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.
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.
- Portable generator sets
- Mobile generator sets for temporary installations
- Mobile generator set for fixed installation for one-off re-supply
- Mobile generator set for fixed installation for re-supply planned at the design stage
- Fixed sets for fixed installations
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.).
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.
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.
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 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.
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.
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 mm2. The installation will then operate as a TN-S or TT system.
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.
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.
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).
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.
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.
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.
Likewise, if circuit breakers are used, the magnetic operation adjustment (short delay) must be set to a low threshold.
Source // Electrical energy supply by Legrand