NAPIT’s Bill Allan answers a common technical query regarding best practice requirements for installations at caravan sites.
A recent enquiry to the NAPIT technical helpline raised an interesting question about sub-main cables with a common neutral conductor that we felt we should share with readers of PE. It was as follows:
I’m working on a caravan site and a previous contractor has installed a 3 core swa sub-main cable for two caravan supply hook ups. The sub-main is run from a single-phase distribution board at the intake position of the site and it contains two line conductors and a common neutral. Each of the line conductors has a separate circuit -breaker. This installation doesn’t seem right to me. Can you direct me to the relevant regulations?
From the information supplied, this installation is a distribution circuit (i.e. a sub-main) which has been intentionally designed with the neutral conductor common to two separate single-phase supplies, each taken from the same phase. Therefore, the neutral conductor is not what is commonly known as a ‘borrowed neutral’.
Although this question pertains to a caravan site, it is of general interest as this type of installation could, in theory, be used in other locations.
Therefore, we will begin by answering this question in a general way. We will then apply this answer specifically to caravan sites.
Installations with a common neutral conductor
Fig. 1 shows the basic arrangement described with the protective conductor and any isolators and RCDs omitted for clarity. We would emphasise that this arrangement cannot comply with BS 7671 and we include it only to help our readers understand the question.
Fig. 1 shows a 3 core sub-main cable with a common neutral conductor shared between two loads. This is not a standard supply system in the UK and raises a number of concerns. Perhaps the most obvious concern is that it does not comply with Section 314, Division of installation, in BS 7671.
Section 314 , Division of installation
Regulations 314.1 and 314.2 include distribution circuits such as those described in the question.
Regulation 314.1 requires that every installation is to be divided into circuits and it gives a number of reasons for this. One reason is to avoid danger and minimize inconvenience in the event of a fault.
Regulation 314.2 states that separate circuits must be provided for parts of the installation which need to be separately controlled in such a way that those circuits are not affected by the failure of other circuits and due account must be taken of the consequences of the operation of any single protective device.
It is evident that Fig. 1 cannot comply with Regulations 314.1 and 314.2.
This means that the common neutral arrangement may not comply with certain basic requirements of BS 7671, such as those for isolation and switching (Chapter 46), overcurrent protection (Chapter 43), protection against electric shock (Chapter 41), the cross-sectional area of neutral conductors (Regulation 524.2) and voltage drop (Section 525).
Where a common neutral conductor is normal
In three- phase and neutral systems, single neutral conductors are normal, even if the three line conductors supply different single- phase loads. An instance of this applying to luminaires is found in Regulation 559.5.5.
Section 708 in BS 7671, Electrical Installations in Caravan/Camping Parks and Similar Locations
We will now turn our attention to the requirements of Section 708. As the type of supply system is not mentioned in the question, we will assume that it is a PME supply to the main intake position of the caravan site.
As the Electricity Safety, Quality and Continuity Regulations (ESQCR) prohibit the connection of a PME earthing facility to caravans, the supply to the caravan pitches will have to be part of a TT system that has a separate connection to Earth independent of the PME earthing. The separation of the earthing can be achieved by installing an earth electrode adjacent to the intake position to make the sub-main distribution to the pitches part of a TT system.
The alternative is to install an earth electrode adjacent to the caravan pitches.
Given the concerns discussed above, one option would be to change the wiring of the 3 core sub-main cable from its present shared neutral arrangement to make the three conductors line, neutral and protective conductor. Identification of the conductors would be required in accordance with Section 514 of BS 7671. If the cross-sectional area of the sub-main cable is adequate, the cable could be used to supply both caravans, or even more (if required). This arrangement is shown in Fig. 2.
Notice that the supply to each socket-outlet which provides a supply to each individual caravan has an overcurrent protective device in accordance with the requirements of Chapter 43 (Regulation 708.533).
Fig. 2 also shows a 30mA RCD capable of disconnecting both line and neutral conductors at each socket-outlet (Regulation 708.415.1).
If the sub-main cable had a shared neutral
If the sub-main cable had a neutral conductor which was shared with another circuit and the overcurrent protective device were to operate, the common neutral conductor would not be broken as the overcurrent device is in the line conductor only. Therefore, although the socket-outlet served by that particular overcurrent device would lose its 230V supply, the neutral conductor, since it has not been isolated, may carry some returning current from the load on the second socket-outlet and present a shock risk at the ‘isolated’ socket-outlet .
If the sub-main cable had a shared neutral conductor, reliable operation of the RCD could not be guaranteed.
Conclusion
There are no doubt other points which could be made and other objections which could be raised against this common neutral arrangement. Space limitation prevents us from going into more detail in this article. Suffice it to state here that this sub-main cable is not suitable and must be changed as soon as possible. As always, the onus is on the installer to confirm that an electrical installation complies with all the relevant requirements of BS 7671.
