be used as an electrode. If welded grids made from wires of smaller diameter (min 5 mm) are used for reinforcement, it can be used as electrodes which should have atleast 4 connections between the terminal lug and grid. The wiring of the electrodes should not go over joints between different parts of larger foundations. At such places, suitable malleable connectors should be installed outside the concrete to provide electrical connections. Concrete embedded foundation earth electrode of single foundations (example large hall) should be connected to other parts of the concrete embedded foundation earth electrode by using suitable earth connectors.
IEC 60364-5-53 does not talk about any specific value of earth resistance. The lower is always better but is depends on various parameters as mentioned above. Special Emphasis should be given for bonding.
Published in Electrical & Power Review, AUGUST 2015