The use of small generating sets is typical on building sites and the like, and may still, less typically, be found within a private dwelling

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Connection of portable generating sets

Connection of portable generating sets
The use of small generating sets is typical on building sites and the like, and may still, less typically, be found within a private dwelling.

Generating sets having power ratings of less than 10 kW are generally single-phase, portable, machines and as such are commonly used to provide power supplies where it is uneconomical or impracticable to obtain a connection to the public supply network.
This article considers the connection of a single-phase portable generating set to provide the sole means of supply for:

The connection of a portable generating set to supply an individual item of equipment
Generators having a power rating of less than 10 kW generally operate as ‘floating’ systems, which means the generator supply is unearthed. As shown in Fig 2, the generator winding is not connected to the frame or to the general mass of Earth, but instead relies on the protective measure electrical separation for protection under earth fault conditions. Therefore, the requirements for electrical separation contained in Section 413 of BS 7671 must be satisfied.

As shown in Fig 2, a protective conductor must connect the exposed conductive- parts of the generator frame (and its enclosure) to the exposed-conductive-parts of the equipment being supplied. No exposed-conductive-part of the separated circuit is to be connected to any protective conductor or exposed conductive- parts of other circuits or to Earth otherwise the protective measure of electrical separation fails (Regulation 413.3.6).
In addition, as required by Regulation 413.3.3, no live parts of the separated circuit should at any point make contact with another circuit or with Earth or a protective conductor. In view of this, Regulation 413.1.2 restricts the use of this protective measure to an individual item of current-using equipment unless the separated circuit is under the supervision of skilled or instructed persons who can confirm the requirements of Regulation Group 418.3 are maintained.
These requirements include, among others, that a warning notice is fixed to every means of access to the generator and area concerned (Regulation 418.3). It should be noted that where more than one item is supplied, Regulation 418.3.8 limits the circuit length to a maximum of 500 m, which is constrained by the product of the nominal voltage and circuit length up to a maximum of 100 000 Vm. This means, for example, that a 230 V nominal voltage would limit the cable run to 434 m.
Whether the separated circuit supplies one or more items of current using equipment use of a separate wiring system is recommended (Regulation 413.3.5). To minimise the risk of electric shock from cable insulation damage, flexible cables such as a type conforming to H07RN-F (BS EN 50525 series) should be installed, and the distance between the generator and the load it supplies should be kept as short possible (clause 7.1.1 of BS 7430: 2011 Code of Practice for Earthing refers). 
The connection of a portable generating set to supply a fixed installation
An unearthed generating set may also be used to supply a fixed installation, however, the protective measure automatic disconnection of supply (ADS) should be employed in accordance with Section 411 of BS 7671, as recommended by clause 7.1.3 of BS 7430: 2011.

As shown in Fig 3, a TN arrangement is configured by connecting one pole of the generator to the main earthing terminal of the installation (MET), and the protective measure ADS is provided by confirming in accordance with BS 7671 that:

  • all exposed-conductive-parts and all extraneous-conductive-parts are connected to the MET,

  • the MET is connected to an earth electro de, which satisfies the requirements of Table 41.5, and,

  • RCDs are installed to provide ADS.

Regulation 551.4.4.2 of BS 7671, requires every circuit supplied by the generator to be provided with additional protection by the means of an RCD having the characteristics specified in Regulation 415.1.1 – having a rated residual operating current (IΔn) not exceeding 30 mA and an operating time not exceeding 40 ms at a residual current of 5 (IΔn).
To check that the requirements of Regulation 411.4.5 for automatic disconnection in the event of a fault are met, the external earth fault loop impedance (Ze) must be determined.
Ze can be found approximately by dividing the nominal line-to-earth voltage (U0) of the generator by the prospective short-circuit current at the generator terminals. As a rule of thumb the short-circuit current may, in the absence of more accurate information, be assumed to be three times the rated output current of the generator. This level of current lasts for only a few cycles for most small generators, after which it quickly decays but this should be long enough to operate a non-delay RCD.
Thus, for example, the Ze value given by a generator with a rated voltage of 230 V and a rated current of 15 A would be approximately equal to 230 ÷ (3 x 15) = 5.1Ω.
The value of Ze must be added to the highest value of R1 + R2 for a circuit to give the value of Zs at the most remote point of the circuit. 
Where such a small-scale generating set is utilised to supply part of an existing installation, it is clear that the external earth fault loop impedance will be significantly larger than that which is normal for a supply from the local DNO.
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