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Earthing of generator set

Generator sets have specific features that must be taken into account for protection against electric shocks. Mobile sets cannot be connected to earth and their connection by means of a flexible cable can be easily damaged.

Neutral earthing systems of generator sets (gensets)
Neutral earthing systems of generator sets (gensets) (photo credit: cat.com)

In general, generator sets have much lower short-circuit levels than transformers (around 3×In instead of 20×In). As a result, the tripping conditions required for protection against indirect contact cannot be provided by devices sized for operation on the normal supply.

Contents:

    1. Portable generator sets
    2. Mobile generator sets for temporary installations
    3. Mobile generator set for fixed installation for one-off re-supply
    4. Mobile generator set for fixed installation for re-supply planned at the design stage
    5. Fixed sets for fixed installations
      1. Conditions for protection against short circuits and indirect contact

1. Portable generator sets

For temporary installations limited to a few kVA, these supply directly a small number of receivers (market stall, kiosk, power supply for portable tools, etc.).

The exposed conductive parts of the set and those of the installation must be linked together by means of a protective conductor. Each outgoing circuit must be protected by a residual current device i∆n ≤ 30 mA.

If the set has one or more power sockets without a protective RCD, there should be one RCD per circuit at a distance of less than 1 m. as earthing is not possible and the neutral pole is not accessible, the installation will operate as an IT system.

Portable generator set neutral earthing system
Figure 1 – Portable generator set neutral earthing system

If the generator set supplies class II devices, the exposed conductive parts are not linked but the provision of one or more RCDs remains mandatory for supplementary protection against direct contact, in particular on the flexible connecting cable.

By the way, class II devices are devices where exposed conductive parts of these devices must not be connected to a protective conductor.

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2. Mobile generator sets for temporary installations

With powers greater than 10 kVA, these supply larger installations (building sites, carousels, circuses, etc.). The exposed conductive parts of the set must be linked to the exposed conductive parts of the devices being used by means of a protective conductor.

Protection against electric shocks is provided by a residual current device i∆n ≤ 30 mA protecting all the outgoing lines, usually incorporated in the set by construction.

If there are requirements for differential discrimination between the circuits supplied, secondary residual current devices i∆n ≤ 30 ma can be installed on each outgoing line, as long as they are at a distance of less than 1 m.

If there is a possibility of establishing a reliable earth connection, the installation can operate in TN-S system mode. The fault current is closed by the neutral or by linking the exposed conductive parts if the neutral is not distributed. This is possible for three-phase loads only, and enables three-pole devices to be used. In this case this is a TN-S system with non-distributed neutral, which should not be confused with a TN-C system.

If there is no earth connection established on the set, the installation will operate as an it system. The breaking and protection devices must have staggered opening of the neutral with protection of all the poles. In addition, the cross-section of the neutral must not be reduced.

NOTE! The installation and setup of generator sets are subject to specific regulations on the characteristics of the areas, the discharge and pollutant levels of the exhaust gases, and the permissible noise. It is advisable to refer to these regulations with the assistance of the manufacturers and competent bodies.

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3. Mobile generator set for fixed installation for one-off re-supply

Temporary one-off re-supply of a fixed installation instead of the mains supply or the usual power supply should only be carried out after isolation.

Manual opening of the main circuit-breaker generally provides this separation, as long as it is held in position (locking, padlocking) or indicated by a warning sign.

In all systems (TT, IT, TN) the exposed conductive parts of the generator set must be interconnected with the earth network of the existing installation. If a local earth connection can be established for the set’s neutral, the earth must be interconnected with the equipotential link of the installation.

If, as is often the case, this operation is not possible or not carried out, the installation will operate as an it system if the generator’s neutral is not accessible.

If the generator’s neutral is accessible, it must be linked to the protection circuit of the fixed installation via a protective conductor (with an identical cross-section) incorporated in the cable or via a separate cable sized for the fault conditions, with a minimum copper cross-section of 16 mm2The installation will then operate as a TN-S or TT system.

Mobile generator set for fixed installation for one-off re-supply
Figure 2 – Mobile generator set for fixed installation for one-off re-supply

Important Caution: – In TN or IT systems, protection against indirect contact may not be provided. in installations that are to be re-supplied by a mobile generator set, a sign must be placed close to the connection point, with the wording:

Minimum power of the set to be installed: x kVa.

In all systems (apart from TN-C), the provision of a residual current protection device is recommended. The residual current toroid sensor must be placed downstream of the earthing of the neutral point (see Figure 2) or on the generator’s neutral point earth conductor.

If the generator is a power supply for safety services, the earthing system used will be the it system.

Mobile generator set for fixed installation
Figure 4 – Mobile generator set for fixed installation

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4. Mobile generator set for fixed installation for re-supply planned at the design stage

When the re-supply of a fixed installation in place of the mains supply or the usual power supply is planned at the design stage, an all-pole supply inverter must be installed.

Irrespective of the neutral earthing system of the fixed installation, it is necessary to interconnect the exposed conductive parts (TT, IT), the neutral point of the set and the exposed conductive parts of the set (TN) to the exposed conductive parts of the existing installation.

If the protection conditions are not met by the overcurrent protection devices (IT and TN systems) or cannot be determined (see Figure 5), a high sensitivity residual current device (30 mA) must be used and the neutral earthed upstream of the RCD (see diagrams below).

Rules for installing a mobile generator set for fixed installations
Figure 5 – Rules for installing a mobile generator set for fixed installations

In TT systems, an RCD must be used in all cases. The part upstream of the residual current device must have double or reinforced insulation. The toroid sensor must be placed on all the live conductors (phase + neutral) or on the conductor linking the neutral point on the alternator to the earth of the installation (TT or TN-S).

This solution is not applicable in TN-C systems.

When a generator set supplies a standalone installation with no power sockets or whose continuity of service is paramount (machine, crane, carousel), it is permissible not to install a residual current device as long as the conditions for protection against indirect contact are met in line with the chosen neutral earthing system.

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5. Fixed sets for fixed installations

If the set is a replacement supply, it must use the same neutral earthing system as the normal supply.

The conditions for protection against indirect contact and tripping for minimum short circuits must be checked, and must be met each time the installation is supplied by the normal supply and by the generator set.

Safety installations should preferably be created with it systems or under TN system conditions.

Caterpillar fixed generator
Figure 6 – Caterpillar fixed generator

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5.1 Conditions for protection against short circuits and indirect contact

The setting or rating of the overcurrent protection devices that provide protection against indirect contact when using a neutral earthing system for a generator set must be chosen with care. the low value of the fault current is not always compatible with the fuse breaking time.

The rated current in of these fuses and that of the generator must be similar and it is essential that the tripping conditions are checked.

Likewise, if circuit breakers are used, the magnetic operation adjustment (short delay) must be set to a low threshold.

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Source // Electrical energy supply by Legrand

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author-pic

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

2 Comments


  1. K. THIRUMAL RAO
    Nov 12, 2018

    The illustration is very brief and understandable.
    Some more details of IT, TT, TNS & TNC may please be covered in the same article.
    Generally temporary DG sets are provided with unbalanced load, providing ELCB may result in spurious tripping leading to commotion during emergency operation.


  2. Myo hlaing
    Nov 12, 2018

    I want a menber.

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